THE UPPER CAMBRIAN ARBAT FORMATION OF THE WESTERN SAYAN

A. G. SIVOV

Introduction

As early as 1925, among the metamorphosed ancient formations of the northern slope of the Western Sayan, the Arbat suite was established [6, 174], composed of intensely altered sandy-argillaceous rocks and rare greenstone effusives. This suite, assigned to the Silurian (?), was correlated with the Shignet sequence, distributed in the southwestern part of the Western Sayan. Somewhat later, deposits synchronous with the Arbat suite, in the basin of the Kyzas River, a left tributary of the Abakan, were described under the name of the second middle Cambrian sequence; moreover, its age was already determined within the upper part of the Lower Cambrian, and it was emphasized that it lies between the first and third sequences of this system, forming with them a single, conformable series [5, 257].

In 1927, Yu. A. Kuznetsov, while studying the Abakan deposit, distinguished a sandstone-graywacke division, closely connected by thin mutual transitions with the underlying melaphyre-limestone-albitophyre division of presumably Middle Cambrian age [4, 27]. At the same time, Yu. A. Kuznetsov, analyzing the composition of the clastic material involved in the structure of the rocks of the sandstone-graywacke division, was compelled to acknowledge that the accumulation of this division proceeded to a significant extent at the expense of erosion of Lower Cambrian and even Precambrian formations [4, 25]. In this connection it is appropriate here to recall the observations of Hausen, who established at the base of the sandstone-graywacke division a basal horizon situated on the eroded surface of limestones [14, 67—68], in which, much later, remains of archaeocyathids of the Sanashtyk-Gol complex were discovered [1; 5, 176].

In 1935 the Arbat suite was distinguished in the Tabat district of the Western Sayan, where it rests on the eroded surface of the effusive Cambrian suite and of the Main intrusion cutting the latter. It was noted that the effusive suite of the named district is synchronous with the lower members of the lower Cambrian sequence of the Kyzas River basin and with the keratophyre division in the environs of the Abakan deposit, while the Arbat suite is synchronous with the second middle Cambrian sequence and the sandstone-graywacke division of these same areas of the Western Sayan. Likewise, the authors who studied the Tabat district sought to show that the deposits of the Arbat suite are overlain by the upper Cambrian suite (the West-Sayan formation), in whose hanging wall lies the algal-trilobite Babakov limestone, resting, according to M. A. Usov, on an eroded surface and on various small folded structures of the Arbat formation or suite [11, 77].

The age of the latter was determined by some investigators as falling within the interval of the upper part of the Lower Cambrian, and by others as the lower part of the Middle Cambrian [11, chart]. This was argued on the basis that deposits of the Arbat-formation type

(strata) in other areas of the Western Sayan (Kysas River) lie stratigraphically below the Sanashtygol Formation, distinguished at the time; the age of the latter, in accordance with the trilobite remains occurring in its deposits, was dated to the second half of the Middle Cambrian [1]. In addition, it was suggested that the Sanashtygol Formation of the Kysas River basin, being younger than the upper Cambrian sequence, is synchronous with the upper members of the lower Cambrian sequence and with the Melaphyro-limestone-albitophyre division. Finally, it was emphasized that the upper Cambrian sequence, characterized by Middle Cambrian fossils, in turn rests with a slight break on the deposits of the Arbat Formation, in which small lenses of limestones with remains of archaeocyaths of Lower Cambrian age had once been found.

However, in establishing the Lower Cambrian age of the Arbat Formation, the authors made a number of serious errors connected with an incorrect determination of the relationships between individual stratigraphic units of the Cambrian system of the Western Sayan in general and its Arbat member in particular. I consider it appropriate to discuss these errors, some of which have been noted in the literature [10], in more detail somewhat later.

For many years, while working on the study of the Cambrian deposits of the Western Sayan, I developed a fundamentally different stratigraphic scheme for this system, which in my view is as follows (from bottom to top):

Lower Cambrian—the Akkol, Chingin, and Babakov formations, which in composition and volume correspond entirely to the upper Cambrian sequence [5] or the West Sayan Formation [11].

Middle Cambrian—the Lower and Upper Monok formations, synchronous with the first lower Cambrian sequence [5].

Upper Cambrian—the Arbat Formation, in composition and volume corresponding completely to the formation of the same name of various authors and to the sandstone-graywacke division of Yu. A. Kuznetsov [4]. Apparently synchronous with the Arbat Formation are the deposits of the Kemterek Formation and the red-colored sequence of the Sayan, whose correlation I discussed in detail in one of my works [8, 211–216].

Fig. 1. Stratigraphic column of the Cambrian of the Western Sayan.

1—sandstones; 2—argillites; 3—siltstones; 4—conglomerates; 5—limestones; 6—effusives; 7—main trondhjemites; 8—hyperbasites; 9—microdiorites; 10—shales and phyllites.

Thus, in my view, the deposits described in the Western Sayan under the name of the Arbat suite or formation are located at the very top of the Cambrian system and correspond to its upper division; I intend to dwell on this in greater detail in the present work (Fig. 1).

But before turning to the clarification of the stratigraphic position and age of the Arbat Formation, it is useful to become acquainted with a number of key sections that make it possible to form an idea of its lithological composition and to establish stratigraphic relationships with the more ancient and younger formations surrounding it.

Description of Sections

Section along the Kair Ridge

The base and middle horizons of the Arbat Formation are exposed on the western slope of the Kair Ridge, which forms the watershed between the middle course of the Chekhan River and the lower course of the Karasibo River (Figs. 2 and 3).

Here, at the head of Sukhoi Log, which flows into the Karasibo River on the left, 2 km above its mouth, upon the heads of deposits of the Chinginskaya Formation (upper Cambrian sequence), which are intensely schistose and steeply inclined to the southeast, there lies a succession of rocks divided into the following stratigraphic members from bottom to top (Fig. 3):

a) conglomeratic, intensely schistose sandstone, greenish-gray in color and coarse-grained in structure, containing pebbles of greenstone effusives, microdiorites, phyllites, schists, hornfels, light-gray marbled limestones, and, more rarely, silicilites. Thickness up to 150 m;

Fig. 2. Location of the reference sections of the Arbat Formation

b) dark-gray finely schistose argillites, replaced in the hanging wall by dark-green medium-grained quartz–feldspar sandstones. Thickness up to 200 m;

c) black fine-grained sandstones rich in carbonaceous matter, consisting of angular, subrounded grains of quartz and feldspar, cemented by a fine-grained, intensely chloritized quartz–feldspar mass. Thickness 30 m;

d) conglomeratic sandstone, consisting of large and small angular fragments of monoclinic pyroxene (augite) and pale-green hornblende, with a rare pebble admixture of greenstone effusives, microdiorites, and silicilites. Thickness up to 30 m;

Fig. 3. Section of the Arbat Formation along the Kair Ridge.

1—conglomeratic sandstones; 2—dark-gray argillites; 3—medium- and coarse-grained sandstones; 4—black sandstones; 5—bluish-green argillites and 6—bluish siltstones of the Arbat Formation; 7—Early Salair microdiorites; 8—phyllites of the Chinginskaya Formation; 9—stratigraphic members of the Arbat Formation

e) this sandstone is overlain by dark-gray argillites with a violet tint, enclosing a bed of bluish-green siltstones, as well as black quartz–feldspar sandstones. Thickness of the member up to 140 m;

f) black fine-grained sandstones overlain by bluish-green argillites, replaced in the hanging wall by bluish siltstones and then by argillites. Thickness 75 m;

ж) a bed of gray coarse-grained quartz–feldspar conglomeratic sandstone, overlain by a member of bluish-green argillites, finely interbedded with siltstones of the same color, which upward acquire predominant importance. Thickness 70 m;

з) light-bluish fine-grained siltstones, interstratified with thin beds of bluish-green argillites. Thickness 75 m.

In the section described, the beds predominantly dip to the southeast, \(140^\circ\), at an angle of \(50^\circ\), and are finely cleaved in the same direction, but with angles of dip of the cleavage planes of about \(70^\circ\). However, at the very summit of the Kair Range a flexure of the beds is observed, and the latter dip to the northwest, \(320^\circ\), at an angle of \(80^\circ\).

Fig. 4. Section of the Arbat Formation along the right bank of the river Chekhan, between the Kurygan sheep farm and the sheep farm.

1—Lower Variscian diorite-diabases; 2—sandstones; 3—gray argillites; 4—bluish-green argillites; 5—banded argillites, 9—diabases; 6—conglomeratic sandstones; 7—alternating sandstones, siltstones of the Arbat Formation; 8—shales of the Chinginsky Formation; a, b, c, d, …—indices of members.

Section along the Chekhan River

Certain features in the structure of the lower and middle horizons of the deposits of the Arbat Formation and in the relations of the latter to more ancient formations are observed in the section along the Chekhan River, in the area between the Kurygan sheep farm and the sheep farm of the Artas collective farm (Figs. 2 and 4).

