On the Stratigraphy of the Precambrian of the Western Sayan

A. G. Sivov

At the stratigraphic conference on the development of unified and correlation stratigraphic schemes for Central Siberia, held in 1964–1965 in Novosibirsk, the following scheme was adopted for subdividing the Precambrian formations of the Western Sayan: the Dzhebash Series below and the Akkol Formation above, of the Upper Proterozoic.

These subdivisions, which I had earlier distinguished under the names Dzhebash and Akkol formations, have been described in detail in the geological literature [7, 8, 9, etc.], so that not much can be added here to what has already been said. In the sections of the Kogdar River, at the visible base of the stratigraphic section of the Dzhebash Series, there occur predominantly ortho- and paragneisses, and higher up amphibole, garnet, garnet-disthene, staurolite crystalline schists, as well as their biotite and plagioclase varieties, with a modest participation of lens-shaped bodies of marbles and quartzites.

The described metamorphic rocks of the lower part (the Kogdar sequence) of the Dzhebash Series, attaining a thickness of more than 2 km, occur in narrow mountain-like wedges, the largest of which occupies an extensive area in the upper reaches of the Abakan River. The section of the series under consideration is capped by muscovite-sericite-albite, albite-quartz, chlorite-sericite, epidote-chlorite-sericite, and other metamorphic paraschists, enclosing beds of ferruginous quartzites of varying thickness and extent, piemontite schists, and lens-shaped bodies of marbles and quartzites. The thickness of these upper members of the Dzhebash Series, assigned by I. N. Kazakov to the Tebin Formation [1], and by A. P. Shcheglov and V. P. Korobeinikov to the Sabin Formation, exceeds 6 km. The stratigraphic relationships between the gneisses and crystalline schists of the Kogdar sequence and the paraschists of the Sabin Formation have not been firmly established. It can only be assumed that the Sabin Formation overlies the Kogdar sequence with a concealed unconformity. Therefore it is quite natural that the plutonic (Kogdar) intrusion of gneiss-granites is so far known only within the field of distribution of the Kogdar sequence.

The Akkol Formation [9, Fig. 3], which has a limited distribution in the Western Sayan, in the type section along the Akkol River, which flows into the Urgun (Urbun) River from the left, is divided into three members. The lower member is composed of albite-sericite-chlorite schists with rare lenses of dark marbles and beds of quartzites. Its thickness is about 1.6 km. The middle member is expressed by prasinite schists (glaucophane varieties are especially characteristic), interbedded with gray micaceous quartzites and containing rare intercalations of graphitized schists and marbles. Its thickness is 1.8 km.

The upper member is represented by gray, more often dark, fine-grained marbles containing ubiquitous remains of oncolites. Its thickness is up to 600 m [9, Fig. 3]. In the Kantegir graben and along the northern slope of the Western Sayan, deposits of the Akkol Formation are not distinguished.

At the same time, the stratigraphic section of the Precambrian of the Western Sayan does not end with the Akkol Formation, but is built up by subdivisions that previously were incorrectly assigned sometimes to the Ordovician, sometimes to the Cambrian. This question requires more detailed treatment.

As early as 1934, in sections of the Nizhnyaya Buiba River, I identified deposits of the Buiba Formation, which make up the main area in the basin of the upper course of the Us River [6]. Later they were studied in sections of the lower courses of the Us, B. Ury, and Yenisei rivers in the area between the B. Ury and Us rivers, on the watershed of the Us and Urgun rivers, and also in the basins of the Tepsel and Golaya rivers. Everywhere the rocks of the Buiba Formation are characterized by a remarkably uniform composition, described in detail by me in 1941 [7]. In typical form it is expressed (Fig. 1) by a thick sequence of interbedded chlorite-sericite, sericite-chlorite, calcite-chlorite, quartz-sericite-chlorite, and more rarely epidote-chlorite-sericite schists, predominantly greenish-gray and green in color. They contain fine-grained sandstones of varying thickness, as well as greenstone-like effusive rocks, apparently of sill facies; both the latter and the former rocks, being intensely schistose, have a bottle-green color.

Probably, at different stratigraphic levels of the Buiba Formation, among its green schists there appear violet siliceous-clayey varieties, reaching thicknesses of more than 200–250 m, as is seen, for example, in the sections of the lower course of the Zolotaya River (opposite the Atyz winter hut), while in the middle parts there are light-gray, slightly pinkish, medium-grained marbles. The latter, in the form of a single lens-shaped body interlayered with chlorite-sericite schists and possessing a thickness of up to 250 m, form the “Bely Kamen” cliff on the right slope of the Yenisei near the mouth of the Nizhny Khadanykh River.

