On the “Lower Silurian” of the Western Sayan

A. G. Sivov

1. Introduction

In the Western Sayan, a thick greenstone sequence, intensely metamorphosed and dislocated, is regionally widespread; a number of researchers assign it to the Lower Silurian. This latter circumstance also found expression on the geological map of the USSR compiled in 1937, where almost the entire territory of the Western Sayan is represented as a field of development of the Lower Silurian. It is true that in the first years of systematic investigations in the aforementioned mountain range, I. K. Bazhenov, having distinguished part of the deposits under consideration as Division II of the metamorphic sequence, was inclined to see in them Precambrian formations; later, however, after assigning to them the less altered sediments of the “variegated” and “Shinet” sequences and establishing between them what appeared to be gradual transitions, he placed them wholly and completely in the Lower Silurian, basing this mainly on the fact that remains of crinoid columnals occur in the “variegated suite” (3, 42).

Recently M. A. Usov, synthesizing materials from the geological survey of the West Siberian Territory, has distinguished in the mountain ranges a West-Altai formation of the lower Ordovician, whose analogues in the Western Sayan correspond approximately to the “Lower Silurian” in the understanding of I. K. Bazhenov.

However, some geologists who worked in the Western Sayan expressed the opinion that I. K. Bazhenov included in the “Lower Silurian” deposits of different ages, which now take part in the structure of a broad zone of folding formed in its main features during the Taconic phase of the Caledonian tectogenesis. Finally, detailed geological investigations carried out in connection with prospecting for rare metals and other useful minerals confirmed the heterochronous character of the “Lower Silurian,” and formations of Ordovician, Upper Cambrian, and even, possibly, Precambrian age began to be distinguished within it.

From 1933 to 1936, participating, on assignment from the West Siberian Geological Trust, in the geological survey of the southwestern slope of the Western Sayan—composed mainly of the “Lower Silurian”—I became thoroughly acquainted with its stratigraphy and composition, and came to the conviction that these formations, although at first glance homogeneous and very uniform, according to a number of features described below, break down clearly into a series of independent formations of different ages.

The subdivision of the “Lower Silurian” of the Western Sayan into natural stratigraphic units, and the description of their composition and interrelations, is the task of the present article.

II. STRATIGRAPHY OF THE GREENSTONE UNIT OF THE “LOWER SILURIAN” OF THE WESTERN SAYAN

As already noted above, the “Lower Silurian” of the Western Sayan is lithologically extremely monotonous, being expressed by metamorphosed, predominantly intensely schistose masses, green in color, more rarely burgundy.

However, upon more detailed study of these formations, they can, according to degree of metamorphism, lithological composition, and conditions of occurrence, readily be subdivided into the following formations, from bottom to top: 1) the Dzhebash Formation, 2) the Kemtеrek Formation, 3) the Buiba Formation, 4) the Shinet Formation, and 5) the Kerem Formation; of these, only the last two units may prove to correspond to the Lower Silurian, whereas the Dzhebash Formation is a more ancient member, possibly Precambrian, the Kemtеrek most likely Upper Cambrian, and the Buiba—for the time being—occupies an indefinite position, fitting within the interval between the Dzhebash and Shinet units.

1. Dzhebash Formation.

The deposits of the Dzhebash Formation are predominantly developed in the basin of the Dzhebash River, a right tributary of the Abakan River, but in places they also appear in the central parts of the Western Sayan, making up the area of Kantegir, Kazyrsuk, and the Amyl basin.

The most characteristic member of the formation consists of crystalline rocks that have undergone sharp diaphthoresis with the formation of a group of chlorite-sericite schists, among which the most widespread are albite-chlorite, albite-sericite-chlorite, chlorite-muscovite-quartz-albite, actinolite-zoisite, and epidote-actinolite-chlorite varieties. Variations in the petrographic composition of the crystalline rocks are not limited to the types listed above, but are more complex; this complication, however, essentially results only from different proportions of the components composing the schists.

In their typical manifestation, the group of chlorite-sericite schists has a dark-green color, with some shades toward lighter tones, coarse, sometimes fine schistosity, and at times a clearly expressed crystallization layering. In a few cases the schists preserve granoblastic, more rarely nemato- and lepidoblastic structures, which, as a rule, have already been considerably altered by subsequent processes of schistosity and become generally cataclastic. In addition, the schists everywhere prove to be penetrated by a dense network of quartz, quartz-albite, albite-calcite, albite, and calcite veinlets, especially readily confined to the planes of schistosity, but not infrequently cutting the latter at a steep angle; this type of hydrothermal formation is later.

In places the veinlets described so finely permeate the rocks that they acquire a characteristic banded, at times clearly gneiss-like structure, caused by an alternation of light and darker bands. The light bands consist of a well-recrystallized albite-calcite mass; the dark ones, of intergrown flakes of bluish-green chlorite and sericite, with a slight presence of epidote and other dark-colored components.

A second characteristic member of the formation consists of hornblende schists, which in places pass into typical amphibolites and apparently constitute,

an independent stratigraphic horizon. The rocks mentioned, consisting essentially of plagioclase, green schistose hornblende, epidote, and chlorite, are often well recrystallized and massive, but in most cases have undergone a distinct diaphthoritic alteration and externally are quite indistinguishable from chlorite-sericite schists. Further, ferruginous quartzites and marbles play a very characteristic role in the composition of the formation. The latter do not form an independent stratigraphic horizon, but occur as thin (1–3 m) interbeds and only in isolated cases reach thicknesses of tens of meters. The marbles are in most cases fine-grained, have a grayish-white, more rarely pinkish, color, and are sometimes dolomitized.

Ferruginous quartzites, confined chiefly to the field of development of the chlorite-sericite schists, form horizons persistent along strike and may serve to some extent as marker members of the formation. In their main mass the ferruginous quartzites have a grayish-black color and a massive, more rarely banded structure, and consist of quartz and iron glance, which alternate with one another in the form of very thin but variably wide bands. Almost always they are cut by numerous veinlets of white pure quartz, adapted predominantly to the schistosity of the rock and at times completely replacing the latter.

Finally, a no less important role in the structure of the formation belongs to biotite schists, hornblende and biotite gneisses, appearing chiefly in the contact aureoles of conformable intrusive masses; but on the SE slope of the Mirsky Range the biotite schists occupy large areas and evidently constitute an independent stratigraphic horizon.

Thus, the Dzhebash Formation, being composed of deeply altered formations, stands out sharply from a series of externally similar deposits and is undoubtedly an independent and the oldest stratigraphic member in the Western Sayan. The degree of depth of alteration of the formation is determined by the almost complete recrystallization of its primarily sedimentary-effusive composition, to which only rarely bear witness locally preserved relicts of clastic and poikilitic structure. In addition, within the field of development of the amphibolites it is possible to distinguish varieties with well-preserved gabbroid and dioritic structures, indicating the participation of intrusive rocks in its formation.