In this area a large anticlinal fold is exposed, in the core of which medium-grained greenish-gray Early Variscian diorite-diabases crop out, while the southeastern limb is composed of a rhythmically bedded conglomerate–sandstone–argillite sequence in the following succession from bottom to top:

а) dark-gray medium-grained, markedly biotitized sandstones, alternating with hornfelsed greenish-gray shales and containing several interbeds of silicified and epidotized limestones. These beds are replaced by fine-grained massive calcareous sandstones, above which greenish-gray silicified siltstones develop, interbedded with bluish-green argillites. Thickness of the member 35 m;

б) a member of complexly alternating gray and bluish-green argillites with a visible thickness of about 75 m. At the base of the member lies a light-

gray medium-grained quartz–feldspar sandstone, with similar rocks also occurring as interbeds within the argillite member. The visible thickness of the sandstone is about 20 m, but it is probably incomplete, since along its contact with the argillites runs Sukhoy Log, which has washed away a considerable part of these rocks. Thickness of the member 95–100 m;

c) coarsely banded argillites of light-green color with a bluish tint. The banding in them is caused by the alternation of broad interbeds, up to 10 cm thick, composed of fine-sandy greenish material, with narrow bands, up to 8 mm thick, consisting of clayey-carbonaceous, slightly silicified material of dark-gray color. These argillites contain several beds of greenish-gray sandstones and two sill-like bodies of diorite-diabase, each 10 m thick. Thickness of the member 200 m;

d) greenish-gray coarse-grained conglomerate-like sandstones, containing varied but relatively sparse pebbles of albitophyres, plagioclase and hornblende porphyrites, quartz-free keratophyres, dark-gray limestones, and dark carbonaceous phyllites. Thickness 30 m;

e) dark-green fine-grained feldspathic sandstones, containing 2 bedded bodies of diorite-diabase, each up to 10 m thick. Thickness 180 m;

f) a bedded body of diorite-diabase of greenish-gray color, consisting of plagioclase, hornblende, and augite. At the contact with the enclosing rocks, the latter undergo noticeable silicification, chloritization, and especially epidotization. Thickness of the intrusive body 200 m;

g) sandstones of the type of member “d,” with several bedded bodies of diorite-diabase, from 1 to 10 m thick. Thickness of the member 120–140 m;

h) dirty-green conglomeratic coarse-grained sandstones, rich in pebbles of quartz albitophyre, dacite, biotitized labradorite porphyrite, marmorized limestone, dark lydites, and dark-green crystalline schists. In the hanging wall of these rocks appear characteristic light-gray medium-grained quartz–feldspar sandstones, consisting almost exclusively of subrounded angular fragments of quartz, acid plagioclase, and felsite, cemented by a quartz–chlorite–epidote mass with an admixture of carbonates. Thickness 30 m;

i) greenish-gray massive fine-grained sandstones, overlain by light-green, bluish-tinged, banded argillites of the type of member “c.” Thickness 75 m;

k) after an unexposed interval of 220 m, banded argillites of the type of member “c” appear. Visible thickness 25 m;

l) then again an unexposed interval of about 250 m, after which appears a complexly alternating member of ash-dark argillites with a characteristic violet tint, interbedded with greenish-gray siltstones, which in the footwall occur as thin interbeds and in the hanging wall occupy the dominant position. At the base of the member lies a conglomeratic sandstone up to 5 m thick, similar in composition to the sandstone of member “h” and differing from the latter in the considerable presence of small pebbles of Maien trochammina. Several thin interbeds of the same sandstone are also found within the argillites. Thickness of the member 200 m;

m) rocks of the same character as in the preceding member, but here the greenish-gray siltstones are situated closer to the footwall, and the ash-dark argillites—to the hanging wall. Here also, at the base of the member, there is a conglomeratic sandstone up to 4 m thick, of the same character as in the preceding member “l.” Thickness of the member 150 m;

h) a thick unit of greenish-gray fine-grained sandstones, rich in boulders of algal limestones lying along the footwall, reaching 0.5 m in diameter, and in small pebbles of albitophyres, greenstone diabases, and Maina trondhjemites. Closer to the hanging wall the sandstones acquire a coarse-grained structure, but then, through thin interbeds, they are replaced by fine-grained varieties, colored greenish gray with a distinct bluish tint. However, at the very top of the member, near the mouth of the Kurlygan creek, pebble accumulations again appear in the sandstones, belonging mainly to metamorphic and intrusive rocks. Within the member there are several bedded diorite-diabases 10 to 20 m thick. The thickness of the member is 300–350 m.

The total thickness of the rocks composing the section on the right bank of the Chekhan River and constituting the Arbat Formation may be estimated at about 1.5 km, excluding, of course, intrusive formations.

The rocks in the section described everywhere dip to the southeast at 140° at an angle of 50°, but closer to the Kurlygan creek the dip angle increases noticeably and reaches 75°.

Above the mouth of the Kurlygan creek, in the very side of the Chekhan River, strongly metamorphosed, intensely schistose dark carbonaceous phyllites appear, enclosing covers of greenstone effusives and layers of lidites, which I distinguish as an independent Chinginskii Formation of middle Lower Cambrian age. Primary bedding in this unit cannot be established, but the schistosity planes dip to the southeast at 140°, with a dip angle of 80°.

Because of the poor exposure of the contact between the Arbat and Chinginskii formations, the relationships between them at the locality described cannot be clarified. However, judging from the elements of the dip direction of the bedding of the Arbat Formation, the rocks of the latter prove to be on the footwall side of the Chinginskii unit, a circumstance caused by overturning of the beds to the northwest.

Section along the Mishikha River

A more complex combination of deposits of the Arbat and Chinginskii formations is found in sections along the Mishikha River, a left tributary of the Karasibo River.

Here, on the left slope of the Mishikha River, 0.3 km above the Konyukhovskii creek, the following section is observed (Figs. 5 and 6):

a) almost at the river’s edge, finely schistose mottled algal calcareous shales crop out; their schistosity planes dip to the southeast at 130° at an angle of 86°. The shales enclose a concordant amphibolite body up to 250 m thick. The contact between the shales and amphibolite is sharp and is expressed in the development of accumulations of epidote and quartz in the enclosing rocks;

b) somewhat higher up the slope and, at the same time, stratigraphically, there is a strongly schistose conglomerate-like sandstone, composed of fragments of dark-green, short-prismatic hornblende, cemented by a calcareous-clayey mass. The sandstone includes rare but varied pebbles of greenstone effusives of labradorite and hornblende porphyrite type, as well as pebbles belonging to amphibolite, trondhjemite, vein albitophyre, microdiorite, and limestone. The sizes of the pebbles range from fractions of a centimeter to 25 cm in diameter, with the largest boulders always belonging to amphibolites and limestones.

The rock described has undergone intense metasomatism, with the formation of such minerals as actinolite, brown biotite, and, more rarely, epidote. The latest metasomatic component is calcite, forming mottled accumulations or thin veinlets.

The thickness of the conglomerate sandstone is about 300 m; the dip direction is northwestern, 320°, angle 60°; and near the Konyukhovsky spring it is southeastern, 140°, angle 30°;

c) below the Konyukhovsky spring, on the same left slope of the Mishikha River, there occur fine-grained light-gray marbled limestones,

Fig. 5. Schematic geological structure of the Kair Ridge in the Western Sayan.

1—Quaternary deposits; 2—Arbat Formation; 3—Maina trondhjemite intrusion; 4—microdiorite intrusion; 5—Chinga Formation; 6—Dzhebash Formation; 7—bedding elements; 8—disjunctive faults

the thickness of which reaches more than 250 m. The dip direction is southeastern, 135°, angle 70°;

d) from beneath the limestones emerge vigorously amphibolitized greenstone hornblende and labrador porphyrites, which, probably,

Fig. 6. Section along the left bank of the Mishikha River.

1—conglomeratic sandstones; 2—dark-green fine-grained and coarse-grained rocks; 3—sandstones of the Arbat Formation, \(Cm_3\); 4—post-Chinga amphibolites; 5—greenstone labrador and pyroxene porphyrites; 6—marbled limestones; 7—algal calcareous shales of the Chinga Formation, \(Cm_1^2\); 8—disjunctive fault

occur in the form of a concordant body up to 200 m thick and have a dip direction likewise to the southeast, with almost the same angle of inclination, i.e. 70°;

d) while maintaining the same dip to the southeast, \(135^\circ\), but with an angle of dip of \(50^\circ\), an alternating member of sandstones and their conglomeratic varieties appears in the outcrops; these consist almost exclusively of pebbles of hornblende porphyries and fragments of amphibole, cemented by an ore mass with a considerable participation of chlorite, actinolite, albite, and, occasionally, epidote and biotite, which often have a clastic character. The thickness of the member is more than 250 m;

e) finally, at the mouth of the Mishikha stream, dark- and light-green sandstones of fine- and medium-grained structure crop out; they also dip to the southeast, \(140^\circ\), at an angle of \(50^\circ\), and have a thickness of 600 m.

Thus, along the Mishikha stream the complex relationships between the Arbat and Chingis formations are exposed; moreover, the latter is cut by amphibolites, which are altered bodies of a microdiorite intrusion.

From the section it is evident that the algal shales, together with the amphibolites that cut them, are unconformably overlain by conglomerate-like sandstones at the base of the Arbat Formation, and that fragments of the former play the most important role in the composition of the latter.

Below the Konyukhovsky settlement, conglomerates of the Arbat Formation proved to be on the footwall side of the effusives and marbleized limestones of the Chingis sequence, which is a usual example of overturned occurrence of these members of the Cambrian system in the Western Sayan.

Section along the Monok River

The deposits of the Arbat Formation that participate in the structure of the section exposed by the Bolshoi Monok River, 1 km above the Terbishcheva stream, where they lie directly on effusives of the Lower Monok Formation or on the lower horizons of the first lower Cambrian sequence, are characterized by a somewhat different composition (Figs. 2 and 7).

At this point, on the right bank of the Bolshoi Monok River, a large anticline is exposed, in the core of which occur hornblende porphyries of the Lower Monok Formation, and in the northwestern limb—conglomerate-limestone rocks of the Upper Monok Formation. In the southeastern limb of the fold, on the eroded surface of the mentioned effusives, rests a sequence of clastic rocks that I assign to the Arbat Formation. These rocks are bedded in the following stratigraphic succession from bottom to top.

Fig. 7. Section along the right bank of the Bolshoi Monok River, above the Terbishcheva settlement

1—conglomeratic and fine-grained (2) sandstones of the Arbat Formation \(Cm_3\); 3—limestones and conglomerates (4) of the Upper Monok Formation \(Cm_2\); 5—effusives of the Lower Monok Formation \(Cm_1^2\) (?)

