A. G. Sivov

Lower Cambrian of the Western Sayan

PUBLISHING HOUSE OF THE POLYTECHNIC INSTITUTE

Tomsk
1953

Introduction

The importance of the Cambrian deposits involved in the geological structure of the Western Sayan can hardly be overestimated. Suffice it to say that various intrusions, differing in their petrographic composition, and the diverse mineralization genetically associated with them, are confined to these formations, which occupy about one-third of the entire area of the Western Sayan.

However, the stratigraphic schemes of the Cambrian system of the Western Sayan, developed at the time on the basis of small-scale geological surveys and without proper paleontological foundations, are far from perfect and are in obvious contradiction with the new data obtained in recent years. A characteristic example of this may be the deposits of the so-called West Sayan formation, or the third upper Cambrian sequence, which were previously regarded as the uppermost members of the Cambrian of the Western Sayan and assigned to the Middle Cambrian, whereas now, according to the materials of the most recent studies, they—at least in the areas of their typical development—prove to be Lower Cambrian and form the base of the named system.

Moreover, these new data also indicate that the West Sayan formation is not homogeneous, but represents a more complex formation consisting of several independent stratigraphic units, separated from one another by clearly expressed tectono-denudational breaks. In the deposits of some of these stratigraphic units, Lower Cambrian organic remains are often found, which substantially refine the age of the fossil-bearing rocks. In this connection, the age of the hyperbasite intrusion of the Western Sayan is determined much more accurately, and the Precambrian age of the well-known Dzhebash formation, widely distributed within the latter, is established with greater confidence. An unequivocal solution of these questions is not only of great theoretical interest, but also of important practical significance.

That is why I considered it possible, first of all, to set forth in the present work materials on the stratigraphy of the Lower Cambrian of the Western Sayan, based chiefly on my personal long-term investigations in the named mountain massif. I was also prompted to do so by the circumstance that a tendency has again appeared in the literature to expand the volume of the Lower Cambrian at the expense of formations alien to it, distributed along the northwestern facies of the Western Sayan \[9, 103–104\] and at one time distinguished as the Lower Monok and Upper Monok formations of the lower part of the Middle Cambrian. Such a combination is not only a step backward in understanding the geology of the Western Sayan, but also disorients practitioners in solving a number of important practical tasks connected with the search for and exploration of mineral resources.

...graphs, and the trilobites that appear only rarely constitute the subject of my special work.

The present work, carried out at the Department of Historical Geology and Paleontology of the S. M. Kirov Tomsk Polytechnic Institute, represents one of the sections of the theme “The Cambrian of the Western Sayan,” approved by the USSR Ministry of Higher Education.

On the West Sayan Formation of the Western Sayan

Before proceeding to consider the factual material on the stratigraphy of the Lower Cambrian, it became necessary to discuss specially the question of the position in the Cambrian stratigraphic scheme of the Western Sayan of the well-known West Sayan formation and of the hyperbasite intrusion characteristic of it.

The motives that prompted the posing of this question, and its significance for elucidating the stratigraphy of the Lower Cambrian, I shall try to show in the subsequent exposition.

As is known, the prototype of the West Sayan formation was established by M. A. Usov as early as 1936, on the basis of materials from geologists who had conducted investigations in the Tabat region of the northern slope of the Western Sayan. The named author believed that the lower horizon of the formation is composed of algal limestones, sometimes with remains of brachiopods and trilobites which, being close to the Upper Torgashino forms, indicate the lower half of the Middle Cambrian [47, 78–79; 43, 61–63]. In the overlying horizon there predominate effusives of intermediate and basic composition, of greenstone aspect, interbedded with dark sandy-argillaceous shales, at times intensely chloritized and consequently possessing a green coloration. In the upper horizon the sandy-argillaceous formations acquire predominant importance, and among them there appear many marly, sometimes dark fetid limestones, often metasomatically replaced by siliceous solutions, with their transformation into siliceous rocks.

In other sectors of the northern slope of the massif, the deposits of the West Sayan formation are more deeply metamorphosed, so that in the Yenisei and Kantegir regions some investigators regarded them as Precambrian and assigned them to the I division of the metamorphic sequence.

At the same time, M. A. Usov considered that the plutons of the hyperbasite intrusion, widely distributed in the Western Sayan, are situated in their main mass within the field of development of the West Sayan formation and are characteristic stratigraphic documents for it [43]. They are expressed by a variegated assemblage of rocks, from peridotites to saussuritized and amphibolitized gabbro, with rare development of their differentiates, such as albitite-aplites (Tabat region) and peculiar hornblendites of the issite type, strongly actinolitized [47, 79–82]. At the same time M. A. Usov emphasized that the silica-rich solutions derived from the hyperbasite magma caused vigorous silicification of the country rocks, with the formation of peculiar metasomatic silicilites [47, 79; 43, 62], so widely represented in the composition of the West Sayan formation.

The sedimentary-pyrogenic West Sayan formation is characterized by a similar composition also on the southeastern slope of the Western Sayan, in particular in the Usinsky and Amyl regions, where it is likewise intruded by comparatively large plutons of hyperbasites.

In the Tabat sheet, the West Sayan formation, in the opinion of M. A. Usov, lies upon the eroded surface of the Arbat formation [47, 77], or suite in the understanding of earlier investigators [29, 176], and is overlain by deeply metamorphosed rocks recognized as Lower Silurian.

The latter, in other areas of the Western Sayan, rest upon hyperbasites without a basal conglomerate and, in connection with sharp tangential movements, were closely planed against serpentinites, which created the idea of the intrusion of hyperbasites into the Lower Silurian sequence, in the Takonian or initial phase of the Erian folding [47, 82; 43, 62]. However, M. A. Usov noted that in the Usa district it had been possible to establish, beneath the Lower Silurian, the presence of an independent sedimentary formation (the Kemterek, in my understanding), which is situated on the eroded surface of the Middle Cambrian with hyperbasites [47, 32 and 83].

These ideas about the Middle Cambrian age and composition of the Western Sayan formation became firmly established in the geological literature and even found a place in a textbook on historical geology [12].

However, as early as 1937 I argued that earlier investigators had erred in determining the stratigraphic position of the upper Cambrian sequence [16, 5], which is one of the analogues of the Western Sayan formation. Somewhat later, using the example of the southwestern part of the Tabat district, convincing materials were presented showing that another analogue of this formation, the Uzunarga sequence, is transgressively and sharply unconformably overlain, with a basal conglomerate at its base, by the Arbat Formation, with which I had previously identified the Kemterek sequence of the southeastern slope of the Western Sayan. Then, having confirmed the earlier observations of Yu. A. Kuznetsov [17, 24] and others, who had established beneath the Arbat Formation a thick pyroclastic-sedimentary sequence—the first lower Cambrian sequence [16, 5]—I subdivided the latter into two independent formations, the Lower Monok and Upper Monok, whose age was at that time determined within the lower part of the second half of the Middle Cambrian [39, 128–134]. Finally, it was shown that the Lower Monok effusives cut the deposits of the Uzunarga Formation, while pebbles of rocks of the latter, with clearly expressed relict tectonic structures [33], are already found in the basal conglomerate of the Upper Monok Formation [39]. This served as the basis for regarding the Uzunarga Formation as the oldest member of the Cambrian, at least not younger than the Lower Cambrian, which was also supported by paleontological evidence. Thus, in limestones of the M. Arbat River, which, as it seemed to me at one time, constituted the lower parts of the Uzunarga Formation, remains of archaeocyaths were found; these were recognized as belonging to the Kameshkov complex, assigned to the upper parts of the Lower Cambrian [38, 136].

Close in composition and age to the archaeocyaths of the M. Arbat River are the archaeocyath associations of the B. Kyzas River, found there in a carbonate horizon situated within the shale rocks of the third upper Cambrian sequence [39, 135]. At the same time, it was known from the literature that the archaeocyath limestones of the M. Arbat River, with some breaks in the outcrops, can be traced to the northeast and, joining there with the trilobite-algal limestones of the Babak Beds (the lower horizon of the prototype of the Western Sayan formation), form one synchronous horizon. In this connection, the Babak algal-trilobite horizon and the two overlying upper components of the prototype of the Western Sayan formation began to be regarded by me as Lower Cambrian, and not Middle Cambrian, as M. A. Usov had determined them on the basis of the trilobite fauna.

Thus, it seemed to have been possible to prove that the prototype of the Western Sayan formation and its analogues—the third upper Cambrian sequence and the Uzunarga Formation—are synchronous formations with an age no younger than the Lower Cambrian, as a result of which the need to retain the last two names disappeared.

Somewhat later, the stratigraphy of the Cambrian of the southern slope of the Western Sayan was refined. As a result of detailed geological investigations of the south-

of the western margin of the Kurtushiba Range, the Uzunarga formation known here, an analogue of the West Sayan formation [34, 406], was subdivided into two independent stratigraphic units separated from one another by unconformities. The lower of these, composed of prasinite and graphitized schists, with members of quartzites and dolomitized algal limestones, was given the name Akkol formation. The upper, represented by terrigenous-effusive, strongly schistose, metamorphosed schists, with subordinate beds of microquartzites and lenses of limestones, sometimes with very poorly preserved remains of algae and archaeocyaths, was named the Chinginskaya formation [39, 136].

In addition, after carrying out short-term investigations in the basin of the upper course of the Elekhem River, a left tributary of the Ulug-Khem, I confirmed earlier observations by Z. A. Lebedeva indicating that the archaeocyathid limestones widespread on the Aktovrak elevations (the watershed between the Temersuk and Elekhem) are cut by hyperbasites and, in contact with the latter, are transformed into a characteristic bluish-gray siliceous rock [24, 120].

The archaeocyaths collected from the above-mentioned limestones, among which were Ajacicyathus otichtaschensis and other forms, proved comparable with the known Kameshkovsky archaeocyath complex. At that time I had no opportunity to establish the nature of the relationships between the Aktovrak limestones and the deposits of the Chinginskaya formation; nevertheless, even then there was no doubt that the former occupy a higher stratigraphic position in the Lower Cambrian section.

On the northern slope of the Western Sayan, synchronous deposits of the Chinginskaya formation were established without particular difficulty. I correlated with it lithologically similar sediments of the Uzunarga formation [38, 27], extending in an unbroken belt from the headwaters of the M. Arbat River in the northeast to the upper reaches of the B. Kyzas in the southwest. On the northern slope, analogues of the Aktovrak limestones were also established on sufficient grounds; with them were correlated the limestones of the M. Arbat and B. Kyzas rivers, both there and here characterized by remains of archaeocyaths of the Kameshkovsky complex, as well as the limestones of Babak Spring, the correlation of the latter being made not only for the reasons set forth above, but chiefly with account of new data that had appeared by that time.

The fact is that in 1946 a collection of archaeocyaths was delivered to the paleontological office of the West Siberian Geological Administration; according to the document attached to it, the collection had been gathered on the direct continuation of the limestones of Babak Spring.

According to the identification by P. S. Krasnopeeva, among the archaeocyaths there was Ajacicyathus otichtaschensis, which, besides the limestones of Babak Spring and the Aktovrak elevations, was also established in the limestones of the B. Kyzas River and was recognized as an important correlative form for the Kameshkovsky archaeocyath complex.

But when correlating the stratigraphic members of the Lower Cambrian of the southern and northern slopes of the Western Sayan, certain contradictions arose which, naturally, required elimination.

The chief contradiction consisted in the fact that on the southern slope the limestones with the Kameshkovsky archaeocyath complex were considered to occur in the upper parts of the Chinginskaya formation (Aktovrak elevations), whereas on the northern slope the relationships between the named members were regarded as the reverse (Babak Spring, M. Arbat River). This contradiction proved easily removable when the indications of K. I. Bogdanovich [5, 274] and other investigators were taken into account, testifying to the overturning of the metamorphic sequences of the northern slope of the Western Sayan to the north.

In accordance with this conception, it could be assumed that the stratigraphic members of the Lower Cambrian (the West Sayan Formation) of the northern slope occur in overturned bedding and that, consequently, the limestones of Babak Spring (the lower horizon of the West Sayan Formation according to M. A. Usov) and of the M. Arbat River, with the Kameshki archaeocyathids, are the very uppermost members of the division, and not the reverse, as M. A. Usov once believed. With such an interpretation of the stratigraphic position of the limestones of Babak Spring it became obvious that the middle and “upper” horizons of the type section of the West Sayan Formation constitute the lower members of the latter, and I began to correlate them with the deposits of the Chingin Formation, or its analogue, the Uzunarginskaya Formation, which, in turn, were recognized as older than the limestones of the M. Arbat River.

Having recognized the limestones with the Kameshki complex of archaeocyathids (the M. Arbat River, Babak Spring, the Aktovrak hills) as synchronous, I began to combine them into a single stratigraphic unit, for which, for a number of reasons, the name West Sayan Formation was retained; moreover, between it and the underlying Chingin Formation the presence of a break and an unconformity was assumed. I shall dwell briefly on the grounds that prompted me to advance the latter supposition.

I recall that in the Aktovrak hills the limestones with the Kameshki archaeocyathids, united into the West Sayan Formation, are cut and metamorphosed by hyperbasites. Then, as early as 1940, on the northern slope of the Western Sayan, in the Kair Range, I distinguished the Kair microdiorite (metadiorite) intrusion, which cuts and metamorphoses the deposits of the Uzunarga (Chingin) Formation [38, 27; 39, 140]. The Kair microdiorite intrusion was tentatively regarded by me as more ancient than the hyperbasite intrusion, which, in turn, was considered older than the Maina trondhjemites of the middle part of the Middle Cambrian.

Analysis of the indicated relationships among the listed intrusive formations inevitably led to the conclusion that during the Lower Cambrian epoch there were two phases of tectogenesis, the earlier of which manifested itself in post-Chingin time and was accompanied by the formation of microdiorites. The later phase occurred in the post–West Sayan age (after the accumulation of the limestones with the Kameshki complex of archaeocyathids), and the emplacement of the hyperbasites was associated with it.

In connection with the foregoing, the supposition of a break and unconformity between the West Sayan and Chingin formations acquired the significance of certainty. This West Sayan Formation, composed only of limestones with remains of archaeocyathids and trilobites, I began to assign to the upper parts of the Lower Cambrian, which was in full accord with ideas concerning the age of the Kameshki complex of archaeocyathids and which the trilobites of Babak Spring, recognized as more ancient than the trilobites of the Sanashtykol limestone—whose age at that time was beginning to be determined within the lower part of the Middle Cambrian—seemed not to contradict.

Such a detailed presentation of certain materials concerning the stratigraphy of the type section of the West Sayan Formation and its analogues is occasioned by the necessity of clarifying the stratigraphic scheme placed in one of my works, published in 1948. In that scheme [39, 136] the type section of the West Sayan Formation was recognized as equal in volume to the Lower Cambrian division, subdivided into three formations, of which the first two lower ones were assigned new names.

For the upper Lower Cambrian member, restricted in volume to the archaeocyathid-trilobite limestones, the name West Sayan Formation was retained on the grounds that it is cut by hyperbasites, which at that time were regarded as the most important stratigraphic documents for this formation [20; 22; 43].

However, in his later work, published in 1950, the former name (West Sayan Formation) was not retained for the upper Lower Cambrian member; instead, it was given a new name—the Babak Formation [40], and at the time it did not seem possible to dwell on the reasons that prompted replacing one name with another.

It is quite natural that, for these reasons, it is necessary to pause here, at least in general terms. A. N. Churakov, relying on a number of general considerations, had already suggested the possible pre-Cambrian age of the hyperbasites of the Western Sayan and of the deposits containing them, which that investigator compared with the well-known Kuten-Buluk Suite [54, 152–153]. The unsoundness of A. N. Churakov’s views on the question under discussion was so obvious that there was no need to dwell on them. I had no doubt that hyperbasites cutting paleontologically characterized Lower Cambrian deposits could not be older than the latter. The error of Z. A. Lebedeva, who insisted on a Caledonian age for the hyperbasites of the southern slope of the Western Sayan, was pointed out in due course by V. A. Kuznetsov [42; 18; 19].

But, in turn, V. A. Kuznetsov cited what at that time seemed to me sufficiently convincing evidence for the Middle Cambrian age of the hyperbasites of the southern slope of the Western Sayan. On the right bank of the Ulukhem River, near the town of Shagonar, he distinguished an effusive-sedimentary sequence with remains of the genus Paradoxides, which, comparing it with the Aktovrak Formation previously established by him, he recognized as an analogue of the prototype of the West Sayan Formation as understood in terms of the volume and composition of the latter by M. A. Usov [20; 22, 28].

Although the erroneousness of correlating the effusive-sedimentary sequence with Paradoxides with the prototype of the West Sayan Formation was obvious to me, there were no grounds to doubt the Middle Cambrian age of the former. Consequently, it remained to acknowledge that in the Western Sayan there are two hyperbasite intrusions of different ages: one cuts the West Sayan (Aktovrak) Formation of Middle Cambrian age, while the other cuts the same West Sayan Formation, but of Lower Cambrian age. It is known that the presence in local stratigraphic nomenclature of several stratigraphic units with identical names but with different volumes, composition, and especially age always gives rise to all kinds of misunderstandings when geological materials are generalized; therefore I considered it possible not to retain the former name—West Sayan Formation—for the upper Lower Cambrian member, and assigned it a new name—the Babak Formation. This renaming also had to be made because the hyperbasites of the Western Sayan, as it seemed to me, being of different ages, could no longer be regarded as the most important stratigraphic documents only for the West Sayan Formation as understood by M. A. Usov in terms of its composition and age.

At the same time, the elimination, by the methods described above, of all particular inaccuracies in the stratigraphy both of the prototype of the West Sayan Formation and of its analogue in the Lower Cambrian made it possible, as it seemed to me for a time, to regard these stratigraphic units not only as commensurate in volume and age, but also to consider the refined section of the first of them as the reference section for the second. But these ideas proved to be in sharp contradiction with views that appeared in the geological literature and revived earlier notions of the Middle Cambrian age of the limestones of the Babak locality, which moreover came to be regarded as the youngest stratigraphic members of the Cambrian of the Western Sayan.

In fact, N. S. Zaitsev and N. V. Pokrovskaya demonstrated that the weakly altered effusive-terrigenous deposits (the authors’ nomenclature)

the Tabat River and the B. Kyzas, and more deeply metamorphosed rocks of the West Sayan formation are supposedly close in age to formations falling within the limits of the Lower Cambrian division. In addition, one of the authors mentioned, N. V. Pokrovskaya, categorically asserted that Middle Cambrian trilobites are allegedly found in the limestones of the Babak klippe, while Lower Cambrian ones occur in the limestones of the Sanashtykgol klippe, and moreover no younger than the Protolenus zone [9, 104].

I had no doubt that N. S. Pokrovskaya had made serious errors in her “new” stratigraphic constructions, combining into the Lower Cambrian division strata differing in degree of metamorphism and regarding the limestones of the Babak klippe, whose position in the section she had not established, as younger than the limestones of the Sanashtykgol klippe. However, realizing that my evidence in this direction, partly set forth above, could not be exhaustive, I set myself the task of personally studying the section of the Tabat River and, on the basis of its study, trying to resolve the new disagreements that had arisen regarding the stratigraphy of the Cambrian of the Western Sayan in general, and of its Lower Cambrian division in particular.

Likewise, it seemed to me that study of the section of the Tabat River would make it possible finally to decide the question of whether the standard section of the prototype of the West Sayan formation could be adopted as a reference section of the Lower Cambrian division in the sense of its volume set forth above.

And so, while taking part in detailed geological investigations carried out in the basin of the upper course of the Tabat River, I collected a sufficiently rich geological material that makes it possible to illuminate the questions of interest to us on a new factual basis.

For this purpose I shall briefly dwell on the description of the section along the Tabat River and characterize in more detail the geological structure of the Babak and Karamysel klippes, where the prototype of the West Sayan formation was first established.

The section along the Tabat River is, on the whole, simple. Along its right slope, upstream from the village of Tabat, the Maina trondhjemites are exposed; 1.5 km below the mouth of the Chazyryk River they cut and metamorphose the keratophyres and porphyrites of the Lower Monok formation, or of the Novoprosvetskaya formation—in M. A. Usov’s understanding [47, 44].

On the Lower Monok effusives, and in places directly on the Maina trondhjemites (headwaters of the Kuskun klippe), with a basal conglomerate at the base, lies a sandy-shaly sequence, formerly called the Arbat formation. The rocks of the latter, at the mouth of the Chazyryk River, are gathered into a steep synclinal fold of ejective type, slightly overturned to the north.

From beneath the Arbat sequence effusives again appear, extending along the right slope of the Chazyryk River as far as the mouth of the Babak klippe, which enters the former from the left, 9 km above the village of Tabat. The effusives, being represented in the lower parts of the section by keratophyres and in the upper parts by diabase porphyrites (spilites), belong to the same Lower Monok (Novoprosvetskaya) formation and are intruded here by the Maina intrusion, small plutons of which are shown on the geological map of previous investigators near the mouth of the Babak klippe.

Along the Babak klippe, my study of the sections was carried out with the participation of K. V. Radugin, and included examination of a large number of mine workings, which often expose the contacts of different orders of stratigraphic members (Fig. 1).

At 200 m above the mouth of the Babak klippe, on its right slope, at a height of 3 m above the valley floor, dark, sometimes agate-black and more rarely greenish lydite crops out, dipping northwest \(350^\circ\) at an angle of \(65^\circ\). On the footwall side of the lydites, exposed by a mine working, there is a bed up to 0.3 m thick of sedimentary breccia, consisting of small angular fragments of gray limestone and well-rounded pebbles of dark phyllites, ze—

...of Paleostone effusives and grains of serpentinite. The breccia, cemented by siliceous material, also dips to the northwest, 350°, at an angle of 65°.

Then, from beneath the breccia, massive light-gray limestones appear, filled with remains of Epiphyton sp., with a very rare presence of trilobites, which occur in beds lacking algae.

At the foot of the slope of Babak Creek, in the limestones once called Babak limestones, bedding is not observed; the planes of sharp fracturing expressed in them dip predominantly steeply to the southeast and strike in an east-northeasterly direction. Upstream along the creek the limestones can be traced for a distance of not less than 265 m; here they are

Fig. 1. Diagram of the relationships of the Babak limestones and Lower Monok effusives along Babak Creek (plan).

1—Early Variscan (?) diorite-diabases; 2—spilites; 3—lydites; and 4—sedimentary breccia of the Lower Monok Formation of the lower part of the Middle Cambrian; 5—limestones of the Babak Formation of the upper part of the Lower Cambrian; 6—contacts traced, and (7) inferred.

cut by a dike of quartz diorite, dipping to the southeast, 135°, at an angle of 65°. In the hanging wall of the dike, diabase porphyrites and their amygdaloidal varieties of dark-greenish color and massive structure are developed. Among the latter, in places, there are also interbeds of lydites, dipping to the southeast, 140°, at an angle of 45–50°. These diabase porphyrites extend upstream along the right slope of Babak Creek up to its headwaters. When tracing the Babak limestones northeastward, almost at the very side of the creek, a distinct bedding is established in them, caused by the alternation of dark-gray finely banded algal limestones and light-gray massive varieties of the latter, lacking algae but sometimes containing remains of trilobites and archaeocyaths.

The bedding planes strike in a latitudinal direction and dip either to the northeast, 65°, or directly east at an angle of 70°. Then, on the watershed between Babak Creek and M. Babak, banded...

lidites, directly overlying the limestones and dipping to the northeast at an angle of 40°. When one proceeds along this watershed toward the mouth of the Babak creek, among the lidites there is a thin cover of amygdaloidal diabase porphyrites, in which rare, small, acute-angled xenoliths of limestone are found, completely identical with the Babak limestone.

Thus, along the Babak creek a steep, asymmetrical anticlinal fold is exposed, in the core of which the Babak limestones occur, while in the limbs—lying unconformably upon them—are lidites alternating with diabase porphyrites, which were taken by M. A. Usov to be the middle horizon of the West Sayan Formation \[47, 79\]. To the northeast of the Babak creek, 0.5 km above its mouth, on the right bank of the Chazryk River, from beneath the diabase porphyrites the Babak limestones reappear, dipping to the southwest at an angle of 55°. From here they extend as a narrow strip along the left bank of the lower course of the Izvestkovyi creek, and then continue along the right slope of the Chazryk River as far as the mouth of the Karamkol creek (Fig. 2). At this point, in limestones dipping southward at an angle of 65°, rare remains of trilobites, archaeocyaths, and brachiopods have been found. In addition, the limestones, here being almost on all sides bordered by diabase porphyrites of the Babak creek type, are, together with the latter, intruded and metamorphosed by Maina trondhjemites, a comparatively large pluton of which is developed 1.5 km north of the Chazryk valley (Fig. 2).

Fig. 2. Diagram of the geological structure of the upper course of the Chazryk River, a tributary of the Tabat River.

1—Quaternary deposits; 2—Early Variscan (?) diorite-diabases; 3—sandstones and conglomerates of the Arbat Formation of the Upper Cambrian; 4—Maina trondhjemites; 5—Avgit porphyrites; 6—spilites; 7—lidites; 8—albitophyres and porphyrites of the Lower Monok Formation; 9—Babak limestones; 10—post-Babak microdiorites; 11—hyperbasites; 12—limestones; 13—clayey and siliceous-clayey shales of the Chingin Formation of the second third of the Lower Cambrian; 14—localities of organic remains; 15—disjunctive contacts.

From the south, adjoining the diabase porphyrites with beds of lidites, there is a unit sharply differing in lithological composition and internal structure, occupying the areas of the middle and upper course of the Karamkol creek.

At the mouth of the latter, there first crops out a bed of lidites dipping southward at an angle of 50°, overlain by sheets of massive, dirty-gray amygdaloidal—

stone porphyrites, intensely calcitized and entirely identical to similar rocks of the Babak stream. In the middle course, the effusives are adjoined by vigorously schistose, noticeably goffered, mottled, greenish-gray aleurolitic and dark carbonaceous, sometimes graphitized schists, as well as phyllites enclosing small lenticular bodies of hyperbasites. Farther upstream along the Karamkol stream, up to its headwaters, there is an alternation of the same schists with bedded bodies of schistose greenstone effusives of the basic series, schistosed beyond recognition. Among the schists, packets of varicolored microquartzite, predominantly gray in color, or of motley marbled limestone—now massive, now thin-bedded, often dolomitized—are quite often observed. In the limestones it was only very rarely possible to find remains of algae belonging to the genera Epiphyton and Osagia. The bedding in the limestones dips predominantly to the southeast, 145°, at an angle of 65–70°; in the same direction dip the schistosity planes, but at a steeper angle of inclination (75–80°).

At the headwaters of the Karamkol and Chazryk streams, within the sequence under consideration there occur numerous sills and dikes of diorite-porphyrites, porphyrites, and especially albitophyres, the latter not infrequently cutting the enclosing rocks across the strike of the schistosity.

This sequence was assigned by M. A. Usov to the “upper” horizon of the West Sayan Formation [47, 78–79]; it (the sequence) is connected, through a clearly expressed disjunctive structure, with the formations adjoining it from the north. In the zone of the disjunctive, observed in outcrops of the Karamkol stream, there are widely developed mylonites and friction breccias, consisting of angular fragments of schists, hyperbasites, and rocks of the footwall, caught up in the orbit of the displacer. The latter dips to the southeast, 155°, at an angle of 70°. In addition, in the zone of the disjunctive, with fine-scale rubbing of the rocks of the footwall against the rocks of the hanging wall, intense schistosity develops, accompanied by goffering, slickensides, zones of crumpling, and grinding.

Thus, as a result of studying the sections along the Babak stream and of detailed mapping of the adjacent nearest areas, it became clear that the effusive-sedimentary deposits developed in this space, previously united into the West Sayan Formation, are represented by rocks differing in degree of metamorphism and in age, which group into independent stratigraphic members, with very complex relationships among them.

Thus, the diabase porphyrites (spilites) and lydite rocks of the Babak stream (the middle horizon according to M. A. Usov), lying with sharp unconformity and with basal breccia at the base upon the Babak limestones, were erroneously assigned to the prototype of the West Sayan Formation. In petrographic composition and stratigraphic position, these porphyrites and lydite rocks constitute one of the characteristic members of the Lower Monok Formation of the lower part of the Middle Cambrian, the rocks of which occupy a broad area on the right—and in places the left—bank of the Chazryk River.

I shall recall that on the right side of the named river, above the Babak stream, and then along the Izvestkovyi stream, diabase porphyrites, here again overlying the Babak limestones, together with the latter are intruded and metamorphosed by the Maina trondhjemites. And, incidentally, I shall note that at the mentioned locality the rocks of the Arbat Formation do not come into contact with the Maina trondhjemites; they were evidently shown erroneously by M. A. Usov on the schematic map he cited [47, 44].

Somewhat more difficult to clarify is the relation of the limestone of the Babak stream to the schistose-effusive siliceous-limestone sequence developed in the middle and upper courses of the Karamkol stream and representing the oldest member of the section under consideration.

of the distribution of the Chinginskaya Formation are cut by microdiorites of the Kair Ridge type and, thus, are older than the latter. In turn, the microdiorites are older than the Maina trondhjemites and the Lower Monok effusives.

Thus, from an analysis of the materials just considered, the following conclusions may be drawn:

1) the limestone of Mt. Babak is not the youngest stratigraphic member of the Cambrian deposits of the basin of the upper reaches of the Tabat River, as N. V. Pokrovskaya believed. This limestone, being older than the Lower Monok and Arbat formations—which were probably regarded by N. S. Zaitsev and N. V. Pokrovskaya as weakly metamorphosed effusive-terrigenous deposits—occupies a lower stratigraphic position in relation to them;

2) M. A. Usov included within the Zapadnosayanskaya Formation stratigraphic members that differ in age, internal structure, and content, and made an error in determining their ascending order;

3) the hyperbasites, occurring only within the field of development of the “upper” horizon of the Zapadnosayanskaya Formation and being older than its other members, cannot be recognized as the most important stratigraphic indicators for it. The hyperbasites will probably prove to be characteristic formations of the Chinginskaya Formation, to which, as was shown above, the “upper” horizon of the prototype of the Zapadnosayanskaya Formation in fact belongs.

Moreover, since the latter has lost its independent stratigraphic significance, the name “Zapadnosayanskaya Formation” can no longer be retained in the local stratigraphic nomenclature;

4) the standard section of the Zapadnosayanskaya Formation, as consisting of deposits of different ages, could not be accepted as a reference section for the Lower Cambrian, and the volume of the latter was erroneously considered by me to be commensurate with the volume of the prototype of the Zapadnosayanskaya Formation.

Therefore, I present the reference section of the Lower Cambrian from the southern slope of the Western Sayan, which in recent years has been the object of very detailed geological investigations.

Stratigraphy of the Lower Cambrian of the Southern Slope of the Western Sayan

The most interesting materials concerning the stratigraphy of the Lower Cambrian were obtained in the southwestern part of the Kurtushibinsky Ridge, where, under my direction, detailed geological mapping was carried out over considerable areas encompassing the territory of the basins of the Urguny River and partly the Elekhema (Ilikhem) River, a left tributary of the Ulukhem (B. Yenisei).

Within the Urguny River basin, Lower Cambrian deposits, previously described here under the name of the Uzunarginskaya or, more often, Zapadnosayanskaya Formation [34, 406; 37, 211; 39, 134—136], appear in the axial part of a slightly northward-overturned anticlinorium, extending in a northeastern direction and complicated by a number of longitudinal disjunctive disturbances. The southwestern limb of this anticlinorium is composed of conglomerate-sandstone-shale deposits of the Kemterek Formation [34, 407; 37, 211], in the basal conglomerate of which there are boulders and pebbles of rocks characteristic of the Lower Cambrian and cut by hyperbasite and microdiorite intrusions.

In the system of the Elekhema and Temirsuk rivers, the deposits of the Kemterek Formation are gathered into gentle folds, in the cores of whose anticlines occur rocks of the known Aktovrak Formation [18; 19; 20, 270], which is one of the members of the Lower Cambrian deposits of the Urguny River basin.

In the deposits of the Kemterek Formation, organic remains are occasionally found which, at one time, owing to their poor preservation, I regarded as Upper Cambrian [37, 214; 35, 304], and later, on the basis of new collections of paleontological remains, as Lower Silurian, as G. A. Kudryavtsev had already indicated earlier [23, 718–719]. On the northern limb of the anticlinorium under consideration there are developed sandy-shaly deposits of the Lower Silurian Shignet Formation, with remains of Calapæcia; in the basin of the Kemchik River (Khemchik) and along the valley of the Yenisei River, above the mouth of the Tepsel’ River, these deposits rest unconformably on rocks of the Kemterek Formation [37, 214–266].