In full harmony with the features of the lithological composition of the Dzhebash Formation is also manifested its tectonic structure, which is very complex and as yet not fully deciphered. The deposits of the formation under consideration are gathered into a series of extremely compressed, at times isoclinal folds with predominant dips to the SE, a persistent NW 50–80° strike, and a tendency to overturning toward the NW. In turn, these principal folded structures are complicated by intense additional small folding of the second and third orders, with an amplitude of swing of the limbs of the latter from one meter and less. Moreover, on the limbs of the microfolded structures mentioned there always develops an extremely intense cleavage, passing into corrugation, bending into the most bizarre forms and sometimes resembling the suture line of ammonites. Adapting themselves to the microcleavage structures, the quartz-carbonate veinlets described above develop, emphasizing with exceptional relief the features of the microtectonics of the formation, so effectively reflected in the photographic image placed in Geology of Siberia, part II, photo 29. (11). Finally, intense fine schistosity, usually developing parallel to the dominant strike—

...of the bedding, with a predominant dip of the schistosity planes to the SW at \(60—90^\circ\) and, more rarely, to the NW at \(60—90^\circ\), completes the characterization of the tectonic structure of the Dzhebash Formation.

Turning to the question of the age of the formation under consideration, it should be emphasized that the deposits composing it are entirely identical with the second division of the metamorphic sequence, which I. K. Bazhenov, at the beginning of his investigations, regarded as a Precambrian formation (1—9, 24), but later, on the basis of a number of poorly substantiated considerations, assigned a Lower Silurian age to the rocks of this division, singling it out as an independent horizon of the lower Ordovician (3, 35—42). However, analysis of the factual material presented below in no way confirms a Lower Silurian age for the Dzhebash Formation (the second division of the metamorphic sequence), but compels one to regard it as a more ancient formation, as is evident above all from consideration of the stratigraphic position of certain intrusive plutons of the Western Sayan spatially associated with it.

Thus, for example, I. K. Bazhenov already noted that among the lower horizon of the “Lower Silurian,” which we correlate with the Dzhebash Formation, there occur concordant bodies of diorites, often intensely amphibolized and even in places transformed into amphibolites (2, 35—36); (3, 80), which, according to my observations, cut not only the sediments of the sequence under consideration, but also deposits of the Middle Cambrian and already occur as pebbles in the conglomerates of the Kemterek Formation, which we provisionally assign to the Upper Cambrian (14).

Similar relationships are also indicated by the trondhjemite intrusive massifs widely distributed in the Western Sayan, known in the literature under the name of the Maina granodiorite, and by plutons of hyperbasites cutting both the lower and middle members of the Cambrian deposits and the crystalline rocks of the Dzhebash Formation; moreover, pebbles of the aforementioned plutonic rocks likewise appear in the conglomerates of the Kemterek sequence noted above. Thus it becomes evident that the deposits of the Dzhebash Formation cannot be younger than the Kemterek Formation, since they are older than the just enumerated pre-Kemterek intrusions. The pre-Upper Cambrian age of the Dzhebash Formation is indicated in turn by the study of the basal conglomerate of the Kemterek sequence, in which, along with rocks identical to those of the Middle Cambrian, there are recorded pebbles of crystalline schists, amphibolites, and fragments of quartzites rich in ferruginous mica—rocks so characteristic of the Dzhebash Formation. The relation of the Dzhebash Formation to Middle Cambrian and younger formations is clarified in the following way. In studying the Maina copper deposit, I. K. Bazhenov emphasized that the most strongly metamorphosed rocks of the second division of the metamorphic sequence (the Dzhebash Formation) are thrust (1, 25) over the less altered formations of the first division, for which it was subsequently possible to prove a Middle Cambrian age (3, 31—34) and even to distinguish it as the independent West Sayan Formation (16, 78—80). At the same time, the crystalline schists of the Dzhebash Formation at a number of points are thrust along a very gently inclined fissure now over deposits of the upper members of the Lower Silurian, as is recorded in the basin of the Maly Tepsel River, a right tributary of the Yenisei River, now over the sediments of the “red-colored Sayan sequence” (the Kemterek Formation), which is established quite clearly along the southern slope of the Borus Range, rising on the right bank of the Yenisei River. In other places where the “red-colored Sayan sequence” is developed, however, it overlaps the deposits of the Dzhebash Formation with a slight angular unconformity (17).

In clarifying the stratigraphic position of the Dzhebash Formation, it is necessary to note the following circumstance, by no means unimportant. I. K. Ba-

...Zhenov, in studying the first lower Cambrian unit of the upper parts of the lower, or the lower parts of the middle, division of the system, recorded that above the effusives of this unit there lies a sandy-conglomeratic horizon, in which pebbles of diorite, quartz, and chlorite schist with veinlets of translucent quartz are found; these appeared in the conglomerate as a result of the erosion of metamorphic rocks (2,17), apparently corresponding to the Dzhebash Formation. The presence in the Lower Cambrian conglomerates of intrusive pebbles also indicates that the time of formation of the Dzhebash Formation corresponded to the formation of deep-seated plutons, to which the granitogneiss and diorite intrusions of the B. Anzas River (2,34 and 35), occurring here among the crystalline rocks of the formation under consideration, could presumably be assigned.

Thus, the Dzhebash Formation is unquestionably older than the Cambrian and, still more so, than the Lower Silurian of the Western Sayan and, consequently, in age can be assigned only to the Precambrian. We are still far from determining a more exact stratigraphic position for the Precambrian of the Western Sayan and from correlating it with well-studied similar formations of neighboring mountain ranges. However, as a first approximation, the formations described could be correlated with the deposits of the upper members of the crystalline-schist formations of the southern slope of the Terekta Range of the Altai Mountains, which N. G. Kassin regards as the upper Proterozoic (6,38—39). It is possible that the Dzhebash Formation will prove to consist of an entire group of independent and variously aged stratigraphic units, which may be very close in age to the Precambrian of Kazakhstan (6).

In establishing the independence of the Dzhebash Formation of Precambrian age, it should be borne in mind that it is precisely with it in the Western Sayan that the belts of ferruginous quartzites are associated; in other regions of the USSR these are of great industrial interest and are confined mainly to Proterozoic formations. This circumstance compels us to treat the Precambrian of the Western Sayan with particular attention, since within its limits discoveries are possible of interesting industrial iron-ore deposits of the Northern Kazakhstan type.

2. Kemterek Formation

In contrast to the Dzhebash sequence, the Kemterek Formation, being represented by coarse-, medium-, and fine-clastic deposits, shows less sharply expressed metamorphism and is characterized by a simpler folded structure.

In its typical development, the sediments of the named formation are distributed over the Kemchik—Kemterek watershed area and along the valley of the Yenisei River, somewhat above the mouth of the Urgun, where a more or less complete section of it is also revealed (Fig. 1). At the indicated localities, at the base of the formation there lies a basal conglomerate up to 35—40 m thick, in places resting upon the crests of steeply cleaved pyroclastic-sedimentary formations of the Middle Cambrian (the West Sayan Formation). The conglomerate is massive, coarsely cleaved, and consists of large and small, well-rounded pebbles of amphibolites, crystalline schists, and ferruginous quartzites, often penetrated by a dense network of quartz-albite veinlets, which develop only within the pebbles themselves and do not affect the cement of the rock at all. In addition, the conglomerate contains, in abundant quantities, pebbles belonging to intensely altered greenstone effusives and dark bituminous limestones—rocks very characteristic of the West Sayan Formation. Finally, in the conglomerate small fragments of chlorite-serpentine, serpen-

Scale: 1 km

A

C

Ю

B

Section along A B

Conventional symbols

1. Devonian monzonites–syenites
2. Devonian deposits
3. Bugunzhul Formation, \(S_1^y\)
4. Kamtas Formation, \(Cm_3\)
5. Zatonsko-Sayan Formation, \(Cm_2\)
6. Elements of schistosity
7. Tectonic lines of disturbance

Fig. 1.

tinite, chlorite, fibrous actinolite, saussuritized gabbro, actinolitized diabase and microdiorite, as well as large pebbles of trondhjemite. The cement of the conglomerate is clayey-sandy; moreover, grains of plagioclase, pyroxene, and actinolite are also noticeable in the sandy material.