For the most part, however, the aforementioned marbles occur in the form of beds of varying extent and thickness, of which 3–4 beds have been established in the upper course of the Us River and one bed in the middle course of the Tozany River, which flows into the Yenisei below the Urgun River. The appearance of the Buiba Formation would be incomplete if one did not dwell on the characteristics of the tectonic structure of its rocks. They occupy an extensive area, which apparently indicates, in general, their gentle occurrence and a small thickness, not exceeding 4–5 km. There are grounds for assuming that the deposits under consideration form a complete fold, the limbs of which are complicated by extremely compressed folds of the second and higher orders, down to crenulation and corrugation, affected by intense schistosity oriented in different directions. At the same time, however, the schistosity planes, coinciding in their strike with the main northeastern strike of the folded structures of the formation, are the most clearly expressed. Its folds of higher order, in plan and section, have an S-shaped form-

Fig. 1. Schematic geological map of the eastern part of the Western Sayan.
1—Dzhoi granitoids of the Late Devonian; 2—Akkol syenite-diorites of the Early Devonian; 3—unsubdivided variegated Devonian deposits; 4—effusive rocks and their pyroclastic derivatives of the Onisifor Formation of the lower part of the Lower Devonian; 5—the Shishtyk clastic-carbonate formation of the upper Silurian; 6—the Kerema (Shignet) sandstone-shale formation (not paleontologically characterized) of the Upper Ordovician–Lower Silurian; 7—the Kemterek conglomerate-shale formation of the upper Cambrian–Lower Ordovician; 8—granitoids of Upper Monok age; 9—hyperbasites of the upper Late Precambrian (?); 10—the Chingin spilite-diabase-chert formation of the upper Late Precambrian; 11–13—Early Precambrian; 11—the Buiba Formation of chlorite-sericite schists; 12—the Akkol Formation of metamorphic rocks; 13—the Dzhebash Series of gneisses and crystalline rocks; 14—faults; 15—stratigraphic boundaries; 16—lines of sections: 1—along the Akkol River, 2—along the Lower Buiba, 3—along the Yenisei, 4—along the Bolshoi Ur, 5–6—along the Chinge and Middle Akkol, 7—along Sukhoy Log, 8—along the Nizhnii Karbulyk River, 9—along the Us River, 10—along the watershed between the Urgun–Us rivers; 11—along the right tributary of the Tikhaya River; 12—along the Bolshoi Khailyk River.

... to which are confined or gently rounded; their hinges are bordered by thin discontinuous lenses of quartz of calcite–albite composition. In addition, milky-white quartz and pinkish-gray calcite constantly occur also in the form of stringers from 1 mm to 5 cm thick, though, to be sure, of extremely variable extent.

In certain features of metamorphism (intense sericitization, chloritization), and chiefly in the characteristics of the tectonic structure, the deposits of the Buiba Formation show a striking similarity to the upper subdivision of the Dzhebash Series, to which they have often been assigned. Most often, however, accumulations of the Buiba Formation, on the basis of their relatively weak metamorphism, were assigned to the Shignet Formation of the upper Ordovician or lower Silurian, although it had long been known that the medium-grained sandstones of the latter, in sections of the lower course of the Us River (opposite the village of Sorzhik), sharply unconformably overlie the intensely foliated chlorite–sericite schists of the former [7]. At the same time, deposits of the Buiba Formation were at one time placed above the Kemtirek Formation [7]; later, singled out as the so-called Amyl Suite of the Middle Cambrian—for example, in the basin of the upper course of the Us River and on the right bank of the Tikhaya River—they were assigned to the Ishkin and Syutkhol suites of conditionally Upper Cambrian age: the right bank of the Yenisei below the Urgun River and the left bank of the Khemchik River at its mouth [1].

However, the Buiba Formation, being an independent stratigraphic subdivision, is not only older than the Kemtirek Formation but also older than the Chingin Formation. Indeed, along the steep Sukhoy Log, which enters the Khemchik River from the left near its mouth, deposits of the Ishkin Suite are sharply unconformably overlain by the shale–sandy–conglomeratic sequence of the Kemtirek Formation; in the dark shales of the latter, on the right slope of the Yenisei, 1.5 km below the Khemchik River, mshanids, hyolithids, trilobites, and original pelecypods were first discovered.