To the southwest the axis of the anticlinorium plunges steeply, as a result of which the rocks composing it disappear from the outcrops and are transgressively overlain by deposits of the Lower Devonian, which form a flat syncline of almost latitudinal extent.

The structure of the region is complicated by the development of numerous small intrusive plutons composed of diorite-syenites, hyperbasites, and microdiorites; of these, the masses of the latter two occur only within the area of distribution of the Lower Cambrian deposits and thus are characteristic stratigraphic indicators for it.

Detailed study of the Lower Cambrian has made it possible, in addition to the hyperbasites and microdiorites, to distinguish within it two more sedimentary-effusive formations, of which we retain the name Akkol for the lower and Chingin for the upper.

Akkol Formation

In their typical development, the deposits of the Akkol Formation are distributed in the basin of the Urgun’ River, a right tributary of the Yenisei River, where they form a belt elongated in the latitudinal direction, up to 35 km long and 3–5 km wide. The composition of the formation is clarified from an almost continuous section extending along the valley of the left tributary of the Urgun’, the Akkol River, which flows almost across the strike of the sequence. Here, 0.5 km above the mouth of the named river, along its right side, adjoining through a thrust fault the deposits of the Lower Silurian Shignet Formation, there extends a sequence of rocks of the Akkol Formation, divisible into the following members from bottom to top:

a) dirty-green, finely laminated albite-chlorite-sericite schists with rare interbeds of dark medium-grained marbles not more than 2–3 m thick. Within the schists, at various intervals, occur beds of quartzites, whose thickness ranges from one to several tens of meters;

b) prasinitic schists, frequently interbedded with quartzites and, more rarely, graphitized rocks, which become predominant in the upper parts of the member and include interbeds of dark marbled limestones and beds of light-gray quartzites;

c) marbled limestones which, in contact with the diorite intrusion, have undergone metasomatic alteration with transformation into silicate “hornfels,” more rarely tactites, marbles, and secondary quartzites.

The alternation of the separate components of the members is shown schematically in the accompanying section (Fig. 3). The carbonate stratigraphic member of the Akkol Formation is adjoined, through a disjunction, by Lower Devonian deposits forming a steep synclinal fold of dejective type, slightly overturned to the northwest.

The apparent thickness of the Akkol Formation, judging from the section of the Akkol River, is not less than 4 km. However, taking into account the possible repetition of beds under isoclinal folding, the true thickness may be somewhat reduced, but it is unlikely to be less than 2–2.5 km. The rocks of the formation exhibit [[unclear: continuation cut off on this page]]

maintained an almost latitudinal extent and a uniform dip to the southeast with angles of inclination of \(70\)—\(80^\circ\). But in the lower course of the Snezhnaya Akkol River, and then in the basin of the B. Saynyk River, in the deposits of the formation there is observed a dip of the beds to the northwest; moreover, its upper carbonate member contains interbeds of graphitized shales.

In the section of the Akkol River the Akkol Formation, being the oldest, is joined to the overlying Chingin sequence by a disjunctive contact, and therefore the position of the hanging wall of the first and the footwall of the latter is not fully revealed. But the primary stratigraphic relations between the members of the Lower Cambrian under consideration are revealed quite well on the Karabuluk River, which is a right tributary of the Urgun, where the strongly metamorphosed formations of the Akkol Formation are directly overlain, through a basal horizon, by sedimentary-effusive rocks of the Chingin Formation. Moreover, as will be shown below, in its place, on the Sr. Akkol River, 1.5 km upstream from the Snezhnaya Akkol River, pebbles and boulders of quartzites characteristic of the Akkol Formation are found in the tuff-conglomerates of the Chingin Formation, which provides some grounds for determining a lower stratigraphic position of the latter in relation to the former. But it is more convenient to dwell on these questions in the description of the Chingin Formation.

Deposits of the Akkol Formation type are not confined, in their areal development, only to the basin of the Urgun River but, undoubtedly, are more widespread. However, the majority of researchers who studied the Kurtushibinsky Range combined them with younger formations into a single stratigraphic unit, which they called either the Uzunargin or the West Sayan formations. Therefore it can only be assumed that the belt, intersected by the Uzyum, Omyl, and other streams of the upper part of the Us River system, composed of prasinitic shales and quartzites, may prove to be the field of development of the Akkol Formation. True, within this field the graphitized shales and marbled dark limestones characteristic of the formation under consideration are not noted, but they may have been missed during route investigations or, more likely, were eroded before the sedimentation of later formations.

Probably the Akkol Formation also includes deposits developed in the middle and upper courses of the Kemterek River (Fig. 4), a left tributary of the Yenisei River, represented here by greenish-gray albite-actinolite shales, quartzites, interbeds of graphitized shales, and thick marbled dark limestones, which I previously assigned to the lower horizons of the Uzunargin Formation.

Fig. 3. Schematic section of the lower member of the Akkol Formation along the Akkol River.

1 — Pantabariskian diorites and their dikes (2); 3 — Chinginian Formation: 4 — marbled, non-graphitized limestones, 5 — prasinites, 6 — graphitized shales, 7 — quartzites, and 8 — albite-sericite shales of the Akkol Formation; 9 — contact-metasomatic rocks; a, b, ... — stratigraphic members of the lower Akkol Formation.

Elements of the petrographic characteristics of the Akkol Formation

For completeness in characterizing the Akkol Formation, one should dwell somewhat on a consideration of its petrographic composition, which reveals with particular clarity the individual features of the formation’s stratotype. As was shown in the example of the section on the Akkol River, its lower members are composed of albite-chlorite-sericite schists. The latter have dense bedding, a greenish-gray color, and perfect schistosity, along the planes of which a fine dusting of sericite and chlorite develops, as well as thin, clearly visible veinlets of quartz and carbonates, sometimes with inclusions of pyrite. Microscopic study of thin sections adds little to the characteristic features of the rock under consideration, revealing in it a lepidoblastic structure formed by albite, chlorite, actinolite, sericite, carbonates, and, more rarely, quartz.

The most interesting rocks of the formation are prazinite schists, which appear approximately in its middle horizons. In outcrops, prazinite schists occur as bed-like bodies from 2 to 100 m thick, regularly alternating with deposits of quartzites, whose thickness

Fig. 4. Diagram of the geological structure of the Kemterek River, a tributary of the Yenisei River.

1—Shishtyk Formation; 2—Shignet Formation; 3—Kemterek Formation; 4—West Sayan hyperbasite intrusion; 5—Main troctolites; 6—Chingin Formation; 7—Akkol Formation.

varies from 1 to 50 m. In the alternation of prazinite schists and quartzites, one may note a definite, though as yet poorly explained, regularity. The point is that in the thickest deposits of prazinites, on the hanging-wall side of the latter, there always appear 2–3 interbeds of quartzites, whose total thickness does not exceed 1–2 m. In turn, on the footwall side of the thickest quartzite beds, interlayers of prazinite schists are found with a total thickness likewise not exceeding 1–2 m (Fig. 5). The contacts between the rocks under consideration are fairly sharp and coincide with the direction of the prevailing schistosity, and at the same time in small additional—

in the additional folds they take part in identical bends, without visible ruptures (Fig. 6).

Among the prasinitic schists two characteristic varieties are distinguished—albite-actinolite-glaucophane and albite-actinolite-zoisite; the differences between them are clearly established only under the microscope. Macroscopically, however, these varieties appear as wearisomely

Fig. 5. Alternation of prasinites (1) and quartzites (2) in one of the outcrops of the Akkol River (section).

monotonous rocks, green in color with various shades, having a finely schistose, microfolded texture with a fibrous structure.

The albite-actinolite-glaucophane schists under the microscope reveal a fibrous-blastic structure; the groundmass is composed of actinolite, with the participation of glaucophane, chlorite, albite, carbonates, quartz, sphene, pyrite, and magnetite.

Actinolite, of finely fibrous habit, with a characteristic pale-green color along \(Ng\), to colorless or slightly greenish-yellow along \(Np\), with an extinction angle \(C : Ng = 18^\circ\), \(Ng - Np = 0.021^\circ\), has the appearance of aggregated strands

Fig. 6. Forms of additional folding in the quartzites of the Akkol Formation.

1—alluvial deposits; 2—quartzites; 3—graphitized schists; 4—prasinites; and 5—breccias of the Akkol Formation.

arranged along the schistosity. In close intergrowth with it occurs glaucophane, distinguished by its bright violet color against the general green background of the thin section. Occasionally individual grains of glaucophane appear in the form of porphyroblasts with barely discernible outlines, but more often they form needle-shaped crystals growing out of the actinolite aggregate. In all this, the glaucophane appears zonal and preserves certain properties of actinolite, such as: extinction, pleochroism, and the magnitude of birefringence. Co-

the amount of glaucophane in the rock is considerable, so that some varieties of the latter are close to glaucophanites.

A significant place in the rock described is occupied by albite, whose fine-grained aggregates are usually scattered in clusters through the field of the thin section, but sometimes it has almost prismatic outlines and is penetrated by fibers of actinolite. In other cases albite appears as lens-shaped accumulations in association with quartz, epidote, and calcite.

Green pennine develops in large patches along amphiboles, replacing individual amphibole grains from the margins and penetrating into them along cracks in the form of veinlets. The coalescence of the latter is accompanied by the formation of a continuous aggregate of chlorite, which is sometimes the principal mineral in the rock relative to the other components.

However, such vigorously chloritized schists are restricted in their development to narrow areas affected by especially intense schistosity.

Albite-actinolite-zoisite schists, in their structural pattern and partly in their mineral composition, are very close to the above-described varieties of prasinites. In them, too, the main fibrous-blastic fabric consists of fine-fibrous pale-green actinolite, replaced to varying degrees by chlorite (pennine). Among the chlorite-actinolite mass are scattered large and small prismatic crystals of $\alpha$-zoisite. In crossed nicols, among the indicated aggregate, mosaic grains of albite with fine polysynthetic twins are visible.

Secondary components—such as sphene, carbonates, quartz, and other rare minerals—behave in both varieties of prasinites in exactly the same way. Thus, sphene, making up 8–10% of the area of the thin section, appears in the form of fine-grained accumulations and is separated as rounded patches or elongated chains, representing strongly cataclased grains. Within the lumpy formations of sphene, small grains of an iron-black mineral, apparently ilmenite, are observed. No less distinctly, against the general pattern of the thin section, ferrous carbonate stands out, concentrating in flaky aggregates rimmed by brown iron oxides. Usually the carbonate is closely associated with actinolite and albite and is genetically connected with them. Another type of carbonate, undoubtedly of later origin, occurring in the form of rhombohedra with fine polysynthetic twins and probably belonging to calcite, appears as branching veinlets or isometric grains. Quartz behaves in an analogous manner, being associated now with albite, now with chlorite.

Among the ore minerals, pyrite is preferentially developed and is readily detected macroscopically. More often it is concentrated in veinlets 1–2 mm thick along parting and schistosity cracks and locally forms stockworks or disseminated inclusions with very regular cubes up to 1.5 cm in diameter.

Distinctive varieties of prasinites are plagioclase-chlorite-zoisite and biotite-chlorite-quartz-albite schists, which externally do not differ in any way from the rocks described earlier. But under the microscope, in the former, a porphyroblastic structure is observed, the porphyroblasts occurring in the form of prismatic crystals and belonging to oligoclase-andesine No. 32 and $\alpha$-zoisite. The main lepidoblastic fabric is composed of pale-green chlorite (pennine) and acicular tremolite. In the windows among the chlorite-tremolite aggregate, mosaic albite grains show through. In addition, sphene, calcite, quartz, pyrite, and magnetite are noted in the composition of the rock.

In the biotite-chlorite-quartz-albite schists, a porphyroblastic structure is likewise discernible under the microscope, but here the porphyroblasts belong to minerals having prismatic outlines, partly replaced by saussurite and, apparently, corresponding to plagioclase. The principal mass of the rock—

is composed of finest-flaked brown biotite, replaced along the edges by sericite (apparently bauerite) and chlorite (pennine). The aggregate of these minerals is closely intergrown, and the transitions from biotite to chlorite are difficult to distinguish. In addition, lenticular areas made up of quartz and albite, as well as veinlets of calcite, are set apart in the field of the thin section.

As is evident from the section presented, within prasinites and dolomitized limestones there occur packets of varying thickness of graphitized schists, which constitute the most characteristic member of the Akkol Formation. The rocks grouped as graphitized schists always contain, to one degree or another, an admixture of earthy-brown, sooty graphite; where it is abundant the rocks acquire a characteristic black color. According to microscopic determinations, two varieties are distinguished among them—quartz-sericite and quartzite schists, which form subtle transitions into one another, only faintly discernible even in individual specimens.

Quartz-sericite schists macroscopically have a dark-gray, sometimes black color, fine banding, and complex crumpling, not infrequently accompanied by platy cleavage. The banding in the rock is caused by the alternation of lenslets and interbeds composed successively of quartz and a micaceous-graphitic aggregate. The thickness of the quartz lenslets varies from 1 to 5 mm, whereas the micaceous graphite interbeds have the appearance of films from fractions to 0.5 mm thick. In addition, along the schistosity and joint cracks there develop thin veinlets of brown carbonate, which is the latest component. It is also characteristic that the platy character of the schists follows exactly all the bends of the crumpling and thereby strongly emphasizes the relict layered texture of the rock. Under the microscope, the latter shows a sharply expressed crystallization-layered texture, accentuated by quartz grains and sericite needles strongly elongated in one direction. The dominant component of the rock is quartz, which appears in thin section as broad bands with an uneven-grained, but in general granoblastic, structure. A fine-grained aggregate of sericite is scattered over the general quartz background in the form of strands bent into small folds and, as a rule, is accompanied by a dense sprinkling of graphite. Pyrite and calcite are also confined to the sericitic bands. The latter often forms independent veinlets cutting the rock at various angles to the schistosity.

Quartzite schists are readily distinguished from the above-described variety of graphitized rocks by their massive texture, light-gray or even white color, and fine-grained structure. However, the massive aspect of the schists under consideration is deceptive, for throughout and side by side they prove to be strongly schistose, take part in small S-shaped folds accompanied by coarse crumpling without visible breaks in continuity. On the schistosity and slickenside planes there are visible flakes of white mica and smears of sooty graphite.

Under the microscope, the crystallization-schistose texture is expressed much more sharply in the quartzite schists; it is caused by the unequal grain size of the quartz bands alternating with rare thin sericite-graphite ribbons. The bands composed of medium-grained quartz have a granoblastic structure and grow at the expense of adjacent lenses of fine-grained quartz. In turn, the quartz lenses develop in place of sericite strands, cutting them by thin veinlets, so that only small fragments remain from the sericite aggregate. In these fragments, sericite is preserved as finest-flaked accumulations or, more rarely, as separate flakes situated between quartz grains. Upon the latter is superposed a dense sprinkling of graphite, whose crumpled ribbons indicate the sericite aggregate that formerly existed in the place of the quartz. With more

with deep recrystallization the slates described acquire a monomineralic quartz composition with a clearly expressed grano- and porphyroblastic structure, and in this case they cannot be distinguished from quartzites, to the characterization of which we now turn.

Quartzites are distributed more or less uniformly through the entire thickness of the formation, within which they appear in the form of bed-like or lens-like bodies and, with relatively consistent thickness, extend for considerable distances.

Not infrequently two adjacent, comparatively thick beds of quartzite unite into one broad band, which then, after some distance, again splits into several lenticular masses, only later to merge once more into a single bedded deposit with a comparatively variable thickness of the latter. In this respect especially indicative is the band of quartzites exposed in the lower course of the Odych River, 3 km above its mouth (Fig. 7).

Fig. 7. Schematic geological structure of the Akkol formation in the lower course of the Odych River.

1—Early Variscan (?) diorite-porphyrites and diorite-diabases (2); 3—Kemterek Formation; 4—marbleized limestones; 5—quartzites; and 6—prasinitic slates of the Akkol Formation.

Odych flows into the Urgun River from the left, approximately 12 km below the Akkol River. At the indicated point the band of quartzites, recorded on the left side of the Odych, extends in both directions. Moving northwest from the valley of the Odych River, within the quartzite band there appears a large number of interbeds and lenses of prasinitic slates, between which the bedding planes, being perfectly parallel and very sharp, are elongated in a northwestern direction and affected by numerous zones of rubbing. Almost across the strike of the bedding planes of the quartzites and prasinites, small disjunctive disturbances develop, along which dikes of diorites and diabases of Early Variscan age are sometimes located.

Outwardly the quartzites are very uniform, being for the most part massive, more rarely coarsely schistose rocks, predominantly fine-grained in structure, light-gray, gray, and dark-gray in color. Microscopically, two varieties can be distinguished among them.

In the first of these the predominant component is quartz, forming mosaic grains elongated in the direction of schistosity, by which the crystalline granoblastic structure is emphasized. Of the secondary minerals, the most widespread are sericite, magnetite, muscovite, and chlorite, often extending in thin discontinuous bands.

In the second variety of quartzites the principal component is also quartz, but the latter is accompanied by a large amount of graphite and, especially, biotite, the flakes of which are situated between quartz grains arranged in regular rows. The presence of rutile in the rock completes its characteristic features.

Marmorized limestones in the deposits of the formation are recorded as thin and, moreover, rare lenses and interbeds, and more rarely as packets or beds up to several tens of meters thick.

Interbeds and lenses of limestones, most often occurring among actinolite-albite and graphitized schists, are usually transformed into calcite-sericite-tremolite schists with an abundant admixture of earthy graphite, situated along the planes of fine schistosity. Owing to the development of graphite, the limestones have a black, coal-like color and a clear medium-grained structure. Under the microscope, in addition to the components indicated, quartz is often present in the limestones, occurring among large calcite crystals, with which it forms mutually uneven boundaries. Tremolite forms idiomorphic-elongate prismatic grains, usually grouped parallel to the schistosity of the rock.

Not infrequently, the limestones of thin beds have undergone only recrystallization, owing to which they prove to be coarse- and fine-grained, consisting of mosaic calcite grains elongated along the schistosity. In the general carbonate pattern, folded-bent ribbons composed of fine-grained graphite are visible. Sometimes thin beds of carbonate rocks are composed of dolomites, which have a fine-grained structure and a dark-gray, almost black color, caused by the presence of abundant graphite.

The marmorized limestones that appear in the upper parts of the Akkol’ Formation are characterized by somewhat different morphological features. They probably constitute a more or less persistent horizon, extending for considerable distances and reaching a thickness of up to 150–200 m, as is seen, for example, in the section of the Snezhnaya Akkol’ River, 0.3 km above its mouth, where they take part in the structure of the closure of an additional synclinal fold, slightly overturned to the northwest. In external appearance, the rocks under consideration in some cases are massive, thin-bedded, almost black formations, occurring as both varieties in the same outcrop. In places these limestones acquire an oolitic structure, which is especially clearly revealed under the microscope. The coloration of the limestones is caused by an abundant admixture of bituminous matter, which is distributed in uneven portions through the rock mass. Under the influence of contact metamorphism the limestones lose their predominantly dark coloration and fine-grained structure and become light-colored, with a crystalline-granular texture, with a sooty coating of graphite concentrated chiefly along the planes of schistosity. This same carbonate horizon is also traceable in the lower course of the Odycha River, where it composes the southwestern limb of an anticlinal fold, the closure of which is formed by prasinites and quartzites of the Akkol’ Formation. At this locality, however, the limestones under consideration exhibit certain distinctive features. They are relatively weakly recrystallized, massive, rather strongly dolomitized, have a dark-gray coloration, and are rich in algal remains, which in their form are close to the oolitic formations of similar limestones of the Snezhnaya Akkol’ River. Among the algae, representatives of the genus Osagia are especially characteristic, probably expressed by several species, including Osagia ovosimilis. Thus, study of the petrographic composition of the deposits of the Akkol’ Formation shows that, among the rocks participating in its composition, dark-colored varieties stand out especially sharply, their color being due to the present…

in them graphite, which is especially readily concentrated in limestones and graphitized shales and somewhat less abundantly—in quartzites.

Chingis Formation

The deposits of the Chingis Formation are characterized by entirely different morphological features; in their typical development they occupy the area of the upper reaches of the Chinga, Elezhem, and the right tributaries of the Ulug-Khem, as well as the basin of the Akkol River and the Bolshoi Khailyk, which flow into the Urgun from the right, and the headwaters of the right lower tributaries of the latter—the B. and N. Karabuluk springs. The lower and middle stratigraphic members of the Chingis Formation are exposed most completely in an almost continuous section of the Srednii Akkol River, extending along its valley approximately 1.5 km above the Snezhnnyi Akkol River (Fig. 8).

In this section it is clearly seen that the deposits of the variegated Devonian are overlain, across a major steeply dipping overthrust, by the lower stratigraphic members of the Chingis Formation, which have the following succession from bottom to top:

a) greenish-gray, intensely schistose tuff-conglomerates and tuff-breccias, in the fragments of which large and small, excellently rounded pebbles of quartzites stand out distinctly. Above them is a bed of limestone up to 10 m thick, overlain by tuffs interbedded with thick layers of microquartzites. The dip direction of the bedding is to the southeast at an angle of 65°; the total thickness of the unit is about 600–700 m;

b) stratigraphically higher lie carbonaceous-aleurolitic shales, overlain by microquartzites interstratified, in turn, with thin beds of the same carbonaceous-aleurolitic shales; the thickness of the unit is about 350–375 m;

c) after this, in the section there appear, probably, the middle members of the formation, expressed by greenstone, coarsely schistose effusives interbedded with rare layers of microquartzites, tuffs, bluish-gray shales, and aleurolites. Near the base of the unit there is a conformable body of microdiorites up to 15 m thick; the total thickness of the unit is more than 1200 m;

d) finally, the upper members of the section are formed by a unit of thinly alternating bluish-gray aleurolites, black and bluish microquartzites, greenstone effusives, and carbonaceous-siliceous shales, dipping at first to the southeast at an angle of 65°, and then to the northwest at an angle of 70°. The thickness of the unit in the exposed part of the section is not less than 150–175 m. In this part of the section as well, in the southeastern limb, apparently of an additional synclinal fold, there is a conformable body of microdiorites up to 20 m thick.

In the section of the Srednii Akkol River, owing to the state of exposure, the position of the hanging wall of the Chingin Formation cannot be established, but in the upper course of the Elekhem River it is determined quite clearly.

Here, in the axial part of an additional anticlinal fold, extending almost latitudinally and slightly overturned to the north, there occur intensely metamorphosed carbonaceous-siltstone shales enclosing beds of microquartzites and sheet-like deposits of greenstone effusives. In the southern limb of the anticline, well exposed in the upper segment of the Elekhem gorge, from beneath the unconformably overlying conglomerate-sandy deposits of the Khemterek Formation of the Lower Silurian, strongly metamorphosed phyllite-carbonaceous-siliceous shales appear, enclosing a lens-shaped body of light-colored, noticeably marbled limestone, noted earlier by E. A. Lebedeva [24, 120]. These limestones are excellently traced eastward along the strike; moreover, on the watershed between Temirsuk and Elekhem they merge into the belt of limestones of the northern slopes of the Aktovrak heights. The limestones, being noticeably hornfelsed by the hyperbasite intrusions cutting them,

Fig. 9. Section diagram along the N. Karabuluk River.

1—prasinites and 2—quartzites of the Akkol Formation; 3—metamorphosed sandstones with lenses of conglomerate; 4—carbonaceous-siliceous shales, 5—greenstone effusives, and 6—microquartzites of the Chingin Formation; 7—sandstones of the Khemterek Formation.

contain abundant and excellently preserved remains of archaeocyaths, among which Dictyocyathus tuvaensis, Archaeocyathus tolli, and Ajacicyathus camptofragmeuc were identified at one time. From here, from my collections, such forms as Ajacicyathus otichtaschen-sis, A. immanis, Orbicyathus mongolicus, Orbicyathus sp., as well as algae belonging to the genus Osagia, were also established.

Deposits of the Chingin Formation are developed also to the northwest of the stratotype Srednii Akkol section, at the headwaters of the lower right tributaries of the Urgun River (the V. and N. Karabuluk rivers), as well as on the left and right banks of the Yenisei and of its left tributary, the Khemterek River, forming here a distinct belt several kilometers wide.

In certain excellent sections of the belt described, the most ancient members of the Chingin Formation are exposed, occurring in clear primary stratigraphic relationships with the underlying Akkol Formation. Thus, in the lower course of the right side of the N. Karabuluk River (Fig. 9), there occurs a unit of sandy-conglomeratic deposits resting with a clear unconformity upon greenish-gray prasinite schists and quartzites that constitute the characteristic components of the Akkol Formation. The almost vertically standing beds of the latter strike in a northwestern direction, whereas the beds of the unit of clastic rocks, striking in a northeastern direction, show a northwestern dip at an angle of only 60–65° (Fig. 10). This unit of clastic rocks, composed of polymict sandstones with interbeds of conglomerates consisting chiefly of well-rounded pebbles of quartzites and, more rarely, actinolite-albite-chlorite schists, I regard as the basal member of the Chingin Formation. The appear-

The greenish-gray siltstones lying stratigraphically above, interbedded with greenstone effusives and dark lydite-like quartzites, probably already represent an analogue of the lower members of the formation described in its stratotype section along the Srednii Akkol River.

The middle and upper members of the Chinginskaya Formation along the N. Karabuluk River are poorly exposed; the isolated outcrops of greenstone effusives, their tuffs, and microquartzites that appear here sporadically, dipping predominantly to the northwest at an angle of 60–65° and more rarely to the southeast with dip angles of 75°, characterize only in general terms the main features of the formation. But

Fig. 10. Schematic diagram of the relations between the Chinginskaya and Akkol formations in the basin of the lower course of the Urgun River.

1—Chingetskaya Formation; 2—Kemterekskaya Formation; 3—Kairskaya (?) microdiorite intrusion; 4—hyperbasite intrusion; 5—greenstone effusives; 6—microquartzites; 7—limestones and 8—sandy-clayey rocks of the Chinginskaya Formation; 9—prazinitic schists and 10—quartzites of the Akkol Formation; 11—stratigraphic contacts, traced (a) and inferred (b); 12—disjunctive.

on the watershed between the N. Karabuluk and V. Khadynnykh Rivers, which are right tributaries of the Yenisei River, these members of the Chinginskaya Formation are exposed in an almost continuous section, in which horizons close in composition to the Upper Lazhem section are especially fully expressed. As in the latter, so here also, clayey-siliceous shales, with an insignificant participation of greenstone effusives and microquartzites, enclose lenticular bodies of limestones of varying thickness, extending for considerable distances. In one of these limestone lenses, stratigraphically the lowest, reaching a thickness of up to 150 m and descending toward the right bank of the N. Karabuluk River, numerous remains of algae were found, belonging to the genera Osagia, Epiphyton and to the species Renalcis granosus. Much more rarely, poorly preserved archaeocyathids occur, among which the genus Orbicyathus sp. was formerly identified.

Finally, one cannot fail to note such important circumstances determining the relationship of the Chingin Formation with the older and younger formations surrounding it. The fact is that 7 km above the mouth of the Kemtеrek River, a left tributary of the Yenisei River, numerous dikes of greenstone effusives, completely identical to those of the Chingin Formation, cutting through the phthanite shales and marbleized limestones of the Akkol Formation, include in great abundance xenoliths of various sizes which, in their composition, are typical of the rocks enclosing the dikes (Fig. 11). In turn, both these formations are transgressively overlain, through a basal conglomerate, by the Lower Silurian Kemtеrek sequence (Fig. 4).

Fig. 11. Dikes of greenstone effusives of the Chingin Formation (2) with xenoliths (3) of albite-chlorite shales (1) of the Akkol Formation.

In addition, chiefly within the Chingin Formation at the mouth of the Kemtеrek River, and especially on the watershed between the N. Karabuluk and V. Khadynnykh rivers, there occur numerous lens-shaped bodies of hyperbasites which, being elongated with their long axes in a north-northeasterly direction, together with the characteristic members of the enclosing rocks, at the contact with the Lower Silurian Shignet Formation, are cut off by a steeply south-dipping overthrust of northwestern strike.

To the northeast of the stratotype Middle Akkol section, the deposits of the Chingin Formation extend as a continuous belt, at least 7 km wide, along the entire northern slope of the Kurtushibinsky Range, appearing here in the axis of an anticlinorium, in the southern limb of which they are overlain by unconformably lying sediments of the Kemtеrek Formation, while in the northern limb—through a major disjunctive dislocation—they are in contact now with formations of the Shignet, now of the Akkol formations.

Throughout this entire extent, the deposits of the Chingin Formation, judging from my earlier work, as well as from the observations of A. N. Vukolov [7], are represented by metamorphosed sandy-conglomeratic sediments alternating with effusives, tuffs, tuffites, microquartzites, enclosing short, rapidly pinching-out lenses of limestones and beds of carbonaceous-aleurolitic and carbonaceous-siliceous shales. In several almost continuous sections, for example along the B. Khailyk, Zolotoy, and other rivers, the lower and middle members of the formation are exposed, almost in every detail analogous to its stratotype occurrence. And only in the extreme northeast, in the headwaters of the Tikhaya River, a left tributary of the Us River, do the higher members of the formation appear, close in composition and structure to the Alakhem section. Thus, in the right tributaries of the Tikhaya River and especially along the watershed area between it and the Anyagus River, there extends a persistent belt of carbonaceous-siliceous shales and greenstone effusives, with which are interbedded thick

lenses of massive limestones with remains of algae, of which Osagia and Epiphyton are the most frequently encountered forms.

Fig. 12. Generalized stratigraphic column of the Chingin Formation, to a considerable degree schematic.

1—metamorphosed sandstones; 2—conglomerates; 3—clayey-siliceous shales; 4—clayey shales; 5—limestones with algae and archaeocyaths of the Karabuluk complex; 6—bluish-gray shales; 7—greenish effusives; 8—tuffs; 9—microquartzites; 10—thin limestones; 11—tuffo-conglomerates and tuffo-breccias; 12—sandstones with interbeds of conglomerates; 13—quartzites; 14—carbonate shales.

Thus, the deposits of the Chingin Formation, occupying considerable areas on the southern slope of the Western Sayan, are distinguished by the complexity of their composition, within which microquartzites and algal-archaeocyath limestones stand out especially clearly, being for it the most important stratigraphic and marker documents. Comparing the numerous partial sections, of which only the most typical are described here, the generalized stratigraphic column of the Chingin Formation may be expressed in the following form (Fig. 12).

At the base of the formation lies a basal horizon, expressed for the most part by a complex sequence of sandy-conglomeratic rocks, overlain by a thick body of tuffs, tuffo-conglomerates, and tuffo-breccias, in places including beds of limestones, numerous layers of microquartzites and carbonaceous-aleurolitic shales, which stratigraphically higher, noticeably decreasing in number and thickness, are intricately interbedded with effusives. In the upper parts of the formation, clastic rocks again predominate, with characteristic carbonaceous-siliceous shales, containing rare and thin beds of microquartzites, bedded bodies of effusives, and thick lenses of limestones, sometimes containing organic remains. The thickness of the formation as a whole may be estimated at no less than 4–4.2 km.

Features of the petrographic composition of the Chingin Formation

The individual features of the Chingin Formation are determined not only by its stratigraphic position, but also by the specific traits of its petrographic composition, on the characterization of which I consider it useful to dwell somewhat.

As was evident from examination of the section along the N. Karabuluk River, at the base of the formation there is a sequence of sandstones with interbeds of conglomerates, which from there extends both to the northeast and to the southwest. More distinctly, the rocks of this sequence crop out east of the Sinderba River, which enters the Us River from the left, where they form a conspicuous belt, traceable almost without interruption to the lower course of the Veselaya River, entering the Tikhaya River from the left. Throughout this enormous extent, the deposits of the sequence under consideration are characterized by a remarkable uniformity of composition. The sandstones, being the principal rock of the sequence, are distin—

are distinguished by their light-gray, more rarely yellowish-gray color, fine- and distinctly granular structure and weakly expressed bedding, always attenuated by fine lamination, the planes of which extend parallel to the bedding. Under the microscope the sandstones described consist of well-rounded fragments of quartz, quartzite, feldspar, green schists, and effusives. The cement is calcite and clayey material, rich in neoformations represented by an aggregate of quartz, sericite, and actinolite.