Conglomeratic sandstones, macroscopically resembling a medium-grained intrusive rock of acidic composition. They are composed almost entirely of unrounded fragments of thick-twinned albite, to which noticeably rounded grains of quartz and small rare pebbles of quartz and trondhjemite are subordinate. The thickness of the sandstone is about 250 m; the direction of dip is southeast \(140^\circ\), angle \(40^\circ\). Upstream, the sandstones under consideration form several gentle small folds and then are overlain by a thick suite of bluish-green sandstones, which are the most widespread rocks of the Arbat Formation.

Section along the Rudnaya Kena River (Fig. 2)

A detailed characterization of the deposits of the Arbat Formation in the vicinity of the Abakan iron-ore deposit was given by Yu. A. Kuznetsov, who assigned them to the sandstone–graywacke division \[4, 24\].

In the section along the Rudnaya Kena River, according to my observations, the deposits of the Arbat Formation rest on the eroded surface of Lower Monok basic effusives, and, according to Gausen, also on limestones, which I regard as one of the members of the Upper Monok Formation \[14, 67—68; 10\]:

a) at the base of the Arbat Formation lies a conglomerate consisting of angular fragments of plagioclase, quartz, felsite, quartz albitophyre, augite and hornblende porphyrites, limestones, and rare pebbles of silicilites, cemented by a medium-grained quartz–feldspar mass. Thickness about 25 m;

b) stratigraphically higher there appear characteristic pinkish-gray, coarse-grained arkosic sandstones, taken by Yu. A. Kuznetsov to be albitophyre tuffs \[4, 23\]. These sandstones are interbedded with thin beds of clayey fine-grained sandstones, brownish argillites, and small lenses of limestone, in one of which remains of trilobites were discovered. Thickness 75—100 m;

c) then, stratigraphically higher, there appear predominantly dark-gray or greenish-gray fine-grained sandstones and siltstones, to which are subordinate beds, from several meters to ten meters thick, of conglomerate consisting of small, well-rounded pebbles of silicilites, keratophyres, banatite-porphyry, and limestones. Thickness up to 500 m;

d) in the upper part of the described section lie fine- and medium-grained grayish-green and dark-green sandstones, in the composition of which, along with quartz–feldspar grains, there are fragments of psammites and argillites. In the lower parts of the horizon under consideration there is a bed of light-gray, weakly marbleized limestone up to 20 m thick. The total thickness of the member is up to 250 m.

The strike direction of the beds is predominantly to the northwest, with an angle of dip up to 70°.

Section along the Arbat River

The relations of the Arbat Formation to the Upper Monok Formation (the upper members of the first lower Cambrian sequence) are revealed quite clearly in an almost continuous section extending along the right bank of the B. Arbat River, which in its lower course flows in one valley with the Abakan.

Here, above the village of Sarasuk (Fig. 9), there crops out a thick sequence of graywacke sandstones of uniform coloration, complexly alternating with coarse and even boulder conglomerates, overlain by claret-green, finely laminated siltstones interbedded with sandstones of the same coloration.

This rock sequence, the details of whose stratification are visible in Fig. 8, is noticeably metamorphosed; moreover, the alteration processes affect the fine- and very fine-grained varieties of the rocks of the sequence especially deeply. Suffice it to say that in the siltstones and fine-grained sandstones the groundmass is filled with large grains of epidote, scales of clinochlore and chlorite, and needles of actinolite, among which, here and there, are visible fragments of spherulitic, intensely chloritized effusive rock, coarse-tabular, strongly epidotized feldspar, and aggregate...

accumulations of leucoxene. In the medium- and coarse-clastic rocks of the unit, metamorphism is expressed somewhat more weakly and is accompanied by the appearance of the same metasomatic components, which develop especially readily in the cement of the rocks.

The rocks of the unit strike in a northeastern direction, \(70^\circ\), and dip south at an angle of \(70\)—\(75^\circ\). In this same direction there develops

Fig. 8. Section along the Bolshoi Arbat River, above the village of Sarasuk.

1—graywacke sandstones; 2—conglomerates; 3—metamorphosed sandstones; 4—conglomerates with boulders; 5—conglomeratic sandstones; 6—siltstones; 7—fine-grained sandstones of the Upper Monok Formation; 8—boulder conglomerate; 9—“albite” sandstones; 10—gray and brown sandstones with interbeds of fine-pebble conglomerates (11) of the Arbat Formation; 12—Devonian effusives; 13—Early Variscan diorite-diabases; 14—disjunctives

the dominant jointing, locally obscured by fractures striking northeast \(35^\circ\), with planes dipping southeast at an angle of inclination up to \(90^\circ\).

On the clearly eroded surface of claret-green siltstones lies a boulder conglomerate (Fig. 8), rich in boulders (up to \(0.4\) m in diameter) belonging to greenish-lilac albitophyres, intensely metamorphosed fine-grained sandstones, gray marbled limestones with remains of archaeocyaths (Loculicyathus tolli Vol), and greenish-gray intrusive rocks of the microdiorite type, very vigorously amphibolitized. Together with the boulders there are small pebbles of the same rocks, with a limited participation of silicilites and claret-green siltstones; the latter are quite similar to the formations occurring in the footwall of the boulder conglomerate. The cement of the conglomerate is either sandy-clayey or sandy; in the latter case it is coarse-grained and weakly chloritized. The thickness of the conglomerate is about \(40\) m.

Stratigraphically above lies the original conglomeratic sandstone, consisting for \(70\%\) of angular fragments of albitized plagioclase, with a minor participation of angular quartz grains, small pebbles of acidic effusives, silicilites, leucocratic granite, claret-green strongly epidotized siltstones and sandstones. The thickness of the sandstone is up to \(30\) m.

Fig. 9. Diagram of the geological structure of the watershed of the Abakan—B. Monok rivers.

1—Quaternary deposits; 2—Devonian effusives; 3—Arbat Formation; 4—limestones and 5—conglomerates of the Upper Monok Formation; 6—effusives of the Lower Monok Formation

The boulder conglomerate and the overlying “albite” sandstone are cut by a dike up to \(6\) m thick of grayish-green, distinctly recrystallized diorite-diabase, striking \(135^\circ\) and dipping to the se—

northeast at an angle of 70°. At the contact with the diorite-diabase the enclosing rocks are slightly ocherized and chloritized, and the limestone boulders are noticeably metamorphosed. The section is crowned by interbedded packets of light-gray, gray, greenish, and brown sandstones and, more rarely, argillites, the thickness of individual beds of which rarely exceeds 1 m. Among the sandstones lie two beds of fine-pebble conglomerate, each of which is situated at the base of sandstone packets characterized by an abrupt change in coloration. These fine- and coarse-grained sandstones, being represented by semi-rounded grains of quartz and, to a lesser degree, plagioclase, with a noticeable accumulation of fragments of chlorite, epidote, and mica flakes, are almost untouched by metamorphism, which is only weakly expressed in direct contact with the diorite-diabase dike cutting the enclosing rocks in a northwesterly direction, with a strike of 290° and dipping southwest at an angle of 65°. The thickness of the sandstone packet is about 450 m.

The beds of conglomerates and sandstones of the upper part of the section under consideration strike in a northwesterly direction and dip southeast at angles from 50 to 70°, with a steeper inclination of the beds observed at the contact with massive Devonian effusives, broken by a number of large fractures along which separate masses of sandstone proved to be drawn in. Somewhat below the mouth of the Matros River, a left tributary of the Arbat River, the effusives lie calmly, being inclined to the horizon at only 15°; while upstream of the Matros River they, in the form of numerous bedded bodies, occur among steeply dipping sandstones, which served as grounds for some investigators to regard such alternation as interbedding.

The rock sequence participating in the structure of the section just described, with the exception of magmatic formations, could easily be combined into a single stratigraphic unit composed of outwardly monotonous, complexly alternating conglomerates, sandstones, and siltstones, inclined at almost one and the same angle to the horizon, with an apparent conformable occurrence. However, a detailed analysis of the section of this sedimentary sequence shows that it consists of two formations separated by a distinct tectono-denudational break.

Indeed, the boulder conglomerate of packet 8 (Fig. 8) of the described section, overlain by “albitic” sandstone, rests on the eroded surface of burgundy siltstones, intensely metamorphosed fragments of which appear both in the composition of the boulder conglomerate and in the “albitic” sandstones.

Moreover, the “albitic” sandstone, outwardly resembling an acidic intrusive rock, when traced northeastward, on the right bank of the Bolshoi Monok River, lies directly on the Lower Monok effusives, and here it forms the base of the Arbat Formation (Figs. 7 and 9).

A completely identical sandstone is recorded within the Arbat Formation in the vicinity of the settlement of Abaza, where it is situated somewhat above the basal conglomerate and includes platy fragments of limestones with remains of Razumovskya uralica Vol.

These circumstances indicate that the boulder conglomerate of the Bolshoi Arbat River section and the “albitic” conglomeratic sandstone overlying it represent a basal stratigraphic member, here also accepted by us as the base of the Arbat Formation. The rocks of this basal member are characterized by a polymictic composition; the fragments, pebbles, and boulders participating in their structure, though noticeably metamorphosed, are situated in an almost unaltered cementing mass.

The sandy-conglomeratic deposits lying stratigraphically above the basal member likewise bear no traces of metamorphism, although they are intensely schistose.