The conglomerates are overlain by an alternating sequence of fine- and thin-grained, yellowish-gray, greenish, steel-gray, dense, often thin-bedded, phyllitized clay shales, among which appear beds of puddingstone formations, coarse-grained gray arkosic sandstones, and green clay shales. The thickness of this stratigraphic unit is 250 m.

In the middle parts of the formation being described there develops a monotonous sequence of green and gray, finely banded, phyllitized clay shales, sometimes dense, sometimes remarkably rhythmically thin-bedded. Subordinate to the shales are beds of green and dirty-green medium-grained sandstones, small lenses of silicilites, thin interbeds 0.5–2 m thick of gray dense limestones, and puddingstones containing a uniform lenticular rare pebble of effusives and pelites, very similar to the analogous formations enclosing the shale puddingstones. The thickness of the sequence is estimated at 500–600 m.

Finally, the formation is crowned by an extremely thick sequence, not less than 500 m, of finely banded dense shales, predominantly dark and dirty-green, among which packets of violet, burgundy, and lilac sandstones and phyllites are especially characteristic; because of their coloration they stand out sharply against the dirty-green tone of the formation. In the middle parts of the horizon under consideration there occur beds of black, graphitized, ringing clay shales and lenses up to 10 m thick of pinkish-gray limestone containing tubular and spherical formations, evidently of organic origin. Confined chiefly to this horizon are sills of dark-green quartz-albite diabases, for the most part intensely altered and in places transformed into chlorite-actinolite schists.

The deposits of the Kemterek Formation differ from the Dzhebash sequence not only in lithological composition, but also in the distinctive forms of folded structures. In the upper part of the Yenisei River, the Kemterek Formation composes a large syncline of latitudinal extent (Fig. 1), the core of which is filled by Devonian and Gotlandian formations. The northwestern limb of the syncline is inclined to the horizon at an angle of only 55–65°, whereas the southeastern limb is sharply overturned, and the beds often stand on end. The development of additional folds is not characteristic of the Kemterek Formation; such folds appear only in places of sharp dynamic junctions of the formation with the sequences surrounding it. Nor is the development of quartz-carbonate veinlets characteristic of it; these appear mainly in the most crumpled parts of the formation, and only when these parts are situated near intrusive massifs. At the same time, the deposits of the formation under consideration have undergone very intensive fine banding, manifested in a number of directions, of which the one parallel to the dominant strike of the bedding is the most distinctly expressed.

The stratigraphic position of the Kemterek Formation among a number of other sequences developed in the Western Sayan is established with sufficient precision. Indeed, it was emphasized above that the deposits of this formation, through a basal conglomerate, rest unconformably upon the formations of the West Sayan sequence of the Middle Cambrian. Moreover, the Kemterek Formation probably lies above the Sanashtyk-Gol limestone, which contains remains of trilobites and archaeocyaths of the middle or even upper parts of the Middle—

of Cambrian (12, 49–51), which is confirmed by the following considerations. In the Tabat nappe of the northern slope of the Western Sayan, O. K. Poletaeva and I. E. Shatrov distinguished an independent Arbat suite (13), which, both in petrographic composition and in stratigraphic position, should be regarded as an analogue of the Kemterek Formation established by us. In fact, in the indicated nappe the Arbat suite, contrary to the authors’ assertions, rests above the West Sayan Formation; this circumstance is proved at least by the fact that the conglomerate of the former contains pebbles of the trondhjemite intrusion (Maina granodiorites), which cuts through, on the SW slope of the Western Sayan, the rocks of the latter.

At the same time, in studying the geological map of the Tabat nappe we clearly discern a very interesting regularity. Hyperbasite plutons, so characteristic and numerous for the lower and upper (West Sayan Formation) strata of the Cambrian in the sense of I. K. Bazhenov, are completely absent from the Arbat suite; this circumstance should by no means be regarded as accidental. It is entirely regular, since the formation of the hyperbasites ended long before the formation of the Arbat suite and its analogue—the Kemterek Formation, in the basal conglomerate of which fragments of hyperbasites already occur. Moreover, judging from the corresponding observations of I. K. Bazhenov and Yu. A. Kuznetsov, in the area of the Abakan iron-ore deposit and in the basin of the Khyzas River, the Arbat suite (and, consequently, also the Kemterek Formation) lies above the Sanashtykol limestone (3, 24–29; 7, 18–26), which contains a fauna of trilobites and archaeocyaths of the middle or upper parts of the Middle Cambrian. At the same time, in the basin of the Urgun River, a right tributary of the Yenisei River, it can be established that within the basal conglomerate of the Shignet Formation, which in age corresponds to the lower or middle members of the Ordovician, pebbles of shales and sandstones are recorded that are extremely similar to analogous formations of the Kemterek Formation. Thus, the Kemterek Formation proves to occupy a position between the upper parts of the Middle Cambrian and the beginning of the Lower Silurian and, apparently, in age should be assigned to the Upper Cambrian.

To this it should be added that deposits of the Upper Cambrian probably are not limited in their distribution to the Usinsk district and the Tabat nappe, but are developed much more widely; and it is above all necessary to assign to this age the “red-colored sequence of the Sayan,” which has many features in common with the Kemterek Formation of the southeastern slope of the Western Sayan.

3. On the Age of the Red-Colored Sequence of the Sayan

In 1924, from the metamorphic sequences of the Western Sayan, I. K. Bazhenov separated a less altered “red-colored sequence of the Sayan,” the typical deposits of which are distributed along the southern slope of the Borus Range, rising on the right bank of the Yenisei River, somewhat above the mouth of the Kantegir River. The intensely dislocated sediments of the named sequence consist of a thick series of interbedded green, red, maroon, and more rarely black phyllitized clayey shales and sandstones, also enclosing interbeds of limestones, in which N. A. Batov discovered crinoid columnals along the Karynsuk River. A very characteristic element of the suite is intraformational conglomerate, consisting mainly of pebbles of the same red-colored sequence, more rarely of effusives and quartz, and occurring in the form of large interbeds. In the area of the Borus Range the “red-colored sequence of the Sayan,” which according to Bazhenov …

an analogue of the lower horizon of the Ordovician, participates in the structure of the southern limb of the anticline, the core of which is made up of rocks of the third upper Cambrian sequence (3,31), or of the West Sayan formation (16,78—80).

The “red-colored sequence of the Sayan” is characterized by precisely the same lithological composition also within the southwestern slope of the Western Sayan, where it is distributed along the northern slopes of the Kurtushibin Range and has been studied by a number of geologists (15). But in sections of the Kurtushibin Range the beds and interbeds of conglomerate that take part in the structure of the “red-colored sequence” consist mainly of the underlying Middle Cambrian formations and of acidic intrusions that cut the latter.