Among the sandstone fragments, quartzites, effusives, and schists deserve special attention. The fragments of the first consist of quartz forming mosaic grains, and a small quantity of sericite, muscovite, and chlorite. The effusive fragments are composed of a rather indeterminate, partly decomposed groundmass with small phenocrysts of clearly twinned plagioclase. The schist fragments are rich in chlorite and sericite. The conglomerate interbeds present among the sandstones consist of well-rounded, fairly large pebbles of dark-gray quartzites rich in graphitized matter and, more rarely, of dark-green actinolite-chlorite schists. The cement of the conglomerate is a fine-sandy mass, intensely chloritized and carbonatized.

No less characteristic formations of the Chinginskaya Formation are carbonaceous-aleurolitic schists, which make up fairly persistent horizons directly overlying the sandy-conglomeratic formations, or else appearing as thin beds among other members of the formation.

Of the carbonaceous-aleurolitic schists, which vary in color from green to gray, their black, finely banded varieties stand out especially sharply. Under the microscope, in the latter there is observed a fine blastopelitic quartz-feldspathic mass, dusted with carbonaceous material. In some varieties of these schists, especially those rich in quartz and carbonaceous matter, round or oval grains (Fig. 13), composed of an aggregate of quartz, are visible; in their structural features they somewhat resemble formations close to the spherolites (radiolaria?) of silicites described by Yu. A. Petrokovich [31, 174]. The schists are rich in neoformations, among which calcite, sericite, and chlorite are constantly present, with a relatively weak participation of actinolite. The substance of the schists is often penetrated by the finest veinlets of quartz, which in individual areas completely replaces the rock, with the formation in the latter of a clearly expressed bridge-like structure.

Fig. 13. Sketch of a thin section of carbonaceous-aleurolitic schist with remains of radiolaria (?); in the drawing the radiolaria are expressed as rounded white bodies situated among a black carbonaceous mass penetrated by veinlets and lenses of quartz. ×30.

Effusives of the Chinginskaya Formation are the most widespread rocks, making up large sheets or bedded bodies situated within the sedimentary formations. The effusive sheets are closely interbedded with tuffs, which outwardly do not differ from the former. In individual outcrops the effusives appear as strongly, coarsely or finely laminated rocks, having a bluish- or greenish-gray color, a fine-grained, more often aphanitic structure. Primary components are best seen in massive, less laminated rocks, in which phenocrysts of plagioclase and, sometimes, of a dark-colored component stand out clearly among a chloritic and sericitic groundmass. Ne-

some varieties of effusives are rich in pyrite, which appears in the form of sparse disseminations or vein-like accumulations readily spreading along the schistosity.

Microscopic studies appreciably supplement the petrographic features of the extrusive formations, represented chiefly by diabase and augite porphyries, the groundmass of which is almost entirely filled with epidote, chlorite, albite, actinolite, accompanied by veinlets of calcite and clinozoisite that developed later than very vigorous cataclasis. Only in individual areas, and not in every thin section, can one see intersertal, amygdaloidal, ophitic, and sometimes variolitic structures, which are revealed by the arrangement of strongly altered relicts of augite pyroxene and twinned plagioclase, intensely crushed and saussuritized. Amygdaloidal diabase porphyrites are noted comparatively more rarely; in them, weakly recrystallized areas of relicts of hyalopilitic and amygdaloidal structures pass into an aggregate of chlorite and carbonate with crystals arranged perpendicular to the curved mesostasis and forming a structure of disintegration. The amygdules are filled with calcite, chlorite, and sometimes epidote and albite.

Thus, the rocks described, which probably formed in an aqueous environment, underwent a greenschist phase of alteration [44, 23—26; 45] and, in accordance with their phase state, should be assigned to the diabases. The effusives are accompanied by tuffs, as well as by tuff breccias, which in places occupy extensive areas (the Middle Akkol River). The tuffs can be readily distinguished only when studied under the microscope. The latter shows that they consist of thin, variously outlined fragments of plagioclase, amphibole, and pyroxene, situated in an intensely chloritized and actinolitized groundmass, in which elongated plagioclase crystals can also sometimes be seen. Through almost imperceptible mutual transitions, the tuffs are replaced by tuff conglomerates and tuff breccias, for which a highly characteristic feature is a hypocrystalline porphyry-breccia structure formed by fragments of plagioclase, amphibole, and pyroxene. The serrated grains of the porphyroblasts are usually torn by fine cracks filled with carbonate or quartz material. The groundmass, being composed of the same components as the porphyroblasts, is rich in neoformations expressed by chlorite, calcite, and quartz, with the latter two components not infrequently occurring in the form of veinlets.

As was said above, in the section along the Middle Akkol River, perfectly rounded pebbles of quartzites, ranging in size from several millimeters to five centimeters in diameter, are found in the tuff conglomerates. These quartzite pebbles, having a light-gray color and a clearly expressed hornfelsic structure, do not differ in any way from similar rocks that constitute the most important component of the Akkol formation. In a number of places the tuffs pass quite gradually into green fine-grained tuffites, which, as the microscope shows, consist of well-rounded grains of pyroxenes and basic plagioclases, cemented by an argillaceous-calcareous mass, almost entirely chloritized and sericitized.

The tuffites are interbedded with dark and green thin-bedded phyllitized and aleurolitic shales, indistinguishable in composition and appearance from similar rocks occurring at the base of the Chingin formation. Such interbedding of extrusive masses with sedimentary formations is quite natural for submarine eruptions.

Microquartzites are the most characteristic rocks of the formation. Spatially, being confined mainly to its effusive-tuffaceous and, more rarely, sedimentary members, they form clearly expressed bed-like bodies with sharp contacts with the enclosing rocks. The thick—

the thickness of the microquartzite beds is subject to appreciable variations and ranges from several tens of meters to several centimeters; in the latter case they appear in the form of small lenses that rapidly pinch out along the strike. However, fairly thick beds of microquartzites also not infrequently pass along the strike into tuffs or tuffites, forming uneven, serrated boundaries in the transition zones.

In both outward appearance and microscopic structure, the microquartzites differ markedly from the quartzites occurring in the underlying Akkol Formation. Possessing predominantly a dark to black, more rarely dirty-green or bluish-gray color, a dense fabric, and a conchoidal fracture with translucent edges, the microquartzites in their outward appearance strikingly resemble lydites. In many cases the microquartzites prove to be broken by an irregular fine system of cracks filled with quartz material, which, spreading out from the cracks, is accompanied by noticeable silicification of the rock, and the latter acquires features of a distinctive hornfelsic structure.

Microscopic study of the microquartzites shows that the fine-grained, typically hornfelsic structure of the main mass is formed by fine grains of quartz, often overlapping one another. Also noticeable are areas composed of coarser quartz grains, appearing in the form of veinlets, though with indistinctly expressed boundaries. Sometimes in individual thin sections it is possible to observe clearly expressed bedding in the microquartzites, caused by an alternation of carbonaceous matter with quartz material possessing a mosaic structure. In other thin sections, however, the carbonaceous matter plays the role of a cement, within which fine quartz grains, always with serrated edges, are situated.

In short, the rocks described, occupying an intermediate position between lydites and quartzites, deserve a special name; and if I retain for them the name microquartzites, this is due to their insufficient study, which requires special investigations.

Limestones play a very minor role in the composition of the formation and appear as short lens-shaped masses that rapidly pinch out along the strike. Spatially, the rocks described are closely associated with the clastic members of the formation and more rarely with the effusive-tuff horizons of the latter. Among the limestones, two varieties are clearly distinguished by morphological and stratigraphic features. The first of these, confined to the lower parts of the formation (the Middle Akkol River), is characterized by a light-gray color, a dense fabric, and clearly expressed fine bedding. The second variety of limestones, grouped in the upper parts of the formation, has a massive structure, dark-gray or light-gray color, and weak recrystallization. Sometimes the limestones are interbedded with calcareous-clayey shales and undergo noticeable silicification with the formation of distinctive “hornfels,” according to the definition of Z. A. Lebedeva [24, 227]. The form of occurrence of these limestones is lenticular, and the thickness of the bodies ranges from several tens of meters to two hundred meters. Limestones of considerable thickness, traced along the strike for more or less significant distances (up to ten kilometers), almost always contain organic remains, among which algae are especially numerous. Very rarely are the latter accompanied by archaeocyaths, which proved to be rather abundant in the limestones extending along the watershed area of the upper reaches of the Elezhem and Temirsuk rivers, right tributaries of the Ulug-Khem. At other localities where similar limestones occur, archaeocyaths were found only on the watershed of the N. Karabuluk and V. Khadannykh rivers, and then in insufficiently good preservation.

In general, the deposits of the Chingin Formation, in lithological composition and thickness, appear to be typical geosynclinal formations,

having undergone intense folding, intensive schistosity, but relatively weak metamorphism, characteristic of the upper parts of the epizone.

The Chingian Formation of the Priuluzhem Region

On the southern slope of the Kurtushibin Range, on the right bank of the Uluzhem River (B. Yenisei), 5 km northwest of the town of Shagonar, V. A. Kuznetsov once distinguished the Aktovrak Formation, which he assigned to the Middle Cambrian \[24\].

According to the studies of V. A. Kuznetsov and to my brief observations in the Priuluzhem region, the unit under consideration participates in the structure of the axis of an anticlinal fold of almost latitudinal strike; on the southern limb of this fold there occur paleontologically characterized deposits of the Upper Silurian, resting, through a basal conglomerate and unconformably, on the underlying rocks of the Aktovrak Formation. On the northern limb of the fold there are widespread sandy-clayey deposits of the Kemterek Formation of the Lower Silurian, at the base of which there is also a basal conglomerate. The deposits of the Aktovrak Formation, being almost unmetamorphosed but steeply dislocated, with limb dips of the fold up to 65° and intensively schistose, are represented by a sequence of clastic rocks interbedded in the lower parts with effusives, more rarely with microquartzites and massive limestones containing remains of algae of the genus Osagia. Among the clastic rocks, in which conglomerates and sandstones predominate, especially characteristic are greenish-gray siltstones appearing in the upper parts of the formation. In the latter, V. A. Kuznetsov also discovered trilobite remains, which have proved to be poorly studied. Among them the new genus Schagonaria Jegor,¹ belonging to the family Paradoxidae, is established with certainty. From my collections of trilobites from the same formation and the same locality, but probably from a different horizon, there are established Redlichina sayanica and Protolenidae, known from Lower Cambrian deposits of the Eastern Sayan. The effusives, belonging to the basic series, are represented by pyroxene porphyrites, occurring in a diagenetized phase of state \[44\] and appearing to be rocks characteristic of the formation. They are massive, have a greenish-gray color, a fine-grained groundmass rich in large (up to 0.5 cm long) segregations of green pyroxene, with very rare participation of plagioclase. No less characteristic are microquartzites, which form thin lenses and interbeds and are readily concentrated in the field of development of the effusives. The microquartzites are distinctly layered rocks, with a dark-gray or greenish-gray color and a clearly expressed clastic structure.

Moving eastward from the locality described, there appears a series of lenticular limestone bodies, whose relationships with the sandy-conglomeratic deposits remained unclear to me. Z. A. Lebedeva believed that these limestones, rich in archaeocyathan remains, were disjunctively wedged into the sandy-conglomeratic formations, which were recognized by the author named as Upper Silurian. But, as already stated, in the particular section of the right bank of the Uluzhem River the sandy-conglomeratic deposits are undoubtedly Lower Cambrian. As for the archaeocyathan limestones, they may prove to be younger than the sandy-conglomeratic rocks, but also Lower Cambrian. The basis for this is the circumstance that the archaeocyaths of the limestones of the Priuluzhem belt, being in composition close to the archaeocyathan assemblage occurring in the limestones of the Chingian Formation of the Upper—

¹ Determinations of the paleontological cabinet of the West Siberian Geological Administration.

of the Elzhem River, differ from it in the clear predominance of pebble species. Limestones containing the pebble complex of archaeocyaths in other districts of the Sayan-Altai region are closely associated with the deposits underlying them and occur in the upper parts of analogues of the Chinginskaya Formation.

Thus, the more weakly metamorphosed effusive-sedimentary deposits of the Aktovrak Formation of the Priuluzhem area were erroneously assigned by V. A. Kuznetsov to the Middle Cambrian [20, 22], as was correctly pointed out earlier by N. S. Zaitsev and N. V. Pokrovskaya [9, 102—103]. These deposits are typically Lower Cambrian; they probably lie stratigraphically above the shale rocks of the upper course of the Elzhem River, which enclose limestones with remains of archaeocyaths of the Gavrilovka complex, but together with the latter participate in the structure of the same type of Chinginskaya Formation. This is proved by the fact that in the deposits of the Priuluzhem district there are microquartzites and pyroxene porphyrites, of which the former are the most characteristic stratigraphic markers for the Chinginskaya Formation, while the latter in other areas of its distribution always occur above the limestone belonging to it with the Gavrilovka complex of archaeocyaths.

In defining the age of the Aktovrak Formation of the Priuluzhem district as Lower Cambrian and regarding it as the upper members of the Chinginskaya Formation, we thereby also clarify the stratigraphic position of the hyperbasites of this district, which at one time I considered younger than the hyperbasites developed in other parts of the Kurtushibinsky Range. In light of the facts cited above, however, there remains no doubt that the hyperbasites of the named range of the Western Sayan are contemporaneous and locally closely associated with the Chinginskaya Formation, whose Lower Cambrian age, as will be shown below, is established with sufficient reliability.

Stratigraphy of the Lower Cambrian of the northern slope of the Western Sayan

Lower Cambrian deposits are also widely distributed along the northern face of the Western Sayan, where at one time they were combined into the first division of the metamorphic sequence [28, 258; 29, 172], and somewhat later—into the third upper Cambrian sequence [28, 258], the West Sayan and Uzunarga formations [47, 79; 38, 27].

The deposits of the division of interest to us, in their petrographic composition and partly in their paleontological content, show an extraordinarily close similarity to the contemporaneous formations of the southern slope of the Western Sayan. The differences consist, perhaps, only in the fact that on the northern slope an analogue of the Akkol Formation has so far not been established; instead, on this slope a new formation appears, which I have named the Babakovskaya.

According to my previous observations [38; 39; 40] and the detailed geological mapping carried out in recent years, the tectonic structure of the northern face of the Western Sayan appears as a large anticlinorium, slightly overturned to the north, with its southern limb, which has a complex structure, extending in the inner parts of the mountain system.

The axial part of the named anticlinorium, elongated in a northeastern direction, is composed of deeply metamorphosed crystalline schists of the Precambrian Dzhebash Formation [39, 138—142; 38, 28], which earlier were taken by some researchers to be Lower Silurian [28, 311; 29, 174].

The northern limb of the anticlinorium, which is at the same time the southern limb of the Minusinsk Basin, extending in an east-northeastern-

direction, is composed of successively alternating, from bottom to top, deposits of the Lower, Middle, and Upper Cambrian, pierced by numerous intrusions of varied petrographic composition and of different ages.

In this limb the Cambrian deposits have been affected by intense, fine, slaty cleavage, which obscures the elements of primary bedding; this bedding is more or less clearly preserved in the Upper Cambrian deposits, but has been almost entirely destroyed in the older rocks.

The elements of bedding and the planes of schistosity are characterized predominantly by a unidirectional steep southerly dip, with the exception of supplementary folds, which have an asymmetrical structure emphasized by the steep dip of the northwestern limbs of anticlines and synclines and by a certain flattening of their opposite parts.

It must be noted that underestimation of the tectonic-structure pattern described for the limb under consideration was one of the causes that gave rise to errors in the stratigraphic constructions of earlier investigators, who, by using the elements of attitude in a straightforward manner, took ancient members for young ones and the latter for ancient ones. This will have to be verified repeatedly in examining individual sections exposing Cambrian deposits.

Among the latter, the most intensely dislocated are the Lower Cambrian deposits adjoining, through a disjunctive structure dipping steeply to the southeast, the crystalline schists of the Dzhebash Formation of the Precambrian. In the belt of Lower Cambrian deposits, on the north connected through a disjunctive structure with the effusives of the Lower Monok Formation of the lower part of the Middle Cambrian, the principal role is played by the deposits of the Chinginskaya Formation, while the sediments of the Babakovskaya Formation are of much lesser importance.

Chinginskaya Formation

The deposits of the Chinginskaya Formation within the northern slope of the Western Sayan are considerably developed, forming a belt of variable width that extends almost without interruption from the middle course of the Amyl River in the northeast to the Kyzylgash stream, which flows into the Abakan River from the right in the southwest.

Over this vast area the deposits of the Chinginskaya Formation are expressed by rather diverse facies, the characteristic features of which could be illustrated by the example of many and thoroughly studied sections. However, I have considered it possible to examine in greater detail only those of them in which the specific features of the formation’s facies are most clearly expressed and in which its primary stratigraphic relationships with the younger Cambrian stratigraphic members are revealed with sufficient clarity.

One such section, which I have taken as the type section, is exposed by the M. Shush River in that stretch of its upper course where, sharply changing its northwestern direction to an almost meridional one, it cuts into the principal spurs of the ridges that make up the northern slope of the Western Sayan. This part of the latter, dissected in the southwest by the valley of the Yenisei River and in the northeast by its large right tributary, the Oya River, studied in its time by N. A. Batov [2] and in recent times by me, is composed chiefly of Cambrian deposits, to which, from the south, along a clearly expressed disjunctive structure, adjoin the crystalline schists of the Precambrian Dzhebash Formation. The Cambrian formations, constituting the limb of a fold slightly overturned to the north, are represented here by the Lower Monok effusives, in places resting with clear parallel unconformity upon the effusive-sedimentary rocks of the Chinginskaya Formation (Fig. 14), which is an analogue of the member of green schists and limestones [2]. The planes of intense...

the alignment of the rocks of the latter, coinciding with the direction of dip of the planes of the primary bedding, of east-northeast strike, dip unilaterally at an angle of 70° to the southeast, while the principal cleavage fissures of the Lower Monok effusives, with the same strike, show now a southeasterly, now a northwesterly dip, with varying angles of inclination from 70 to 50°.

Taking account of this motif of the tectonic structure of the area, it is not difficult to determine also the succession of the stratigraphic members of the Chingin Formation, which appear in ascending order when moving from south to north along the section.

Indeed, on the right bank of the M. Shush River, at the mouth of its right tributary, the Babakovsky Creek, there occur intensely metamorphosed and finely schistose crystalline schists of the Dzhebash Formation of the Precambrian,

Fig. 14. Relationships of the effusives of the Lower Monok Formation with deposits of the Chingin Formation along the M. Shush River in its middle course.

1—Lower Monok effusives; 2—microquartzites;
3—limestones and 4—metamorphosed sandstones of the Chingin Formation.

the planes of schistosity of which dip to the southeast 160° at an angle of 75–80°. Farther downstream, adjoining them along a clearly expressed disjunctive contact, which likewise dips steeply to the southeast, there appear the characteristic components of the Chingin Formation in the following succession from south to north (from bottom to top) and in the following alternation with one another (Fig. 15):

a) carbonaceous-siliceous schists of dark-gray color, in places noticeably pyritized; visible thickness about 500 m;

b) phyllites of light greenish color; thickness 370 m;

c) microquartzites, dark gray, in places black, often alternating with carbonaceous-siliceous schists; thickness up to 2800 m;

d) greenstone, intensely schistose effusives of the diabase-augite porphyrite type; thickness 850 m;

d) metamorphosed greenish-gray sandstones; thickness about 940 m;

e) massive dark-gray limestones, alternating with microquartzites and containing sills of albitophyre. At the contact with the albitophyres the limestones proved to be markedly recrystallized to the state of fine-grained marbles;

zh) albitophyres, to which, from the north across a disjunctive fault, light siliceous shales (lydites) adjoin; these in turn are overlain by albitophyres of a clearly cover facies. The latter, southwest of the valley of the M. Shush River, occur as thin sheets on various stratigraphic members of the Chingin Formation, and somewhat farther north merge into a comparatively broad belt of effusive rocks of the Lower Monok Formation.

The total thickness of the rocks of the Chingin Formation in the Malo-Shush section is determined at 5.5–6 km; moreover, the stratigraphic members distinguished within it are more or less persistent along the strike when moving southwest from the valley of the M. Shush River, whereas

Fig. 15. Schematic section along the M. Shush River.

1—effusives and 2—siliceous shales of the Lower Monok Formation; 3—microquartzites; 4—limestones; 5—metamorphosed sandstones; 6—greenstone effusives; 7—carbonaceous-siliceous shales and 8—phyllites of the Chingin Formation; 9—crystalline shales and 10—marbles of the Dzhebash Formation; 11—disjunctives; a, b, v...—stratigraphic members of the Chingin Formation.

toward the northeast from it many of them gradually disappear from the sections. Thus, in the direction toward the B. Shush River, and then the Baraksan River, microquartzites play an ever smaller role in the composition of the formation under consideration; here they appear in the form of numerous beds, from several meters to tens of meters thick, while in the basin of the Rudnaya River, a tributary of the Oya River, only two beds of microquartzites have been noted. Here they occur among dark-green, intensely schistose clayey shales, alternating with sandstones of the same color but of different grain size, with carbonaceous-siliceous shales, and also with limestones. The latter, having a dense structure, light-gray color, and extremely fine lamination, form short lens-shaped bodies, no more than 150 m thick. In the headwaters of the Rudnaya River, in one such limestone body, remains of algae are found, among them Renalcis granosus. Effusives are encountered here very rarely; they are represented by greenstone diabase porphyrites appearing in the form of concordant bodies.

Along the left bank of the Oya River, 0.5 km below the mouth of the Mangatov spring, there occur higher stratigraphic members of the formation, at the base of which lies a characteristic conglomerate horizon. It consists of well-rounded pebbles of pyroxene porphyrites, fragments of gray, bedded limestone, sandstone, and shale, cemented by a tuffaceous-clayey mass. From beneath the conglomerate, dipping to the southeast, there emerge clay shales with remarkably pronounced fine lamination, caused by the alternation of broad bands composed of greenish-colored aleurolitic material and narrow ribbons of light-gray color formed by pelitic material. Shales of this type, in places interbedded with their violet and maroon varieties, contain beds of fine-grained sandstones and rare sheets (?) of green, strongly schistose pyroxene porphyrites. The sequence ends with massive, slightly marbled limestones, which at the mouth of the Rudnaya River form a lens-shaped body up to 200 m thick, and in which N. A. Batov found isolated specimens of archaeocyathids [2, 42].

These upper members of the Chinginsky Formation, expressed essentially by violet and maroon shales containing thin sheets of pyroxene porphyrites, are repeated also in the section of the Naryssa River, 6 km above the mouth of its right tributary, the Takhtaya River (Fig. 16). There the formations under consideration adjoin, across a disjunctive fault, the crystalline shales of the Dzhebash Formation and, likewise across a disjunctive fault, are coupled with the effusives of the Lower Monok Formation. Within the latter there are beds of fine-pebble conglomerate composed of fragments of microquartzites, gray bedded limestones, violet shales, greenstone effusives, and other rocks—products of erosion of the Chinginsky Formation. The sedimentary formations of the Lower Monok Formation exhibit a steep dip to the southeast, which is also maintained in the deposits of the Chinginsky Formation, something entirely natural given the overturned bedding of the rocks of the latter.

Fig. 16. Scheme of the section of the Chinginsky Formation along the Naryssa River

1—green clayey shales; 2—clayey limestone shales; 3—violet and maroon clayey shales; 4—sandstones; 5—conglomerates; 6—pyroxene porphyrites; 7—microquartzites; 8—disjunctives.

To the southwest from the M. Shush River, the belt of the Chinginsky Formation, almost without changing in width, extends continuously to the headwaters of the Bey River, a right tributary of the Abakan River.

In this belt, as in the section of the M. Shush River, the rocks of the formation in the south are disjunctively joined with the crystalline schists of the lower horizon of the “Lower Silurian,” or Division II of the metamorphic sequence, assigned by me to the Dzhebash Formation [37; 38; 39], and by A. N. Churakov to the Kutenbuluk suite of Precambrian age (52, 191—192). From the north, the deposits of the Chingin Formation are likewise, probably over vast areas, separated by a disjunction from younger members of the Cambrian, in particular from the Lower Monok Formation.

Within the indicated belt, the section of the Chingin Formation is perhaps most fully represented along the left slope of the Yenisei valley, beginning 250—300 m above the mouth of the Ui River. Here, in the terrace scarp and on its surface, green and rusty-green metamorphosed rocks crop out, interbedded with graphitized schists from a few centimeters to two or three meters thick. The direction of dip of the clearly expressed banding, coinciding with fine schistosity, is southeasterly, and the angles of inclination vary within 75—80°. The visible thickness of the described member is probably not less than 150—200 m.

Then, on the left slope of the Ui River, there rises a series of large outcrops composed of dark, marbled and dolomitized limestones. In the limestones, interbeds of banded schists are visible, striking east-northeast and dipping almost due south at an angle of 75—80°. Intense schistosity, cutting the planes of primary bedding at an acute angle, strikes in the same direction as the bedding and dips southeast at an angle of 50°. Farther on, already on the left side of the Yenisei River and in the scarp of its first floodplain terrace, banded schists appear, in which the light bands are composed of quartz and calcite, and the dark bands are formed by chlorite, tremolite, epidote, and magnetite. The total thickness of the member is about 1.5 km; the attitude elements are the same as in the carbonate member of the formation.¹ Then characteristic light-gray, faintly banded microquartzites crop out, interbedded with a member of siliceous-clayey carbonaceous schists.

Finally, the microquartzites are replaced by a complex sequence of dirty-gray, thinly schistose aleurolitic sandstones, interbedded with clayey schists of the same color and containing two horizons of gray limestones. The total thickness of the described stratigraphic member is not less than 1.3 km; its rocks dip southeast at an angle of 60—65°. Adjoining it across a disjunction are effusive rocks of the Lower Monok Formation, cut through by albitites of the Main intrusion. If one were to use in a straightforward way the attitude elements chiefly of the schistosity planes, which in places coincide with the bedding elements and dip uniformly to the southeast, it could be supposed that the described members of the Chingin Formation appear successively in ascending order when moving from north to south along the section, in the same order in which they were at one time distinguished as part of the so-called Division I of the metamorphic sequence, or the third upper Cambrian sequence [29, 172; 47, 80]. However, this supposition must immediately be abandoned if one takes into account that in other parts of the Western Sayan the effusive rocks of the Lower Monok Formation are younger than the Chingin Formation. In the present section the latter is cut across the strike by numerous dikes of albitophyres, representing the roots of Lower Monok lavas. This circumstance indicates that in the area under consideration as well, the Cambrian stratigraphic members are in an overturned position, and their ascending order should be followed when moving from south to north along the section, in the southern segment of which the deposits of the Chingin Formation are situated, stratigraphically overlain by pyrogenic—

¹ These parts of the Yenisei section may possibly represent an analogue of the Akkal Formation.

rocks—the Nizhnemonok Formation (Fig. 17). The above-noted interformational break, probably having the character of a steep reverse fault, somewhat complicates the relationships between the formations, but does not disturb their general sequential order.

In strict and regular connection with the described character of the structure is the vigorous schistosity, the planes of which dip predominantly unilaterally to the southeast, but with clearly flattening angles of dip in the direction of overturning of the masses; in this same direction a weakening of metamorphism in the rocks of the formations is also observed.

In light of the circumstances set forth, it may be assumed with a sufficient degree of certainty that the stratigraphic members of the Chingin Formation as well, whose apparent thickness may be estimated at 3.4 km, successively replace one another in ascending order when moving from south to north along the section.

Fig. 17. Schematic section along the Yenisei River below the mouth of the Ui River

1—Maina trondhjemites; 2—effusives of the Nizhnemonok Formation; 3—algal limestones; 4—clayey shales; 5—sandstones; 6—bedded limestones; 7—carbonaceous-siliceous shales; 8—microquartzites; 9—banded shales and 10—banded shales with interbeds of graphitized shales of the Chingin Formation; 11—disjunctive fault; a, b, c … stratigraphic units of the Chingin Formation.

In the Yenisei section, the greenstone effusives synchronous with it drop out completely from the composition of the Chingin Formation, and its facies profile is determined essentially by fine-clastic, but strongly metamorphosed rocks, in a subordinate relation to which occur microquartzites, as well as limestones morphologically quite identical to the limestones of the Rudnaya River. However, in the basin of the middle course of the Siza River, a right tributary of the Yenisei, the limestones acquire a massive structure, and here, alternating with greenstone effusives and microquartzites, they contain remains of archaeocyaths.

In the area of the Tabat and Chekhan rivers the deposits of the Chingin Formation, changing somewhat in facies composition, on their hanging-wall side enter into complex relations with the Arbat Formation of the Upper Cambrian.

Thus, in the basin of the M. Arbat River, along its left side, 0.3 km above the Gryazny spring, the following section may be observed:

a) greenish-gray sandstones with interbeds of conglomerate crowded with pebbles of greenstone effusives, more rarely with fragments of phyllitized shales, pebbles of microdiorite, and also boulders of gray and pinkish-gray limestones; the direction of dip is southeast, 65–70°. Farther eastward there appears a narrow band b) of carbonaceous-siliceous shales not more than 50 m thick, followed by c) limestones of light-gray, sometimes reddish color with rare remains of algae and archaeocyaths. Among the latter, Ajacicyathus proskuriakovi, Archaeocyathus tolli, Coscinocyathus calatus [39, 136] were at one time identified; to this list must be added Ajacicyathus tomicus, Coscinocyathus roikovi, and the alga Epiphyton fasciculatum. The general composition of the archaeocyath fauna is closest of all to the Gavrilovka complex, and not to the Kameshki complex, as I had previously supposed [39; 40]. From the presence of banding in the limestones it may be established that they dip to the southeast at an angle of 70°. After this, beyond about 75 m of poorly exposed ground, there extend d) dark-gray, strongly schistose phyllites,

siliceous-argillaceous shales containing small interbedded bodies of microdiorites; the dip of the schistosity planes is to the southeast at an angle of 70–75°.

If one were guided only by the elements of occurrence, one might assume that the members of the described section, differing in lithologic composition, form a homocline with a one-sided dip to the south, the separate members following one another in an apparently ascending order, beginning with bed “a” and ending with bed “g.” However, a careful study of the other geological features of the section clearly shows that the members participating in its structure are in an overturned position. Indeed, in the conglomerate of bed “a” there occur pebbles of rocks characteristic of the shale-phyllite unit and of the limestones occurring among them; the conglomerates contain pebbles of microdiorites that cut the shale-phyllite rocks.

Obviously, the youngest formations of the section are the deposits of bed “a,” which represent the basal horizons of the Arbat formation (Fig. 18). The shale-phyllite rocks containing limestones with the fauna of archaeocyathids of the Gavrilovka complex are more ancient members of the section, forming components of the Chingin formation.

Fig. 18. Schematic geological structure of the interfluve of the M. Arbat and Dzhebash rivers.

1 — valley deposits and poorly exposed areas; 2 — effusives of the Pechnovka formation of the Middle Devonian; 3 — green thin-bedded sandstones; 4 — bed of sandstones with interbeds of conglomerates; 5 — basal conglomerate of the Arbat formation of the Upper Cambrian; 6 — Kair microdiorites of the upper part of the Lower Cambrian; 7 — archaeocyathid-algal limestones; 8 — phyllites and carbonaceous-siliceous shales of the Chingin formation of the second third of the Lower Cambrian.

On the southwestern continuation of the limestones of the M. Arbat River, in the basin of the Dzhebash River, along its right tributary the Chistobai River, limestones again appear, likewise occurring among the shales of the Chingin formation. But in these limestones the remains of archaeocyathids proved to be poorly preserved, and therefore their attribution to the Gavrilovka complex remained unclarified.