The sequence of sandy-conglomeratic formations located beneath the basal member of the Arbat Formation I unite into an independent Upper Monok Formation, which in the section of the Bolshoi Arbat River is synchronous with the upper horizons of the lower Cambrian sequence \[5\]. This is proved above all by the fact that, moving northeastward from the Arbat section within the sandy-conglomeratic deposits of the Upper Monok Formation, on the watershed of Maskalak Spring and the Bolshoi Monok River, limestone lenses up to 10 m thick appear, with organic remains typical of them from the Sanashtykol complex. Among the latter there is also Razumovskya uralica Vol., found in turn in fragments of “albite” sandstones from the vicinity of the settlement of Abaza.

Examination of a number of sections exposing the deposits of the Arbat Formation shows that at its base there is a thick basal conglomerate, overlying either the pyroclastic-sedimentary formations of the Chinginsk Formation or the effusives of the Lower Monok Formation. In all these cases the boundary between the Arbat Formation and the underlying rocks is very distinct, being expressed by a surface of erosion and unconformity, as well as by the development of a basal horizon at its base.

This boundary is established less clearly between the Arbat and Upper Monok formations, certain horizons of which, being petrographically similar, create apparent transitions between them, and, where they occur in parallel bedding, also a picture of apparent tectonic conformity.

Not infrequently the deposits of the Arbat Formation occur in overturned bedding, and then in specific sections they are situated at the base of older formations, as is seen, for example, in the sections of the Mishiha River, and also along the Maly Arbat and Chekhan rivers, below Kurlygan Spring.

Elements of the stratigraphy and petrographic composition of the Arbat Formation

The petrographic composition of the Arbat Formation is extremely uniform, being represented essentially by clastic rocks, among which sandstones of varying grain size predominate; argillites and conglomerates are less common. Only in isolated cases among the clastic rocks of the formation are small limestone lenses of limited extent recorded.

In the distribution of the individual petrographic varieties of the rocks of the formation a certain rhythmic regularity is observed, by means of which it is possible to distinguish within the formation smaller stratigraphic units, with boundaries between them that are, of course, conventional, and with a volume corresponding to members.

The base of the formation, composed essentially of molasse formations 150 m to 200 m thick, with interbeds of sandstones and siltstones, may be combined into the basal or lower member. Deposits represented predominantly by argillites and siltstones with packets of sandstones and conglomerates constitute the middle member, whose thickness is not less than 1300 m. And, finally, flysch-like formations represented by fine- and medium-grained, noticeably shaly sandstones, among which siltstones and argillites occur as thin (from 4 to 5 cm) interbeds, make up the upper member. The lower boundary of this member could be marked by conglomeratic sandstones with large boulders of limestone, in particular those exposed in the Chekhan section, near the mouth ...

cl. Kurlygan (member “n”), along the Dzhebash River, 2 km below the Chekhan River, and at other points. However, we are unable to present a detailed section of the formation under consideration, since throughout its entire extent it is composed of extremely monotonous fine-grained sandstones, gathered into small tight folds, with the amplitude of the limbs of the latter amounting to several tens of meters.

In addition, the absence of natural continuous sections exposing this formation, and the absence among the formations composing it of marker horizons, constitute additional difficulties in its stratification. Therefore the thickness of this formation, established in studying the poorly exposed section along the Dzhebash River at 500–600 m, must be regarded as a conditional value (Fig. 10).

In accordance with the latter, it does not seem possible to present a summary stratigraphic column of the Arbat Formation, whose thickness has tentatively been taken as 2 km.

In the distribution of the mineral composition of the rocks of the formation under consideration, a certain regularity may be noted, manifested both in the vertical and in the horizontal profiles; this is readily clarified by analyzing the material composition of each of its stratigraphic members, the characterization of which it is useful to dwell upon briefly.

Lower basal formation

The accumulation of this formation, represented by conglomerates, conglomeratic sandstones including rare interbeds of argillites and siltstones, proceeded essentially at the expense of erosion of the rocks of the underlying basement and, probably, with insignificant activity of rock-forming organisms, which left their traces in the form of small lenses of limestone, sometimes with paleontological content. This circumstance is especially clearly expressed in the analysis of the clastic material of the conglomeratic formations of the formation, the composition of which is entirely determined by the underlying parent rocks.

Fig. 10. Scheme of the geological structure of the lower course of the Dzhebash River.

1—Quaternary; 2—Early Variscan intrusion; 3—Lower Devonian effusives; 4—Arbat Upper Cambrian Formation; 5—Chingis Lower Cambrian Formation; 6—stratigraphic boundaries; 7—disjunctives.

Thus, in the Kair Range and along the Myshikha River, in the conglomerates of the formation, a major role is played by fragments of greenstone effusives, metamorphosed sandstones, shales, limestones, silicilites, and, more rarely, intrusives, whose similarity to the rocks that produced them—the rocks of the Chingis Formation—is most sharply revealed by microscopic comparison.

Among pebbles of effusive rocks, the following varieties are most characteristic:

a) biotitized and chloritized labrador porphyrite, in which large laths of intensely saussuritized plagioclase are situated within a vigorously altered groundmass, with a relict diabase texture preserved in places, passing here and there

into diabasic. The main mass of the porphyrite consists of weakly saussuritized labradorite, in the interstices between the grains of which abundant green chlorite is developed, replacing yellowish-brown biotite;

b) epidotized hornblende porphyrite, in which, besides weakly saussuritized segregations of labradorite, there have been preserved phenocrysts of hornblende in the form of spots, replaced by biotite, colorless epidote, and carbonate. The character of alteration of the groundmass has the same features as in the effusive described earlier;

c) hornfelsed diabase, with a distinctly expressed diabasic structure of the groundmass, composed of weakly saussuritized labradorite situated amid an aggregate of biotite, epidote, and leucoxene;

d) biotitized andesite, possessing a holocrystalline, granular porphyritic structure and consisting of “ovoid” zonal grains of plagioclase—andesine, immersed in a very fine-grained andesitic groundmass. The latter, as well as the porphyritic segregations of andesine, are intensely biotitized and are cut along fractures by thin veinlets of quartz.

Among the pebbles of intrusive rocks present in the conglomerates, microdiorite and amphibolite are most often found; moreover, the microdiorite pebble, almost entirely converted to epidosite, is cut by numerous veinlets of quartz that do not extend into the cement of the rock. In some places, in parts of the microdiorite that have escaped alteration, a dioritic structure is visible, formed by albitized, saussuritized plagioclase, hornblende replaced by calcite, and brownish-green biotite passing into chlorite. In addition, individual portions of this rock are vigorously biotitized, silicified, albitized, and represent a typical hornfels aggregate. In rare cases the microdiorite is less metamorphosed, and then its primary features appear more distinctly. The amphibolite pebble found in the conglomerate consists of amphibole, pyroxene, with a subordinate participation of plagioclase, which have undergone intense biotitization. Together with the intrusive pebbles, fragments and boulders belonging to contact-metasomatic rocks are associated in the conglomerate; among these, calcite–feldspar–epidote–biotite hornfelses with a clearly expressed banded texture, not characteristic of the cement, are the most typical formations.

In addition, in the conglomerates under consideration one may constantly note the presence of pebbles of black and other-colored carbonaceous-aleurolitic shales, greenish-gray metamorphosed sandstones, dark, sometimes banded lydite, and light-gray marbled limestones. It is interesting to emphasize that this essentially clastic material composes the conglomeratic sandstones of the suite, distributed along the B. Kyzas River, at the mouth of the Vesely Creek; here large boulders of limestone are often found, sometimes with remains of Archaeocyathus ijizkii Toll. These boulders at one time were mistakenly taken for limestone lenses syngenetic with the enclosing rock.

Much more rarely, in the conglomerates of the described localities, fragments and pebbles are found that belong to keratophyres, albitophyres, hornblende porphyrites and their amygdaloidal varieties, as well as to Maina trondhjemites; fragments of the latter play an important role in the rocks of the suite where these directly cover the effusives of the Lower Monok Formation, in many places enclosing plutons of the Maina intrusion.

In this respect, the conglomerates distributed over the watershed area of the Kyzas and B. Karakol rivers and the interfluve of the Ana—right Kirginek, which is a right tributary of the Abakan, are typical. At these localities, basal—

member, resting on the uneven surface of the Lower Monok effusives, is composed almost entirely of products of erosion of the latter.

In this connection, pebbles of effusives, which have a violet, more rarely pinkish, light- or dark-gray coloration, with numerous phenocrysts of plagioclase, isometric quartz grains, more rarely pyroxene and hornblende, set in a fine-crystalline or lithoid groundmass, preserve the typical features of the parent rocks that supplied the pebbles. Somewhat less common are large boulders of light-gray and pinkish-gray limestones, sometimes containing remains of archaeocyaths belonging to Archaeocyathus ijizkii Toll, Coscinocyathus dianthus Born.

Pebbles of greenstone effusives, black shales, and metamorphosed sandstones are found sporadically and are almost always accompanied by small pebbles of banded silicilites.

In most cases (the Kurchep, Rudnaya Kenya, B. Arbat, and B. Monok rivers), the basal suite rests on heterochronous formations of the Middle Cambrian, the products of erosion of which also served as the source for the formation of the characteristic conglomerate-like sandstone at the base of the suite (Fig. 11).

Fig. 11. Diagram of the geological structure of the Karakol Range.

1—Quaternary deposits; 2—Devonian deposits; 3—post-Cambrian effusives; 4—Arbat Formation; 5—conglomerates of the Upper Monok Formation; 6—limestones of the Upper Monok Formation; 7—effusives of the Lower Monok Formation; 8—elements of bedding; 9—points of characteristic outcrops.

This sandstone consists of large, up to 1 cm in diameter, completely unrounded fragments of pinkish-yellow, light-gray, or gray albite, which has preserved isometric or elongate-prismatic outlines.