Thus, the “red-colored sequence of the Sayan,” wherever it has been more or less fully studied, has a remarkably persistent lithological composition; it begins with conglomerates or coarse-clastic formations, which in composition are often basal, and rests predominantly with a sharp unconformity, transgressively, on the West Sayan formation. It is true that east of the Kantegir River the “red-colored sequence of the Sayan,” according to N. A. Batov, quite conformably covers deposits of the lower horizon of the “Lower Silurian,” having clear gradual transitions with it, with which I. K. Bazhenov agrees (3,42—46). But A. P. Schmidt, who took the closest part in the study of the Kantegir massif, showed that there can be no question of mutual transitions between these suites, for the lower horizon of the “Ordovician” underwent slight folding long before the sedimentation of the “red-colored sequence of the Sayan.” Moreover, according to Schmidt, the crystalline schists of the lower horizon of the “Ordovician,” easily comparable with the Dzhebash formation described above, arose from deposits of various ages, and, perhaps, some part of them will prove to be much older than the Cambrian (17).

But why, in that case, is the basal conglomerate, so characteristic of its analogues distributed along the southwestern slope of the Western Sayan, absent at the base of the red-colored sequence of the Kantegir district? To this question we obtain an answer from the works of Bazhenov, who assumes the presence of a tectonic fault passing along the southern limb of the anticline of the Borus Range, as a result of which the lower members of the red-colored sequence were cut off, or, possibly, the latter simply underwent a facies change, owing to which coarse-clastic formations could be replaced by medium- and fine-clastic varieties (3,45).

In accordance with the foregoing, a tectonic-denudational break clearly emerges between the time of formation of the “red-colored sequence of the Sayan” and the underlying ancient formations of the Cambrian and, possibly, Precambrian. The presence of a long break is recorded between the sequence under consideration and the “Shignet suite” in the understanding of I. K. Bazhenov. The latter exhibits much simpler folded structures than the “red-colored sequence of the Sayan,” which is gathered into isoclinal folds (4,12). If to this one adds that the Shignet formation, in our understanding, lies unconformably on the underlying sequences, and that in its basal conglomerate pebbles of rocks already occur that are close to the “red-colored sequence of the Sayan,” then it is obvious that the latter is also separated from the former by a major break.

Thus, the “red-colored sequence of the Sayan,” considered by a number of authors as the lower member of the Ordovician, occupies an independent stratigraphic position, being separated by a sharp unconformity from the latter and from the underlying West Sayan formation; and in age, apparently, it will correspond to the Upper Cambrian, as is established for its analogue—the Kemterek formation. And in this sense one likewise cannot agree with the assumptions of V. A. Kuznetsov (8,60) and V. A. Obruchev (11,312), who place the “red-colored sequence of the Sayan” in the Upper Got-

...land, and all the more so with A. G. Vologdin (5, 8), who equated this suite with the red-colored deposits of the Lower Devonian of the Minusinsk Basin.

In establishing the synchronism of the “red-colored Sayan sequence,” the Kemterek Formation, and the Arbat Suite with the Upper Cambrian, we by no means forget the occurrence, within the latter suite, of original archaeocyathids, and in the former of crinoid columnals. But this circumstance cannot in any way serve as an obstacle to synchronizing the formations mentioned and assigning them to the Upper Cambrian, since remains of original archaeocyathids, according to our observations, are recorded in the limestones of Orlinaya Gora together with Illaenurus, Kingstonia, and Agnostus cyclopyge, while crinoid columnals are found in abundance also in unquestionably Middle Cambrian formations of the same locality of Salair.

Thus, the deposits of the “red-colored Sayan sequence” and of the Arbat Suite, described in the literature as of different ages, are regarded by us as synchronous formations and are correlated with the Kemterek Formation, developed along the southwestern slope of the Western Sayan, which in age is tentatively assigned to the Upper Cambrian.

4. The Buiba Formation.

The rocks composing the Buiba Formation are, externally, very similar to the above-described formations of the Dzhebash sequence and differ from the latter in a somewhat different petrographic composition, much less pronounced metamorphism, and a less complex internal structure.

In its typical development, the Buiba Formation is represented by a thick sequence of alternating light- and gray-green, rarely violet, phyllitized argillaceous shales, among which there sporadically appear beds of considerable thickness of intensely chloritized, dark- and dirty-green sandstones and subordinate calcareous-chlorite-sericite shales, likewise predominantly green in color. This monotonous composition of the formation is supplemented by the appearance in its lower horizons of light-gray, pink, and light fine-grained marbles, occurring in the form of very thick lens-shaped bodies, as is established along the right bank of the Yenisei River and the Toznykh River, a right tributary of the Yenisei, or else in the form of persistent beds, which is especially characteristic of the headwaters of the Us River. In addition, among the rocks of the formation there are recorded rare, thin sill-like bodies of greenstone effusives, outwardly almost indistinguishable from strongly chloritized sandstones. The Buiba Formation is characterized by such a composition along the Lower Buiba River, which flows into the Us River from the right, and along the M. Ur River, which enters the Yenisei from the left.

In other cases the composition of the formation is complicated by the appearance of calcareous-sericite, quartz-albite-chlorite, and even actinolite schists, which, however, do not constitute lithologically persistent horizons, but most readily develop in places of the strongest stresses of folded structures or are situated in zones of intense dynamic contacts.

Thus, in lithological composition, the Buiba Formation shows certain differences from the outwardly similar Dzhebash sequence; moreover, these differences are most sharply manifested in the degree of metamorphism and especially in the character of the primary composition of the formations.

The primary nature of the crystalline schists and amphibolites of the Dzhebash Formation, owing to their deep metamorphism, cannot be established with proper precision. There are grounds to believe that both magmatic and sedimentary for-

formations, but made faceless by deep metamorphism, perhaps long before the deposition of the Cambrian system. The conditions of sedimentation of the deposits of the Buiba Formation, however, are recognized with the utmost clarity, and their primary sedimentary nature is well revealed not only microscopically but quite often megascopically as well.

Thus, in the sandstones a blastopsammitic structure is almost always clearly manifested, connected through a whole series of transitions with a blastopelitic one, which is quite natural for rocks displaying fine bedding, well recognizable even megascopically. Among the mineral components making up the named rocks, angular, slightly serrated grains of quartz enjoy predominant development, scattered amid a strongly chloritized groundmass. In subordinate quantity to the quartz there are weakly rounded grains of plagioclase, in most cases strongly sericitized. Alongside the minerals mentioned, grains of calcite participate, distributed almost uniformly in the field of the thin section and appearing as a kind of cementing substance. Among the new formations, zoisite is characteristic, which, together with chlorite, sericite, and then clayey-carbonaceous matter, constitutes the groundmass of the rock.

Phyllitized clayey shales are characterized by approximately the same composition; their groundmass consists of pelitic or aleuritic-pelitic material; in it are submerged individual grains of angular quartz, flakes of sericite, with a noticeable development of chlorite and, more rarely, acicular actinolite. Moreover, even in such more or less altered rocks as quartz-albite-chlorite and actinolite schists, a clastic structure is noticeable, although it is often preserved in the form of a relict one.