The Maly Arbat section, owing to poor exposure, by no means fully reflects the features of the facies composition of the Chingin formation within the belt of its distribution under consideration. This composition is best revealed from the almost continuous section of the Chekhan River, along the right slope of which, from the Karasuk River to the Kuganakov winter quarters, increasingly more ancient members of the Chingin formation appear, grouping into enlarged beds in the following sequence—

sequences from top to bottom, taking into account, of course, the overturning of the masses to the north (Fig. 19):

a) dark and gray, markedly chloritized phyllites, often interbedded with dark-gray, metamorphosed sandstones rich in carbonaceous matter and of quartz–feldspar composition. In the lower parts of the unit there appear concordant bodies of greenstone diabase porphyrites, almost completely altered to the state of sericite–biotite–albite schists. Dip direction—175°, angle of dip 75°; thickness of the unit about 750 m; interruption in outcrops for a distance of about 2 km;

b) a sheet of greenstone effusives, the rocks of which at the periphery have proved to be intensely schistose and transformed into albite–biotite–chlorite–epidote schists. In the central parts of the sheet the rocks are better preserved and are expressed as gray, cryptocrystalline, silicified and uralitized diabase porphyrites. On the hanging-wall side of the sheet lie beds of gray bedded limestones, dark phyllites, and sandstones; on the footwall side—gray calcareous phyllites and fine-grained sandstones of the same color; thickness of the unit about 820 m;

Fig. 19. Schematic section along the right bank of the Chakan from the mouth of the Krasu River to the village of Kutanakov.

1—metamorphosed sandstones; 2—phyllites; 3—greenstone diabase porphyrites; 4—beds of limestones; 5—hornfelsed sandstones and phyllites; 6—porphyrites; 7—microquartzites and 8—felsites of the Chingis formation; 9—amphibolites of the Kairlyk intrusive formation; 10—Maine trondhjemites; 11—Devonian crystalline schists; a, b, c... stratigraphic units.

c) biotite–sericite–chlorite, carbonatized hornfelses, alternating with hornfelsed sandstones of various grain sizes. The latter are in places pierced by numerous thin veinlets of quartz–calcite–tourmaline composition, from 2 to 10 cm thick. Some sandstone beds include lens-like, thin bodies of dark microquartzites, sometimes thin-bedded. Within the unit occur beds of strongly hornfelsed porphyrites and their tuffs, as well as sills of Maine trondhjemites; dip direction—southeast 170°, angle of dip 75°; thickness of the unit up to 900 m;

d) predominantly weakly hornfelsed, medium-grained sandstones, only locally interbedded with similarly hornfelsed phyllites; dip direction—southeast 165°, angle of dip 80°; thickness of the unit about 500–600 m;

e) light-gray, dense, finely schistose, sericitized and calcitized felsites, coarsely interbedded with dark phyllites, enclosing a concordant body of amphibolite and a sill of Maine trondhjemite; thickness of the unit up to 750 m;

f) dark, intensely hornfelsed porphyritic tuffs, interbedded with biotitized greenish-gray phyllites and carbonatized hornfelses, and enclosing several concordant bodies of Maine trondhjemites of varying thickness; thickness of the unit about 1 km.

Then a zone of crumpling about 100 m thick appears sharply, after which, in the outcrops, intensely platy, deeply metamorphosed, dark-green crystalline schists occur; I assign these to the Dzhebash Formation of Precambrian age.

The visible thickness of the deposits of the Chingin Formation in the section along the Chekhan River reaches no less than 6.5–7 km, and the increase in this thickness, in comparison with the Maloush section, clearly results from the development of effusive rocks, among which acid varieties are very characteristic. The sedimentary-effusive deposits of the section under consideration contain numerous concordant intrusive bodies, close in composition to the Maina trondhjemite, which caused intense metamorphism in the surrounding rocks. At the same time, in this section microquartzites are almost absent and limestones are poorly represented; but both of these rocks appear in considerable masses to the northeast of the valley of the Chekhan River, for example, along the Karasuk River and in the basin of the Dzhebash River.

Fig. 20. Scheme of the relationships of the Cambrian formations in the basin of the B. Kyzas River.

1—Quaternary deposits; 2—Arbat Formation; 3—limestones and conglomerates of the (4) Verkhne-Monok Formation; 5—effusive rocks of the Nizhne-Monok Formation; 6—limestones; 7—schists and 8—metamorphosed sandstones of the Chingin Formation.

In the direction of movement toward the Kair Range, the Mishikha River, a left tributary of the Karasibo River, the deposits of the Chingin Formation, being unconformably overlain by sediments of the Arbat Formation [40, 9], in their composition remain close to the Chakhan section. The differences consist only in the fact that in the section of the Mishikha River, in the upper part of the Chingin Formation, there are two clearly separated horizons of carbonate rocks, the lower of which is exceptionally rich in remains of algae. The upper horizon of limestones is overlain by a cover of characteristic pyroxene porphyrites of exactly the same composition as in the sections of the On and Naryssa rivers.

From the headwaters of the Mishikha River the belt of the Chingin Formation extends into the basin of the B. Kyzas River, where substantial facies changes again take place within it and where it enters into alternately complex relationships with the Arbat and with the Verkhne-Monok formations. The relations of the Chingin Formation with the Verkhne-Monok Formation of the lower part of the Middle Cambrian are distinctly revealed from the most complete exposed section of the B. Kyzas River, in the area from the mouth of the Levy Kyzas to Mount Troitskaya (Fig. 20).

Along this stretch natural outcrops are not numerous, but a fairly large number of various workings makes up for what nature itself did not provide. In any case, the artificial exposures, in combination with the natural ones, make it possible to form an idea of the successive change, in the stratigraphic section, of the most important members of the formation of interest to us, and, most importantly, of its relationships with the younger Cambrian deposits (Fig. 21).

a) On the right slope of the valley of the Poperechny Kyzas River, near its mouth, conglomerates are exposed; their well-rounded coarse pebbles consist almost entirely of albitophyres and porphyrites typical of the Nizhne-Monok Formation of the lower part of the Middle Cambrian. More rarely, in these conglomerates, pebbles of limestones, dark siliceous clayey schists, and me—

metamorphosed sandstones, the rocks of which are characteristic of the Lower Cambrian deposits. Within the conglomerate beds there occur “lens-shaped bodies”¹ of limestones, packed with remains of archaeocyaths and algae of the Sanashtyk-Gol (?) complex—(Ajacicyathus sanas chtycolensis). The dip of the conglomerate-carbonate sequence is southeast, at an angle of 50°.

b) Then, immediately in the junction between the Left and Transverse Kyzas, dark-gray massive, noticeably marbleized limestones appear, interbedded in the upper part with marls. At the visible base the limestones are intensely crumpled, finely laminated, and in places bear features of calcareous schists. Conversely, in the upper part they are more massive and appear somewhat sandy and clayey. The thickness of the limestones in the exposure is more than 250 m. The dip direction of the bedding, well emphasized by interbeds of marl, is southeast, at an angle of 70°. Almost in the middle part of the limestones, rare remains of archaeocyaths, not especially well preserved, were found.

c) After this, across an interval of only 10 m of unexposed space, on the right side of the Left Kyzas River, intensely laminated and crumpled carbonaceous-aleurolitic shales appear, the cement of which is noticeably recrystallized and rich in new formations consisting of chlorite, sericite, brown feldspar, and more rarely epidote. The thickness in the exposure is about 200 m. The dip direction is southeast, at an angle of 70°.

Fig. 21. Schematic section along the right slope of the Left Kyzas River

1—conglomerates; 2—limestones with the Sanashtyk-Gol complex of archaeocyaths; and 3—sandstones of the Upper Monok Formation of the lower part of the Middle Cambrian; 4—limestones with the Gavrilovka complex of archaeocyaths; 5—marly limestones; and 6—carbonaceous-siliceous shales of the Chingin Formation of the second third of the Lower Cambrian; 7—conglomeratic sandstones of the Arbat Formation of the Upper Cambrian.

d) Finally, along the same right slope of the Left Kyzas, across an unexposed space of about 100 m, coarse-grained conglomeratic sandstones with interbeds of conglomerate appear. In the composition of the latter, together with rare pebbles of albitophyres, porphyrites, and intrusive rocks, there are large, poorly rounded limestone boulders and fragments of carbonaceous-aleurolitic shales, extremely similar to analogous rocks exposed in the junction of the Left and Transverse Kyzas. These conglomerate-sandy formations, dipping southwest at 220° at an angle of 50°, I regard as the base of the Arbat Formation. It is interesting to emphasize that at the mouth of the Left Kyzas, on its left slope, limestones with archaeocyath fauna are absent, and here in the outcrops there occur conglomeratic sandstones and even conglomerates interbedded with arkosic greenish-gray sandstones and clayey shales, which constitute typical components of the Arbat Formation, or of the second Cambrian sequence [16, 5; 39; 40]. In the conglomerates and sandstones of this formation, exposed opposite the mouth of the Vesely Creek, a right tributary of the Left Kyzas, large, poorly rounded, elliptical boulders of slightly marbleized limestones are found, in which remains of archaeocyaths have repeatedly been observed, recognized as belonging to the Kamensk complex [40, 16].

d) South of the Veselyi Brook, a right tributary of the B. Kyzas (Fig. 20), from beneath the basal conglomerate of the Arbaty Formation, the middle members of the Chingin Formation apparently crop out. They are represented by greenish-gray clayey shales, often alternating with fine-grained sandstones, and by carbonaceous-aleurolitic shales containing an interbed of conglomerate composed of flat pebbles of black clayey shale and, more rarely, quartzite. The cement of the conglomerate is coarse sandy and intensely sericitized. Along the Bezymyannyi Brook, clayey dark-gray limestones with remains of algae of the genera Osagia and Renalcis granosus, as well as microquartzites, appearing in the form of short and thin lenses that rapidly disappear along strike, are assigned to this stratigraphic member.

The direction of dip in the described packet of the Chingin Formation changes: near the mouth of the Veselyi Brook the beds predominantly dip to the southeast, \(140^\circ\), at angles of dip of \(65\text{–}70^\circ\); then the direction of dip changes to northwestern, while the angles of dip become noticeably gentler, reaching on average no more than \(45^\circ\).

e) Finally, from beneath the described stratigraphic member there emerge intensely pyritized sandstones of rusty-gray color, of medium- or fine-grained structure. The sandstones consist of a variable amount of subrounded or angular grains of quartz, plagioclase, and, more rarely, quartzites. The cement is clayey-siliceous, rather strongly sericitized, weakly chloritized and epidotized.

Thus, in the section of the B. Kyzas, deposits of different ages take part, gathered into relatively simple asymmetric folds distinctly inclined to the northwest. Of these deposits, the youngest are the sediments of the Arbaty Formation, which fill the lock of a synclinal fold (Fig. 20). At the base of the Arbaty Formation, whose age I determine as Upper Cambrian, lies a thick basal conglomerate, resting unconformably and transgressively on various stratigraphic members of the underlying sequences.

On the left bank of the B. Kyzas, from beneath the Arbaty Formation there crop out conglomerate-sandstone rocks of the Upper Monok Formation, containing lenses of limestones with remains of archaeocyaths of the Sanashtykgol complex (packet “a”). In the conglomerates of the formation, which are also arranged transgressively (Fig. 21), pebbles and boulders occur of rocks in no way differing from the rocks of the still more ancient sequence previously united into the Uzunarga Formation or the upper Cambrian sequence. I have recognized this stratigraphic member as an analogue of the Chingin Formation. At its visible base there appear characteristic pyritized sandstones with an incomplete thickness of about 150–175 m; stratigraphically above lie carbonaceous-siliceous shales and phyllites with interbeds of microquartzites and conglomerates. The thickness of this packet, taking into account additional folding, is determined within 450–475 m. Within the carbonaceous-siliceous shales there are two limestone horizons, of which in the upper one (packet “b”) archaeocyaths occur, and among them the following forms were at one time identified: Ajacicyathus cf. otichtaschensis, A. dissepimentalis, A. ijizkii, A. immanis, Coscinocyathus dianthus, Protopharetra simplicissima. This limestone horizon is well traced along the watershed between Poperechnyi and Srednii Kyzas, and there it lies within the shaly members of the formation, dipping to the southeast, \(140^\circ\), at an angle of \(65^\circ\). Archaeocyaths are present in the limestones, among which there are many forms in common with the species found in the limestones exposed in the core of the B. Kyzas (packet “b”). These common forms—Ajacicyathus otichtaschensis, A. immanis, Coscinocyathus dianthus—are accompanied by Archaeocyathus tolli, Labyrinthomorpha lata, Spirocyathus densus, Tercyathus batovi, and also by algae—Epiphyton fasciculatum, E. grande, Edelsteinia cylindrica, Renalcis granosus, Osagia ovosimilis.

On the whole, the composition of the archaeocyathan fauna of the horizon described, previously combined into the Kameshkovsky complex [11, 135], in accordance with the new data, is distinguished as the Gavrilovsky complex. The second horizon of limestones—the lower one—is characterized by the presence of algae—Osagia, Renalcis granosus.

The apparent, far from complete, thickness of the Chingin Formation in the Kyzass area reaches not less than 800–900 m; moreover, its deposits, while rather strongly dislocated and intensely schistose, prove to be relatively weakly metamorphosed.

Some features of the lithological composition and facial expression of the Chingin Formation of the northern slope of the Western Sayan

An analysis of the sections described above shows that the facies profile of the Chingin Formation of the northern flank of the Western Sayan does not remain constant, but changes markedly both horizontally and vertically; at the same time, however, some of its characteristic components, such as, for example, microquartzites, pyroxene porphyrites, and especially algal-archaeocyathan limestones, can be traced from section to section with striking regularity, appearing in them only in differing quantities and in beds of different thickness. Other members of the formation are less persistent, in particular the clastic and effusive-tuffaceous ones. Among the clastic facies, an important place in the composition of the formation is occupied by medium- and fine-grained sandstones, grouped chiefly in the southwestern belt of its distribution and gradually disappearing from the sections with movement toward the northeast. In any case, in the sections of the Oi River and, especially, the Naryssa River, sandstones appear in the form of thin beds situated within the lower schist horizons. It should be emphasized, moreover, that in mineralogical composition and even in degree of metamorphism the sandstones, regardless of their stratigraphic position, prove everywhere to be more or less homogeneous. They always retain a clearly expressed psammitoblastic structure, formed by angular, more rarely subrounded, grains of plagioclase, with a noticeable participation of quartz and a relatively rare presence of angular fragments of albitophyre and quartzite. Only in some varieties of sandstones, alongside the two minerals noted, are grains of pyroxene sometimes found, as well as subrounded, somewhat elongated small fragments of chloritized, sericitized, and, rarely, graphitized schist, and larger angular fragments of chloritized but generally fresh andesite. Of the newly formed minerals, epidote and biotite are especially abundant, the latter component being constantly replaced by chlorite and sericite. The cementing carbonate-pelitic, more rarely quartz-feldspathic mass is deeply chloritized, less sericitized, calcitized, and epidotized.

Coarse-clastic rocks appear very rarely within the formation. Expressed as thin interbedded conglomerates or conglomeratic sandstones, in the area of the Naryssa–Rudnaya rivers they probably constitute two independent horizons, of which the lower seems to be positioned stratigraphically above the algal thin-bedded limestones. This lower conglomerate horizon, established in the upper reaches of the Naryssa River (Fig. 16), extends also on the left bank of the Oi River, below the Mangatov winter hut, and is lost somewhere on the watershed between the latter and the Rudnaya River.

At all the noted points the conglomerate under consideration is remarkably consistent in composition, being made up predominantly of fine and well-rounded pebbles of effusives, schists, and fragments of layered limestones, cemented by a tuffaceous-clayey mass, strongly chloritized—

Taking into account the composition of the conglomerate, it may be regarded as one and the same bed, situated at a single stratigraphic level and separating the upper members of the formation from the lower ones.

These upper members in the basins of the Naryssa and Oi rivers are represented chiefly by shale formations, including their burgundy and violet varieties, containing thin flows of pyroxene porphyrites, rare beds of carbonaceous-siliceous shales, and interbeds of lydite; the thickness is increased by a horizon of archaeocyathid limestone.

The upper members of the Chinginskaya Formation are characterized by a similar composition also in the basin of the B. Kyzas River, where they are separated from the lower components of the formation by a member of carbonaceous-siliceous rocks containing lenses of conglomerate (Fig. 22).

This is not the place to dwell on the characteristics of the shale formations, since they are extremely variable facies-wise in their composition and differ little from similar rocks participating in the structure of the stratotype section of the formation established along the Akkol River. It should only be noted that the pelitomorphic components, both in the Akkol River basin and on the northern slope of the Western Sayan, are grouped predominantly in the upper parts of the Chinginskaya Formation.

Effusives, concentrated in the Shushensko-Chekhanskaya belt of development of the formation, are completely absent in the southwestern continuation of the latter (B. Kyzas River) and appear as rare interstratal bodies along the northeastern extent of the belt (basins of the Oi and Naryssa rivers).

In the Shushensko-Chekhanskaya belt, the effusives, forming mainly interstratal bodies and much more rarely flows of varying thickness, are sometimes accompanied by tuffs or by their tuffogenic varieties. The differences in mode of occurrence do not affect the petrographic composition of the effusives, represented chiefly by diabase, labradorite, and, more rarely, pyroxene porphyrites, which have undergone fine fissility and profound greenstone alteration. In connection with the latter, only a relict porphyritic texture is preserved in the rocks listed, in a fine-grained, more rarely dense, greenish-gray groundmass, in which distinct segregations of plagioclase, pyroxene, and hornblende are situated.

Segregations of plagioclase are rarely preserved in an unaltered state; more often its prisms are cataclased, fragmented, lose their regular outlines, and zoisite develops along them, less commonly chlorite and albite. In other cases, plagioclase segregations are almost completely replaced by chlorite, calcite, and especially muscovite, or else are completely albitized.

Secondary alterations affect much more sharply the segregations of the dark-colored components; their well-expressed prisms undergo intense actinolitization or are replaced by an aggregate of calcite, zoisite, and, more rarely, biotite.

Among this type of effusives, especially characteristic are the pyroxene porphyrites, which occupy the highest stratigraphic position in the column of the Chinginskaya Formation. In the basin of the Mishikha River (Fig. 22) they form a small flow resting on marbled limestones correlated with the archaeocyathid limestones of the B. Kyzas River.

Similar effusives have also been identified along the Naryssa and Oi rivers; there, moreover, the pyroxene porphyrites are repeatedly interbedded with clay shales, again in the upper parts of the Chinginskaya Formation. At all the localities indicated, the pyroxene porphyrites are remarkably consistent in composition. They possess a clearly expressed porphyritic texture with a pilotaxitic, and sometimes andesitic, groundmass. Among the segregations, broad prisms of pale-green hornblende, of medium and large size (up to 1 cm in length), formed by replacement of pyroxene, predominate. The main—

I r. Alelek
II Watershed of the Rudnaya River
III r. M. Izyr?
IV Middle course of the Sizaya River
V Yenisei River near the settlement of Uy
VI Upper course of the M. Arbat River
VII r. Chekhan
VIII r. Minusa
IX r. B. Irba?

Kudasskaya Formation

Maloshushenskaya Formation

Dzhebashskaya Formation, Pt₂

Fig. 22. Types of sections of the Chinginskaya Formation on the northern slope of the Western Sayan

1—conglomerates; 2—fine-grained bedded sandstones; 3—diabaseized effusives of dyke and sheet facies; 4—violet shales; 5—green bedded shales; 6—microquartzites; 7—metamorphosed sandstones; 8—massive limestones; 9—bedded limestones; 10—massive shales; 11—thin-bedded shales; 12—phyllites; 13—greenstone effusives; 14—porphyroblastic shales; 15—banded shales with interbeds of graphitized shales; 16—metamorphosed banded shales; 17—Kairian microdiorites; 18—Maina trachytes; 19—pyroxene porphyrites; 20—green-stone acid effusives; 21—tuffs; 22—phyllitized shales; 23—crystalline schists and gneisses; 24—contacts: a) transgressive and b) disjunctive.

The groundmass is a fine-crystalline aggregate formed by prisms of pyroxene, generally weakly amphibolized, and by flakes of feldspar, strongly saussuritized. Chlorite and magnetite are abundant in it; these are associated with calcite and epidote, forming accumulations or diffuse aggregates.

Effusives that have fallen within the sphere of influence of a microdiorite intrusion undergo additional changes connected with their hornfelsing. In the groundmass of such hornfelsed porphyrites, consisting of small prisms of saussuritized plagioclase and large flakes of chlorite, quartz appears; its xenomorphic grains are situated in the interstices between the above-mentioned minerals, in amounts reaching no less than 5–6%. In addition, biotite develops especially abundantly in the rocks, often passing into chlorite, as do sericite, more rarely actinolite and calcite, and also magnetite, almost entirely replaced by brown or red iron oxide.

But irrespective of intrusive influence, in the effusives—especially those composing small conformable bodies—the processes of alteration go so far that they almost completely efface the primary nature of the rocks, and the latter acquire the typical features of schists. In such rocks a hetero-, more rarely homeo- or nematoblastic structure is clearly manifested, formed by prisms of albite; flakes of green chlorite replacing brown biotite; sericite; quartz; epidote; zoisite; and more rarely actinolite. The latest secondary minerals are chlorite and carbonates, which most often develop in the form of veinlets arranged along the schistosity. Such strongly altered quartz–albite–zoisite–actinolite rocks are most often spatially confined to the lower parts of the Chingis Formation, where they are involved in zones of major disjunctive disturbances. In the basin of the Tabat and Dzhebash rivers the rocks under consideration, adjoining the disjunctive disturbance that separates the deposits of the Dzhebash Formation from the rocks of the Chingis Formation, have proved so strongly altered that they show a complete external resemblance to the crystalline schists of the Dzhebash Formation. However, at some distance from the disjunctive contact, in quartz–albite–zoisite¹–actinolite rocks, relics of an effusive structure can always be detected under the microscope, something that is never found in the crystalline schists of the Dzhebash Formation. Moreover, quartz–albite–zoisite–actinolite schists are not infrequently also found along the periphery of large sheets of effusives occurring in relatively high horizons of the Chingis Formation.

Within the latter, acidic varieties of effusives have been established, represented by felsites and “keratophyres”; the former underwent a less profound greenstone alteration than the basic effusives.

The felsites, identified in the section of the Chekhan River, where they occur as conformable bodies, have a light-gray coloration with a very faint greenish tint, a dense structure, and a fine lamination, along the planes of which an abundant dissemination of sericite develops. Under the microscope the rock shows a fine-grained felsitic quartz–feldspar mass, in which the segregations of fine albite are only barely discernible. Chlorite, sericite, calcite, and more rarely actinolite are abundant in the groundmass. In addition, the felsites underwent slight silicification with the formation of accumulations of chalcedony.

“Keratophyres,” widespread in the Kalganovsky Range, are very distinctive rocks. They are rather strongly laminated, have a dark-green color and a dense groundmass. Under the microscope they display features of deep metamorphism, almost completely

destroying the primary composition of the rock, crowded with new formations belonging to chlorite, calcite, epidote, albite, and actinolite. Only in rare cases among such altered rocks was it possible to detect varieties in which areas of a vaguely recrystallized groundmass, resembling a felsitic one, and segregations of large melted-looking grains of albite No. 4 are visible.

Tuffs are spatially closely associated with the effusives, and especially with their basic varieties; together with them—and most often with the participation of siliceous-argillaceous shales and phyllites—they determine, in some areas of the distribution of the Chingin Formation, its facies profile (the Tabat River).

As already noted, microquartzites and reefogenic limestones are the most important marker formations of the Chingin Formation; they appear everywhere and independently of the character of its facies profile.

Microquartzites occur in the form of relatively persistent beds extending for considerable distances, but with sharply variable thickness. In these rocks, which are noticeably metamorphosed and characterized by their dark or even black, more rarely greenish coloration, a clastic-laminated structure is often established under the microscope, formed either by an alternation of dusty-carbonaceous matter and fine quartz material, or by a distinct sorting of the latter.

In the first variety of structure, its laminated features are emphasized by the alternation of thin, dark bands composed of carbonaceous matter and light and broader bands formed by quartz material. Quartz grains with noticeably serrated edges closely adjoin one another, and thus the structure of this band is close to hornfelsic. However, fine grains of quartz with clearly clastic outlines are also located within the carbonaceous mass of the band, concentrating in it at its base.

In addition, among the carbonaceous-quartz material, large and noticeably rounded fragments of quartz stand out distinctly, thinly enveloped by the cementing mass (Fig. 23).

In the second variety of structure, its laminated character is determined by rhythmic sorting of the clastic material, expressed mainly by quartz, with comparatively weak participation of carbonaceous matter, which stretches out in the form of a cord in the upper part of the band. The quartz grains, in the main part of their mass, proved to be recrystallized and thus lose their primary clastic form. But in individual grains the latter is preserved in the form of a dotted outline, clearly visible at higher magnifications (Fig. 24). Thus, from the description given it becomes evident that microquartzites are primary sedimentary rocks and, being characteristic only of the Chingin Formation, represent for it characteristic stratigraphic elements.

Opinions have constantly been expressed in the literature that microquartzites are metasomatic formations that arose through the alteration of limestones, shales, and even effusives. Hyperbasites, from which siliceous solutions emanated, causing partial silicification of the enclosing sequence, were considered the sources of metasomatism [47, 79].

Such metasomatic formations, especially widespread in the Kalganovka Range and the Tabat district, were at one time called “hornfelses.” But, probably, not all these “hornfelses” are metasomatic; among them there are also primary sedimentary representatives, such as, for example, the siliceous shales indicated by A. N. Churakov [52, 190–191]. He emphasized that in the Kalganovka Range the siliceous shales (“hornfelses”), occurring among “keratophyres”

and dark oolitic limestones, are separated from the latter quite sharply and, together with them, are schistose in an east-northeasterly direction.

To this we add that, at the locality under consideration, hyperbasites are absent, while the small plutons of Maina trondhjemites widespread here cut through the deposits of microquartzites. It is obvious that the formation of the microquartzites of the Kalganov Range was not connected with intrusive activity and that they, as in other parts of the Western Sayan, for the most part arose through recrystallization of originally sedimentary siliceous rocks.

No less characteristic marker elements of the Chingin Formation are limestones which, like the microquartzites, in their spatial distribution are confined to all facies of the formation,

Fig. 23. Drawing of a thin section with a large quartz fragment (1), enveloped by thin strands of carbonaceous mass (2) ×60

Fig. 24. Drawing of a thin section of microquartzite with relict rhythmic bedding, M. Shush River, middle course ×60.

and, probably, in its upper parts two independent horizons are grouped, differing from one another in morphological features and paleontological content.

One of these horizons, established in the basin of the B. Kyzas River, is located at the base of the schistose components of the formation (Fig. 22). It reaches a thickness of about 150 m and is composed of dark-gray, slightly clayey limestones, relatively rich in remains of algae, previously taken for concentric rollers of inorganic origin.

In the basin of the Mishikha River this same, probably, horizon is already composed of algal calcareous shales up to 250 m thick [40, 9], and along the Yenisei River at the mouth of the Sizaya River it is apparently expressed by thin-bedded, slightly silicified limestones, again with remains of algae. The latter are almost everywhere represented by members of the genera Osagia, Renalcis, and Epiphyton.

Another horizon (Fig. 22) of limestones, apparently occurring at the very top of the formation, is typically represented in the basin of the Kyzas River, at the confluence of its tributaries the Poperechnyi and Srednii Kyzas, as well as along the watershed area between the named rivers.

This horizon is characterized by sharply distinct morphological features. The limestones composing it are for the most part massive, dense, more rarely fine-grained, dark-gray and gray in color; much more rarely there appear white, yellowish, or pinkish varieties, in which most often

and remains of archaeocyaths and algae are found, the composition of which I have given in the appropriate places in the descriptions of the sections.

Probably limestones of this same horizon crop out along the Mishikha River, below the Konyukhovsky spring, the Karasuk spring, the right tributary of the Chekhan River, in the basin of the M. Arbat River, in the middle reaches of the Sizaya and M. Shushi rivers, and also along the Rudnaya River, near its mouth. At all the points listed, the limestones, being morphologically close to the limestones of B. Kyzas, sometimes reach thicknesses of more than 300 m and are often characterized by the presence of algae. As for archaeocyaths, the latter, in poor preservation, were established by N. A. Batov [2] along the Rudnaya River, then by me in the upper reaches of the M. Arbat River, 0.3 km above the Gryazny spring. Finally, well-preserved archaeocyaths were also found in the middle reaches of the Sizaya River, but, unfortunately, a small collection of them was lost for a number of reasons.

Limestones are also characteristic of the older members of the Chingin Formation; examples of this may be the sections along the Yenisei River (Fig. 22), the Kalganovsky Range, and the Tabat River. At all the indicated points the limestones are characterized for the most part by a massive structure, gray to black coloration, and an abundance of anthroconitic material concentrated in separate portions. Oolitic varieties of limestones containing remains of algae of the Osagia type are noted. In the basin of the Rudnaya River (Fig. 22), undoubtedly, in the lower horizons of the formation there occur lenses of thin-bedded limestones with remains of Renalcis granosus.

At the same time, from an analysis of the sections cited above, the primary stratigraphic relationships of the Chingin Formation with the surrounding younger Cambrian deposits are revealed with exceptional clarity (Fig. 22).

Thus, in the example of the sections of the M. Arbat River and then the Left Kyzas River, it is clearly established that the Chingin deposits are overlain with sharp unconformity and a basal conglomerate at the base by rocks of the Arbat and Upper Monok formations respectively, on which I have dwelt in detail in one of my works [40].

In the basin of the middle reaches of the M. Shushi River, low-thickness flows of Lower Monok effusives lie transgressively on the eroded surface of the Chingin sequence. But this locality is so far the only one where the primary stratigraphic relations between the Chingin and Lower Monok formations, which are most often in disjunctive contact with one another, have been preserved. Yet even with such relationships, the successive ascending order of the formations under consideration is established quite reliably when a number of features are taken into account, of which the following, in my opinion, are the most important. In the deposits of the Chingin Formation, almost throughout the area of its distribution, sills and dikes of albitophyres are found, representing feeder channels of the Lower Monok effusives. In the dikes of albitophyres, especially numerous in the belt of development of the Chingin Formation adjoining the valley of the Yenisei River, there are included, of various outlines, fragments of microquartzites and greenstone effusives, with clear relict tectonostructures, torn out by the dikes during their intrusion into the sequence of the Chingin Formation.

This is especially effectively expressed in the Kalganovsky Range, where stock-like masses of greenstone basic effusives, strongly schistose, are cut by almost unschistose dikes of albitophyres, filled with large xenoliths of the rocks enclosing the dikes. In addition, among the lavas of the Lower Monok effusives, at many points, beds of fragmental rocks have been established, in places filled with pebbles of greenstone effusives and, among them, pyroxene porphyrites, schists of various coloration, and microquartzites, in their petrographic composition entirely identical

similar components making up the Chinginskaya Formation. All these facts leave no doubt that the deposits of the Chinginskaya Formation are older than the Nizhnemonok effusives.

Finally, from an analysis of the same sections it follows with complete clarity that the general stratigraphic column of the Chinginskaya Formation on the northern slope of the Western Sayan is in many details strikingly close to the column of the formation of the same name, expressed in its stratotype section along the middle Akkol River and in the lectotype section of the Elzhemu River (Figs. 12 and 22). This similarity is especially emphasized by the fact that, both on the northern slope of the Western Sayan and on its southern slope, large masses of reefogenic limestones with nearly identical forms of archaeocyaths and algae are confined to the upper members of the Chinginskaya Formation, represented chiefly by clastic rocks. The similarity of the formation is also indicated by microquartzites, which appear in the deposits regardless of facies and are, in their main mass, primary sedimentary formations, though they have undergone varying degrees of recrystallization. All these similar features testify to the simultaneity and identical sedimentation conditions of the deposits of the Chinginskaya Formation on the southern and northern slopes of the Western Sayan. Therefore the predominant association with them of masses of hyperbasites, whose character and age I shall discuss in the appropriate place, is not accidental.

Babakovskaya Formation

The column of the Lower Cambrian division of the northern slope of the Western Sayan is not exhausted by the Chinginskaya Formation, but is extended by one more stratigraphic member, which I have named the Babakovskaya Formation.

In its stratotype manifestation this formation, expressed in the upper course of the Tabat River, along Babak Creek, is composed of limestones, unconformably and with a basal breccia at the base overlain by the effusives of the Nizhnemonok Formation of the lower Middle Cambrian. As already indicated in the appropriate place (Figs. 1 and 2), the footwall of the Babakovskaya Formation is not exposed by the present denudation surface, and therefore it was not possible to establish its relations with the underlying Chinginskaya sequence.

It can only be supposed that the limestones of the Babakovskaya Formation rest, with a break and unconformity, upon the more strongly dislocated and intensely metamorphosed rocks of the Chinginskaya Formation. This assumption is based on the fact that in other regions of the Sayan-Altai area, deposits synchronous with the Babakovskaya Formation, with a basal conglomerate at the base, rest unconformably upon rocks composing the lowest members of the Cambrian and even of the Precambrian (the Kayanchinskaya Formation of the Gorny Altai; the Erbinskaya—village of Potekhino, Kuznetsk Alatau).