The rock contains many rounded fine quartz grains and hornblende prisms, associated with small and well-rounded pebbles of albitophyre, pyroxene porphyrite, granitoids of the Maina trondhjemite type, and siliceous rocks (lydites). The fragments in the rock adjoin one another closely, so that the calcareous-clayey cement is macroscopically quite invisible, owing to which the rock externally very much resembles

a coarse-grained intrusive rock of intermediate basicity. Small fragments and comparatively large pebbles of gray and pinkish-gray limestone are often noted in the rock, sometimes with remains of algae—Razumowskya uralica Vol. (Rudnaya Kenya River).

Pebbles of crystalline schists of the prazinite type and of albite-quartz-sericite formations, whose characteristic representatives take part in the structure of the Akkol Formation, occur very rarely in the conglomerates.

The concentration and dimensions of the pebble material in the rocks of the basal formation under consideration are subject to considerable variation. In places the rock is almost entirely composed of rather well-rounded, variegated pebbles of nearly uniform size, cemented by a coarse-grained quartz-feldspar mass rich in calcite-dolomite material, in which numerous flakes of chloritized biotite, chlorite, fragments of hornblende, brown ironstone, and leucoxene are present. For the most part, however, the rock consists of angular, more rarely well-rounded fragments of feldspars, among which grains of andesine-labrador are not uncommon. The feldspars, together with the accompanying angular grains of quartz, fragments of hornblende, and, rarely, pyroxene, are cemented by a clayey-calcareous mass, weakly chloritized and sericitized. But sometimes (Mishikha River, above Konyukhovskii Spring) the rock is literally crowded with large fragments, completely unrounded and unsorted, of short-prismatic hornblende, bound by calcareous material with rare fine fragments of quartz and feldspar. In either case the pebble material is present in the form of small accumulations or single specimens; its dimensions range from the size of a pinhead to 25 cm in diameter. The shape of the pebbles is usually angular, with the exception of intrusive rocks and silicilites, whose pebbles are most often rounded or elliptically elongated.

The conglomeratic rocks distributed at the head of Kurlygan Spring, which enters the Chekhan River from the right, 200 m below the ulus of N. Artas, are characterized by a sharply different composition. Here the rocks are filled with completely unsorted large angular fragments and grains of vein quartz, which also occurs in the form of rare but well-rounded pebbles up to 5 cm in diameter. It is interesting to emphasize that inclusions of greenish-gray, thin-schistose chlorite-sericite schist can sometimes be seen in the quartz pebble, and in the cement, which also consists almost entirely of fine quartz grains, there are flakes of chlorite, probably of clastic origin.

As a result of such granulometric inconstancy of the clastic material, the petrographic aspect of the conglomeratic formations is extremely variable and changes almost at every step from coarse- to medium-clastic rocks, both in the vertical and in the horizontal directions. These rocks, being massive and sometimes thick-bedded, are characterized chiefly by a dirty-green, more rarely light-gray and brown coloration.

Within the conglomeratic formations of the formation described, interbeds or even beds of greenish-gray siltstones and argillites occur not infrequently; they are rich in a fine-grained quartz-feldspar mass cemented by clayey-calcareous material. Only at one point, namely along the Rudnaya Kenya River, on its right bank, at a distance of 3 km from the mouth, among the basal conglomeratic sandstones, lenses were found at least 10 m long and up to 0.7 m thick of greenish-gray limestone with remains of trilobites, from which I identified the form Abakanopleura kenensis Siw.

Middle Formation

This formation, most typically represented in sections between the mouth of the Chekhan River and its right tributary, the Kurlygan spring, in the Kair Range, as well as in the middle course of the Tabat River and along the B. Kyzas River, above the Prav. Kyzas, is characterized by a predominance of argillites and siltstones over sandstones and conglomeratic formations, which appear as thin beds or packets within pelitic rocks. The latter, differing from one another only in the character of their coloration, which varies from bluish, greenish, and ash-dark tones, occur in coarse and fine alternation with one another and make up rhythmically bedded packets.

In many cases, at the base of almost every packet there is a conglomeratic or coarse-grained sandstone, stratigraphically sharply replaced by siltstone-argillite formations.

The sandstones occurring within the pelitic rocks are extremely uniform. They are predominantly greenish and bluish, with various shades, in color and have a fine—more rarely medium-grained—texture. These rocks are usually composed of angular, more often subrounded quartz grains, weakly sericitized plagioclase, with a noticeable participation of magnetite, brown iron ore, and leucoxene. The listed components are cemented by a calcareous-dolomitic mass, noticeably chloritized and epidotized. However, epidote is apparently in part detrital, since some of its grains have a rounded form and are cemented by carbonate. The thickness of psammitic beds usually does not exceed two tens of meters, and is sometimes much less.

Among the uniform psammites of the formation, black and dark-green sandstones stand out sharply by their distinctive features; they constitute persistent, well-traceable beds extending over considerable distances and attaining a thickness of ten meters.

The former, being close in composition to the described varieties of sandstones, are distinguished above all by their characteristic coloration, and then by a fine-grained quartz-feldspathic, slightly chloritized cement rich in carbonaceous matter, distributed in patches within the groundmass.

Dark-green sandstones, of medium- and more rarely coarse-grained texture, are distinguished by a peculiar composition of the clastic material. The latter is almost entirely represented by large and small fragments of monoclinic pyroxene (augite), with a relatively small content of hornblende. The dark-colored components are enveloped by a spotted fine-grained aggregate consisting of calcite, chlorite, actinolite, and zoisite. The biotite present (iddingsite?), replacing pyroxene and hornblende, undoubtedly formed earlier than the above-mentioned aggregate neoformations. The described sandstones, up to 30 m thick, apparently appear at the base of the argillite-siltstone formations of the formation, as can be established in sections of the Kair Range, the Chekhan River, and the Tabat River [11; 12].

In the described sandstones, regardless of the degree of their grain size, one can always note an accumulation of interpebble, highly angular material, which gives the rocks a conglomeratic appearance. These latter not infrequently form independent beds of varying thickness, appearing among the argillite-siltstone rocks without transitional members, or else they are connected with them through a clearly expressed rhythm from medium- to fine-grained and siltstone formations. Individual-

dual features of the conglomeratic rocks are determined by the content of clastic material participating in their structure.

Thus, the conglomeratic sandstone encountered in the system of the Tabat River contains predominantly pebbles and boulders of Maina trondhjemite, with a comparatively rare presence of fragments of keratophyre and porphyrite [11]. In the lower course of the Chekhana River, the conglomeratic interbeds are more variegated in composition, being composed of fragments and pebbles of feldspar porphyrite-andesinites, albitophyres, hornblende porphyrites, quartz-free and quartz keratophyres. Much more rarely there occur pebbles of Maina trondhjemite, as well as greenstone effusives, dark-gray thin-schistose phyllites, light-gray marbled limestones, and silicilites that are very variable in color.

In the lower horizons of the formation under consideration, thin lenses of limestone of light-gray color and fine-grained structure are found. The thickness of the carbonate interbeds does not exceed 0.5 m, and their extent is 50–60 m. No more than three such interbeds have been noted in the section of the Chekhana River.

Upper Formation

The rocks of the named formation, composing the upper members of the Arbat Formation, are represented chiefly by fine-grained, more rarely medium-grained sandstones with a characteristic dark- and light-green coloration, sometimes with bluish and violet shades. Somewhat less frequently, sandstones of brown and especially claret coloration are found, and then outwardly they are very similar to analogous rocks of Devonian age.

Apparently, the base of this formation is made up of a conglomeratic sandstone original in composition, including numerous pebbles and boulders of limestone, varying in size from 0.7 m to fractions of a cm in diameter, situated now in a fine-, now in a coarse-grained mass composed of subrounded grains of quartz and plagioclase, with the participation of numerous small pebbles of diagenetized effusives, intrusives, and silicilites. The limestone boulders in many cases have a rounded or angular and much more rarely elliptical form; in the latter case the diameter of the short axis of the boulder does not exceed 0.3 m, whereas the long one reaches 0.75 m. These elliptical boulders, being arranged with their long axis parallel to the bedding of the sandstone, produce the apparent impression of small lenses syngenetic with the enclosing rock, for which some investigators have taken them [12]. However, the ideal roundness of the boulders under consideration, the presence of a cementing mass, and especially of relict tectonostructures expressed in the development of an entire system of fractures filled with quartz-epidote or quartz-carbonate veinlets, not passing into the cement of the conglomerate, testify to the clastic nature of these formations. Moreover, in a large exposure (Figs. 5 and 12), occurring on the left side of the Chekhana River, 400 m below the mouth of the Karasibo River, some of the numerous limestone boulders are penetrated by veinlets of sealing-wax-red jasper, again not extending into the cement of the conglomerate. Fragments of limestone do not always appear in the form of bouldery and pebble forms, but not infrequently as sharply angular or even slablike formations, the sizes of which vary within relatively wide intervals (from fractions of a centimeter to 0.25 m). The degree of concentration of clastic carbonate material in the rock is very variable and appears now in the form of isolated specimens, now as comparatively considerable and, moreover, chaotic accumulations.

In boulders, pebbles, and other fragments of limestone having a light-gray, sometimes pinkish or sugar-white coloration, there are rarely found ...

remains of archaeocyathids and algae are encountered; among them, at one time, the following representatives were identified—Archaeocyathus ijizkii Toll., A. proskurjakovi Toll.

Only in a single specimen did I find a representative of brachiopods of the type Bagenovia sajanica Rad., and several specimens of sponges, which have so far remained unstudied. The most important localities of paleontological remains in limestone fragments included in the conglomeratic sandstones of the Arbat Formation were indicated more than once at the time; moreover, these carbonate clastic masses were, without grounds,

Fig. 12. Schematic section of the Arbat Formation along the left bank of the Chekhan River, 200 m above the mouth of the N. Artas stream.