In lithological composition and degree of metamorphism, the rocks of the Buiba Formation somewhat approach the formations of the Kemterek sequence, but in the type of their internal structure they differ sharply from it, and by this feature most closely resemble the deposits of the Dzhebash sequence. Thus, the characteristic foliation and corrugation proper to the sediments of the Dzhebash sequence are manifested to the same degree and with the same clarity in the Buiba Formation as well; but here the microstructures are usually accompanied by extremely compressed secondary folds of the second order, which both in plan and in section have an S-shaped form, while their hinge parts are smoothly rounded and the axes plunge steeply in the most varied directions. However, despite such intensely visible folded structures, the formation as a whole lies at gentle angles of dip, composing large areas elongated in a NE direction, a circumstance also characteristic of the Dzhebash sequence.

A second characteristic feature bringing the Buiba Formation closer to the Dzhebash Formation is the development, in both, of an enormous quantity of quartz-calcite, calcite, and albite-quartz thin, more rarely more or less large, veinlets, usually confined to microfolded structures, but in places cutting across the latter and one another in the most varied directions.

Thus, the Buiba Formation that we distinguish, only in the character of the internal structure and coloration of the rocks, somewhat resembles the deposits of the Dzhebash sequence, while differing noticeably from the latter in petrographic composition, degree of metamorphism, and, chiefly, spatial distribution, being closely connected in this case with the formations of the Shignet Formation described below. Thus, along the M. Ury River, a left tributary of the Yenisei River, where the bank sections rise above the river level by at least 1 km, relatively narrow bands of strongly schistose rocks of the Bui-

![Figure 2]

Diagram of the geological structure of the valley of the Us River below the spring of B. Saizhik

Scale

Conventional symbols:

1. Quaternary deposits
2. Shinet Formation \(S_1^{\mathrm{sh}}\)
3. Burin Formation \(S_1^{\mathrm{b}}\)
4. Elements of bedding

Fig. 2.

![Figure 3]

Scale

A—B

Section along A—B

Conditional b. Khainak [[unclear: handwritten locality name]]

Conventional symbols:

1. [[symbol: dotted pattern]]
2. [[symbol: chevrons]]
3. [[symbol: crosses]]
4. [[symbol: diagonal hatching]]

Fig. 3.

of the Bui-Ba formation. Conversely, as is recorded in the middle and upper courses of the Us River, within the broad field of the Bui-Ba formation it is possible to distinguish weakly metamorphosed and quietly bedded rocks of the Shignet formation, drawn into the cores of deep synclines of the underlying formations.

Unfortunately, we do not have at our disposal more or less convincing materials that would strictly establish the stratigraphic position of the Bui-Ba formation, and therefore we place it quite provisionally above the Kemterek formation and, in age, bring it close to the Shignet formation, which, as will be shown below, corresponds to the Ordovician. The Bui-Ba formation, however, may prove to be an older stratigraphic member of the Western Sayan than the Lower Silurian, perhaps close in age to the Dzhebash formation and representing only a less altered facies of the latter. This becomes all the more probable because, under conditions in which the named formations are distributed at the same geographic points, it is almost impossible to divide them into independent stratigraphic units.

5. The Shignet Formation¹

The Shignet formation consists of remarkably uniform fine- and medium-grained, dirty-green sandstones, noticeably epidotized, among which thin (15–20 cm and less) interbeds and packets of phyllitized clay shales are recorded; these are colored predominantly green, with various shades, and more rarely acquire violet, claret, and gray coloration. Only in individual cases do small lenses of limestones appear in its composition, often transformed into calcareous shales or marbles. The Shignet formation, through a basal horizon of complex composition, transgressively overlies the underlying deposits, resting either directly on deposits of the Bui-Ba formation (Fig. 2), beginning here with a basal sandstone (lower course of the Us River), or with a fine-pebble conglomerate (middle course of the M. Ur River), or else it covers the rocks of the Western Sayan formation (the B. Khailyk watershed, right tributary of the Urguni River) (Fig. 3), forming here at the base a thick unit of basal conglomerate. The basal horizon of the formation is a very characteristic rock and, in its composition, displays marked facies variability, depending chiefly on the composition of the underlying strata. Thus, on the watershed of the B. and M. Khailyk rivers (Fig. 3), where the basal horizon covers deposits of the Western Sayan formation of the Middle Cambrian, it begins with a conglomerate in which pebbles of strongly compressed greenstone effusives and metamorphic shales, petrographically identical to similar rocks of the underlying formation, acquire predominant importance. Then, in the conglomerate, large pebbles of black marbleized limestones, banatites, gabbro-pyroxenites, and peridotites—rocks typical of the same Middle Cambrian—are recorded. Finally, rarely, pebbles of black graphitized, green, and even claret shales are found in the conglomerate, externally in no way distinguishable from similar rocks of the Kemterek formation of the Upper Cambrian. In those same cases, when the basal horizon covers deposits of the Bui-Ba formation, its composition changes sharply; moreover, in the basin of the M. Ur it is composed of a fine-pebble conglomerate formed by vein quartz, effusi-

Fig. 4.

Visible labels: NW; SE; 4 m; [[unclear: vertical scale label, possibly “6”]].

Fig. 5.

Visible labels: NW; NE; 1; 2; 3.

... by green sandstones and phyllitized shales, while in the lower course of the Us River it consists of quartz–feldspar sandstone, in which weakly rounded and angular fragments of the underlying phyllitized shales are recorded (Fig. 2).

No less characteristic is the composition of the sandstones of the Shignet Formation, which make up 90% of its structure. Megascopically, one can constantly observe in them angular grains of gray quartz, and along the plane of schistosity, scales of chlorite and sericite. Under the microscope, besides quartz, grains of plagioclase, alkali feldspar, flakes of white mica, fragments of microquartzite, and effusives, very intensely actinolitized, are present in appreciable quantities. The cement of the sandstones is a clayey-calcareous mass, in which scales of chlorite and needles of actinolite are visible. In most cases the sandstones are massive, roughly schistose, and usually have a greenish-gray color, which at the surface is sometimes obscured by a dirty-white weathering crust. In other cases, a brownish and claret coloration is recorded in the sandstones, apparently of secondary origin, since rocks of such coloration are not persistent either in thickness or along strike. Moreover, in some places it is clearly established that sandstones of green color are variegated by a considerable number of brown bands, arranged either parallel to the bedding or cutting the latter obliquely. Finally, in places the sandstones prove to be strongly crushed and pass into shales, but these altered formations are confined to individual parts of the formation that have been drawn into zones of major interformational displacements or that participate in a tense and complex internal folded structure. Adapting themselves chiefly to these zones of crumpling of the formation, relatively numerous quartz–carbonate veinlets develop, which in other cases are comparatively scarce.