In the basin of the Tabat River the Babakovskaya Formation extends as a narrow and discontinuous belt from the upper course of the Chazyryk River in the northeast to the left bank of Chistogudezh Creek, which flows into the Chazyryk from the left, in the southwest. Along this entire extent the formation is composed of highly characteristic limestones. They have a light, light-gray, sometimes siliceous-white color, a predominantly massive structure, more rarely fine bedding, especially well expressed in the middle parts of the limestone sequence.

In the massive varieties of limestones, algae of the Osagia type, especially Epiphyton sp., are constantly found, and much more rarely trilobites. The latter were identified by me at several points, but the trilobites proved to be especially abundant in a small lens of shell limestone situated among the massive limestones composing the cliffs on the watershed between the Chistogudezh and Babak creeks (Fig. 2).

From the mentioned lens I identified the following representatives of trilobites: Granularia obrutchevi Pol., Erbia gigantea Jegor., Tabatopygellina n. gen.

Proerbia Lerm., Arbataspis n. gen. Chondragraulos Lerm. The first two forms were also found in the limestones on the right bank of the Chistogudezh Creek, Babak Creek, and on the right bank of the Chazryk River, 0.3 km above Izvestkovyi Creek. Here the trilobites occurred in association with archaeocyaths, with brachiopods present only rarely. A small collection of archaeocyaths, gathered as early as 1946, was studied by P. S. Krasnopeeva, who identified the following forms: Ajacicyathus sp. nov., A. cf. otichtaschensis, Sibericyathus dissepimentalis, Bicyathus crassisimurus, Ajacicyathus simplex, Tersia sp.

The listed composition of the archaeocyath fauna, being characterized by a predominance of sanashtykol species over kameshkov species, is distinctive, but very close to the so-called Obruchev archaeocyath complex [11, 38]. Among the new collections of archaeocyaths from the same locality, so far Coscinocyathus dianthus and Ethmophyllum gracilis have been identified; moreover, these forms proved to occur in an association of archaeocyaths filling the limestones of Gerasimov Creek, a right tributary of the M. Monok River.

The algae studied by P. S. Krasnopeeva proved to be represented by ordinary species, such as: Epiphyton fruticosum, E. fasciculatum, Rasumovskya ramosa.

I assign to the Babak Formation the limestones that appear in the area of distribution of the effusives of the Lower Monok Formation, typically manifested in the basin of the V. Monok River, which flows into the Abakan from the right near the village of Monok. These limestones, occurring there in the same relationships as the trilobite–archaeocyath limestones of the Chazryk River basin, are especially closely allied to the latter in the composition of the archaeocyath fauna.

But in order to verify what has been said, let us examine the section along Gerasimov Creek, where the relationships between the limestones of interest to us and the Lower Monok effusives are expressed most clearly (Fig. 25).

Fig. 25. Section along Gerasimov Creek 1.2 km above its mouth.
1—fine-clastic conglomerates; 2—pebble conglomerate and 3—gravelites of the Upper Monok Formation; 4—Mainian trondhjemites; 5—tuffs and 6—tuff-conglomerates with rare flows of effusives of the Lower Monok Formation; 7—limestones with the Obruchev archaeocyath complex of the lower(?) Babak Formation.

I first studied the section along Gerasimov Creek as early as 1940, during the preparation of a medium-scale geological map. In 1951 I studied this section under conditions of detailed geological mapping, with the use of mine workings and with tracing of characteristic horizons along the strike. As a result of these detailed investigations it became clear that the limestones developed in the lower course of Gerasimov Creek had earlier been erroneously included among the deposits of the Upper Monok Formation [39, 130], since the latter, as will be shown below, are undoubtedly younger than the former.

The limestones mentioned, 1.2 km above the mouth of Gerasimov Creek, compose a narrow strip elongated in a sublatitudinal direction and bordered on almost all sides by effusives of the Lower Monok Formation. The limestones, having a light-gray, gray, and sometimes cherry-red coloration and a massive structure, contain abundant remains of archaeocyaths. From the inclination of the cups of the latter it is established that the limestones predominantly dip to the southeast, $150^\circ$, at an angle of $70^\circ$. At their northwestern contact, bedding is established in them, caused by the appearance of thin interbeds of yellowish marls dipping to the northwest at an angle of $65^\circ$. Evidently,

The limestones of the belt described form a steep anticlinal fold of sublatitudinal strike, of which only the southeastern limb has been most fully preserved.

In the limestones several systems of fractures are clearly expressed, the principal ones being oriented in accordance with the strike of the bedding, but dipping at a steeper angle to the southeast and more gently (25–30°) to the northwest; fractures are also noted that extend in a sublongitudinal direction and dip to the northeast at an angle of 70–75°. These steeply dipping fractures are, in places and to a considerable depth, filled with a violet-gray mass containing numerous fragments of whitish albite and gray quartz, or with a reddish, and sometimes yellowish-gray, dense clayey material, always with a slight admixture of fine-sandy matter. The products filling the fractures are typical sedimentary dikes (Fig. 26).

Fig. 26. Limestones of the Babakovo Formation (1), enclosing sedimentary dikes (2); sketch from a photograph; Kl. Gerasimov 1.2 km upstream from its mouth.

In the southeastern limb the limestones are cut by a small stock of quartz albite, which is an analogue of the Maina trondhjemite. In the steep exocontact with the quartz albite, the limestones are intensely silicified and pyritized, acquire a brown color, and in places are penetrated by veinlets of epidote associated with chlorite, calcite, and pyrite. The width of the zone of contact-altered rocks is very small and measures only a few tens of centimeters.

Conglomerates rest directly on the quartz albitites; on their footwall side lies a thin bed of gravelite, consisting almost entirely of grus of quartz albitite cemented by a siliceous-clayey mass. The gravelites and the overlying boulder conglomerates, dipping to the southeast at \(150^\circ\) at an angle of \(55—60^\circ\), constitute the base of the Upper Monok Middle Cambrian formation [38; 39, 129].

In the northwestern limb the limestones under consideration are overlain by a member of grayish-lilac tuff-conglomerates, which in turn are overlain by albitophyres of the same coloration, alternating with their amygdaloidal varieties and tuffs. In the lavas of the albitophyres, large (up to 10 cm in diameter), acute-angled xenoliths of limestones with rare archaeocyath remains are occasionally found. In this pyroclastic unit, bedding is established in a number of places, dipping to the northwest, \(345^\circ\), at an angle of \(60—65^\circ\). Moving eastward from the valley of Gerasimov Creek, on the watershed between it and the M. Monok River, the limestones plunge beneath the pyroclastic unit, on which, in the upper reaches of the M. Monok River, the basal conglomerate of the Upper Monok formation rests.

From the section presented (Fig. 27) it is clearly evident that Gerasimov Creek, in its lower course, cuts an anticlinal fold, in the core of which limestones are exposed that, by their stratigraphic position and especially by the composition of the archaeocyaths occurring in them, correspond to the archaeocyath-trilobite limestones of Babak Creek.

Fig. 27. Schematic section along Gerasimov Creek.

1—Early Variscan (?) diorite-diabase intrusion; 2—conglomeratic sandstones; 3—sandy limestones with rare archaeocyaths; 4—sandstones; 5—limestones with archaeocyaths of the Sanashtyk-Gol complex; 6—limestones with siderite interbeds; 7—conglomerates, and 8—gravelites of the Upper Monok formation; 9—Main trondhjemite; 10—limestones of the lower (?) Babak formation; 11—tuffs with rare quartz porphyry clasts; 12—tuffs, tuff-conglomerates with sandstone interbeds; 13—albitophyres; and 14—lydites with remains of sponge spicules of the Lower Monok formation.

In fact, in the limestones of the lower course of Gerasimov Creek, exposed in the core of the anticlinal fold described, remains of archaeocyaths were collected by me at various times; only some of them were studied by S. V. Chernysheva, paleontologist of the West Siberian Geological Administration, who identified the following species: Ajacicyathus ijizkii, Ajacicyathus cf. immanis, A. proscuriakovi, A. cf. uricus, Coscinocyathus dianthus Born var. stabilis Krasn., Coscinocyathus sp., Ethmophyllum cf. gracilis, Ethmophyllum ratum, Ethmophyllum ulmensis. In addition, it is possible that the following forms were also at one time determined from this same limestone—Ajacicyathus dwighti, A. pelvinulus, A. disporvallum, A. (Rhabdocyathus) crassimurus, Carinocyathus bagenovi, Coscinocyathus dianthus, C. convexus, C. calatus, Cyclocyathus tersiensis, Thalamocyathus (Cyclocyathus) kunetzovi [11, 39]. The listed composition of the fauna is characterized by the predominance of Kameshki forms, with a noticeable development of species united in the Sanashtyk-Gol archaeocyath complex, which is also characteristic of the limestones of the Chazryk River basin. The presence of Sanashtyk-Gol species in the limestones of the lower course of Gerasimov Creek was the reason why I assigned their enclosing rocks to the Upper Monok formation. But now these ideas must be abandoned, since the limestones under consideration are not only older than the Upper Monok formation, but also older than the Lower Monok.

Indeed, in the northeastern limb of the anticlinal fold (Fig. 27) these limestones, with their characteristic assemblage of archaeocyaths, are unconformably overlain by a pyroclastic sequence, which I take as the type of the Lower Monok Formation. Between the latter and the limestones underlying it there was a major hiatus, during which denudation affected especially strongly the northeastern limb of the anticline, where the limestones proved to have been almost completely eroded even before the volcanic activity of Lower Monok time. In addition, during the hiatus the limestones underwent intensive karstification, as is evidenced by the sedimentary dikes preserved in them. To this it may be added that xenoliths of limestones with clear relict tectonic structures and remains of archaeocyaths, which unfortunately have not been studied, are found in the Lower Monok effusives. All the facts cited above indicate that the limestones of the lower course of Gerasimov Creek occupy a definite stratigraphic position in the Lower Cambrian section; moreover, by the composition of the archaeocyathan remains occurring in them, they may be included in the Babak Formation as its oldest member relative to the limestones of Babak Creek.

In addition to the limestones of Gerasimov Creek, the Babak Formation should probably include the limestones of the Terbishcheva River, which extend as a narrow strip along its left slope in the direction of the right side of B. Monok, and from there into the middle course of the creek flowing into B. Monok somewhat below the Terbishcheva River. Throughout this area the limestones of interest to us, just as along Gerasimov Creek, occur in the core of an anticlinal fold, on the limbs of which Lower Monok effusives rest with a basal conglomerate at their base. The limestones mentioned, being characterized by archaeocyathan remains with Sanashtykol species predominating over Kameshkov forms, for the same reasons that were noted when considering the limestones of the lower course of Gerasimov Creek, were formerly mistakenly assigned to the Upper Monok Formation [40, 10—12].

These limestones, being, like the limestones of Babak Creek, pre–Lower Monok and characterized by approximately the same association of archaeocyaths, now united in the Obruchev complex, are synchronous and therefore belong to the Babak Formation.

Composition and structure of the Lower Cambrian deposits of the axial part of the Western Sayan

Lower Cambrian deposits are distributed not only along the northern and southern slopes of the Western Sayan, but are also developed in its axial part, where they form a narrow belt extending along the basin of the Kantegir River in the southwest, and the Borus and Berezovy ranges in the northeast.

The Lower Cambrian deposits have been studied by me in more or less detail within the Berezovy Range, where they were combined by N. A. Batov into a division of green schists, limestones, and quartzites, and also in an almost continuous section of the Yenisei River valley from the mouth of the Talovka River in the south to the Kantegir River in the north; here they are known under the name of Division I of the metamorphic sequence, or the Kantegir Suite (A. P. Schmidt).

Along the Yenisei River the Lower Cambrian section, according to my observations, is represented in the following form when proceeding from south to north (from bottom to top):

a) crystalline schists of the Dzhebash Formation;

b) a sequence of greenish-gray and green chlorite schists, enclosing beds of marbleized limestones, quartzites, packets of strongly metamorphosed “sandstones,” and sills of diabases, rather intensely altered; thickness about 2 km;

c) siliceous–carbonaceous–clayey schists interbedded with black, light-colored, and pinkish microquartzites;

c) hyperbasites; thickness up to 5 km;

d) greenish-gray quartz–feldspar sandstones interbedded with phyllites and shales;

e) weakly altered amygdaloidal porphyrites, green chloritized shales, often alternating with dark microquartzites and limestones. The latter are sometimes accompanied by tectonic breccia, in which fragments of effusives, microquartzites, and limestones are cemented by a calcareous mass.

The rocks participating in the structure of the Yenisei section are gathered into compressed folds, slightly overturned to the northwest; their limbs, being intensely and finely schistose, show a unilateral dip to the southeast at 155–165°, with angles of dip varying from 80° in the southern to 65° in the northern parts of the section. The direction of dip of the schistosity planes coincides with the direction of dip of the bedding, which can be established without error only in individual outcrops composed of packets of shales and limestones.

The overturning of the folded structures in the Yenisei section to the northwest was not taken into account by A. P. Schmidt, who distinguished the Kantegir suite; its detailed composite section, cited by M. A. Usov as one of the types of the West Sayan formation [47, 80], should be considered in reverse order, i.e., from top to bottom, and then the less metamorphosed rocks will constitute the hanging wall of the formation, while the more deeply metamorphosed formations will prove to be in its footwall, as is in fact observed in the section along the Yenisei River. At the same time, in the Yenisei section the West Sayan formation is probably composite, uniting deposits of the Akkol and Chingin formations, whose specific features are clearly manifested in petrographic analysis of the rocks participating in their structure and in the degree of metamorphism of the latter, which clearly increases in accordance with stratigraphic depth.

Akkol Formation

The Akkol Formation of the belt of Lower Cambrian under consideration should perhaps include the oldest members of the Yenisei section of the West Sayan formation, exposed at the mouth of the Talovka River and forming a relatively narrow belt traced only along the valley of the Yenisei River. This belt is composed of chlorite shales, represented by actinolite–albite, sericite-“arkose”-actinolite, and actinolite varieties, with a noticeable participation of marmorized limestones, as well as quartzites (unit “b” of the section), whose petrographic features are clearly manifested in their microscopic study.

Actinolite–albite shales of light-green color and dense, almost aphanitic texture are massive, coarsely schistose rocks, in which under the microscope one sees elongated needles of green actinolite, irregular grains of epidote, and gray albite, not infrequently arranged in the form of thin interlayers up to 1–2 mm thick.

Sericite-“arkose”-actinolite shales differ from the varieties described above by their dark-green color and more vigorous schistosity. They consist of more or less well-rounded grains of quartz and feldspar enclosed in a sericite–actinolite mass. In contact aureoles with hyperbasites, new formations in these shales are manifested especially intensely and are accompanied by the development of sericite, chlorite, actinolite, and, especially, epidote and quartz; moreover, when the last three components predominate, peculiar quartz–epidote–actinolite hornfelses arise, and when chlorite and muscovite participate in them,

arranged in parallel bands, original quartz-albite-chlorite-muscovite gneisses are formed. The latter border the hyperbasite pluton from the southwest.

Actinolite schists are dark-green, dense rocks composed of needles and hexagonal-prismatic green hornblende, up to 4 mm long, with a noticeable participation of grayish feldspar grains. In other cases, in the schists, along with actinolite, glaucophane is present, accompanied by chlorite, albite, quartz, carbonate, sphene, magnetite, and pyrite; thus these rocks prove to be close in composition to prasinites.

In close spatial association with the actinolite and actinolite-glaucophane schists, quartzites develop. These rocks, which have a light-gray and pinkish-gray color, a fine-grained fabric, and a clearly expressed granoblastic structure, are composed of quartz, with which albite, chlorite, and an ore component of dust-like structure are associated in subordinate amounts.

Finally, lens-shaped bodies of marbled limestones of oolitic structure, noticeably dolomitized, usually dark in color because of the presence of graphite and giving off a smell of hydrogen sulfide when struck with a hammer, occur predominantly here as well.

Thus, in the Yenisei section the deposits of the Akkol Formation are deeply metamorphosed and, in petrographic composition, differ little from the typical rocks of this formation, which is distributed in the stratotype section, with the exception of the graphitized schists, which, probably, along the Yenisei River were cut off by disjunctive disturbances so numerous in this part of the Western Sayan.

In the Berezovyi Range, it is possible that strongly metamorphosed actinolite-albite schists, extending as a narrow strip along the right bank of the Berezovaya River, beginning 1 km above the mouth of the M. Kashkarek River and continuing as far as the mouth of the B. Kashkarek River, should be assigned to the Akkol Formation. Somewhat below the mouth of the latter, the actinolite-albite schists contain a large number of quartzites, which, together with strongly marbled dark limestones appearing there as well, emphasize the typical aspect of this formation.

Chinginskaya Formation

The deposits of the named formation, distributed in the axial part of the Western Sayan, are individualized with the utmost distinctness.

In the Yenisei section its composition includes siliceous-clayey-carbonaceous schists, quartz-feldspar sandstones, microquartzites, and limestones (members “b,” “d,” and “e” of the section), which, with an astonishing stratigraphic consistency, are repeated also over the area of the Berezovyi Range, where they likewise enclose large plutons of hyperbasites, intruded by Maina (?) albitites, which caused noticeable metasomatism in the rocks surrounding them.

However, despite the energetic metasomatism of the rocks of the Chinginskaya Formation, their primary features are preserved fairly distinctly and persist over the entire area of their distribution. Thus, the siliceous-clayey-carbonaceous schists, which are the predominant rocks of the Chinginskaya Formation in the Berezovyi Range and play a secondary role in the Yenisei section, have a dark-gray color and clearly expressed banding, caused by the alternation of dark and light bands. The dark bands consist of a weakly polarizing substance rich in dust of magnetite and hematite, while the light ones are formed by fine fragments of quartz cemented by pelito-

material including the finest flakes of sericite and fine carbonaceous dust.

The sandstones, which form thick members in the Yenisei section and are weakly expressed in the Berezovyi Range, are characterized by a greenish-gray color, a fine- to medium-grained texture, and a quartz–feldspar composition. Under the microscope one can always observe in them a psammitic structure, formed by small, up to 1.5 mm in diameter, well-rounded and subrounded grains of quartz and sericitized plagioclase, with a notable participation of strongly chloritized fragments of effusives and chlorite-sericite schists, situated in a calcareous-chloritic groundmass.

In the Berezovyi Range the sandstones described occur within phyllites, among which dark-colored varieties predominate, with a well-developed pelitic structure made up of small grains elongated parallel to the schistosity, with considerable participation of metasomatic chlorite, sericite, actinolite, biotite, and carbonaceous pigment.

Microquartzites are of considerable interest. In the Berezovyi Range they make up thick beds and are grouped near hyperbasite plutons, whereas on the Yenisei River microquartzites appear within the lower and upper members of the formation as lenses, rapidly pinching out along strike and no more than 5–6 m thick. However, both here and there the microquartzites are characterized by a remarkably constant composition. Possessing a gray, greenish- and pinkish-gray, and for the most part dark to black color and a dense fabric, the microquartzites always reveal the presence of relict bedding, caused by the alternation of broad bands composed of carbonaceous matter and thinner laminae filled with the finest, often weakly polarizing quartz grains, situated in a pelitic mass.

Effusives are common rocks for the Chinginskaya Formation, represented by a cover facies, as is indicated by the tuffs interbedded with them, most widely distributed at the head of the M. Berezovaya River.

Finally, limestones are the most interesting stratigraphic component of the Chinginskaya Formation, widespread in the axial part of the Western Sayan. As established in the section along the Yenisei River, where they form short, relatively thin and rare lenses, they are characterized by a light-gray or dark-gray color and, in places, by clearly expressed banding. In moving toward the Berezovyi Range, the limestones become more massive, lose their bedding, and along the B. Kashkarek River, a right tributary of the Berezovaya River, reach a thickness of 40–50 m. In the limestones of this locality, occurring among phyllites and siliceous-carbonaceous schists, N. A. Batov had already noted the presence of archaeocyaths, which, according to my collections, proved to belong to new species of the genera Orbicyathus and Asterocyathus?

Southwest of the valley of the Yenisei River, in the direction of the basin of the upper reaches of the Karasibo and Izyngul rivers, the Lower Cambrian deposits have not been subdivided into their constituent geological formations because this territory of the Western Sayan has been poorly studied. It is possible that within the indicated area deposits of the Chinginskaya Formation are chiefly widespread, judging by the broad distribution of effusive-siliceous rocks on Mount Kopen and carbonaceous-siliceous schists with remains of radiolarians at the head of the Karasibo River. This becomes all the more probable since on Mount Kopen, rising in the basin of the Arasuk River, limestones are developed that are rich in hemispherical formations up to 2 cm in diameter, some of which have been recognized as glabellae of trilobites; and in the upper reaches of the Pyurogarak River, which flows into the Karasibo River from the left, calcareous schists with remains of algae are known.

Intrusive Formations of the Lower Cambrian

Within the Western Sayan, contemporaneous intrusive formations, varied in petrographic composition, are widely distributed. Among them, massifs composed of microdiorites and hyperbasites, spatially confined chiefly to Lower Cambrian deposits, play a significant role. The petrographic composition of the hyperbasites and microdiorites is so characteristic that they are easily distinguished from the complex of other deep-seated formations and, in this respect, may have guiding significance, provided, of course, that their age is precisely established. But before turning to consideration of their age position, it is necessary at least in general outline to dwell on their petrographic characteristics, giving principal attention to the microdiorite intrusion, whose distinctive features sharply distinguish it from similar formations connected with later intrusive activity.

Western Sayan Hyperbasite Intrusion (Formation)

Hyperbasites, which are relatively widespread in the Western Sayan, have for understandable reasons attracted the attention of many investigators [18; 19; 20; 32; 43; 34]. Thanks to the work of the latter, there is now no need to doubt that the hyperbasites, being regularly distributed in the field of development mainly of Lower Cambrian deposits, constitute an important prospecting criterion and are characteristic stratigraphic documents. However, despite the very wide distribution of hyperbasites in the Western Sayan and the significance they acquire in stratigraphy, their petrographic composition remains almost unknown. Therefore, in order to fill this gap to some extent, it will probably not be superfluous to dwell on the characteristics of the best-studied Kurtushiba pluton and, using its example, to show the diversity of rocks that formed through the development of an ultrabasic magma genetically independent and not connected with a mafic magma of another composition [50, 289].

Kurtushiba Pluton

The Kurtushiba pluton, a brief characterization of which has been given in several works [36; 34, 415—417; 43; 47], is located in the headwaters of the Koarda, Oresh, Omyla, and Idzhim rivers of the Us River system. It forms a linear body elongated along the axis of an anticline composed of deposits of the Chinginskaya Formation, extending in a northeastern direction.

The central part of the pluton is occupied by peridotites, which toward the periphery are replaced by pyroxenites.

Among the peridotites, characterized by the presence on the weathered surface of a thin rusty-brown weathering crust that arose as a result of the decomposition of ferruginous-magnesian silicates with the formation of iron hydroxides, two types of rocks are readily distinguished externally—saxonites and lherzolites.

Saxonites (harzburgites) have a very limited distribution and occur as lenticular bodies with diffuse outlines inside the massif, grouped predominantly in its central parts. Macroscopically, the saxonites are grass-green rocks with distinctly expressed banding, caused by the alternation of darker and lighter components. In the solid fine-grained mass of the rock only idiomorphic grains of picotite are distinguishable, often taking…

replaced by chromite. In the schlieren, besides the ore mineral, an essential component is olivine, to which rhombic pyroxene and abundant serpentine, formed through alteration of the first two components, are subordinate.

Olivine, occurring in the form of polygonal or rounded grains that are completely colorless and show no signs of cleavage, has high relief, bright interference colors, and optical constants \(2v + 86;\ Ng - Np = 0.049\). More often, however, individual crystals of olivine are broken up by a fine system of cracks into an aggregate of lenticularly arranged grains, with the same optical orientation and simultaneous extinction, while crystals that have escaped crushing show sharply expressed undulatory extinction.

Rhombic pyroxene (enstatite), with optical constants \(2v + 78;\ Ng - Np = 0.009\), is present in an amount of not more than 10% and likewise constantly bears traces of intense cataclasis. The latter is sometimes accompanied by the grinding into the prismatic pyroxene grains of small grains of olivine, subsequently replaced by serpentine. This grinding-in follows fine cracks obliquely oriented with respect to the elongation of the axis of the pyroxene crystal. More often, however, as a result of cataclasis along the margins of large pyroxene crystals, small grains of it become mixed with grains of olivine.

Serpentine, represented by light-green chrysotile, shows a veinlet-like, fine-fibrous structure, with fibers growing perpendicular to the direction of the cracks, in the middle parts of which small grains of magnetite appear. With rare exceptions, the orientation of the serpentine veinlets follows the schistosity of the rock, which is manifested very strongly in the latter.

As a result of the deep cataclasis experienced by the rock, its primary panidiomorphic structure is preserved only rarely as relict, against the general background of the cataclastic structure.

Lherzolites are the most widespread rocks of the massif and at the same time are closely linked by mutual transitions with saxonites. Externally, lherzolites differ from the latter in their characteristic dark-gray color with a greenish tint, fine-grained composition, and the presence of large olive-green porphyritic segregations of pyroxene, possessing a pearly luster and perfect cleavage. The main groundmass of the rock is noticeably serpentinized. Under the microscope it is clearly seen that the essential component in the rock is olivine, to which are subordinate pyroxene, represented by monoclinic and rhombic varieties, as well as chromite and magnetite, with abundant participation of serpentine, which is a secondary autometamorphic mineral.

Olivine, displaying the same morphological and optical properties as in the saxonites, is present in the lherzolites only in somewhat greater amounts relative to pyroxene, expressed by enstatite and diallage.

The enstatite of the lherzolite proved to be almost entirely serpentinized, altered to bastite, and is recognized only by the weakly preserved rim, along the edges of bastitized grains, of the primary mineral, with its optical constants \(2v = +78;\ Ng - Np = 0.009\). Diallage is present in approximately equal amount, with optical constants \(2v = +56;\ Ng - Np = 0.025\), appearing in grains xenomorphic with respect to olivine and not infrequently forming twins. Diallage is colorless, with a faint greenish tint and well-expressed cleavage. In comparison with enstatite, it is the more stable component and contains almost no new formations, of which only serpentine develops along parting and cleavage cracks.

Chromite, present in small amounts, forms irregular, lobate grains, often with poikilitic inclusions of olivine. As a product of the alteration of the latter and of pyroxene, serpentine occupies a substantial place in the rock, represented by fibrous chrysotile with the characteristic features usual for it. In the central parts of the chrysotile veinlets there is magnetite, whose small grains have a wormlike form.

The primary structure of the rock has been wholly lost and is typically cataclastic.

Pyroxenites in the form of a broad border fringe the peripheral parts of the pluton, being in close mutual transitions with the peridotites described above, which is especially clearly seen in the upper course of the Koyarda River. The pyroxenites are medium-grained rocks of greenish-gray color, with clearly distinguishable dark-green pyroxene crystals 3–4 mm in size, oriented in one direction and thereby giving it a gneiss-like character. Under the microscope the predominant mineral in the rock is pyroxene, appearing as isometric grains with distinct cleavage and coarse parting. In transmitted light the grains of the component under consideration appear cloudy. In terms of optical constants, \(2v = +60^\circ;\ Ng - Np = 0.028\), it corresponds most closely to diopside. Usually, along the cleavage and parting of the diopside, serpentine develops, which sometimes completely replaces the pyroxene with the formation of bastite. In subordinate amount to diopside there is rhombic pyroxene of enstatite character, with optical properties \(2v = +64^\circ;\ Ng - Np = 0.004\). It appears as prismatic grains, colorless in transmitted light, in which cleavage in two directions is distinctly visible. Plagioclase occupies an entirely insignificant place in the rock; its strongly destroyed prismatic grains are almost entirely replaced by saussurite. The interstices between the indicated components are filled with fibrous chrysotile and flakes of talc. In very limited quantity, accumulations of magnetite occur, and in one thin section, in addition, small grains of quartz were found.

Serpentinites within the Kurtyushiba massif are exceptionally widespread, composing within it lenticular bodies of varying size, which toward the periphery acquire the character of almost continuous fields. It should be noted that examination of thin sections of serpentinites and serpentinized rocks showed only the presence of one variety of serpentine—chrysotile—which, of course, does not exclude the presence of its other variety—antigorite—that may by chance not have been represented in the collected material.

Chrysotile serpentinites macroscopically are almost black rocks with a greenish tint, of dense texture, with a flat splintery fracture and a thin crust of serpophite developing on slickenside surfaces. Under the microscope, a dense network of veinlets, filled with fine-fibrous chrysotile arranged perpendicular to the length of the cracks and in the form of thin fibers penetrating the remnants of olivine and pyroxene grains enclosed in the meshes of the network, is sharply conspicuous. The middle part of the veinlets is filled with a semi-amorphous substance, evidently the same chrysotile, in which magnetite occurs in the form of aggregate clusters and grains of irregular outline. The mentioned chrysotile veinlets, intersecting at various angles, form now a simple, now a complex reticulate structure. In the latter case the veinlets consist of several parallel cords of chrysotile, possessing one and the same structure. In the center of the veinlet there is a semi-amorphous mass of chrysotile and worm-shaped grains of magnetite. The refractive index and interference color, as a rule, differ in different veinlets and decrease along

as one approaches the center of the vein. It is interesting to note that the chrysotile fibers are arranged strictly symmetrically on both sides of the primary fracture, perpendicular to which, and in the direction toward the centers of the cells, the process of serpentinization proceeded.

However, sometimes the fine-fibrous structure of individual veinlets can be observed only at high magnifications; in that case the veinlets, consisting of a series of parallel thin chrysotile fibers, appear as a plate.

In addition to chrysotile, serpentinites contain individual grains of olivine and pyroxene that have not yet been completely transformed into chrysotile. The latter is preserved much more often, since it is a more resistant mineral and is less readily subject to the processes of serpentinization.

Finally, in the belts of development of serpentinites, thin veinlets of asbestos are noted, with fiber lengths of 0.3–0.5 cm, as well as light-green, parallel, prismatic metaxite up to 8 cm thick.

Contact metamorphism of the hyperbasite intrusion is manifested in the form of the development of a rather broad zone of altered rocks, reaching in places not less than 0.5 km in width. In the rocks of the exocontact, represented by effusive-sedimentary deposits of the Chinginskaya Formation, intensive amphibolitization is observed, which led to the formation of amphibolite schists and even amphibolites. Serpentinization is expressed much more weakly; it is manifested in the appearance of serpentine veinlets in the country rocks immediately adjoining the pluton. It is probable that some silicified and silicified-carbonatized rocks confined to the exocontact of the pluton are metasomatic formations that arose through replacement of schists and limestones by siliceous solutions emanating from the hyperbasite magma.

Metasomatic processes are manifested more intensively in the northeastern contact of the pluton, where a series of large apophyses branches off from it, occurring in prasinite schists of the Akkol (?) Formation. In direct contact with the aforementioned bodies, the schists are impregnated with carbonates, with which garnet and sphene are associated, as is seen along Klyuch Prokop’ev, the right tributary of the Tikhaya River, which flows into the Us River from the left. Near this same body, but along Klyuch Razvedochnyi, original green and brownish-yellow skarns develop; individual portions of these consist exclusively of diopside, whereas others consist of green amphibole, and it is established that the first mineral formed somewhat later than the second.

Let us briefly consider the characteristics of the numerous small hyperbasite plutons situated either along the continuation of, or parallel to, the long axis of the Kurtushibinskii massif, and occurring predominantly in effusive-clastic deposits of the Chinginskaya Formation. According to Z. A. Lebedeva [24], only on the southern slope of the Kurtushibinskii Range, in the area of the Aktovrak and Karatag heights, on the right bank of the Ulukhem River in the first case and on the left bank in the second, small hyperbasite bodies are situated among archaeocyathid limestones, probably also belonging to the Chinginskaya Formation of the Lower Cambrian. The rocks of these small plutons proved to be entirely serpentinized and transformed into typical serpentinites, often with asbestos veinlets. In contact with the serpentinites, the limestones underwent vigorous silicification, with the formation in them of distinctive siliceous rocks (“hornfelses”) of dark-gray or bluish, more rarely brown and reddish, coloration [24, 126, 120, 209, 227]. Deep serpentinization is not only characteristic of the Aktovrak and Karatag massifs, but is in general a characteristic feature of almost all small hyperbasite plutons of the Kurtushibinskii Range; moreover, the serpentinites and serpentinized rocks composing them are characterized by the presence chiefly of chrysotile varieties. The latter circumstance should be ex—

This may be explained by the fact that serpentinization apparently affects rocks of the peridotite type, including, perhaps, dunites, since sometimes in serpentinites one can see only relics of olivine and large grains of chromite preserved in the mesh structure of chrysotile veinlets, as was noted in its time by M. K. Korovin [13]. Among the hyperbasites of the Kurtushiba Range there may possibly be plutons composed of pyroxenites, or at least with a considerable participation of the latter. Such plutons are probably the Ashpan and Idzhim plutons, which were studied in detail by N. D. Merkur’ev in connection with the search for asbestos deposits [30, 420—422].