1—medium-grained sandstone, 2—fine-grained sandstone, and 3—limestone conglomerate of the upper member; 4—phyllitized siltstone of the middle member; 5—disjunctive structure.

taken to be lenses syngenetic with the enclosing rocks [4]. To these I may also add the following localities: the right bank of the Chekhan River, 300 m below the Kurlygan stream; the middle course of the N. Kanzhul stream, which flows from the left into the Karasibo River, 1 km above the mouth of the former; and the B. Arbat stream in its lower course.

The conglomeratic sandstone described probably constitutes a persistent stratigraphic member, traceable for several tens of kilometers from the Chekhan River to the northeast, as far as the mouth of the M. Abakan—to the southwest.

The mineralogical content of the other psammitic rocks of the upper member is very uniform and is limited to the presence of grains of quartz of various sizes, but predominantly fine, and very rarely feldspar, cemented by an argillaceous-calcareous mass rich in flakes of chlorite and, at times, epidote and even hornblende, which possibly also occur in a clastic state. In this respect, a certain exception is formed by sandstones possessing brown, burgundy, and light-gray coloration. They occur as thin beds; the first two of these varieties are most often composed of quartz grains of medium size, with an abundant presence of fine fragments of sandstone, argillite, chlorite, and biotite, with a relatively modest participation of plagioclase and fine-grained accumulations belonging to red siltstone, psammite, limestone, and fine-grained granitoid. The rock is cemented by a fine-grained quartz mass. In the light-gray sandstones, however, quartz again predominates, but together with it fragments of light felsites and grains of acid plagioclase play a notable role.

Among the generally psammitic rocks of the upper member, pelitic formations occur extremely rarely and almost always in the form of very thin interbeds, the thickness of which never exceeds 1 m. They, possessing bluish-greenish-gray and more rarely violet coloration and being situated at the top of a layered packet, as it were complete a finished rhythm of sedimentation, which to some extent can be observed in the section of the small Sukhoi Log, which flows into the Karasibo River from the left, 1 km below the mouth of the Chapsar stream.

To the characterization of the upper member of the Arbat Formation it may be added that within it, just as in the preceding members, limestones occur extremely rarely. Over a comparatively broad area of development of the upper member, a limestone bed 20 m thick, white and light-gray in color, dense and homogeneous in structure, without any signs of organic remains, has been recorded only at the mouth of the Rudnaya Keni River. This limestone has been described by many investigators, so that its morphological features have been covered rather fully [4, 25].

The above-described members of the Arbat Formation differ from one another not only in petrographic composition, but also in the character of bedding, as well as in the degree of sorting and rounding of the clastic material. Thus, in the first two lower members the clastic material is almost completely unsorted and very poorly rounded; in places it is true angular debris, little reworked by water and cemented by a quartz-feldspar mass that is just as poorly sorted. Moreover, even in the argillite and siltstone formations, sometimes thin-bedded and banded, accumulations and individual specimens of coarse-clastic material can be seen, chaotically arranged with respect to the stratification. In these two members, manifestations of cross-bedding are more often observed, especially well expressed in fine-grained sandstones.

In the upper member, however, the clastic material of the rocks is more completely reworked by water, and therefore in many cases it remains homogeneous in its magnitude and size and is arranged in the rocks in accordance with sorting, often producing in them a clearly expressed bedding. In this connection the latter becomes parallel and at times rhythmically persistent, the rhythm retaining its features both within an individual bed and between beds. Nevertheless, in the sandstones of this member accumulations of pebble-boulder material of limestones, and more rarely of other rocks, are sometimes observed; most often they are concentrated in its lower horizons.

In short, accumulation of the sediments of the Arbat Formation proceeded chiefly at the expense of terrigenous material under conditions of short transport and, probably, a severe climate. As a result, such clastic components of the rocks as plagioclase, albite, and some dark-colored minerals appear well preserved and bear only slight traces of decomposition.

On the age of the Arbat Formation

The age position of the Arbat Formation is clarified both from direct observations of its relationships with older and younger stratigraphic units and from analysis of the organic remains found in the limestones of its basal horizon.

In the description of the sections presented above it was shown that the deposits of the Arbat Formation rest sharply transgressively, and with a basal conglomerate at the base, on rocks of the basement—either on metamorphosed formations of the Chingis Formation (Kair Range, summit of Kly. Kurlugana, Mishikha River), or on diagenetized effusives of the Lower Monok Formation (watershed of the M. and B. Karakol rivers, right tributaries of the Abakan River; B. Monok River, 5 km above its mouth), and on the sedimentary-carbonate members of the Upper Monok Formation (B. Monok River, settlement of Abakanskii Zavod, and B. Arbat River).

The conglomerates of the Arbat Formation consist of pebbles of diagenetized, sometimes hornfelsed keratophyres, albitophyres, and porphyrites, often with distinct traces of relict tectonic structures caused by the development of systems of fractures filled with epidote-quartz veinlets.

or sealing-wax-red jasper that do not extend into the cement of the conglomerate. This complex of clastic material, in its petrographic composition, is completely identical to the rocks of the lower Man beds of the Middle Cambrian formation. The pebbles and boulders of trondhjemite, albitite, and quartz diorite widely distributed in the conglomerate correspond petrographically exactly to the Main intrusion, as was noted in due course by some investigators of the Western Sayan [10; 11]. Furthermore, the numerous fragments and boulders of limestones occurring in the clastic rocks of the Arbat Formation sometimes contain remains of archaeocyaths—Coscinocyathus dianthus Born., Archaeocyathus ijizkii Toll., algae, and sometimes brachiopods, in particular Bagenovia sajanica Rad. and others. The first two forms are constantly found in the Babakovo limestones of the Kyzas River, which I assign to the upper part of the Lower Cambrian, whereas the latter is so far known from the Sanashtykol horizon of the Upper Man Formation of the first half of the Middle Cambrian.

At the same time, in the conglomerates of the Arbat Formation there are pebbles and fragments of intensely compressed, metasomatically altered, and partly hornfelsed greenstone effusives, metamorphosed sandstones, shales, lydites, microdiorites, often passing into amphibolites. This clastic material of the conglomerate is petrographically quite analogous to the rocks participating in the structure of the Chingino Formation of the first half of the Lower Cambrian and of the microdiorite intrusion cutting the latter. Pebbles of crystalline schists are found more rarely in the conglomerates; among them especially characteristic are prasinitic and albite–quartz–sericite varieties, as well as quartzites, which are the most typical rocks of the most ancient formations of the Western Sayan.

Thus, the position of the footwall of the Arbat Formation is determined with sufficient precision by the transgressive overlap of its basal horizon onto deposits of the Upper Man Formation of the first half of the Middle Cambrian and onto formations of more ancient stratigraphic units. Sedimentation of the deposits of the Arbat Formation took place after a prolonged hiatus that became established at the end of the first half of the Middle Cambrian, during which energetic denudation affected not only the Middle Cambrian rocks but also the most ancient formations of the Western Sayan, such as the crystalline schists.

As for the stratigraphic position of the hanging wall of the Arbat Formation, it is established by the discordant overlap, upon the deposits of the latter, of effusive rocks of the Lower Devonian, on whose eroded surface in turn lie variegated sediments with remains of Asteroxylon elberfeldense Kraus et Weyl. [9, 27]. These relations are excellently established from sections of the lower course of the Dzhebash River, where the steeply southeast-dipping fine-grained greenish-gray sandstones of the Arbat Formation are overlain by the gently lying effusives of the Lower Devonian Kharadzhul Formation, dipping to the northwest at an angle of only 15–20°. Moreover, not infrequently in the footwall of the Kharadzhul effusives, among tuffaceous fragments, one can see pebbles of sandstones petrographically indistinguishable from the rocks of the Arbat Formation. Likewise, the deposits of the latter are cut by numerous dikes of brown and brownish-gray augite and labradorite porphyrites and light- or dark-gray diabases, probably constituting the roots of the above-mentioned Kharadzhul extrusive sheets, and by no means syngenetic formations with the enclosing sequence. In direct contact with these dikes the deposits of the Arbat Formation undergo noticeable contact alteration, expressed in a considerable darkening of the sandstones, their hornfelsing, and the appearance in them of

bladed hornblende. In the dikes one can sometimes see incorporated fragments of sandstones, which usually acquire a whitish-pink coloration owing to the development of clinozoisite and albite in a recrystallized mosaic aggregate of quartz.

On the basis of the stratigraphic relationships described above, the age of the Arbat Formation is determined within a considerable interval of time—the second half of the Middle Cambrian and the end of the Upper Silurian.

However, this age range of the Arbat Formation is considerably narrowed by analysis of the paleontological remains occurring in its deposits. It was indicated above that in limestone lenses along the Rudnaya Kena River, which flows into the Abakan River from the left, near the settlement of the Abakan Plant (Abaza), trilobites were discovered, represented only by endemic forms, among which I have established a new genus, Abakanopleura Siw., of the family Asaphiscidae Raym. Some species of this genus, for example Abakanopleura kenensis Siw., are close to species of the known generic representatives Orloviella Lerm. and Kaninia Walc and Resser, established in the deposits of the Saldair Formation of the town of Orlina, Salair Ridge [14, 257—301]. In addition, trilobites have been found in the limestones of the Arbat Formation that show considerable similarity to forms belonging to the genera Dolgaieila Siw. and Maladiopsis Siw. of the Gornoshorsk Formation of the Kuznetsk Alatau [14].

The Upper Cambrian age of the Saldair and Gornoshorsk formations is not in doubt. However, the stratigraphic succession of these formations is established far from reliably. There is reason to believe that the first of them, contrary to previous determinations [7, 305], will prove to be older than the second. If this assumption is confirmed by subsequent investigations, then in that case the Saldair Formation, in whose deposits mixed Middle and Upper Cambrian faunas are found [7, 301—304], should be regarded as the base of the Upper Cambrian of the Sayan-Altai region. In connection with this, the deposits of the Arbat Formation may also prove to be the oldest members of the Upper Cambrian, since they contain some trilobites displaying close kinship with trilobites of the Saldair Formation. The latter is all the more probable because, in petrographic composition as well, they show considerable similarity to one another, although the conditions of their accumulation were not the same. If the sediments of the Saldair Formation are typically platformal, then the deposits of the Arbat Formation are undoubtedly geosynclinal.