The independence of the Shignet Formation is revealed much more distinctly in the study of its internal structure, which is simpler in relation to the previously described strata. In general, the rocks of the Shignet Formation take part in the structure of flat and broad brachiostructures, elongated now to the NW, now in a latitudinal direction, with their comparatively gentle bedding at 30°–35° (Fig. 8). However, these principal folded structures are almost always complicated by small additional folding of various orders, which ultimately is what catches the eye in individual outcrops. These secondary folded structures are usually inclined to the horizon at an angle of 50°–60° and show amplitudes of fold limbs of 3–4 m, so that in one and the same large outcrop several folds at once may fit, at times rising to a considerable height (for example, folds on the right bank of the Us River, below the mouth of the B. Sorzhik). But the forms of the folds mentioned are characterized by surprisingly regular features, and their morphological peculiarities are very distinctive. They usually have strongly drawn-out and slightly sharp-crested hinge parts (Fig. 4), which are not infrequently complicated by thrust disturbances with a very insignificant amplitude of displacement. It is interesting to note that in most cases the structures under consideration are only very weakly complicated by platy cleavage and corrugation, which develop especially intensely only when the rocks of the Shignet Formation are drawn into zones of the most intense deformation. But within these zones the folded structures under consideration also become more complex, at times assuming a scaly structure, or else their hinge parts are accompanied by ruptures of continuity, leading to the thrusting of the gentle NE limbs of the folds onto their opposite steep parts (Fig. 5). The tendency of folds to overturn toward the NE is revealed not only near zones of dina-

tectonic disturbances, but is in general a characteristic leitmotif of the structure of the formation, for, as observations on the elements of bedding show, it is everywhere possible to record steeper angles of dip of the NW and SE limbs of folds than of their opposite parts. The described structures of the Shignet Formation should by no means be regarded as the result of a single tangential compression; rather, their visible features and types were formed under the influence of the cumulative effect of a series of tectonic movements, which proceeded with particular vigor in post-Devonian time. The latter is evident from the fact that the folded structures of the formation near contacts with post-Devonian marginal intrusions prove to be simpler and less strained, and, conversely, display very complex forms at some distance from these magmatic plutons (Fig. 6).

In contrast to the Shignet Formation, the above-described structures of the Buiba sequence were apparently formed, in their main features, long before the emplacement of the post-Devonian intrusives, for the magmatic solutions of the latter, in the form of injections, were already strictly adapted to its microfolded structures. Thus, the lithological composition of the Shignet Formation, the degree of metamorphism of its rocks, and the character of its internal structure indicate that this formation is an independent stratigraphic unit, sharply distinct from the greenstone sequences characterized earlier.

The stratigraphic position of the Shignet Formation is determined with sufficient precision. As was evident from its description, it is unquestionably younger than the West Sayan and Khemterek formations, overlying the latter transgressively through a basal conglomerate that includes the whole range of Cambrian rocks. At the same time the Shignet Formation is undoubtedly younger than the Buiba sequence as well, which it covers in places through a conglomerate and in places through a basal sandstone. However, the Shignet Formation is indisputably older than the Kerem sequence and especially than the Gotlandian deposits, which rest upon it transgressively, unconformably, with a basal conglomerate at the base (Fig. 8).

As for the age of the Shignet Formation, this question is more conveniently considered after the characterization of the Kerem sequence, since the latter ultimately provides some reference materials for establishing it.

6. Kerem Formation.

The deposits of the Kerem Formation, in petrographic composition and in the almost complete absence of metamorphism, differ so sharply from the formations described below that they were not infrequently taken for Upper Silurian sediments and, without special grounds, were combined with the latter, as, for example, was done by A. S. Mitropolsky (10). At the same time, lithologically the Kerem Formation astonishingly closely resembles the deposits of the Shignet sequence and in many cases, in surface observations (without tracing the contacts between them), it is very difficult to separate them from one another. The deposits of the Kerem Formation usually occur in the cores of deep synclines of the Shignet Formation, on which they rest with a concealed unconformity and on an eroded surface (Fig. 7). In other cases the sediments of the Kerem Formation form small, discontinuous belts, almost always bordering the Upper Silurian formations, which overlie them with a distinct angular unconformity and a basal conglomerate at the base (Fig. 8).

The Kerem Formation is composed of a thick sequence of psammites, to which interbeds and members of clayey shales and, more rarely, calcareous rocks are subordinate. An idea of the structure of the Kerem Formation may be obtained by studying the section along the M. Uru River, a left tributary of the Yenisei River. Here, immediately above the mouth of the Irkhyr spring, the following section is revealed.

At the base of the section lie fine- and very fine-grained, massive, greenish-gray sandstones, including characteristic poorly rounded, brown grains of jasper-like rocks. Among these sandstones appear interbeds and beds of brown psammites, which not infrequently pass into pudding-like and conglomerate-like formations. In the puddingstones, flattened pebbles predominate, belonging to gray, weakly chloritized sandstones and shales, very similar to analogous rocks of the Shignet Formation. The thickness of the sequence may be estimated at 100–150 m.

Stratigraphically higher there rest interbedded greenish-gray, violet, and burgundy clay shales, containing sandstone beds of the same coloration. Thickness 10–15 m.

The shales are overlain by brown, medium-grained sandstones containing flattened pebbles of greenish-gray psammites, up to 4–5 cm in diameter. Thickness 15–20 m. Higher in the section, greenish-gray fine-grained sandstones again develop, containing small fragments of jasper-like rocks and ferruginous mica and including numerous thin interbeds of burgundy and green shales. Thickness 100 m.

Finally, the formation is capped by a sequence of reddish-brown, burgundy, and violet sandstones and shales, among which characteristic thin beds of bluish-gray shales, pistachio-green psammites, and thin lenses of calcarenites are recorded. The thickness of the horizon reaches 250 m. Thus, the rocks composing the Kerem Formation show noticeable differences from the rocks of the Shignet sequence chiefly in that the former, on the whole, are characterized by burgundy and violet tones of coloration, whereas the latter are marked by a green color.

Along with this, in mineralogical composition the psammites of the Kerem Formation differ somewhat from similar rocks of the Shignet sequence.

Thus, in their principal mass the sandstones of the formation described have a coarse- and medium-grained structure, often show oblique bedding, and in most cases are massive or coarsely shaly. Macroscopically, in the sandstones only angular grains of quartz are readily recognized, and in some varieties, in addition to these, reddish fragments of jasper-like rocks and flakes of ferruginous mica, scattered in a fine-grained mass of other components difficult to determine. Under the microscope, the minerals composing the sandstones are weakly rounded but noticeably sorted and, in addition to the above-mentioned megascopically visible components, contain slightly kaolinized and sericitized plagioclase, pyroxene, silicilite, and fragments of acidic and basic effusives. In addition, rare flat grains of chloritized psammite and calcite are observed. The cementing mass of the sandstones is clayey-calcareous, slightly sericitized and chloritized, impregnated with finely dispersed hematite, the ore dust of which was probably deposited simultaneously with the formation of the sandy sediment.

Clay shales, appearing as interbeds up to 10 cm thick or as entire packets up to 2–3 m thick, are also very characteristic rocks. In most cases these are thin-bedded formations of burgundy, more rarely dark-green color, in which the process of metamorphism has manifested itself to a very slight degree, being expressed chiefly in the appearance of a fine sprinkling of sericite developing along the planes of intense, fine schistosity. At the same time, the internal structure of the Kerem sequence is surprisingly simple, a circumstance especially striking when studying the sediments composing it.

As is revealed from the continuous sections of the M. Ur River, especially in the area above the Chernaya River, the deposits of the formation under consideration form a large, relatively gentle syncline of general latitudinal strike. It is of interest to note in this connection that the NW limb of the mentioned syncli-

Level of the Yenisei River

Fig. 8. Diagram of folded deformations of the Shipenta Formation (left bank of the Yenisei River near the mouth of the Bolshaya Kerma River).