The Kantegir Pluton

The largest hyperbasite massif of the Western Sayan is undoubtedly the Kantegir massif, which forms the core of the Borus Range and of the bald uplands adjoining it from the northeast and southwest. This linearly elongated massif, concordantly occurring in the deposits of the Chingin and, possibly, the Akkol formations, is bounded on the south and north by faults, with the fault planes dipping to the southeast at an angle of 85—80°.

A number of researchers have studied the massif; among them A. N. Zlobin and A. P. Schmidt gave the most detailed description of it, which may be consulted in condensed form in the generalizing works of M. A. Usov [43; 47].

The study of a large collection of rocks gathered by me along the Yenisei River, whose valley cuts the western part of the massif of the Borus Range, and especially of material obtained as a result of mapping the massifs of the Berezovyi Range, shows that the petrographic composition of the Kantegir pluton, even in a number of details, is close in composition to the Kurtushiba massif. As in the latter, the central part of the Kantegir pluton is composed of peridotites that have undergone vigorous serpentinization, with the formation of typical serpentinites, which locally, and in the endocontacts constantly, are the most widespread rocks.

Among the peridotites the principal place belongs to lherzolites, which have a gray or greenish-gray color and a medium-, more rarely coarse-grained texture, with a relict panidiomorphic structure preserved here and there. This rock consists, in decreasing order of quantitative abundance, of diallage, bronzite, and olivine, which are to a considerable degree serpentinized, and locally have passed entirely into chrysotile, antigorite, and bastite serpentine. Magnetite and chromite are present in small amounts.

A lesser role in the structure of the pluton is played by saxonites (harzburgites), which are connected by gradual transitions with the lherzolites and differ from the latter only by a somewhat increased content of olivine relative to the rhombic and monoclinic pyroxene prevailing in the rock. However, in some varieties of peridotites olivine decidedly predominates over pyroxenes, and the rock approaches olivinites in composition. Among the newly formed minerals, in addition to antigorite and chrysotile serpentine, which especially readily replace olivine, tremolite and talc are present in noticeable amounts; the former occurs as radiating prisms with an extinction angle of 14 to 19°, and the latter as flakes. It should be noted that tremolite arises somewhat later than antigorite and bastite and earlier than veinlet chrysotile.

A significant place by volume in the massif is occupied by dark-gray pyroxenites, among whose cryptocrystalline or fine-grained groundmass are visible gray and golden-yellow grains of pyroxene, and sometimes thin veinlets of ore-bearing

mineral. In the rock, monoclinic pyroxene (diopside) predominates; subordinate to it is enstatite \((2v = +78^\circ)\) and, more rarely, hypersthene with optical constants \(2v = -62^\circ;\ Ng - Np =\) up to \(0.018;\ < c : Ng = 0^\circ\). Of the accessory minerals, magnetite and chromite are present in variable amounts. The spaces between pyroxene grains are filled with radiating chrysotile, less commonly with antigorite and structureless serpophyte.

Diopsidites, websterites, and, possibly, bronzitites and hypersthenites are the principal varieties of pyroxenites in the massif.

Among the serpentinites, antigorite varieties predominate; asbestos veinlets sometimes appear in association with them, as do accumulations of magnetite, talc, and veinlets of calcite and chromite.

Chrysotile serpentinites are encountered somewhat less often; moreover, in close association with the antigorite varieties there arises an intermediate group of serpentinites consisting of a tangled aggregate of antigorite and chrysotile with relicts of diopside and rhombic pyroxene. It is noteworthy that when monoclinic pyroxene predominates in the rock, chrysotile acquires the chief importance, whereas rhombic pyroxene favors the formation of antigorite.

Contact metamorphism is manifested in the very same features that are characteristic of the Kurtushibin massif. It should only be noted that M. A. Usov is inclined to explain the occurrence, among the rocks of the Chingin Formation, of widely developed siliceous rocks as the result of metasomatism by hyperbasite magma.

I shall not dwell on the characterization of the hyperbasites distributed within the field of the Lower Cambrian deposits of the northern slope of the Western Sayan, since the well-known small pluton of the Tabat River basin proved to be so strongly serpentinized that the primary nature of the rocks composing it can be reconstructed only with considerable difficulty.

Several somewhat larger, lens-shaped massifs are distributed east of the Yenisei River valley, where they occur chiefly among deposits of the Chingin and, more rarely, the Dzhebash Formations, and are characterized by certain features noted earlier by N. A. Batov [2], and later by me. These features consist in the fact that the plutons under consideration underwent deep serpentinization and are almost completely represented by serpentinites, often intensely schistose, which in places acquire the features of serpentinite and talc schists.

The serpentinites, having a light-green or dark-green coloration, are cut by numerous veinlets of quartz and epidote, which intersect one another, as well as by veinlets of calcite and magnetite. In the serpentinites one can constantly see rare veinlets of asbestos with fibers up to 2–3 mm long.

Despite such intense serpentinization of the hyperbasites described, in many thin sections it is possible to see relicts of the primary minerals, of which olivine is preserved in the rocks participating in the structure of the central parts of the massifs, and pyroxene in the peripheral parts. Evidently, before serpentinization the primary rocks of the hyperbasites were peridotites, among which representatives rich in olivine apparently played a considerable role. However, in the Malokogdar massif the rocks composing it have been preserved much better, and among them dunites are clearly distinguished, occupying a relatively large area and thus constituting the second locality of firmly established dunite segregations in the Western Sayan [32, 949].

It is impossible not to note a certain peculiarity of the contact effect of the hyperbasites on the surrounding rocks. It is expressed in intense hornfelsing, as well as in actinolitization; moreover, actinolite crystals, occurring together with diopside, quite often reach no less than 15–20 cm in length. The hyperbasites are sometimes bordered by broad bands of limestones, which,

in addition to silicification, undergo vigorous listvenitization with the formation of listvenites of the Ural type.

As was evident from the preceding account, the main mass of hyperbasites is grouped in the southern marginal zone of the Western Sayan, where, being confined to the crests of anticlines, they form a long garland of large and small massifs distributed over almost the entire belt of Lower Cambrian geosynclinal deposits. In the northern marginal zone of the Western Sayan, however, the hyperbasites form two branches. In one of them, the shorter one, extending from the middle course of the Shadat River almost to the M. Shushi River, they form thin but sharply elongated massifs. In the other, longer branch, situated to the south (the Berezovyi and Borus ranges) of the first and joining it in the basin of the Oi River, the hyperbasites constitute short but thick plutons of lenticular form, arranged en echelon one behind another. Within these branches the hyperbasite plutons are for the most part confined to the sutures of deep faults that arose between the rocks of the Lower Cambrian and the Precambrian. Southwest of the Yenisei River valley, the hyperbasites, appearing at first as separate isolated massifs, then disappear altogether with further movement into the systems of the upper reaches of the Kantegir and Abakan.

By contrast, in this area small massifs of microdiorites are widely developed in the form of sill-like bodies, occurring predominantly in the zone of distribution of the Lower Cambrian.

Such a spatial arrangement of the hyperbasite massifs is probably due to their confinement to deep faults, which proved to be the most favorable structures for the penetration of ultrabasic magma into the zones of deeply submerged parts of the geosyncline during the first phase of its subsidence [50, 286, 288].

In all cases of development of hyperbasites, they are of the same type; they are characterized by the complexity of their petrographic composition, caused by partial differentiation of peridotitic magma, which proceeded in the following probable order—dunites, harzburgites, lherzolites, and pyroxenites.

The postmagmatic stage in the evolution of the peridotitic magma was accompanied by abundant release of hydrothermal solutions, which caused vigorous autometamorphism of the rocks of the plutons, with the appearance in the rocks of newly formed minerals belonging to uralite, chlorite, sossurite, talc, and serpentine. In all probability, the formation of uralite, replacing pyroxene, occurred first, after which chlorite appeared, probably arising under conditions of lower temperatures and pressures than uralite. The subsequent life of the plutons proceeded under conditions of intense pressures, accompanied by energetic crushing of the rocks composing them, with the appearance in them of the finest system of cracks; these were used by solutions that deposited serpentine, often growing along the margins of uralite and chlorite.

The process of serpentinization in the ultrabasic rocks apparently proceeded in the very same order that was once established by B. P. Krotov on material from the peridotites of the Urals [15]. Certain details noted in our material do not introduce serious changes into the established conceptions of this process, and therefore there is no need to dwell on its consideration.

In the final stage of formation of the hyperbasite pluton, magmatic activity develops in the form of its hysteromagmatic phase, as a result of which the rocks surrounding the massif became intensely silicified, and in its endo- and exocontacts there arose thin veins and veinlets of quartz, sometimes with sulfides.

It should be noted, however, that not all silicilites of the Lower Cambrian arose as a result of contact processes of ultrabasic magma, as,

this is believed by some researchers [47, 79; 26], or the formation of the latter occurred much later than the formation of other varieties of silicilites.

In fact, in a number of cases the hyperbasite plutons cut the masses of silicilites at an acute angle, and fragments of the latter, in the form of small xenoliths, are clearly established in some varieties of hyperbasite rocks (Borus Range).

Kair microdiorite intrusion (formation)

The individual features of the microdiorite intrusion may be examined using the example of small plutons distributed in the Kair Range, which forms the watershed of the Chekhan and Karasibo rivers on the northern slope of the Western Sayan [40, 9; 39]. Here the linearly elongated plutons of this intrusion break through the deposits of the Chinginskaya Formation, which extend in a northeastern direction. Still more often, the microdiorites occur in the form of thin sill-like bodies, often densely saturating the formations enclosing them, at times with a distance between individual bodies of no more than 3–5 m.

As a rule, the rocks of the intrusion are vigorously schistose; moreover, the intensity of schistosity is more sharply manifested in the small bedded bodies and less intensely in the central parts of the larger massifs. As a result of these causes, the outward appearance of the rocks does not remain constant, but changes depending on the degree of schistosity.

Thus, some parts of the more or less large plutons, weakly affected by schistosity, are composed of a massive greenish-gray, medium-grained, distinctly gneiss-like rock, in which dirty-yellow plagioclase and a dark-colored mineral, often forming large segregations, are macroscopically clearly distinguishable. But microscopic study of thin sections shows that only occasionally is a porphyritic structure preserved in the rock, in which the porphyritic segregations are represented by prismatic and tabular hornblende and, more rarely, by plagioclase of intermediate composition.

Still more rarely can relics of a hypidiomorphic structure be seen, formed by prisms of distinctly zoned and polysynthetically twinned andesine, very weakly saussuritized, but in some places intensely sericitized. The interstices between the plagioclase are filled with quartz, which is always almost xenomorphic in relation to the plagioclase, except for certain parts of the thin section where sharply idiomorphic quartz grains are visible. Myrmekite develops along the margins of the plagioclase, although it is not especially abundant.

For the most part, however, the structure of the rock is a fine-grained hornfelsic aggregate consisting of quartz, albite, chlorite, sericite, epidote, and, very rarely, hornblende. In the hornfelsic aggregate, quartz is sometimes present in the form of very large grains, and plagioclase in the form of crushed prisms or small fragments; this circumstance is due to incomplete recrystallization of these minerals, which still retain signs of their primary nature.

Closer to the periphery of the plutons, alteration in the rock increases markedly, as a result of which the plagioclase almost entirely passes into zoisite, albite, and chlorite, or is replaced by a fine-grained aggregate consisting of chlorite, epidote, and scaly muscovite. Along the dark-colored component—pyroxene—there develops an aggregate of green chlorite, brownish-green hornblende, uralite, and more rarely biotite, which, incidentally, also replaces along the margins the green hornblende, apparently a partly primary component. Along with the biotitization of the green hornblende, by means of

green actinolite (uralite) and chlorite develop along it, and at the contact with plagioclase, epidote.

Most often the primary dark-colored component is not preserved at all, but is replaced by an aggregate of chlorite and reddish-brown biotite, at the expense of which chlorite also forms. The interstices between the primarily preserved components—plagioclase and hornblende—are filled with quartz and even micropegmatite.

Among the ore components in the rock, magnetite and titanite are present in small quantities, the former of which is probably a metasomatic mineral, since it forms vein-like accumulations.

Processes of alteration are most sharply manifested in the peripheral parts of the massifs, usually composed of rocks which in outward appearance resemble medium- and fine-grained plagioclase amphibolites of greenish-gray color, consisting of green hornblende and feldspar. Under the microscope a relict diorite structure can occasionally be observed, which, as a rule, assumes the character of a mylonitic blastoporphyritic one. Plagioclase, an essential component, is represented by labradorite, forming sharply idiomorphic-zonal prismatic crystals. However, in some thin sections this mineral, owing to saussuritization, sericitization, and especially albitization, passes into albite-oligoclase, although in the outlines of its crystals it most likely resembles andesine-labradorite. The dark-colored component clearly occupies a subordinate position in the rock and, being represented by hornblende, is characterized by a pale-green color and an acicular-foliated form, sharply fringed at the ends of its crystals, passing into acicular actinolite. The latter sometimes forms a tangled fibrous aggregate with a small admixture of chlorite. Most often the dark-colored component is preserved as relics, being replaced by epidote, chlorite, and a blue-green very fine-grained amphibole. Among the accessory minerals, magnetite and titanite are present. The mylonitic parts of the rock undergo intense saussuritization (epidote and sericite after plagioclase), chloritization, calcitization, and subsequently silicification. The latter is especially sharply expressed in the rock and is accompanied by the development of accumulations or veinlets of quartz, with a considerable participation in them of carbonate, appearing in the latest stage of transformations in the rock.

The extreme degree of alteration of the microdiorite intrusion is accompanied by the appearance of amphibolites, represented by two varieties, the first of which, represented by plagioclase amphibolites, is confined mainly to the endocontacts of small plutons.

These amphibolites have a greenish-gray color, a fine-grained massive structure, and consist of dark-green hornblende, to which small pale-greenish crystals of plagioclase are subordinate. Under the microscope an uneven structure is established in the rock, varying from hetero- to homeo- or idio- and even porphyro- and nematoblastic forms of its expression. The predominant mineral—green columnar hornblende—is present in an amount of not less than 70–78% and is characterized by intense pleochroism from yellowish-green to blue-green. Hornblende forms for the most part large (appearing in this case in the form of porphyroblasts), sometimes short, sometimes markedly elongated prisms, in the latter case resembling fibers of actinolite. Often individual grains of amphibole, or parts of them, are replaced by clinozoisite, fine xenoblastic calcite with high interference colors, or else by colorless zoisite, appearing in the form of small, more rarely large grains. Epidote, colorless muscovite, and chlorite, which often develops in the form of the finest veinlets, can occasionally be recorded in the rock. The leucocratic part of the rock is made up of small grains of albite, with an insignificant participation of quartz appearing in the interstices between the albite.

The second variety of amphibolites, composing small independent bodies, is macroscopically similar to hornblendites. They have a dark-green coloration and consist exclusively of hornblende, the large crystals of which often have a fibrous structure. Under the microscope one constantly observes a porphyro-, more rarely hetero- or nematoblastic structure of the rock, the porphyroblasts in which are represented by large pale-green and broadly prismatic crystals of amphibole, often possessing serrated or acicular margins owing to the transition of this mineral into actinolite. The porphyroblasts are situated in a tangled fibrous aggregate of pale-green hornblende, albite, chlorite, and calcite, the last three components occurring in variable proportions relative to one another.

Contact metamorphism, caused by the microdiorite intrusion, is manifested with exceptional sharpness. In the inner zones of the exocontact belt there develop gneiss-like quartz–hornblende hornfelses, having a greenish-gray coloration and a clearly expressed hornfelsic structure formed by spots—bands of fine-grained hornblende. The latter is characterized by a dense green color, strong pleochroism, and prismatic, more rarely hexagonal, outlines of its crystals. The mineral under consideration is situated among a very fine-grained hornfelsic aggregate composed of quartz, with a minor participation of green hornblende, brown biotite, relict feldspar, and zoisite.

In the outer belt of the exocontact, hornfelses of another type appear, in which, together with quartz, albite is present, forming with the first mineral a mosaic structure. Yellowish-green, acicular, more rarely finely prismatic amphibole, together with brown platy biotite and green chlorite, are distributed evenly throughout the entire rock.

However, here too there are noted hornfelses consisting chiefly of quartz with a characteristic mosaic structure, to which scales of chlorite are subordinate, formed in part through replacement of biotite. The latter is sometimes accompanied by muscovite, with which it rarely forms single scales. Finally, at a considerable distance from the contact there arises a rock with a clearly porphyroblastic structure, the porphyroblasts in it belonging to hexagonal, often idioblastic prisms of blue-green hornblende. The main mass of the rock is composed of a microhornfelsic aggregate consisting of quartz, albite, brown biotite with an admixture of amphibole. These hornfelses are associated here with actinolite schists, possessing a gneiss-like texture and a sieve-like structure formed by small grains of quartz, acid plagioclase, calcite, and large-scaled biotite with inclusions of quartz. Actinolite is abundant, developing in the form of elongated platy and hexagonal crystals, often with inclusions of quartz, calcite, and here and there biotite.

The hornfelses just described undergo intense silicification, probably due to metasomatism connected with later intrusive volcanism. Silicification is accompanied by the development of numerous and thin veinlets of quartz or its accumulations, concentrated along the schistosity planes of the rocks bordering the microdiorite plutons.

Within the Kair Range, on the eroded surface of the microdiorite massifs, there lies the basal conglomerate of the Arbat Formation, in whose pebbles the entire range of rocks of this intrusion and its contact-metamorphic hornfelses is found [40, 9 and 16; 39, 136].

As already noted above, the rocks of the described intrusion especially often appear in the form of sills, which are most widely distributed to the northeast of the Kair Range, where they often assume the character of an actinolitized, finely gneissose green rock, macroscopically difficult to distinguish from the enclosing schists.

Probable analogues of the Kair intrusion, represented by intensely milonitized quartz and augite diorites, as well as by gabbro, are also widely distributed in the Kurtushiba Range, where they are not infrequently found in close spatial association with a large hyperbasite pluton cutting the deposits of the Chingin Formation.

The diorites, described in one of my works, form small masses of linear-elongate or isometric shape, most typically expressed in the upper course of the Tikhaya River, a left tributary of the Us River [34, 411]. Much more often the diorites appear in the form of beds or dike-like bodies, composed of deeply altered rocks, in which only occasionally can small grains of plagioclase, hornblende, or pyroxene be seen, replaced by chlorite, epidote, and actinolite. With the fine-grained structure of these rocks and with more intense schistosity they bear the features of epidote-chlorite or actinolite-epidote schists.

However, the study of a larger number of thin sections under the microscope has made it possible somewhat to clarify their primary intrusive nature. Thus, in thin sections it is sometimes possible to see, among a matted-fibrous aggregate of actinolite, with a considerable presence of chlorite, epidote, sericite, and uralite, signs of actinolitized hornblende arranged parallel to the schistosity of the rock, which gives the latter a gneiss-like structure. In addition, in individual thin sections relics of ophitic structure are preserved, formed by saussuritized plagioclase and monoclinic pyroxene, replaced at the margins by uralitic hornblende.

Gabbro, which I formerly regarded as differentiates of ultrabasic magma [34, 415], forms oval-shaped stocks, thick beds, and veins, not infrequently cutting the Kurtushiba hyperbasite massif. In its most typical occurrence the gabbro is expressed at the head of the Omyla River, a tributary of the Us River, where it composes a stock of isometric outline, with a diameter of no more than 0.5 km. The gabbro has a fine-grained composition, dark-gray, more rarely gray coloration, and consists of a predominant dark-colored component and subordinate plagioclase, which displays a parallel arrangement of its grains and gives the rock a gneiss-like appearance. Almost across the strike of the gneissosity, the latter is cut by numerous veinlets filled with grayish quartz. Under the microscope it is established that monoclinic pyroxene takes an essential part in the structure of the rock; its isometric crystals are noticeably amphibolized and in places are entirely replaced by uralitic hornblende, along the margins of which chrysotile grows. Strongly destroyed laths of basic plagioclase, transformed into a mixture of saussurite, have an evidently subordinate importance. Of the ore minerals, magnetite is present, accumulating along the periphery of the uralitized hornblende. Numerous veinlets cutting the rock, in addition to quartz, are filled with saussurite, which, incidentally, also cuts grains of uralite.

Vein gabbro displays certain specific features. This rock, light gray in color and of medium-grained composition, consists of equal amounts of plagioclase and pyroxene, the former of these minerals being almost entirely replaced by calcite, zoisite, and epidote. The isometric plates of pyroxene, however, are intensely amphibolized and in places transformed into uralitic hornblende. Cataclasis is sharply manifested in the rock, expressed in the bending and even twisting of many plates of uralitic hornblende and in the intense crushing of other components of the rock, whose structure becomes typically cataclastic.

Plutons of the type of the Kair intrusion are also significantly widespread in the axial belt of development of the Lower Cambrian, where they again form small isometric stocks or sills, judging from my observations and those of A. P. Shmidt, who described it under the name of the Katushka intrusion.

In the axial belt, the rocks of this intrusion are likewise deeply altered, so that macroscopically they appear as chlorite–amphibole schists. Under the microscope, in the latter, the chief minerals visible are products of neoformation—quartz, actinolite, and chlorite, with a minor presence of plagioclase (albite–oligoclase) and green hornblende, which probably are the primary components of the intrusive rock. Sometimes in the schists, together with intermediate plagioclase and hornblende, potassic feldspar is observed; its isometric grains consistently pass into sericite. The plagioclase grains, in turn, are replaced along their margins by carbonate, and internally by saussurite. Along the hornblende prisms an indigo-blue chlorite develops, which not infrequently acquires a brown coloration and in this case appears in the form of veinlets.

Along with the rocks described, which before the processes of metamorphism were evidently close in composition to diorites, hornblendites are markedly widespread. The latter macroscopically are dark green, more rarely almost black, dense, sometimes fine- and medium-grained, somewhat banded rocks, consisting of black and green hornblende, abundant chlorite, and rare grains of pyrite. Under the microscope, the main component in the rocks is greenish-brown hornblende, whose greenish tint is apparently due to intense chloritization. Then, in a small amount, plagioclase (labradorite) and monoclinic pyroxene—augite—are present, the latter usually passing into uralitic hornblende, as well as idiomorphic pink titanite and, sometimes, magnetite. The hornblende is markedly chloritized and epidotized, and along its margins passes into a green component. The plagioclase is intensely saussuritized, and in its place epidote, zoisite, chlorite, and, in addition, carbonate are present. Of the neoformations, generally richly represented in the rock, chlorite plays the exceptional role and is expressed in two varieties. The first variety, of indigo-blue interference color, arises at the expense of brown hornblende; the second, brown in color and later, appears as thin crosscutting veinlets. In a word, the rock, evidently belonging in primary composition to issite [25, 134], bears features of deep metamorphism, and outwardly it is completely indistinguishable from the prasinite-type schists enclosing it.

On the Age of the Lower Cambrian Geological Formations of the Western Sayan

Determining the age of the above-described sedimentary-effusive geological formations of the Lower Cambrian, which I have named the Akkol, Chinga, and Babak formations, is attended by considerable difficulties. The latter are due mainly to the fact that in the deposits of these formations organic remains—belonging predominantly to algae, archaeocyathids, and, more rarely, trilobites—are not encountered very often and, moreover, are characterized by certain specific features.

Thus, archaeocyathids and algae, being represented by forms with a fairly wide horizontal distribution but with a far from established range of vertical development, are not reliable documents for determining the age of the deposits. Trilobites, characterized chiefly by new species and new genera, remain, in most cases, purely local associations.

However, despite such a strongly expressed endemic composition of the paleontological remains, it is difficult to overestimate their significance as correlative documents, and for the following reasons.

Each of the above-named Lower Cambrian geological formations is characterized by a special assemblage of fossils which, being not

sharply separated from the complex of the immediately underlying and overlying formation, bears elements of noticeable renewal. In time it follows manifestations of tectonic movements, documented in the geological record by surfaces of hiatuses, whose role in drawing boundaries between stratigraphic units of different rank was assessed fairly long ago [8, 158—159; 12; 47]. Under the specific conditions of the Western Sayan, these breaks determine not only the volume of the Lower Cambrian series, but also of the subordinate stratigraphic units characterized, as was said, by distinctly individual complexes of fossils. Among them, and especially among the trilobite fauna, species and even genera are found that are grouped around previously known corresponding taxonomic units as their derivatives or, sometimes, predecessors.

The presence of these paleontological forms in the Lower Cambrian geological formations of the Western Sayan makes it possible to compare the latter not only within the named massif, but also, with some confidence, to correlate them with known standards of the series distributed in other regions of the Soviet Union and partly in foreign countries.

From what has been said it follows that, by individualizing the complexes of organic remains, identifying among them the forms most characteristic for purposes of correlation, and then controlling the vertical distribution of the latter by the stratigraphic-tectonic method, it is possible with a considerable degree of probability to solve the problem connected with determining the Lower Cambrian age of the series of interest to us and to outline ways of possible dating of the stratigraphic units subordinate to it.

There is no need to dwell on the determination of the scope of the Lower Cambrian series of the Western Sayan, established from its stratigraphic relations with the underlying and overlying strata.

It is enough to recall that on the northern slope of the Western Sayan the Lower Monok effusives of the lower Middle Cambrian [40, scheme] lie transgressively on various stratigraphic units of the Lower Cambrian, which in turn, on the southern slope of the massif, unconformably, with a basal conglomerate at the base, cover the Kemterek shales of the lower Lower Silurian. The pebbles of the conglomerate, with bright features of relict tectonostructures [33], consist almost entirely of Lower Cambrian rocks. This surface of tectono-denudational break adequately determines the position of the hanging wall of the Lower Cambrian, which cannot be said with regard to its footwall, most of which has been truncated by disjunctive disturbances of a complex type. Thus, along the Yenisei River south of the mouths of the Uy and Golubaya, and then in the Kalganov Range, the crystalline shales of the Dzhebash formation, previously united into the second division of the metamorphic sequence, being thrust up onto Lower Cambrian deposits, cut off a large part of its footwall.

Along the thrust, extending in a northeastern direction and dipping to the southeast at an angle of 60—65°, zones of crushing, breccias, and secondary fine folding are developed everywhere, especially sharply expressed in the hanging wall of the disturbance.

Farther, to the southwest of the Kalganov Range, in the basin of the Abakan River, and then east of the Yenisei River, in the lower course of the Shadat Creek, the Lower Cambrian rocks are completely truncated by the disturbance, and the Dzhebash crystalline shales enter into disjunctive contact with younger stratigraphic units of the Middle Cambrian and even of the Devonian.

The described disturbance, which I have called the Maloshushensky thrust, is characterized by a long history of its development, as was already indicated by A. N. Churakov [52, 192—193]. Having arisen long before the formation of the Lower Monok effusives, the Maloshushensky thrust in the subsequent geologic

…history repeatedly resumed, but already in a noticeably weakened form of its manifestation. In any case, the dike and sill facies of the Nizhnemonok Formation, cutting through Lower Cambrian deposits and lying within the orbit of the thrust, are far less affected by deformation.

Likewise, in the axial part of the Western Sayan, the Lower Cambrian deposits are connected with the crystalline schists of the Dzhebash Formation by means of complex disjunctive disturbances which, in the opinion of I. A. Molchanov, have the character of normal faults [27, 13].

Only within a volume bounded by an ancient fault—by the Precambrian crystalline schists on the footwall side and by an unconformity surface from the Lower Monok effusives of the lower Middle Cambrian in the hanging wall—are the Akkol, Chinga, and Babak formations, intruded by hyperbasites and microdiorites, strictly individualized in the Lower Cambrian division; I intend to dwell on the determination of their age.

The Akkol Formation, forming the base of the Lower Cambrian in its type development in the Kurtushibinsky Range, consists of a thickness of deeply metamorphosed and intensely dislocated rocks represented by greenish-gray albite-chlorite-sericite schists, containing beds of prasinite and graphitized varieties of these schists, as well as layers of algal, somewhat carbonaceous, marblized and dolomitized limestones, with abundant gray and, more rarely, dark quartzites. The lithological features of the formation are so characteristic that its deposits, even under conditions of some facial variability, are distinguished without particular difficulty in other parts of the Western Sayan as well. Thus, in the axial part of the named massif (Yenisei River), the Akkol Formation, being expressed mainly by albite-actinolite schists, is unerringly distinguished from the composition of rocks similar to it thanks to the presence of quartzites, “oolitic” marblized limestones, and especially graphitized schists. The average apparent thickness of the Akkol Formation is probably not less than 3.5–4 km.

Organic remains occur very rarely in the deposits of the formation under consideration; moreover, since they are represented by algae belonging to several new species of the genus Osagia, they are not reliable documents for correlation and still less for age determination. It is known that representatives of the genus Osagia are characterized by a broad vertical distribution, occurring both in Upper Proterozoic and in Lower and even Upper Paleozoic deposits [54, 114; 28, 103, 104]. Obviously, relying only on paleontological documents, the age of the Akkol Formation could be determined within a broad interval of time—from the Proterozoic to the first half of the Upper Paleozoic. However, taking into account the position of the formation of interest to us in the stratigraphic section, its age can most probably be regarded as close to the very base of the Lower Cambrian, for the following reasons.

It has been shown in its place that the Akkol Formation is a younger stratigraphic member than the crystalline schists of the Precambrian Dzhebash Formation, and that between the times of their formation there undoubtedly occurred a major tectono-denudational break. The range of the latter is determined by the fact that, prior to sedimentation of the Akkol Formation, the deposits of the Dzhebash Formation had been folded, schistosed, and then, in connection with the emplacement of a granite intrusion, had in places been transformed into metasomatic gneisses. Meanwhile, the deposits of the Akkol Formation were not involved in this complex chain of geological events and, being less metamorphosed, in a number of localities proved to be overlain along a relatively steep thrust by the Precambrian crystalline schists.

In turn, the deposits of the Akkol Formation are older than the rocks of the Chingin Formation, and there was also a break between them, which is very clearly established along the N. Karabuluk River, a right tributary of the Urgun River (Figs. 8 and 9). Here the almost vertical prasinite schists and quartzites of the Akkol Formation are overlain by a member of medium- and fine-clastic rocks with interbeds of conglomerate, extending in a north-easterly direction and dipping to the northwest at an angle of 65–70°. This member of relatively gently bedded rocks was taken as the basal member of the Chingin Formation; moreover, it became clear that, in the composition of its medium-clastic rocks, alongside the constantly occurring quartz and feldspar, there are almost always fragments of quartzites and greenish, strongly altered rocks, the primary nature of which is close to effusives. The conglomerate interbeds consist entirely of well-rounded and comparatively large pebbles of quartzites, as well as chlorite-sericite schists, typical of those rocks on which the basal member of the Chingin Formation rests directly. It is evident that during the accumulation of the latter, strata in which strongly metamorphosed schists and quartzites played a large role were subjected to erosion. One of such strata was undoubtedly the Akkol Formation, whose rocks by the time of sedimentation of the Chingin Formation had already become metamorphosed, as can readily be verified on the basis of further observations of this kind.

The deposits of the Akkol Formation in many places contain concordant or cross-cutting bodies of diabases (the Kantegir district, the Kemterek locality, etc.), which in composition do not differ in any way from the pyrogenic rocks of the Chingin Formation. And it turns out that in the diabase dikes xenoliths of actinolite schists and marbles, characteristic of the enclosing rocks, are not infrequently found (Fig. 10). But the Akkol Formation, occupying an intermediate position between the Dzhebash and Chingin formations, is nevertheless, in age, closer to the latter of them. This is seen from the fact that the deposits of the formations mentioned, while differing somewhat from one another in degree of metamorphism, show a noticeable similarity in petrographic composition. This similarity is especially sharply emphasized by the silicilites, which in the Akkol Formation appear as more metamorphosed rocks—quartzites, and in the Chingin Formation as less altered microquartzites. The main mass of the silicilites under consideration is of sedimentary origin, and their formation probably proceeded under identical climatic environmental conditions. Therefore it is hardly likely that these conditions at the time of sedimentation of the deposits of the Chingin and Akkol formations changed in any serious way. In any case, there was no “climatic unconformity,” as A. N. Churakov understands it [5, 57—58; 54, 159], between these formations.