In short, without definitively resolving the question of the stratigraphic position of the Arbat Formation within the Upper Cambrian, it may be considered fully proved that it is the youngest stratigraphic member of the Cambrian system of the Western Sayan.

Our conclusions on the stratigraphic position of the Arbat Formation substantially correct earlier ideas about its age, which had been formed on the basis of the works of other investigators of the Western Sayan [5, 171—176].

They believed that the Arbat Formation (suite) is situated between the deposits of the “first lower” and “third upper” Cambrian strata [6, 257].

In one of my works I showed that the “upper Cambrian stratum,” which is the analogue of the West Sayan Formation [11], constitutes the base of the Cambrian system of the Western Sayan and, in turn, is subdivided into three independent stratigraphic units (from bottom to top): the Akkol, Chingis, and Babakovo formations, the age of the last two of which cannot be younger than the top of the Lower Cambrian, whereas the first may possibly prove to be an older formation—even Precambrian. Rav-

Thus, I established that the deposits of the “first lower Cambrian sequence,” consisting of two independent formations—the Lower Monok (lower) and Upper Monok (upper)—lie transgressively and unconformably upon the formations of the “upper Cambrian sequence,” i.e., the Chinginskaya and Babakovskaya formations, as I understand them. Fossils of archaeocyaths and trilobites occurring in the (Sanashtykol) limestones of the Upper Monok Formation indicate that they belong to the first half of the Middle Cambrian. I discussed above the stratigraphic position and age of the Arbat Formation, or the second Cambrian sequence, and showed that it is the uppermost member of the Cambrian of the Western Sayan. It should be noted that M. A. Usov believed that the archaeocyath–trilobite Babakov limestones of the third Cambrian sequence (the Western Sayan Formation) rest transgressively and on an eroded surface of the Arbat Formation [11; 12]. However, having studied a number of sections that reveal the relations between the Babakov limestones and the deposits of the Arbat Formation, I showed that these sequences are often found in an overturned position, as is seen, for example, in the section along the Mishikha River (Fig. 6). In an overturned position are the deposits of the Arbat Formation in the Tabat district, something that was not understood by the investigators who at one time studied that district.

I discussed in detail the relations between the Babakov limestones and the Arbat Formation in the Tabat district in one of my papers, and therefore I consider it possible not to dwell on them here. I shall only point out that, in the best sections along the Gryaznyi Spring, a tributary of the Maly Arbat River, conglomerates of the Arbat Formation are situated in the footwall of the Babakov limestones; in their clastic material large boulders of limestones with remains of fossils typical of the Babakov Formation are found.

Thus, from all that has been set forth above it follows quite evidently that the deposits described in the Western Sayan under the name of the Arbat suite and the second upper Cambrian sequence constitute an independent stratigraphic unit, for which I retain the name Arbat Formation, proposed by M. A. Usov [11; 12], since the volume and content established for it by that author raise no objections on my part. At the same time, however, I consider it proven that the Arbat Formation, contrary to M. A. Usov’s opinion, must be assigned not to the first half of the Middle Cambrian but to the Upper Series of the system, and that it is the youngest stratigraphic member of the Cambrian of the Western Sayan.

Elements of the Tectonic Structure of the Arbat Formation

The independence of the Arbat Formation is determined not only by the relations considered above between it and the older and younger formations surrounding it, but also by certain specific features of its tectonic structure.

The deposits of the Arbat Formation constitute two large synclinal folds isolated from one another, lying along the same northeastern strike. One of these synclinal structures, occupying the areas of the upper reaches of the Bolshoi Kyzas River and the lower and middle course of the Maly Abakan River, I have named the Kurchan structure (Fig. 11), while the other, extending along the interfluve of the Ana–Dzhebash and continuing farther northeast into the basin of the Tabat River, is the Chekhan structure.

The synclinal structure of these two structural forms is expressed not only by the change in dip of the beds on their opposite limbs, but also by the upwarping of older deposits from beneath the formations of the Arbat ...

formation. Thus, in the south, from beneath the deposits of the latter emerge rocks of the Chinginskaya sequence of the middle Lower Cambrian, while in the north there occur alternately the sediments of the Upper Monok and the older Lower Monok formations of the first half of the Middle Cambrian. The dip angle of the limbs of the synclinal structures does not remain everywhere the same. The northern limbs of these structures, over almost their entire extent, dip southward at an angle of only 35–40°, whereas the southern limbs dip steeply northward at an angle of 80–85°, in places being clearly overturned in the same northerly direction. Overturned bedding of the deposits of the Arbat Formation is observed at many points where they adjoin rocks of the Chinginskaya sequence and is probably due to thrusting of the rocks of the latter over the deposits of the former. Because of these circumstances, near the thrust fault the beds of the Arbat Formation dip at an angle of 70–75° to the southeast, beneath the deposits of the Chinginskaya Formation.

Thus, in the outcrops of the Levyi Kyzas River, below the village of Veselyi, then the Mishikha, below the village of Konyukhovskii, and at other points, conglomerates of the Arbat Formation dip at an angle of 65–70° beneath the strongly metamorphosed and intensely dislocated rocks of the Chinginskaya Formation; this circumstance served for some investigators as proof of the higher stratigraphic position of the latter sequence. However, analysis of the composition of the conglomerate of the Arbat Formation, in which fragments and well-rounded pebbles are represented chiefly by rocks characteristic of the Chinginskaya Formation and by the intrusion that cuts it, made it possible to determine unmistakably the true relationships between these units of different ages.

Both the northern and southern limbs of the main synclines of the Arbat Formation under consideration are complicated by an entire series of small additional folds of the second and even third order, with limb amplitudes from several meters to ten meters.

The forms of these secondary folds are very characteristic. In most cases they are strongly compressed, with sharply pointed hinge zones, complicated by disjunctive disturbances of the reverse-fault type (Fig. 12).

At the same time, the deposits of the Arbat Formation are very vigorously cleaved; moreover, the dominant strike direction of the cleavage coincides with the strike direction of the bedding. The cleavage planes, always dipping somewhat more steeply than the bedding, often deliberately mask the latter, as a result of which in the deposits of the Arbat Formation one more often observes a one-sided, predominantly southeasterly dip of the cleavage, with dip angles of up to 80°.

The cleavage clearly has selective features, being finer and more intense in fine-clastic, homogeneous rocks and near disjunctive disturbances, and less vigorous and even coarse in coarse-fragmental rocks, especially in those cases when the latter are not drawn into the zone of rupture dislocations.

It is appropriate to emphasize here that the degree of intensity of cleavage, its strike, and the angle of dip of the cleavage planes observed in Devonian deposits associated with the Sayan thrust completely coincide with similar elements found in the sediments of the Arbat Formation. This circumstance is probably due to the simultaneity of the movements that affected the deposits of the Arbat Formation and the Devonian and caused in them the formation of identical forms of cleavage.

The deposits of the Arbat Formation contain numerous plutons of early Variscan diorite-diabases, occurring predominantly in the form of concordant bodies or small stocks. The emplacement of the diorite-diabase magma took place much later than the time of the main cleavage of the rocks of the Arbat Formation and was accompanied by vigorous metasomatism of the latter—

Hydrothermal jets were directed along the most weakened structural elements and were localized selectively in the rocks of the continent, concentrating in the exocontacts of plutons and noticeably dispersing at some distance from them. Among the newly formed minerals in the rocks of the formation, chlorite and epidote are the most widespread; they densely replace the cement of the rock and occur more rarely in the form of thin veinlets, which is most characteristic of the latter component. Of the other metasomatic minerals, carbonates, quartz, and, very rarely, albite are observed; these are the latest formations relative to the above-mentioned green components.

It may be that the predominance of the green coloration of the rocks of the Arbat Formation should be placed in causal connection with the development of metasomatic neoformations generated by intrusive activity of Early Variscan time. The latter seems to be confirmed by the fact that sharply expressed green coloration is localized in the most intensely schistose and fine-grained rocks, which moreover form the exocontacts of plutons or participate in zones of disjunctive disturbances.

However, the presence in the rocks even of such green minerals as epidote and chlorite cannot be connected only with metasomatic processes; it is possible that these components are of clastic origin and arose through the destruction of ancient greenstone formations.

In any case, in many varieties of rocks of the Arbat Formation, chlorite and epidote of clastic origin are noted not so very rarely. In short, metasomatic manifestations did not cause deep changes in the deposits of the formation under consideration, whose primary sedimentary nature is established with complete clarity.

Main Conclusions

Thus, analysis of the material described above shows that, within the Cambrian deposits of the Western Sayan, conglomerate-sandy-argillitic sediments are stratigraphically and lithologically readily distinguished; these sediments were at one time combined into the Arbat suite, and by M. A. Usov and myself into the formation of the same name. I have determined the age of the latter as Upper Cambrian.

The Upper Cambrian deposits of the Arbat Formation lie with sharp unconformity, with a basal conglomerate at the base, and transgressively on the rocks of the basement, covering either the Upper Monok conglomerate-limestone sediments of the lower half of the Middle Cambrian, or the Lower Monok effusive rocks of the lower part of the Middle Cambrian, or the Babakov limestones of the upper part of the Lower Cambrian, and finally the effusive-sedimentary formations of the Chingis strata of the middle Lower Cambrian (Fig. 1).