Scale

Fig. 6.

Diagram of the geological structure of the upper reaches of the Bolshoi Ur River

N

Scale: 0–2–4–6 km

Conventional symbols

Fig. 7.

were inclined to the horizon at an angle of only 30–35°, whereas the SW limb dips at an angle of 50–60°, thereby indicating an overturning of the fold to the NW. In other cases (B. Ur, 2 km above the Adyksuk River) the folds of the Kerema Formation have a symmetrical form, with smoothly and roundly closing crest portions and fairly gentle angles of dip, measured at only 15–20°. However, near zones of major disjunctive disturbances, an unusually intense cleavage develops in the sequence, affecting chiefly the thin interbeds of argillaceous shales enclosed between massive and thick sandstones. But here the cleaved microstructures, unlike similar forms of the Buiba Formation, are characterized by the development of flexure-like forms; moreover, the places of pinching are affected by fine ruptural disturbances of the thrust type (Fig. 10).

Thus, in the degree of dislocation and in the simplicity of their structures, the deposits of the Kerema Formation differ very sharply from the more complexly deformed Shignet and especially Buiba sequences; only a sharply expressed dominant jointing, manifested here and there with equal intensity and running parallel to the bedding, somewhat brings together these heterogeneous and heterochronous formations.

We must now clarify the question of the stratigraphic position of the Kerema Formation. It should be borne in mind that its contacts with the underlying and overlying sequences at a number of points are tectonically forced, as a result of which, in individual outcrops, it is possible to see seemingly close mutual transitions between these formations. However, upon detailed tracing of the contacts between the Kerema and Shignet formations it is always found that the deposits of the former participate in the structure of the cores of synclines, the limbs of which are composed of Shignet deposits. Along with this, in places it is possible to record (for example, at the mouth of the Kerema River, a left tributary of the Yenisei River) the normal overlap of the Kerema sequence by rocks of the Shignet Formation; moreover, at its base there not infrequently occur coarse-grained sandstones containing lenses of conglomerate composed of well-rounded pebbles of psammites and shales, externally very strongly resembling similar deposits of the underlying formation. Finally, the conspicuous weak metamorphism and relatively simple tectonic structure of the Kerema Formation cannot but serve as features indicating its younger age in comparison with the underlying deposits.

At the same time, the deposits of the Kerema Formation are indisputably older than the faunistically characterized Gotlandian. The latter, at the Shignet-Uzunsuk watershed, rests unconformably, through a basal conglomerate, upon the underlying sequences (Fig. 8), and within the conglomerate there are recorded well-rounded large pebbles of green, greenish-gray, violet, and brown sandstones and shales, very similar to the corresponding deposits of the Shignet and Kerema formations.

7. On the age of the Shignet and Kerema formations.

As was evident from the description, the deposits of the Shignet and Kerema formations that we have distinguished and characterized occupy a quite definite stratigraphic position, being situated between the Kemterek sequence of the Upper Cambrian and the Gotlandian deposits, and in terms of age, apparently, must be assigned wholly to the Ordovician. The latter follows not only from purely stratigraphic constructions, but also from the analysis of the paleontological documents collected by A. S. Mitropolsky from the formation of “marly limestones, shales, and sandstones,” distributed—

Plan of the geological structure of the Uzun-Suk–A. Ur area
Scale: 0–3 km

Conventional symbols:

• Upper granodiorites
• Graptolitic deposits
• Kerem Formation, \(S_1^k\)
• Shignet Formation, \(S_1^{sh}\)
• Buibinsk Formation, \(S_1^b\)
• Elements of bedding

Fig. 8.

Microdisplacements in the Kerem Formation

Fig. 9.

...identified by B. Ur above the Alyk-Suk River and assigned to the Gotlandian and the Lower Devonian (10).

It should be taken into account here that Mitropolsky included in the composition of the named formation also the deposits of the Kerem beds that we have distinguished (18—map), and, in his opinion, on the basis of the organic remains occurring in it, this formation corresponds to the middle Gotlandian. However, somewhat above we emphasized that the Gotlandian deposits, containing remains of Pycnostylus guelfensis and Halysites catenularia (which, among others, occur together) and a number of other forms, unconformably, through a basal conglomerate, cover the underlying sediments, resting now on the Kerem, now on the Shignet and even on more ancient formations (Figs. 8 and 7; 18—map), and, thus, the latter cannot be younger than the basal conglomerate of the Upper Silurian. Obviously, the formation of “marly limestones, shales and sandstones,” distinguished by A. S. Mitropolsky, is complex, undoubtedly of different ages, and the organic remains recorded within it—Pycnostylus guelfensis Whit., Favosites spinigerus Hall, Palaeofavosites aspera d’Orb, Conchidium aff. crassiplica Hall, Pachypora aff. dilacerata, Hipparionyx (?) proximus Vanixem var., Spirifer cnf. varuxemi Hall—are unlikely to prove wholly corresponding to the Gotlandian, since many of them are determined quite conditionally. In addition, Mitropolsky does not tie the listed paleontological remains precisely to the section, and therefore it is very difficult to form an idea of the distribution of these forms through the section and to ascertain whether they occur together at all localities, or whether they are grouped in some other way.

True, Mitropolsky notes that the last two species, which have a Lower Devonian age, occur together with typically Gotlandian forms in the upper parts of the section, while the others are grouped mainly in the lower and middle horizons of the formation that he distinguished. However, even such an explanation does not eliminate the shortcomings noted above, for the lower horizons with remains of pentamerids and tabulates of Mitropolsky’s formation of “marly limestones, shales and sandstones,” developed in the upper course of the B. Ur River (18, map; Fig. 7) and regarded by us as members of the Kerem formation, extending into the basin of the Bagazey River, are overlain by the basal conglomerate of the Gotlandian, on which, in turn, limestones and marls containing remains of Pycnostylus guelfensis, Favosites spinigerus, Halysites catenularia and other typically Upper Silurian forms rest completely conformably (Fig. 7). Exactly the same relations between the Kerem formation and the Gotlandian are also revealed on the divide of the Uzun-Suk–Shignet rivers (Fig. 8); but Mitropolsky at this point, too, combines these different-aged and independent beds into a single division of the Upper Silurian (18, map). Thus, the deposits of the Kerem formation, in my understanding, and, apparently, the lower members of the formation of “limestones, shales and sandstones” in Mitropolsky’s interpretation, are contemporaneous formations which, in our opinion, are separated from the Gotlandian by a clearly expressed break and unconformity. We are fully agreed that the middle and upper horizons of the formation of “limestones, shales and sandstones” do indeed correspond to the Upper Silurian, but in the upper reaches of the B. Ur River the latter are in tectonically forced relations with the Kerem beds (Fig. 7), and since these beds are lithologically extremely close to one another, it is natural that, in cursory observations, combining them into one formation is quite admissible.