At the same time, the algal remains found in the Akkol Formation contain forms of Osagia ovosimilis, which are also characteristic of the Chingin Formation, bringing them still closer in age.

As will be shown in its proper place, the assignment of the deposits of the latter to the Lower Cambrian division, and moreover to its lowest members, gives rise to no doubts. In connection with this I considered it possible to assign the deposits of the Akkol Formation as well to the Lower Cambrian. Such a determination of the stratigraphic and age position of the Akkol Formation, in my opinion, is further justified by the fact that the break separating it from the Chingin Formation was probably caused by oscillatory movements and was not prolonged in time.

The Chingin Formation, in its stratotypical expression, is again represented in the Kurtushiba Range, where it is composed of metamorphosed greenish-gray quartz–feldspar sandstones, dark gray, with various shades in color, siliceous-clayey, carbo-

with carbonaceous-aleurolitic shales and greenish-gray phyllites. Among them, in the form of now thick beds, now thin lenses, light-gray and bluish microquartzites occasionally appear, as well as limestones and interbedded bodies of greenstone effusives. The cover facies of the effusives in places almost entirely displace the sedimentary components of the formation and, sometimes assuming a mandelstone character, are often interbedded with tuffs enclosing thick beds of microquartzites and rare beds of shales and phyllites. The latter, in the upper parts of the formation, acquire predominant importance; moreover, masses of limestones with remains of algae and archaeocyaths are here intercalated with them, among which the genus Orbicyathus and the species Ajacicyathus immanis (Elehem River) are especially characteristic. The Chingin Formation is characterized by an almost identical composition, in relation to its stratotype development, also within the axial part of the Western Sayan, as well as on its northern slope. This similarity is not only lithological but also age-related, since, as on the southern slope of the Western Sayan, in the axial and northern belts of distribution of the deposits under consideration, correlative forms of archaeocyaths are found in them, for example, Orbicyathus (B. Kashkarek River), Ajacicyathus immanis (B. Kyzas River), everywhere associated with algal remains.

The Chingin Formation, reaching a thickness of 5–6 km, is distinctly divided into two members, which, according to the place of their typical manifestation, are assigned their own names: the Maloshushenskaya—the lower—and the Kyzasskaya—the upper.

The Maloshushenskaya Member, well expressed in the stratotype section of the formation along the Sr. Akkol River and in the lectotype section along the M. Shush River, is composed predominantly of greenstone effusives, microquartzites, carbonaceous-siliceous shales, with a relatively small participation of medium-clastic accumulations and especially limestones, invariably thin-bedded and poor in organic remains, represented mainly by algae.

The Kyzasskaya Member, typically manifested in sections of the Kyzas, Oi, and Naryssa rivers, consists essentially of pelitomorphic formations, with an increasing role of archaeocyathid-algal limestones, but with a weak development of microquartzites, as well as effusives, among which pyroxene porphyries are especially characteristic. At the base of this member there locally lies a conglomerate, forming in the basin of the Oi and Naryssa rivers a persistent lithological horizon.

On the southern slope of the Western Sayan, analogues of the Kyzasskaya Member are best represented on the right bank of the Ulug-Khem River, 3–4 km northeast of the town of Shagonar, and in the spurs of the Aktovrak hills, located on the watershed between the Temirsuk and Elehem rivers. At the indicated points, as also in the basin of the Naryssa River, the Kyzasskaya Member is dominated by clastic rocks containing covers of pyroxene porphyries and, by analogy with the section of the Kyzas River, includes two horizons of archaeocyathid-algal limestones. But in contrast to the section of the Kyzas River, in the Priulugkhem region, between the horizons of archaeocyathid-algal limestones, there are beds of greenish-gray shales with remains of trilobites.

Although the Maloshushenskaya and Kyzasskaya members differ from one another in lithological composition and even paleontological content, they participate in the tectonic structure of the Western Sayan as a single whole, forming one Chingin Formation. It is sufficient to recall that the deposits of the latter, neither in the degree of schistosity nor, still more, in the character of metamorphism, exhibit any differences depending on stratigraphic depth. All its members, from the smallest to the largest, are structurally most closely connected with one another, and some of them are the most important marker horizons. These include, first of all, the microquartzites, which are easily identified thanks to their speci-

…physical petrographic features and which are distributed in the horizontal and vertical sections of the formation independently of its facies. Under conditions of detailed geological mapping, the use of horizons of microquartzites and other marker units described above made it possible reliably to correlate individual sections of the Chinta Formation, which is, on the whole, relatively poor in paleontological documents; among these, archaeocyathids and algae occur much more frequently than trilobites.

It was shown above that archaeocyathids have so far been found as single specimens in the limestones of the N. Karabuluk and V. Khadannykh watershed, along the Kashkarek River, and in much greater quantities—in the limestones of the Elegem, Kyzas, and M. Arbat rivers. The stratigraphic position of the limestones mentioned and the distribution within them of individual forms of the archaeocyathid fauna were given in the descriptions of the corresponding sections. Therefore here it is possible to confine ourselves to giving a general list of archaeocyathid species, of which the following are the most characteristic: Ajacicyathus immanis, A. otichtaschensis, A. jijzki, A. proscurjakovi, A. camptophragmeus, A. dissepimentalis, Archaeocyathus tolli, Coscinocyathus dianthus, C. papillatus, Dictyocyathus tuvaensis, Orbicyathus mongolicus, Orbicyathus sp., Protopharetra simplicissima, Spirocyathus tolli, Tercyathus batovi.

Of the species listed, Ajacicyathus immanis is common to both the Kyzas and Elegem sections of the formation; moreover, at the latter locality it is associated with the genus Orbicyathus, which, in turn, being established in the limestones of the Karabuluk–Khadannykh watershed and along the B. Kashkarek River, may prove to be the most important correlative form for the Chinta Formation as a whole. It can only be regretted that the archaeocyathids discovered by me in the limestones of the Sivaya River remained unstudied, while the archaeocyathids found earlier by N. A. Batov in the limestones of the Rudnaya River proved to be poorly preserved.

Nevertheless, the forms of the archaeocyathid fauna cited above, belonging to the so-called Gavrilovka complex [11, 40], make it possible to date the age of the rocks containing the fauna within the narrow limits of the lower Lower Cambrian. Trilobites found in the Priulukhem belt of distribution of the Chinta Formation, in shales probably overlying, stratigraphically, the limestones with the Gavrilovka complex of archaeocyathids, also indicate this age. Among the trilobites the following have been established: the new genus Schagonaria from the family Paradoxidae, the new species Redlichina sayanica, associated with several genera from the family Protolenidae, including Bergeroniellus. It is known that representatives of the latter family are characteristic only of Lower Cambrian deposits [65, 165–166].

The genus Redlichina Lerm., as E. V. Lermontova noted at the time, shows a close similarity to the genus Redlichia Cossmatt [1, 133], which, according to the latest summary by Li (Lee), also occurs in the Lower Cambrian deposits of China and, in particular, in the classic section of Shandong Province (Schantung), where the Monto Formation with Redlichia chinensis Walc. was first distinguished [55, 84–88].

In beds of the same age, Redlichia appears in Korea, Indo-China, India, and Australia [4, 80–81; 1; 55]. But in the Sayano–Altai region, for example, on the southwestern margin of the Eastern Sayan, near the village of Kameshki, representatives of the genus Redlichina, together with other forms, have been found in shales lying stratigraphically below the limestones with the Kameshki complex of archaeocyathids, the age of which is generally recognized as Lower Cambrian [11, 35 and 36].

The latter contains many genera and species characteristic of the Gavrilovka complex of archaeocyathids, so that distinguishing them can be carried out only with very detailed studies.

Therefore, at the present stage in the study of the Cambrian fauna of the Western Sayan and of other regions of the Sayan–Altai area, it will not be a major error to assign the aforementioned archaeocyathan assemblages to one large biostratigraphic subdivision of the Lower Cambrian.

The presence in the trilobite fauna of the Chingin Formation of the genus Schagonaria from the family Paradoxidae cannot contradict this conclusion, since archaic representatives of the named family are now known also in the Lower Cambrian division.

Thus, the Chingin Formation of the Western Sayan, characterized by remains of the Gavrilov assemblage of archaeocyaths and by trilobites, united by me into the Chingin assemblage, must unquestionably be assigned to the Lower Cambrian division and, moreover, by no means to its very uppermost parts.

In fact, it was shown in the appropriate place that in the Western Sayan, along its northern slope, stratigraphically above the deposits of the Chingin Formation lies the Babak Formation, composed exclusively of limestones rich in remains of trilobites, archaeocyaths, algae, and more rarely brachiopods. Among the trilobites found along the Babak R., there have been established Erbia gigantea Jegorova, Erbia siberica (Schm.) Lerm., Granularia obrutchevi, new species from the previously known genera Kooteniella (?), Proerbia, Chondragraulos, and the new genera Tabatopygellina, Arbataspis. Associated with the trilobites are the algae Epiphyton fasciculatum, E. fruticosum, Rasumovskya ramosa, as well as archaeocyaths, the species composition of which was given in the description of the limestones of the Babak R. The latter, on the basis of archaeocyaths and algae, proved to be entirely comparable with the limestones of the lower course of the Gerasimov R. and the Terbisheva R., tributaries of the Monok.

The generalized composition of the archaeocyathan fauna of the Babak Formation is so far exhausted by the following species: Ajacicyathus ijizkii, A. davighi, A. pelminiulus, A. disporwallum, A. (Rhabdocyathus) crassimurus, A. procurjakovi, A. cf. uricus, A. cf. otichtaschensis, A. simplex, A. sp. nov., Bicyathus crassimurus, Carinocyathus bagenovi, Coscinocyathus dianthus, C. convexus, C. dianthus Born. var. stabilis, C. calatus, Cyclocyathus tersiensis, Ethmophyllum gracilis, E. ratum, E. ulmensis, Sibericyathu dissepimentalis, Thalamocyathus (Cyclocyathus) kusnetzovi, Tersia sp.

Among the listed forms, species prevail that are grouped in the Sanashtykgol and Obruchev assemblages of archaeocyaths, with a comparatively large participation of the Kameshkov assemblage. On the whole, it is most correct to identify the archaeocyathan fauna of the Babak Formation with the Obruchev assemblage, since in the composition of the Sanashtykgol assemblage representatives of the Kameshkov assemblage are almost entirely absent. The Obruchev assemblage of archaeocyaths, also the Policyaith horizon, was considered, in age relation, transitional from the Lower Cambrian to the Middle and was correlated with the so-called Upper Kutorgina horizon of the base of the Middle Cambrian [11, 38; 65, 147—148, table].

E. V. Lermontova considered the Policyaith horizon younger than the Upper Kutorgina, though not prejudging the question of the age of the first of them [65, table]. However, study of the stratigraphic relationships and paleontological content of deposits of the Sayan–Altai area equivalent to the Upper Kutorgina and Policyaith horizons shows that both these horizons form a single biostratigraphic unit, the age of which cannot be younger than the very top of the Lower Cambrian. Indeed, the Proerbia present in the Babak Formation is a most typical Late Lower Cambrian trilobite, known only from the upper parts of the Kutorgina sequence of the well-studied Lower Cambrian section of Central Siberia [65, 109 and table]. The genus Tabatopygellina is an intermediate form between Kootenia and Bonnia, showing the greatest similarity to the latter of these.

Bonnia, as is known, is characteristic of the Lower Cambrian [56], and in the classical section of the same Central Siberia it again occurs in the upper parts of the Kutorgina sequence [65, table].

In addition, the vertical range of the genus Granularia is probably confined only to the limits of the Lower Cambrian. In any case, the limestones of the Bidzha River, flowing down from the Batenev Range on the eastern slope of the Kuznetsk Alatau, where the typical species of the genus Granularia was found, were erroneously assigned by E. V. Lermontova to the lower Middle Cambrian [65, 147–148 and table]. From what follows it will be seen that the limestones mentioned, being synchronous with the limestones of Mt. Babak, are likewise Lower Cambrian.

At the same time, the presence in the latter of Erbia gigantea and Chondragraulos only confirms the Late Lower Cambrian age of the Babak Formation, for the named trilobite forms have recently been found in association with Bergeroniellus, Bonnia, and Pagetina in the Kayancha Formation of the Gorny Altai, where, among other things, they are accompanied by Granularia, Arbataspis, and archaeocyaths. Among them P. S. Krasnopeeva established the species Erbocyathus heterovallum, characteristic of the Obruchev (Poliellatovyi) assemblage, in association with many species of the genus Ethmophyllum, as well as species of the Kameshki assemblage, such as, for example, Ajacicyathus cf. proscurjakovi, A. dwigthi, established also in the Babak Formation of the Western Sayan, where they are accompanied by representatives of the genus Ethmophyllum.

Anticipating somewhat, I shall point out that the polyciatid limestones of the village of Potekhino (Mt. Dolgii Mys) are, if not contemporaneous, then very close in age to the Babak limestones.

In light of the foregoing, it becomes obvious that the Babak Formation, containing a distinctive trilobite assemblage named by me the Babak assemblage, as well as the Obruchev (polyciatid) assemblage of archaeocyaths accompanied by algae, is clearly correlated with the upper Kutorgina, i.e., the polyciatid horizon, of the age of the very uppermost Lower Cambrian. This conclusion is based not only on paleontological evidence but also on stratigraphic observations.

There is no doubt that the Babak Formation lies stratigraphically above the Chinginskaya Formation, characterized by the Gavrilovka assemblage of archaeocyaths and the Chinginskaya assemblage of trilobites. Moreover, these formations are probably separated by an unconformity surface that arose as a result of the manifestation of a phase of tectogenesis that took place at the end of Chinginskaya time. It is true that at the base of the limestones of the Babak Formation no basal terrigenous accumulations have been established, but their presence is not obligatory, since in the geological literature on Western Siberia repeated examples have been cited in which a transgressive sequence composed of limestones is not accompanied by nearshore facies [53, 124; 43]. The break and unconformity between the Chinginskaya and Babak formations are evidenced by the fact that the latter is not intruded by hyperbasite plutons, the formation of which probably ended long before the time of accumulation of the limestones of the Babak Formation. In other areas of the Sayano-Altai region, its equivalent deposits rest with a clear unconformity upon the underlying rocks (Gorny Altai, village of Potekhino, eastern slope of the Kuznetsk Alatau).

In the Western Sayan, the Babak Formation is overlain with angular unconformity by effusives of the Lower Monok Formation, and both are cut by Maina trondhjemites. Pebbles of the latter fill the conglomerates of the Upper Monok Formation, which contains limestone horizons with archaeocyaths of the Sanashtyk-Gol assemblage and a trilobite assemblage of the same name from the lower Middle Cambrian [39; 40].

In this connection I note that N. S. Zaitsev and N. V. Pokrovskaya make a serious error in assigning the limestones of Mt. Babak to the “Middle Cambrian” [9, 104];

they, being Lower Cambrian, are undoubtedly older than the Sanashtykol limestones.

It should always be borne in mind that, without precise establishment of the ascending stratigraphic order of paleontologically characterized deposits, the straightforward application of the paleontological method for purposes of correlation and age determination cannot guarantee against errors arising from the still insufficient study of the organic remains of the Cambrian of the Sayano-Altai region in general and of the Western Sayan in particular. A sufficient number of examples of this could be cited, but I shall limit myself to only two.

It is known that, in her time, E. V. Lermontova, guided by East Asian Cambrian standards, believed that beds with the genus Redlichina constitute the lower part of the Middle Cambrian [1, 133—134; 65] and lie above the protolenid deposits of Central Siberia, as well as above the limestones with the Kameshki archaeocyathid complex of the village of Kameshki in the Eastern Sayan [3, 84—85]. Now, however, after detailed studies carried out in the second of the indicated regions, there remains no doubt that the Kameshki archaeocyathid limestones overlie the deposits with Redlichina.

Further, the idea has become firmly rooted in the literature that representatives of the family Paradoxidae Emmrich are characteristic only of the Middle Cambrian; however, in the Sayano-Altai region one of the archaic genera of this family has been established in the Lower Cambrian Chinginskaya Formation together with Redlichina, and in Central Siberia (Yakutia)—in deposits of the same age in association with forms typical of the family Protolenidae.

Having established the age of the effusive-sedimentary stratigraphic units of the Lower Cambrian of the Western Sayan, it is also possible, with sufficient justification, to determine the age of the hyperbasite and microdiorite intrusive formations characteristic of this division, whose plutons are situated predominantly within the area of distribution of the Chinginskaya Formation. Both the former and the latter masses of the mentioned intrusions are cut by and metamorphosed by Main trodhjemites, and thus the latter are younger than the former. In addition, K. V. Radugin, on the basis of study of the Berezovyi (Berezovskii) Range, within the northern slope of the Western Sayan, proves that pebbles of hyperbasites occur in the conglomerate-breccias of the Lidite unit, which underlies the deposits of the Verkhnemonok Formation. It may now be considered established that the Lidite unit represents one of the members of the Nizhnemonok Formation, expressed, for example, along the Babak River. There, at the base of a similar unit, lies a breccia containing fragments and grains of serpentinites, which indicates a pre-Nizhnemonok age for the hyperbasites.

Certain difficulties arise in determining the age of the Kair microdiorite intrusion, and for the following reason. In the opinion of some investigators (A. Shmidt), in the Borus Range, and then in the lower reaches of the Kantegir River, strongly altered rocks of plutons of medium basicity, which I combine into the Kair intrusion, are supposedly cut through and metamorphosed by hyperbasites. I have not had the opportunity to verify these observations of A. Shmidt. However, in the Kurtushibinsky Range the hyperbasites are cut by thick dikes of gabbro and microdiorites, as was recently confirmed also by G. V. Pinus [32, 950]. At the same time, in the same Kurtushibinsky Range, in the basal conglomerate of the Kentereskaya Formation of the lower Lower Silurian, pebbles of serpentinites and of gabbro and microdiorites cutting them are found. And in the Kair Range (on the northern slope of the Western Sayan) plutons of the microdiorite intrusion are directly overlain by the basal conglomerate of the Upper Cambrian Arbat Formation, in whose composition pebbles of microdiorites play no small role [40, 9].

Finally, in the Tabat–Dzhebash region of the same northern slope of the Western Sayan, gabbroid rocks, cutting and metamorphosing the Babak limestones of the upper Lower Cambrian (Tabat River basin), occur together with microdiorites already in the pebbles of the conglomerate of the Lower Monok Formation, which I had previously considered to belong to the Upper Monok Formation [Monok River, 40, 10].

The facts cited indicate that the rocks of the basic magma, which I have combined into an independent Kair microdiorite intrusive formation, while unquestionably Lower Cambrian, may possibly prove to be of different ages with respect to the time of their emplacement. However, the final resolution of this question depends on clarification of the nature of those original hornblendites and amphibolites which, in the Borus Range, as a result of the alleged contact effect of hyperbasites upon them, are transformed into issites. It may be that the metamorphism of the latter is caused not by the hyperbasite intrusion, but by the Main (?) albitites, whose plutons are so widely distributed in the named range.

Assuming that the resolution of this question is a matter for more detailed studies, one can be certain only that after the emplacement of the hyperbasites and, undoubtedly, at the end of the Lower Cambrian, dioritic magma was intruded, the differentiation of which was accompanied by the formation of rather diverse rocks, from gabbro to diorites. The plutons of the latter, under the influence of the Main trondhjemites, underwent intense alteration (chiefly amphibolization) and, in this altered form, ended up in the pebbles of the conglomerate of the above-mentioned Lower Monok and later formations.

On the correlation of the Lower Cambrian geological formations of the Western Sayan and adjacent regions

In comparing the Lower Cambrian formations of the Western Sayan with equivalent deposits distributed in other regions of the Sayan-Altai area, importance was attached not only to the paleontological content found in the strata being compared, but also to their stratigraphic position in the section, as well as to the presence in some of them of characteristic facies-identical lithological components.

The Lower Cambrian deposits of the Sayan-Altai area have not everywhere been studied with the same degree of detail, and therefore, in correlating them, I relied chiefly on those regions within which I had personally carried out geological investigations of one kind or another. It is quite understandable that in these comparisons I sought to use, as fully as possible, the materials of my predecessors and especially of later investigators.

Among the local stratigraphic units of the Lower Cambrian, distinguished at different times by different authors in the Sayan-Altai area, analogues of the Babak Formation and partly of the Chingis Formation are most clearly recognizable, and to a lesser degree those of the Akkol Formation, which is natural, since in the Western Sayan it is paleontologically the least well characterized, and its lower side there proved either not to have been exposed by the present denudation surface, or to have been cut off by disjunctive disturbances. Therefore, and also because throughout the Sayan-Altai area deposits older than the middle of the Lower Cambrian are weakly subdivided, the determination of analogues of the Akkol Formation encounters considerable difficulties. These analogues are very provisionally distinguished by me only in Mountain Shoria, in the basin of the upper course of the Mrassy River. In the indicated region, in the headwaters of the Matur and Uzas rivers, the Matur Formation is distinguished (A. G. Sivov, 1941), composed of intensely dislocated metamorphosed carbonate rocks, interbedded with red-

quartzites, graphitized shales, enclosing thin sheets (?) of greenstone effusives of the basic series. The position of its footwall remained undetermined, for to the northeast, along a disjunctive contact, there adjoin it, probably, still more ancient carbonate formations, assigned by K. V. Radugin to the Proterozoic. In its hanging wall, in the basin of the Uzas River, above a conglomerate there lies a terrigenous-effusive-carbonate sequence, which will be discussed somewhat below, with remains of archaeocyaths, including Ajacicyathus immanis. In the marbled limestones of the Matur Formation, remains of algae are constantly found, among which Osagia ovosimilis, Conophyton, and Collenia are especially characteristic; in places they are accompanied by archaeocyaths (?) with an extremely primitive cup structure (Sr. Ada River, in its lower course).

The composition of the organic remains of the Matur Formation differs sharply from the composition of the algae occurring in the more ancient carbonate sequences of Gornaya Shoriya. The presence in the deposits of the Matur Formation of Osagia ovosimilis, on the one hand, and its position in the section, on the other, served as grounds for correlating it with the Akkol Formation of the Western Sayan, although facies-wise they differ sharply from one another. The presence in the former of remains of archaeocyaths (?) was an important support for assigning these formations to the Lower Cambrian.

Analogues of the Chinginskaya Formation have a wider distribution in a number of regions of the Sayan–Altai area and are identified there with much greater certainty than analogues of the Akkol Formation (fig. 28).

Thus, on the southwestern margin of the Eastern Sayan, in the basins of the lower course of the Kazyr River, in the vicinity of the village of N. Kuryaty, a sequence is distinguished of aleurolitic, siliceous-clayey shales, interlayered with greenstone effusives, enclosing thick beds of microquartzites and horizons of gray and pinkish-gray limestones with abundant remains of archaeocyaths. Among the latter, forms have recently been established whose belonging to the Gavrilovsko-Kameshkovsky complex is beyond doubt. On the northwestern continuation of the sequence of the village of N. Kuryaty there is the reference section of the Lower Cambrian for the Eastern Sayan, which in the past was interpreted differently by the same investigators, and the age of the rocks composing it was determined from Precambrian to Upper Cambrian, excluding igneous formations [28, 262—264]. My study of this section showed that it is composed predominantly of Lower Cambrian deposits, here united into the Kameshkovskaya suite, the volume of which corresponds to all three suites of previous investigators [28, 263—264]. Since disagreements still exist on the question of interpreting the section of the village of Kameshki, it was naturally necessary to dwell somewhat on its characterization.

One kilometer northwest of the named settlement, on the right slope of Kameshki Creek, conglomerates crop out, composed of boulders of archaeocyathan limestones, pebbles of effusive rocks, including greenstone pyroxene porphyrites; the conglomerates, up to 10 m thick, dip northwest at an angle of 75–80°. From beneath them are exposed massive, slightly banded, dark-gray marbled limestones, dipping northwest at an angle of 75–80° and reaching a thickness of 25–30 m. Maintaining the same dip, from beneath the limestones there appears a member of thin-bedded, dark-gray shales with remains of trilobites, replaced stratigraphically below by their green varieties, including interbeds of microquartzite, with well-preserved sponge spicules. The thickness of this member is not less than 70 m. Among the trilobites mentioned above I identified Redlichina vologdini, R. sayanica n. sp., Pagetina rjonsnitzkii, P. primaeva, Pagetia ornatus n. sp., which have been adopted as the standard of the Chinginskaya complex.

Comparison with the section of the Middle Sayan

Biostratigraphic subdivisions

Correlation of the Lower Cambrian deposits of the Sayano-Altai region

Left-hand stratigraphic scale:

• Lena Stage
• Sinskaya Formation
• Pestrotsvetnaya Formation
• Beds with Obolella
• Aldan Stage (?)

Subdivisions shown in the comparison column:

• Dermodtov horizon
• Sinyutyktygol horizon
• Vashinatonian (?)
• Upper Torgashino Cm₁
• Lower Torgashino
• Sinskaya Formation
• Pestrotsvetnaya Formation
• Beds with Obolella

Biostratigraphic column:

• Name of paleontological complexes
• Sinyutyktygol archaeocyathan and trilobite complexes
• Bazhaikha complex: trilobites Schistometopus, Fallotaspis, Redlichia, Pagetia, Sbautia; algal complex; archaeocyaths — Coscinocyathus
• Kameshkovskii complex of archaeocyaths: Aptocyathus astuta, A. purginoides, Coscinocyathus, Alconthus and a number of trilobites, algae
• Chinzhinskii complex: trilobites Redlichia, Pagetia, Pagetina, Schagonnus, Bergetonellus
• Gavrilovskii complex of trilobites: Abadiella, Anomocare, Asterocyclus, subtilis, Oelungthus, mongolicus, Tersella nodosa; sometimes trilobites, algae, brachiopods, Eoobolus
• Algae, Obolella, Obolus, Coleolo and archaeocyaths (?)

Regional columns and visible formation names:

• Gorny Altai (middle reaches of the Katun River)
• Ust-Seminskaya Formation
• Kamlakskaya Formation
• Inya–Sary-Koksha
• Ust-Sema Formation
• Kamlakskaya Formation
• Elandinskaya Formation
• Upper Markakol Formation

• Adinskaya Formation

• Salair (Gur’evsk district)

• Pecherkinskaya Formation

• Gavrilovskaya Formation

• Gornaya Shoria (upper reaches of the Mrassu River)

• Pecherkinskaya Formation
• Manasskaya–Bertonskaya Formation
• Kachinskaya and Ust-Sema formations
• Upper Mrassu–Malaya Mrassu Formation and Adinskaya Formation

• Mazurskaya Formation

• Eastern slope of the Kuznetsk Alatau (Kiya–Barsa area)

• Potekhinskaya Formation
• Fominskaya Formation

• Emskenskaya Formation

• Western Sayan

• Nizhne-Kondomskaya Formation
• Balakhtinskaya Formation
• Verkhnemonokskie limestones
• Azasskaya Formation
• Shunetinskaya Formation

• Ak-Kolskaya Formation

• Eastern Sayan

• Area of the villages of Kamenka and N. Kuragino
  • Nizhne-Kondomskaya Formation
  • Kamenskie limestones
  • Chinzhinskaya Formation
• Biryusinsko–Torgashinskii district
  • Bazhaikha Formation
  • Verkhnetorgashinskii limestone
  • Nizhnetorgashinskii limestone
  • Beds of clayey-carbonate shales
  • Biryusinskii limestone
  • Limestone–graywacke Formation
  • Effusive Formation

Side labels:

• Chinzhinskaya Formation
• Torgashinskaya Formation

Fig. 28. Legend

1—metamorphosed sandstones; 2—clayey shales; 3—marmorized limestones; 4—conglomerates and gravelites; 5—clayey-siliceous shales; 6—microquartzites; 7—tuffs; 8—sandy limestones; 9—greenstone effusives of intermediate composition; 10—phyllites; 11—pyroxene porphyrites; 12—graywackes and siltstones; 13—graptolitic shales; 14—effusives of acid composition; 15—quartzites; 16—intrusive rocks of granitic composition; 17—hyperbasites; 18—paleontological remains; 19—transgressive contacts.

Farther on, almost to the very lowest part of the Kameshki River, exposures are absent; after this, in small ravines falling into the stream of the same name on the right, there appear, undoubtedly, bedrock outcrops of shales, with rare and poorly preserved fossils represented by trilobites, from which Pagetina primaeva is reliably established. This member of the section is of interest not only because it contains trilobite remains, but chiefly because in it a distinct bedding is discernible, dipping to the southeast at an angle of not less than 55°. Stratigraphically higher lie limestones, divisible by textural features into several units, also dipping to the southeast, but at an angle of 45–50°. The lower two units, with a thickness of up to 150 m, are composed of massive, strongly recrystallized limestones with sparsely distributed archaeocyathid remains, interbedded with clayey limestones containing rare Protolenidae. The upper units, up to 325 m thick, are expressed by reddish-gray and yellowish varieties of limestones, with abundant archaeocyathids, among which Ajacicyathus elongatus, A. tomicus, A. ijizkii, A. (Rhabdocyathus) sibiricus, Urcyathus (Ajacicyathus) intracostatus, Coscinocyathus cornucopiae, C. calatus, Cyclocyathus tersiensis, Thalamocyathus (Cyclocyathus) nikitini had previously been established; algae, including Epiphyton fasciculatum, are associated with the archaeocyathids. The listed archaeocyathid fauna represents the standard of the Kameshki complex. Then, east of Kameshki Stream, sandy-conglomeratic formations of the preceding type again become widespread, but they dip to the southeast at an angle of 75°. Finally, in the headwaters of Kameshki Stream, on its right side, effusives appear, strongly schistose, probably forming a small cover within the carbonate rocks.

From analysis of the section described it becomes clear that its youngest member is the sandy-conglomeratic sequence forming the limbs of a slightly overturned to the northwest anticlinal fold. However, contrary to the opinions of earlier investigators [28, 263], I could not recognize this sequence as Lower Devonian; in my view, by the character of its lithological composition, tectonic structure, and stratigraphic position, it represents an analogue of the Upper Monok or, at least, the Arbat formation of the Western Sayan. The carbonate-shale rocks participating in the structure of the closure of the anticlinal fold, on the basis of the fossils occurring in them, must be assigned entirely to the Lower Cambrian; moreover, its shale members, characterized by trilobite remains, represent its oldest horizon, and not the reverse, as was recognized by earlier investigators [28, 263]. The Lower Cambrian deposits of the Kameshki suite are confidently correlated with the carbonate-shale sequence of the N. Kuryata River, which, in turn, to the southwest, in the basin of the middle course of the Amyla River, directly enters the field of distribution of the Chinginskaya formation of the Western Sayan. I note in this connection that the comparable formations in all the listed localities are unconformably overlain by covers of acidic and intermediate-basic effusives, petrographically showing a striking similarity to the Lower Monok formation of the Western Sayan. At the same time, one cannot fail to note the circumstance that in the Lower Cambrian section of the Kameshki River a new marker horizon is distinguished—limestones with the Kameshki complex of archaeocyathids [11], lying stratigraphically above the shale members with Redlichina. These limestones may prove to be synchronous with those archaeocyathid limestones which, in the Priyul’khem belt of development of the Chinginskaya formation, apparently also lie above deposits with Redlichina.

The Biryusinsk–Torgoshin region of the Eastern Sayan is the second important locality where an analogue of the Chinginskaya formation is revealed on sufficiently firm grounds. It should be pointed out that I had no opportunity to become personally acquainted with the geology of this region, and the stratigraphy of the widespread

there, the oldest deposits, is the subject of lively discussions [70, 42–51]. However, an analysis of the literature sources and a study of materials placed at my disposal by student trainees of the Geological Prospecting Faculty of the Tomsk Polytechnic Institute make it possible, with good reason, to correlate the Tyubil Formation developed there, or, what is the same, the limestone–graywacke formation [63; 69], and the closely connected Biryusa limestones with archaeocyathan fauna, with the Chingin Formation of the Western Sayan.