In the clastic rocks of the Arbat Formation, especially in its basal horizon, fragments and pebbles of rocks typical of the basement are abundantly represented, including its intrusions of different ages: the microdiorite of Poslechingin age and the trondhjemite of pre–Upper Monok age. Many pebbles and fragments of rocks participating in the conglomerates of the Arbat Formation are intensely metamorphosed and preserve vivid traces of relict tectonic structures. These documents, together with the surface of sharp unconformity between the basement formations and the deposits of the Arbat Formation, clearly testify to the manifestation of an energetic Salairian diastrophism, one phase of which occurred before the accumulation of the rocks of the Arbat Formation. For this phase of tectogenesis, which took place after the sedimentation of the deposits of the Upper Monok Formation of the first half of the Middle Cambrian, the name first proposed for it by M. A. Usov should be retained—the Tabat phase.

[11—scheme]. In this connection it is necessary to bear in mind that the Ancheshevskaya and Novoprosvetovskaya formations distinguished by M. A. Usov within the Western Sayan have been combined by me into the Upper Monokian sequence, with the exception of the effusive formations alien to it, which constitute characteristic members of the Lower Monokian formation.

The Tabat phase, being one of the powerful phases of the Salair diastrophism, was accompanied not only by folding deformations of the Upper Monokian deposits, but was also the cause of the emergence of a prolonged break within the Western Sayan, during which, in the neighboring folded zones of the Kuznetsk Alatau and Salair, accumulation took place of a thick sequence of sediments of the Mondybash and Orlinogorsk formations [11—scheme], synchronous with the paradoxid beds of Norway (upper half of the Middle Cambrian). Despite the energetic manifestation of the Tabat phase of tectogenesis in the territory of the Western Sayan, it apparently was not accompanied by intrusive activity, for among the intrusive plutons of this mountain massif it is not yet possible with due confidence to distinguish intrusions of the end of the first half of the Middle Cambrian. It is true that V. A. Kuznetsov insists on the Middle Cambrian age of the hyperbasites of the Western Sayan and Tuva, but the argumentation he gives is not entirely substantiated, for in the deposits enclosing the hyperbasite intrusion the organic remains proved to be not Middle Cambrian, but more ancient, most likely Kameshkovian [23, 27—28]; evidently the Bayangol suite, distinguished by V. A. Kuznetsov, whose synchronism with the Upper Monokian formation is beyond doubt, cannot be regarded as identical with the deposits enclosing the hyperbasites [2, 94—95].

At the same time, the mountainous land that arose as a result of the Tabat phase of tectogenesis was subjected to intense denudation, which had exposed by the beginning of accumulation of the Arbat Formation the deepest horizons of the earth’s crust, namely the crystalline schists of the Precambrian and partly of the lower Cambrian.

Accumulation of sediments of the Arbat Formation proceeded with abundant supply and short transport of terrigenous material from the adjoining continent, which possessed a strongly dissected high-mountain surface relief. This, probably, explains the molasse character of the sediments of the Arbat Formation, especially sharply expressed at its base, composed of conglomerates. The latter are made up of poorly sorted and slightly rounded, but compositionally and dimensionally diverse clastic material, chaotically arranged in a rock that outwardly resembles tillite, as which it was taken by some investigators [13, 52—53; 14]. It is true that in these rocks I was unable to establish the presence of boulders, pebbles, or other fragments with striation characteristic of the activity of glacier ice. However, the abundance in these rocks of feldspars and ferromagnesian minerals, characterized by exceptional freshness, poor sorting, and angularity, testifies to the relatively severe climatic conditions that prevailed during their accumulation.

However, by the time of accumulation of the flysch sediments of the Arbat Formation, when the continent that served as the source of supply of clastic material had become to a considerable degree flat, the climatic conditions evidently changed toward a dry and probably hot climate. In connection with this, from the continent there came chiefly fine- and thin-clastic material, with occasional removal of large limestone boulders, now chaotically scattered among the fine-grained sandstones of the formation, in whose coloration green, claret, and violet colors predominate, while in composition quartz particles predominate, fairly well rounded and sorted.

After all the processes associated with the accumulation of sediments of the Arbat Formation, the latter proved to have been drawn into the sphere of tectonic movements, as a result of which the complex structural forms described by me above were formed. We do not have sufficiently convincing materials at our disposal that determine the upper age boundary of the post-Arbat tectogenesis, since throughout the entire area of distribution of the Arbat Formation its steeply dislocated deposits are overlain by gently lying sheets of effusives of the uppermost Upper Silurian or lowermost Lower Devonian.

However, there is likewise no doubt that these movements, associated with the Salair diastrophism, ended, probably, either at the end of the Upper Cambrian or even at the beginning of the Lower Silurian. Indeed, in the Western Sayan, on its southeastern margin, deposits composing the Shignet Formation are widely distributed; both in stratigraphic position and in lithological composition they show a close similarity to the West Altai (green-violet) formation of other regions of the Sayan-Altai area [8; 11; 12]. The deposits of the Shignet Formation lie on an eroded surface, with sharp unconformity, on the conglomerate-shale formations of the Kemterek Formation, which I identified with the Arbat sequence [8, 211—216]¹). In turn, in the upper horizons of the deposits of the Shignet Formation, remains of pentamerids are found, whose age cannot be younger than the uppermost Lower Silurian. It is interesting here to emphasize that the folded structures of the Kemterek Formation show an exceptional similarity to the folded forms of the Arbat sequence, forming large brachystructures in plan, the limbs of which are complicated by intense additional folding of various orders and are vigorously cleaved in a direction concordant with the strike of folding.

By contrast, the deposits of the Shignet Formation are gathered into gentle folds with dips of no more than 30—35° and are comparatively weakly cleaved. Thus, between the Kemterek and Shignet formations an angular unconformity is clearly outlined, caused by the manifestation of the latest phase of the Salair tectogenesis, which should be given the name Kemterek, provided, of course, that the Shignet Formation actually proves to be of post–Lower Silurian age, as it is accepted to be in the Western Sayan [8, 225—228].

The independence of the Kemterek phase was expressed not only in the formation of folded structures in the Arbat-Kemterek complex of deposits, but also in the emplacement of an intrusion of granodioritic composition. Plutons of this intrusion, in the form of linearly elongated bodies, are located essentially among the deposits of the Kemterek and older formations (the plutons of the Oya-Kazyrsuk watershed), with the exception of the deposits of the Arbat Formation, in the area of distribution of which plutons of this intrusion have nowhere been noted. The age of the granodiorite intrusion is established within comparatively narrow intervals of time. It is unquestionably younger than the hyperbasites of the Western Sayan and older than the diorite-diabase intrusion, whose post-Sedovian age is beyond doubt.

It may be that the profound metasomatism of the rocks of the Arbat-Kemterek complex, accompanied by the development of such new formations as chlorite, epidote and

¹) After the work had been set in type, many new materials were obtained indicating that the Arbat Formation cannot be identified with the Kemterek Formation, for in the deposits of the latter organic remains of pelycypods and trilobites were discovered, probably of Late Upper Cambrian or even Early Lower Silurian age. (In this connection it should be assumed that the Arbat Formation underwent an independent phase of tectogenesis at a time earlier than the Kemterek phase.)

more rarely actinolite and albite. However, metasomatic changes in the Arbat–Kemterek complex undoubtedly intensified also in connection with the manifestation of later intrusive activity, especially the Early Variscan diorite–diabase intrusion, whose plutons are widely distributed within this complex.

It may be supposed that, in connection with the latest tectonic movements (Early Variscan), the rocks of the complex underwent a complication of folded structures and an intensification of foliation, which assumed the most strained forms under conditions of disjunctive junctions of the complex with deposits of younger and older age.

LITERATURE

1. Atlas of the guiding forms of fossil faunas of the USSR, Cambrian I, Gos. Geolizdat, 1940.

2. Kuznetsov V. A.—New data on the geological structure of Tuva, Izv. AN SSSR, geological series, no. 5, 1946.

3. The same.—On the question of the stratigraphic position of the hyperbasites of Tuva and the Western Sayan, Third Geological Conference in Memory of Acad. M. A. Usov, Abstracts of Reports, Tomsk Polytechnic Institute, Tomsk, 1948.

4. Kuznetsov Yu. A.—Geological structure of the Abakan iron-ore deposit, Izv. Sib. otd. geol. kom., vol. VIII, issue 3, 1929.

5. Obruchev V. A.—Geology of Siberia, vols. I and II, 1935–36.

6. The same.—History of the geological investigation of Siberia, fifth period (1918–1944). Altai–Sayan Region, AN SSSR, issue IV, 1948.

7. Sivov A. G.—Upper Cambrian of the Sayan–Altai region, Proceedings of the First Scientific Conference on the Study and Development of the Productive Forces of Siberia, Tomsk State University, vol. II, 1940.

8. The same.—On the “Lower Silurian” of the Western Sayan, Izv. Tomsk. Order of the Red Banner of Labor Industrial Institute named after S. M. Kirov, vol. 62, issue 1, 1942.

9. The same and Lyubertsev A. A.—The character of mineralization along the Sayan overthrust in the Western Sayan, Vestn. Zap.-Sib. geol. upr., no. 3, 1941.

10. Sivov A. G.—The Cambrian and Precambrian of the Western Sayan, Proceedings of the Mining-Geological Institute, ZSFAN, issue 2, 1948.

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

12. The same.—The hyperbasite formation of Western Siberia, Collection dedicated to Acad. V. A. Obruchev, AN SSSR, 1939.

13. Churakov A. N.—Traces of Proterozoic glaciation in the south of central Siberia, Collection dedicated to Acad. V. A. Obruchev, AN SSSR, 1938.

14. Hausen H. The upper Yenissei drainage Area (Territory of Urinkhai): Acta geogr. Societas geographica Fenniae, Helsingfors, 1927, no. 1, 67.