As for the age of the Kerem formation, this question cannot yet be resolved with complete certainty. According to Mitropolsky, on the basis of the organic remains occurring in it, it should be assigned to the Gotlandian, and even to its middle parts; from our point...

it should be regarded as the upper part of the Lower Silurian, for the following reasons. First, the Kerem Formation lies below faunistically characterized limestones which, judging from the composition of the coral remains, may be correlated with the Wenlock and partly Valentian stages of Western Europe; in that case the basal conglomerates may possibly prove to be the lower beds of the Valentian stage. Second, during the study of the Kemterek klippe, on the Adyk-su River, the right tributary of the B. Ur, opposite the Bagazey brook, from a bed of sandy limestone whose relation to Upper Silurian deposits has not been established with certainty, I collected organic remains, admittedly not very well preserved. However, P. S. Krasnopeeva succeeded in identifying among them representatives of Fenestella sp. and Dicranopora sp.; the remains of the latter are characteristic both of the Ordovician and of the Gotlandian and, more rarely, of the Devonian.

At the same time, the genus Palaeofavosites and certain representatives of pentamerids from Mitropolsky’s collection may prove to be Lower Silurian, since identical forms were also found in the sandy limestone of the Adyk-suk brook, together with Streptelasma and Calapaecia¹, which are characteristic (especially the latter) exclusively of the Ordovician. Unfortunately, the relation of the sandy limestone of the Adyksuk River, with remains of Calapaecia, to the Gotlandian deposits has remained unclear, and therefore its belonging to the Kerem Formation cannot yet be considered proven. Nevertheless, the facts set forth above are quite sufficient for assigning the Kerem Formation to the upper part of the Ordovician, if one takes into account that it lies below the lower beds of the Valentian stage and that the sandy limestone with Lower Silurian fossils most likely belongs precisely to this formation.

If our conclusions regarding the age of the Kerem Formation prove correct, then in that case there is no great difficulty in determining the age of the underlying Shignet sequence. The latter, it seems to us, may with full justification be assigned to the Lower Silurian, without yet determining more precisely its position in M. A. Usov’s scheme (16). It is possible that the deposits of the Shignet Formation will prove to be synchronous with the sandy-clayey formation developed in the Gornaya Shoria in the basin of the Bolshaya Rechka near the Viktoryevka mine, in which remains of trilobites and brachiopods of the family Orthidae have been found and which, like the Shignet Formation, underlie deposits of the upper Lower Silurian.

III. CONCLUSION

Thus, the greenstone deposits of the Western Sayan, regarded by previous investigators as “Lower Silurian,” prove to have a more complex structure and to be formations of different ages, clearly falling into the following formations:

1. The Dzhebash Formation—composed of crystalline schists, gneisses, and amphibolites, to which beds of ferruginous quartzites and marbles are subordinate, and corresponding in age to the Precambrian.

2. The Kemterek Formation—essentially expressed by conglomerates, green schists, and sandstones, which in the upper parts acquire a violet and burgundy tone, giving the formation a reddish tint. The deposits of the Kemterek Formation will evidently prove, in age, to be very close to the Upper Cambrian.

3. The Buybin Formation—represented by phyllitized greenish-gray schists, chloritized sandstones of the same color, containing—

...thick beds of marbelized limestones, and a thick mass of greenstone basic effusives. The indicated formation at present occupies an undefined stratigraphic position and is provisionally assigned by us to the Lower Silurian, although its pre–Lower Silurian age is also possible.

1. The Shignet Formation, expressed by greenish-gray conglomerates and sandstones, to which very thin interbeds and packets of clayey shales and, more rarely, limestones are subordinate.

2. The Kerema Formation—consisting of maroon, lilac, and green sandstones and shales, including thin interbeds of marly rocks and calciphyres with remains of brachiopods and corals, as, for example, A. S. Mitropolsky records on the B. Ur River. These last two formations, evidently, fit entirely within the Lower Silurian and stratigraphically probably constitute its different stages.

The proposed subdivision of the greenstone deposits of the Western Sayan should be regarded as a working scheme, still requiring appropriate refinement.

Thus, for the time being, the independence of the Buiba Formation cannot yet be regarded as strictly proven; it may turn out to be an analogue of the Dzhebash strata, only less metamorphosed. Likewise, the stratigraphic position of the Kerema strata has not been clarified, although establishing its age should present the least difficulty, since it contains fossils either of the upper Ordovician or of the Gotlandian. The deposits of the Kemcherek and even the Buiba formations are also not hopeless with respect to the search for fossils, for in the marly rocks of the former some tubular formations and crinoid columnals have been discovered, while in the limestones of the latter, exposed at the mouth of the Adanart River and in the middle course of the Tozanykh River, right tributaries of the Yenisei River, there are spherical formations resembling algae.

LIST OF REFERENCES USED

1. Bazhenov, I. K.—Report on geological investigations in the area of the Maina copper deposit, Minusinsk uezd. Publ. SOGK, 1924, vol. IV, issue 1.

2. The same.—Geological structure of the junction between the Western Sayan and the Kuznetsk Alatau. Izv. Zap.-Sib. Dept. Geolkom, 1930, vol. V, issue 3.

3. The same.—The Western Sayan. Essays on the Geology of Siberia. Publishing House of the Academy of Sciences, 1934.

4. The same.—Preliminary report on geological investigations in 1926 in the southwestern Sayans. Izv. SOGK, 1927, vol. VII, issue 1.

5. Vologdin, A. G.—On the age of the Yenisei Series. Geological Bulletin, 1930, vol. VII.

6. Kassin, N. G.—The Precambrian of Kazakhstan. Soviet Geology, No. 11, 1938.

7. Kuznetsov, Yu. A.—Geological structure of the Abakan iron-ore deposit. Publ. Sib. Dept. Geol. Com., vol. VIII, issue 3, 1929.

8. Kuznetsov, V. A.—New data on the geology of the Western Sayan. Bulletin of ZSGGRT, issue 2, 1934.

9. Lebedeva, Z. A.—Principal features of the geology of Tuva. Transactions of the Mongolian Commission, Academy of Sciences of the USSR, issue 2, 1938.

10. Mitropolsky, I. I.—Some new data on the stratigraphy of the Meso-Paleozoic of the Western Sayan. Bulletin of ZSGT, No. 5, 1937.

11. Obruchev, V. A.—Geology of Siberia; vols. I and II. 1935, 1936.

12. Poletaeva, O. K.—Fauna of Cambrian trilobites of the Sanashtykol Limestone of the Western Sayan. Materials on the Geology of Zapsibkrai, issue 35, 1936.

13. Poletaeva, O. K. and Shatrov, I. B.—Geological structure of the northern slope of the Western Sayan in the area of the B. and M. Arbat Rivers. Materials on the Geology of Zapsibkrai, issue 33, 1936.

14. Sivov, A. G.—Geology and age of the intrusions of the southeastern slope of the Western Sayan. Proceedings of the Scientific Conference on the Study and Development of the Productive Forces of Siberia, vol. II, 1940.

15. Tomashpolskaya, V. D. and Bazhenov, I. K.—The new Usinsk asbestos-bearing district. Bulletin of ZSGGGT, issue 2, 1934.

16. Usov, M. A.—Phases and cycles of tectogenesis of the West Siberian Territory, Tomsk, 1936.

17. Shmidt, A. P.—Volcanism and metamorphism of the area of the Kantegir hyperbasite massif in the Western Sayan. Diploma thesis at TII, 1935.

18. Derbikov, I. V.—The Variscan intrusive complex of the Western Sayan. Bulletin of ZSGT, issue 2, 1938.

1. According to the determination of K. V. Radugin. ↩↩