Indeed, the Tyubil Formation, sharply separated by a basal conglomerate [69, 187–193] from the underlying strata (possibly, in part, of the Akkol Formation), is composed essentially of fine-grained, greenish-gray sandstones interbedded with clay shales, beds of dark siliceous-clayey and carbonaceous-siliceous rocks, and black, more rarely bluish-gray limestones. In the latter, student trainees found algae, among which I identified Osagia and Epiphyton, and which were later also discovered by K. V. Radugin (oral communication). He also states that, in the formation of interest to us, characteristic covers of greenstone pyroxene porphyrites are widespread; according to earlier observations by Yu. A. Kuznetsov, it is intruded by hyperbasites, a small pluton of which crops out, for example, along the Kashtak ravine [63, 19]. Irrespective of the true stratigraphic relationships [63, 15], the Biryusa limestones undoubtedly belong to it, for archaeocyathids occur in them, and among them I have identified, from student collections, Ajacicyathus, Asterocyathus (?) with forms close to the Gavrilovka complex. The archaeocyathids are accompanied by algae, in particular Epiphyton.

Thus, by expanding the volume of the Tyubil Formation at the expense of the Biryusa limestones, it shows, in its lithological composition and stratigraphic position, an exceptional similarity to the Chingin Formation of the Western Sayan. This similarity is further emphasized by the fact that both formations are characterized in their lower parts by remains of Osagia and Epiphyton, and in their upper parts by archaeocyathids of the Gavrilovka complex. This similarity is also indicated by the hyperbasites characteristic of both the Tyubil and Chingin formations and occurring predominantly among the terrigenous-sedimentary units of the latter.

In my opinion, the so-called Lower Torgoshin limestones [11, 37], with a typical complex of Kameshki archaeocyathids—Ajacicyathus acutus, A. proskurjakovi, A. patulus, Coscinocyathus dianthus, C. corbicula, C. elongatus, and others [11, 27],¹ should also be included in the Tyubil Formation. The question of the nature of the relationships of these limestones with the limestone-graywacke units, although important, cannot be considered unequivocally resolved. There is reason to suppose that the basal conglomerates identified within the Torgoshin limestones [70, 45–46] may not constitute the base of the Lower Torgoshin archaeocyathid limestones proper. These conglomerates may possibly underlie the Upper Torgoshin limestones, characterized by trilobites.

In any case, Yu. A. Kuznetsov, who studied the Torgoshin limestone sequence, does not note the presence at its base of basal-type conglomerates [63, 12–13]. They apparently appear stratigraphically higher and are overlain by limestones with trilobites, whose age, incidentally, is not Middle Cambrian [70, 45], but Lower Cambrian. Then, in the section of Mt. Kameshki, limestones synchronous with the Lower Torgoshin limestones, struc—

¹ It should be borne in mind that the listed archaeocyathid forms were identified by Toll only from modern boulders and pebbles collected by Proskuryakov from an imprecisely established point. Consequently, the position in the section of the Lower Torgoshin horizon (limestone?) remained unclear at that time; this horizon has not been identified at the present time either.

closely connected with the graywacke sequence, comparable with the similar sequence of the Biryusinsk–Torgoshino district of the Eastern Sayan and with the Chinga Formation of the Western Sayan.

Finally, as will be shown below, on the eastern slope of the Kuznetsk Alatau, in the vicinity of the village of Potekhino, limestones synchronous with the same Lower Torgoshino limestones are in fact separated by a conglomerate from limestones whose correlation with the Upper Torgoshino limestones is beyond doubt. Obviously, the section of the limestone sequence developed in the area of the village of Torgoshino requires special study in order to determine the possibility of subdividing it into independent stratigraphic units and of establishing the position in the section of the conglomerates that interest us.

In the Kuznetsk Alatau, analogues of the Chinga Formation are established in two independent districts, the geology of which is known to me not only from the literature but also from personal investigations. One of these districts occupies an extensive area situated on the eastern slope of the Kuznetsk Alatau, between the Erba River in the northeast and the Askiz River in the southwest.

At the northeastern margin of the outlined area, in the vicinity of the village of Potekhino, on the well-known Mount Chesnokovaya there, from beneath unconformably overlying limestones with an archaeocyathan fauna of the Obruchev Complex there emerges another, very thick carbonate sequence, which I have called the Chesnokovaya sequence. The composition of the latter is not homogeneous, but changes markedly both horizontally and vertically. In its lower parts there are predominantly grouped massive, sometimes bedded or flaggy, marmorized limestones, steeply dislocated and cut by numerous fractures healed now by veinlets of calcite, now by quartz. The limestones everywhere are dolomitized to one degree or another; moreover, among them, at various stratigraphic levels, occur light-gray and gray quartzites, as well as intensely schistose sill bodies of greenstone effusives, probably representing the roots of younger extrusive outpourings. In the dolomitized limestones, which smell of hydrogen sulfide when struck, algae are constantly found, belonging chiefly to Osagia, Collenia, and much more rarely to Epiphyton. In places the algal limestones form thick packages traceable over considerable distances. The described part of the Chesnokovaya sequence, in its lithologic composition and stratigraphic position, can readily be correlated with the Yenisei Formation, which A. N. Churakov persistently regards as Precambrian [51, 54] and which, in the area described, occupies extensive tracts, as may be judged from the geological map compiled by the above-mentioned investigator [51, map]. But in the vicinity of the village of Potekhino, Epiphyton is present in a similar limestone sequence, and in the basin of the Askiz River, along its left tributary, the Kyukh Creek, archaeocyathans occur together with algae, and moreover in the most ancient parts of the limestone formation developed there. The lower members of the Chesnokovaya sequence everywhere quite conformably, and even with gradual transitions, are overlain by its upper members, which are likewise composed of limestones, but are predominantly light-gray, bluish, and pinkish-gray in color and massive in structure. Dark limestones that smell of hydrogen sulfide when struck, and siliceous rocks, occur much more rarely in the upper member than in the lower member, but dolomites and dolomitized limestones are encountered in the former just as often as in the latter. Effusives are also confined to the upper member of the Chesnokovaya sequence; they are represented by greenstone, intensely schistose pyroxene porphyrites, sometimes including masses of limestone with remains of the alga Osagia. The effusives form thin sheets, situated at the most varied stratigraphic levels of the upper part of the Chesnokovaya sequence. Near the village of Potekhino they crop out

in the closure of an anticlinal fold, and in the Kamyshtinsko-Askiz region they not infrequently crown the limestone sequence.

The latter, at a number of localities, are filled with remains of archaeocyaths, which are especially well preserved in the limestones of the Chesnokovaya and Martyukhina mountains, at the springs of the Bidzha River, and in the middle courses of the B. Syra and Baza rivers.

At the first of the named localities, according to determinations by P. S. Krasnopeeva, the following forms of archaeocyaths have been established: Ajacicyathus patulus, A. tornicus, A. aequivallum, A. spinosus, A. arteintervallum [11, 38], A. regularis, accompanied by the algae Epiphyton fasciculatum, E. fruticosum, Renalcis granosus, and others [61, 42].

From the Baza River the archaeocyaths were preliminarily studied by me; there, the following forms occur: Ajacicyathus proscurfacovi, A. petalus, A. jizkii, several species of Ethmophyllum, and algae of the Epiphyton group.

The limestones containing this typically Kameshkov archaeocyathan fauna, at least in the area of the village of Potekhino and especially within the Syra–Baza river basin, are structurally most closely connected with the limestones of the lower part of the Chesnokov sequence. In it there also occur archaeocyaths (Kyukh River), which, on the basis of the presence of the species Ajacicyathus immanis (my determinations), may be regarded as belonging to the Gavrilovka complex. Obviously, the Chesnokov sequence, characterized by the Kameshkov and Gavrilovka (?) complexes of archaeocyaths, being Lower Cambrian, can to a certain extent be correlated with the Chingin Formation of the Western Sayan. In establishing the Lower Cambrian age of the Chesnokov sequence, I by no means wish to say that on the eastern slope of the Kuznetsk Alatau there are no older formations lithologically showing a close similarity to the above-named sequence, and especially to its lower components.

Another region of the Kuznetsk Alatau where equivalents of the Chingin Formation are widespread is in Mountain Shoria, in the basin of the upper course of the Mrassy River. Among the carbonate–effusive–terrigenous accumulations widely developed there, undoubtedly of Lower Cambrian age, M. A. Usov, on the basis of K. V. Radugin’s materials [67], at one time distinguished the following “formations,” in ascending order: the Mrassy, Upper Mrassy, Gavrilovka (also Karchitka), and Uzas formations [47]. But, as more detailed investigations show, many of the listed “formations” probably do not represent independent subdivisions, but are only facies of a single stratigraphic unit with a complex vertical and horizontal structure [60, 46–49]. This conclusion is not contradicted by the paleontological remains found in the Mrassy, Upper Mrassy, and Gavrilovka “formations.” Of these, contrary to the opinion of M. A. Usov, the Upper Mrassy “formation” is the oldest. It, being composed of members of conglomerates, sandstones, and argillaceous-siliceous shales containing effusive diabases with pillow texture, is overlain by limestones with an archaeocyathan fauna, of which Archaeocyathus tolli, Ajacicyathus proscurjacovi, and Rhabdocyathus sp. will most likely prove to be close to the Gavrilovka complex, and for the following reasons. In the indicated limestones K. V. Radugin discovered the only specimen of a trilobite, which, according to my determinations, although it belongs to the family Protolenidae, is characterized by a more primitive structure of the cranidium. In addition, deposits of the type of the Upper Mrassy “formation” extend also into the area of the Uzas River basin, where they are described under the name of the sedimentary-effusive sequence [58, 72], within which in 1941 I established a conglomerate with pebbles of algal limestones very similar to the limestones of the Matur Formation. The sedimentary-effusive sequence along the Uzas River is overlain by limestones which, in accordance with views then prevailing on the age of archaeocyaths [51, 34–36; 62, 178; 64, 1097], were mistakenly assigned to the Middle Cambrian [58, 75]. But now the main mass of these limestones must be acknowl-

Lower Cambrian, for it contains archaeocyathids, including such a form characteristic of the Gavrilovka complex as Ajacicyathus immanis.

It is interesting to emphasize that K. V. Radugin, while studying collections assembled by me together with G. P. Bolgov from the headwaters of the Uzas River, found in them a limestone with a trilobite, which has remained unstudied to the present day.

As for the Gavrilovka (Karchit) “formation,” it, composed predominantly of limestones and divided into five members, is characterized by the typical Kameshkovka complex of archaeocyathids, among them Ajacicyathus acutus, A. proscurjacovi, A. ijizkii, A. artintervalum, A. minussiensis, A. primitivus, Ethmophyllum aff. parvum, and other new species, accompanied by algal remains from the Epiphyton group, ordinary for the Cambrian [67]. The listed assemblage of organic remains is characteristic of the upper parts of the “formation,” having an average thickness of up to 300 m; the rest of its components, in the section along the Mrassy River, reaching 1.5 km in thickness, are devoid of fossils. These lower members of the Gavrilovka “formation” should possibly be correlated with the deposits of the Upper Mrassy “formation,” as more ancient relative to the limestones with the Kameshkovka complex of archaeocyathids.

The Mrassy “formation,” formed essentially by clastic accumulations with members of limestones containing remains of Archaeocyathus tolli, Ajacicyathus ijizkii, A. amplus, Labyrinthomorpha tolli, and the Rhabdocyathus Gavrilovka complex (?), will probably prove to be only a facies of the Upper Mrassy “formation,” if, of course, G. V. Kurganov’s observations are confirmed that the deposits of the first of these near the Sr. Chelei ulus pass beneath carbonate strata [60, 48]. The Uzas “formation,” as a volcanogenic-sedimentary one, may not have independent significance, and its rocks are probably situated at different stratigraphic levels, as, among others, G. V. Kurganov also emphasizes [60, 47].

With such an interpretation of the stratigraphy of the deposits containing horizons with the Kameshkovka and Gavrilovka complexes of archaeocyathids, they (the deposits), not only on the basis of paleontological content but also of lithological composition, show close similarity to the Chinginskaya Formation of the Western Sayan. This is especially clearly revealed thanks to the presence, in the compared deposits, of siliceous and effusive rocks, whose facial appearance is maintained there and here down to the details.

On the Salair, the Gavrilovka Formation, widespread there, is clearly correlated with the Chinginskaya Formation; it is composed of marbled limestones with rare effusives and is characterized by the presence of a typically expressed Gavrilovka complex of archaeocyathids, including Ajacicyathus immanis, Orbicyathus mongolicus, Tersia nodosa, Asterocyathus salairicus, A. latus, Goscinocyathus papillatus, and others [11, 40–41]. Its visible thickness is estimated at 2.5 km; moreover, being correlated with the Chinginskaya Formation, it is easily identified with the Upper Mrassy and, probably, will be correlative with the lower members of the Gavrilovka “formation” of Gornaya Shoria. In this connection, in order to avoid the well-known misunderstandings that may arise in stratigraphic correlation, it is advisable to retain for the Karchit limestone sequence the name given to it by K. V. Radugin—the Karchit “formation” [67, 34], and not M. A. Usov’s Gavrilovka “formation.”

In Gorny Altai, analogues of the Chinginskaya “formation” are revealed only as a first approximation to reality. A whole group of “formations,” distinguished by K. V. Radugin in the Chemal region essentially by analogy with Gornaya Shoria, may be conditionally correlated with it. These “formations,” in ascending order, are as follows: the Adyak, Upper Mrassy, Elanda, Karchit, and Uzas [68, 260–270]. They are not paleontologically characterized, and their independent separation elicited sharp objection from V. P. Nekhoroshev [66, 20–22], and only in the basin

on the Sarykoksha River it is possible, with much greater confidence, to identify analogues of the formation of interest to us, where they are expressed by a carbonate unit with undoubtedly Lower Cambrian fossils (archaeocyaths) and with an angular unconformity of the underlying beds with Prohedinia.

Equivalent deposits of the Babakovo Formation are reliably established in the Gorny Altai, on the eastern slope of the Kuznetsk Alatau, on the northwestern margin of the Eastern Sayan, and also in Gornaya Shoria.

In the first of the named regions, the analogue of the formation of interest to us is the Kayancha Formation, typically developed along the Katun River between the Ust-Sema River and the settlement of Cheposh. According to the detailed investigations of M. K. Vinkman [59, 76–78], the Kayancha Formation there rests with a clear unconformity on underlying strata assigned to the Precambrian. At its base there is a basal conglomerate overlain by thin-bedded limestones interbedded with sandstones and argillaceous shales. In the upper parts of the formation, massive, marbled limestones of light-gray, sometimes pale-pink color are predominantly developed. Trilobites occur in them, associated with archaeocyaths and algae [59, 77]. During the preparation of materials for the handbook on the paleontology of Western Siberia, I was able personally to examine this trilobite fauna, among which L. I. Egorova identified Erbia gigantea, Bergeroniellus, Chondragraulos, Granularia, Bonnia, and I, in addition, Arbataspis.

Among the archaeocyaths P. S. Krasnopeeva identifies Ajacicyathus cf. proscurjacovi, A. dwighti, Erbocyathus heterovallum, Ethmophyllum cavequadratum, E. diploperforatum, E. edelsteini, and others, accompanied by several species of algae belonging to the genera Epiphyton, Renalcis, and Collenia.

On the eastern slope of the Kuznetsk Alatau the Erba Formation, which I have distinguished and which is distributed over the area between the Erba River and the headwaters of the Bidzhi River, is correlated with the Babakovo Formation. The Erba Formation is composed chiefly of terrigenous deposits, among which conglomerates and sandstones, rather intensely dislocated and metamorphosed, play a significant role. Limestones within the formation, occurring as lenses of varying thickness and length, are probably situated at different stratigraphic levels. Its separate parts, apparently, under conditions of complex tectonics, were also taken for the so-called Kuten-Buluk Formation. In many places the Erba Formation rests on various members of the Chesnokovo sequence through a basal terrigenous horizon, and at some localities through a basal limestone breccia no more than ten centimeters thick. Such relationships, for example, between the Erba Formation and the Chesnokovo limestones may be observed on Mount Chesnokovaya and in the headwaters of the Bidzhi River. At the first of these localities, in the limestones of the Erba Formation, P. S. Krasnopeeva collected archaeocyaths, among which she identified Erbocyathus obrutchevi, E. heterovallum. In the vicinity of Mount Dolgii Mys (northeast of the village of Potekhino), Erbocyathus obrutchevi, E. heterovallum are associated with trilobites [11], among which E. V. Lermontova had already identified the following forms: Kooteniella slatkowskii, Erbia sibirica [1].

In the headwaters of the Bidzhi River the limestones of the Erba Formation contain mainly remains of trilobites and only very rarely archaeocyaths. Among the trilobites I identified Granularia obrutchevi, Proerbia (?) bidzenia, Arbataspis. The archaeocyath fauna has remained unstudied.

It is appropriate to emphasize here that in the conglomerates of the Erba Formation there are sometimes found pebbles of marbled limestone with clearly expressed relict tectonostructures and remains of archaeocyaths of the Kameshki complex.

On the northwestern margin of the Eastern Sayan, the Verkhnetorgashino limestone, repeatedly ... may be accepted as an analogue of the Babakovo Formation.

described in the geological literature [63, map; 11, 37], but not separated from the underlying limestone sequence with the Kameshkovskii archaeocyathid complex.¹) In the Verkhnetorgoshinskii limestone, E. V. Lermontova identified the following trilobites: Kooteniella slatkowskii, Erbia sibirica [64, 65], and I, in addition, Chondragraulos, accompanied by brachiopods, and among them Kutorgina cingulataeformis. The archaeocyathids from this limestone are under study.

In Gornaya Shoriya, the Berezovskaya Formation of light-colored limestones, distinguished by K. V. Radugin along the Amzas (Emzas) River, with Erbocyathus heterovallum and Ajacicyathus clarus [67, 41], represents an analogue of the Babakovo Formation. It should only be borne in mind that K. V. Radugin erroneously considered the Berezovskaya Formation to lie stratigraphically above the sedimentary-effusive accumulations with Agraulos [67, 41]. There are now entirely precise data that the latter, additionally characterized by remains of Paradoxides suboelandicus, overlie the limestones of the Berezovskaya Formation. K. V. Radugin correlated with it the limestones distributed in the basin of the Mrassa River, along its tributary, the Mazas River, which he distinguished as the Mazas Formation [67, 40]. As the above-mentioned investigator indicated, it undoubtedly lies above the Karchitskie limestones and is overlain by effusives [67, 41—42], occurring in a diagenetized state and represented by acidic and basic varieties close in petrographic composition to the Pecherkinskaya (Lower Monokskaya) Formation. From the Mazas Formation the following forms of archaeocyathids are known: Ajacicyathus clarus, A. primitivus, A. regularis, Tumulocyathus pustulatus, Archaeocyathus artus, Ethmophyllum gracilis, Spirocyathus, accompanied by the algae Epiphyton fasciculatum, Razumovskya uralica.

Thus, the relatively heterofacial formations considered above, on the basis of the presence in them of the Babakovo complex of trilobites and the Obruchev complex of archaeocyathids, occurring together or separately, are reliably correlated with one another and constitute a single biostratigraphic unit, previously called either the Upper Kutorgina or the Polycyathus horizon [1; 11, 65].

Thus, the Mazas Formation, through Ajacicyathus clarus, is correlated with the Berezovskaya; the latter, in turn, through Erbocyathus heterovallum—with the Erbinskaya; and the latter, through Kooteniella slatkowskii, Erbia sibirica—with the Verkhnetorgoshinskii limestone, and through Erbocyathus heterovallum—with the Kayanchinskaya Formation; and, finally, the latter, through a series of forms, is correlated with the Babakovo Formation.

To this I shall add that the local stratigraphic units described are almost everywhere separated from the underlying sequences by an unconformity, of either angular or parallel type [57, 45—51]. Moreover, as was indicated in its proper place, the Babakovo Formation is cut by a microdiorite intrusion, which in turn cuts the deposits of the Erbinskaya Formation. These microdiorites in the Potekhino-Askyz area are older than the acidic granite intrusion, which A. I. Aleksandrov compares with the Main intrusion and which in the Western Sayan is also younger than the microdiorite intrusion.

Analogues of the Babakovo Formation in a number of areas of their development with a clear parallel unconformity (Gorny Altai, Gornaya Shoriya, Western Sayan) are overlain by effusive-sedimentary formations of the lower part of the Middle Cambrian, which only rarely contain limestones with remains of Edelsteinaspis (Gorny Altai, Ust-Sema Formation). On the eastern slope of the Kuznetsk Alatau, the Erbinskaya Formation is likewise overlain unconformably by limestones with remains of Poliellina and Dinesus (left bank of Loshchenkov Log). Incidentally

¹) K. V. Radugin has proved that the Torgashino limestones, with a basal conglomerate and unconformably, rest on the underlying sequences; the rich archaeocyathid fauna of these limestones is represented predominantly by species of the Obruchev complex.

I shall point out that the limestones with Dorjuga sp. in the vicinity of Bey-Buluk, described at one time by A. N. Churakov, do indeed lie discordantly, with a sedimentary breccia at the base, upon a terrigenous-carbonate sequence, which he assigned to the Kutenbuluk Formation (53, 68–69). But I have recognized the latter as the Erba Formation, since it contains interbeds of limestone with archaeocyaths that have remained unstudied.

On the biostratigraphic subdivision of the Lower Cambrian deposits of the Sayan-Altai region

The paleontological assemblages established within the Lower Cambrian of the Sayan-Altai region, successively replacing one another in time and characterized by a relatively narrow vertical and broad horizontal distribution, form the basis for subdividing the enclosing rocks into biostratigraphic subdivisions, each of which may prove equal in stratigraphic range to a stage.

It is known that, up to the present time, the Lower Cambrian has commonly been subdivided into zones, the number of which is established as from 4 to 5 and which are more or less fully manifested only in individual parts of the North Atlantic province [6]. Recently, on the basis of the study of Central Siberia, grounds were given for distinguishing in the platform Lower Cambrian two stages—the lower, Karpheian, and the upper, Lenian or Protolenus [41]. But this relatively well-studied standard proved difficult to correlate with the section of the geosynclinal deposits of the Lower Cambrian of the Sayan-Altai region, owing to the fact that in the latter the succession of paleontological assemblages, probably closely related to the faunistic groups of the Karpheian and Lenian stages, did not always coincide in time. Furthermore, in the named mountainous country the renewal of the composition of each successive paleontological assemblage took place following tectonic movements, documented in the geological record by surfaces of unconformities and breaks, which clearly determine the boundaries between stages. Finally, in the Sayan-Altai region there appear new biostratigraphic elements, now more ancient, now younger in relation to Central Siberia, and thus the Lower Cambrian column of the former has proved considerably longer in comparison with the column of the latter. These circumstances could not be disregarded in subdividing the Lower Cambrian deposits of the Sayan-Altai region into stages, of which three are distinguished here and whose names coincide with the names of the formations established as stratotypes for them in the Western Sayan, with the exception of the upper stage, called Kayanchin for the reasons given below.

The Akkol Stage \((\mathrm{Cm}_1^1?)\) is distinguished provisionally, since its paleontological content is so far limited only to remains of algae, including Osagia ovosimilis, whose significance for stratigraphic correlation still remains not entirely clear. The independence of this member is determined by the fact that it is separated from the overlying stage by a distinct unconformity. Analogues of the Akkol Stage in the standard section of Central Siberia are probably absent.

The Chingin Stage \((\mathrm{Cm}_1^2)\) is relatively rich in organic remains, and among them archaeocyaths, algae, and, more rarely, trilobites are diverse. The first of these are grouped into the Gavrilovka and Kameshkovka assemblages, and the latter into the Chingin assemblage, the deposits enclosing them lying in the upper part of the stage. Through the algae associated with the archaeocyaths, these parts of the stage are closely connected with its lower members, characterized by remains of algae, for example, Epiphyton sp., Renalcis granosus, Osagia.

The Chinginsky Stage proved difficult to correlate not only with remote standards of the Lower Cambrian, but even with the section of Central Siberia. The point is that the Gavrilovsky archaeocyathan assemblage present in this stage, despite its distinctiveness, shows a marked similarity to South Australian forms. It is known that in South Australia archaeocyathan limestones lie at the base of the Lower Cambrian, separated from the underlying algal limestones, assigned to the Precambrian, by a sharp break [71]. The archaeocyathan limestones there are overlain by deposits with Protolenus in the lower parts and Redlichia in the upper parts. In Central Siberia, at the base of the Lower Cambrian there are likewise archaeocyathan limestones, lying, undoubtedly, below the Lena Stage (the Protolenus zone according to E. V. Lermontova), but probably within beds with Olenellidae (the Kerphey Stage according to N. P. Suvorova). Beds with Redlichia have not been established in the area under consideration, and its Lower Cambrian section ends with the Upper Kutorghina horizon, if, of course, the transitional sequence does indeed prove to belong to the lower parts of the Middle Cambrian [65, 157].

But in the Western Sayan the horizon with the Chinginsky trilobite assemblage contains both Protolenidae and Redlichina, and it probably lies above the limestone with archaeocyathans that show affinity with South Australian forms (the Gavrilovsky assemblage). To this we add that in the Eastern Sayan the horizon with Protolenidae, Redlichina, Pagetina rjonsnitzkii is overlain by limestones with the Kameshkovsky archaeocyathan assemblage, closely related to South European forms (Sardinia). Rare remains of Bergeroniellus have been found in this limestone.

In view of the foregoing, it becomes evident that the Chinginsky Stage includes both elements of the Kerphey and almost the entire Lena Stage of Central Siberia, with the exception, probably, of the Upper Kutorghina horizon, as having been mistakenly assigned to the latter. This is why the Chinginsky Stage is also distinguished as the second stage ($Cm_1^2$) within the Lower Cambrian of the Sayano-Altai region.

The Kayanchinsky Stage is the uppermost component of the Lower Cambrian of the area under consideration. The Kayanchinsky Formation of the Gorny Altai was adopted as the stratotype of this stage because the position of its footwall is established much more clearly in comparison with its analogue—the Babakovsky Formation of the Western Sayan. The Kayanchinsky Formation transgressively overlies, through a basal conglomerate, deposits assigned to the Precambrian. The Erbinsky, Berezovsky, and Mazassky formations of the Kuznetsk Alatau and the Upper Torgoshinsky limestone of the Eastern Sayan are clearly correlated with it. The Kayanchinsky Stage as a whole is characterized by the Babakovsky trilobite assemblage and the Obruchevsky (polycyathan) archaeocyathan assemblage (Erbocyathus heterovallum).

The general aspect of the fauna of the first of these preserves features characteristic of the Lower Cambrian (Proerbia, Bergeroniellus, Granularia), but with a noticeable presence of forms that play some role in the lower parts of the Middle Cambrian (Erbia, Chondragraulos). The composition of the assemblage, with some exceptions, is markedly distinctive and difficult to correlate with the faunas of known standards throughout the world. The Obruchevsky archaeocyathan assemblage is characterized by the same features. Both show a certain affinity with the Pacific zoogeographic province.

Because of the presence in the Kayanchinsky Stage of elements of forms that developed more intensively later, it could be assigned to the Middle Cambrian. However, throughout the Sayano-Altai region the stage under consideration is sharply separated from the overlying sequences by an unconformity, in which fossils of the lower parts of the Middle Cambrian are present and forms from the upper parts of the Lower Cambrian continue to persist, although in greatly reduced numbers. In accordance with this, it is reasonable to regard the Kayanchinsky Stage

the third biostratigraphic member \((Cm_1^3)\) of the Lower Cambrian, so far characteristic only of the Sayano-Altai region.

Thus, the deposits of the biostratigraphic stages of the Lower Cambrian of the Sayano-Altai region, neither in their sedimentation conditions nor, still more, in their paleontological content, are equivalent to the zones, horizons, or even stages distinguished in the eponymous division of Central Siberia. In my view, as is also shown in the appended scheme (Fig. 28), they enter as subordinate subdivisions into the stages of the Sayano-Altai region. In the latter, within each stage there are united local stratigraphic units of different facies profiles, possessing entirely different stratigraphic ranges, but containing fossil assemblages relatively close in composition (Fig. 28). These local stratigraphic units may be retained on a geological map in the form of facies differentiations of the corresponding stage, represented by special conventional signs.

Some conclusions

Thus, in the Western Sayan, deposits formerly united into the third upper sequence of the Cambrian, the West Sayan formation, and other stratigraphic units referred to the Middle Cambrian and bearing variegated local names, are clearly subdivided into three independent effusive-sedimentary formations—the Akkol, Chingin, and Babakovo formations—the lithological composition and position of which in the stratigraphic section are shown in the column presented above (Fig. 28).

1. Each of these formations, differing sharply in lithological composition, is characterized by a strictly distinct assemblage of organic remains, which date the age of the enclosing paleontological documents in relatively narrow intervals of Early Cambrian time, for which it is proposed to adopt a three-member division.

In accordance with the latter, the Akkol formation, chiefly on the basis of its stratigraphic position and partly of algal remains, among which several new species of the genus Osagia are established, associated with the previously known species Osagia ovosimilis, is assigned to the very base of the Lower Cambrian \(Cm_1^1\) (?).

The Chingin formation, characterized by the presence of the Gavrilov archaeocyathid assemblage and, among them, such important forms as Ajacicyathus immanis, Coscinocyathus papillatus, Orbicyathus mongolicus, with the participation of the Chingin trilobite assemblage of the previously known genus Bergeroniellus, a new species of the genus Redlichina, and also the new genus Schagonaria, is assigned to the second third of the Lower Cambrian \(Cm_1^2\). And, finally, the Babakovo formation, for which the Babakovo trilobite assemblage is characteristic, represented by Granularia obrutchevi, Erbia slatkovskii, a new species of the genus Proerbia, the new genera Tabatopygellina and Arbataspis, and a new species belonging to the previously known genus Chondragraulos, in association with the Obruchev (polycyathid) archaeocyathid assemblage, is assigned to the very top of the Lower Cambrian \(Cm_1^3\) and is regarded as the youngest stratigraphic member of the division.

1. Among the deposits of the Akkol and Chingin formations, the most characteristic rocks are silicilites, which are only weakly distributed in the more ancient and younger stratigraphic members of the Western Sayan. Pebbles of silicilites are constantly found in the basal conglomerate of the Verkhne-Monok formation of the lower Middle Cambrian. Such a stratigraphic position of the silicilites permits them to be regarded as guide elements for the lower half of the Lower Cambrian. Without denying the metasomatic genesis of some varieties of silicilites, it should be considered proven that in their main mass they are primary sedimen...

… those that arose before the emplacement of the hyperbasite intrusion and are not genetically connected with it.

1. The Lower Cambrian geological formations of the Western Sayan proved to be fully comparable with the equivalent formations of the entire Sayano-Altai region; moreover, in the latter they are clearly subdivided into three stages: the Akkol stage—Cm₁¹ (?), the Chingin stage—Cm₁², and the Kayancha stage—Cm₁³. Each of the named stages is characterized by a complex facies profile and by strictly individual complexes of paleontological associations, among which both trilobite complexes and archaeocyaths are of great importance for purposes of correlation.

2. The deposits of the Chingin and Kayancha stages are separated from one another by a clearly expressed unconformity surface, which arose in connection with the manifestation of the early phase of the Salair tectogenesis, for which the previously proposed name—the West Sayan phase—could be retained. As a result of this phase, complex Alpine-type folds were formed in the deposits drawn into its orbit, including those of the Akkol stage. Probably, as early as the stage of maximum subsidence of the Lower Cambrian geosynclinal trough, the hyperbasite intrusion characteristic of the Sayano-Altai region was emplaced, and deep faults also formed, one of which is clearly expressed within the Western Sayan, where it is known as the Maloushenskii thrust, separating Precambrian deposits from the Lower Cambrian.

3. The second phase of the same Salair tectogenesis, which could be given the name Babakovo, took place after all the processes connected with the accumulation of deposits of the Kayancha stage. This phase was expressed not only in folding phenomena, but its product was a microdiorite intrusion of the Kair type, established in the Western Sayan and on the eastern slope of the Kuznetsk Alatau. The Babakovo phase of tectogenesis, which led to the uplift of the formations it affected, with their subsequent erosion, concludes the epoch of the Lower Cambrian in the Sayano-Altai region, the deposits of which are everywhere separated from Middle Cambrian deposits by a surface of parallel or angular unconformity.

LITERATURE

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1. Possibly these deeply metamorphosed rocks will correspond to analogues of the Akkol Formation. ↩↩