This fauna determines the age of the enclosing rocks as within the interval from the uppermost Upper Cambrian to the lower Ordovician. Thus, the pre–Upper Cambrian age of the Buiba Formation (the Ishkin Suite) is evident. In turn, its deposits, previously assigned to the Shignet Formation [9], along the Akkol River (a left tributary of the Urgun) take part in the structure of the northern limb of a slightly northward-overturned anticlinal fold, the central part of which is composed of actinolite–gneiss–chlorite schists enclosing a bed of quartzite of the Akkol Formation. In the section described, at the base of the Buiba Formation lies a bed of coarse-grained, green sandstone packed with fragments of the rocks on which it rests directly, i.e., of the Akkol Formation. Upsection, the basal sandstone, up to 25 m thick, is overlain by strongly foliated and intensely chloritized fine-grained sandstones, which in turn are overlain by chlorite–sericite, sericite–chlorite, and other schists typical of the Buiba Formation. They occur in almost continuous exposures in the valleys of the right tributaries of the Urgun River and the left tributaries of the Us River (Figs. 1 and 2).

At the same time, it has now been established that at a number of localities on the southeastern slope of the Western Sayan the deposits of the Buiba Formation are unconformably overlain by spilite–diabase–siliceous rocks of the Chingin Formation, the details of which I shall discuss later. Here I shall only emphasize that the rocks of the Buiba Formation arose through greenschist alteration of originally sedimentary, predominantly fine-clastic accumulations of flyschoid type. Their clastic nature is clearly established in the study of many specimens under the microscope.

Fig. 2. Schematic geological map of the Urgun–B. Khailyk rivers. 1—hyperbasites of the upper Late Precambrian (?); 2—spilite–diabase–shale rocks of the Chingin Formation of the upper Late Precambrian. Early Precambrian: 3—Suibinskaya Formation: a—chlorite–sericite, sericite–chlorite, and other schists, b—medium- and coarse-grained chloritized sandstones; 4—Akkol Formation: a—actinolite–albite–chlorite schists, b—quartzites; 5—Urgun Fault; 6—stratigraphic boundaries; 7—bedding elements.

Analysis of the accumulated material shows that in the stratotypic section along the Srednii Akkol River the Chingin Formation is expressed only by its lower beds, with a thickness of up to 2 km. As is known, they are represented by greenstone-appearing spilites, spilitized diabases, and variolites, interbedded with their pyroclastic accumulations, siliceous-clayey and carbonaceous-siliceous shales, black microquartzites, tuffaceous sandstones, tuffaceous gravelites, and one bed of light-gray bedded limestone. In the type section, the higher members of the lower part of the Chingin Formation, composed of characteristic bluish-gray thin-bedded (“ribbon”) argillites, reach a thickness of no more than 50 m [9, fig. 8]. These “ribbon” aleurolites are traced farther to the northeast, and in the middle course of the B. Khailykta River (3 km from the stratotype), with a thickness of 200 m, together with the overlying siliceous-clayey shales, aleurolites, aphanitic diabases, variolites, and their pyroclastics, with a modest participation of microquartzites, they cap the section of the lower parts of the Chingin Formation, whose total thickness may be estimated at 4–4.5 km and which, possibly, should be combined into the Khalyk sequence.

In sections of the same B. Khailyk River, the Khailyk sequence is overlain by an effusive-sedimentary sequence of somewhat different composition, up to 2 km thick and deserving a special geographic name (the Karakhem, or Malokhailyk, sequence). At the base of the Karakhem sequence there are thick packets of siliceous-clayey shales (up to 500 m), replaced stratigraphically higher by spilitized diabases and their pyroclastics, interbedded with felsites, keratophyres and their pyroclastic accumulations, very rarely with microquartzites, carbonaceous-siliceous shales, sandstones, and enclosing lens-shaped bodies of limestones with remains of oncolites. Limestones of this upper sequence of the Chingin Formation are especially common in the Karabulyuk syncline (N. Karabulyuk River), where they contain not only oncolites but also catagraphs [9, fig. 10].

The clastic-effusive deposits of the Elekhem–Temirsuk rivers, which in their time were assigned to the Chingin Formation [9], belong to it far from in full. In any case, the sandstones, conglomerates, gravelites, siliceous-clayey shales with rare sheets of subalkaline diabases, lenses of limestones with remains of algae and archaeocyaths (Fig. 1), widespread on the divide of the Temirsuk and Elekhem rivers (Mount Karatag), represent an independent isochronous unit—apparently the Ezhim suite [2].

The conglomerates contain pebbles of greenstone effusives, keratophyres, dark limestones, and black microquartzites—rocks so characteristic of the Chingin Formation. It is quite possible that stratigraphic analogues of the Ezhim suite are also developed on the northern slope of the Kurtushibin Range, if one takes into account that in the limestones occurring among the clastic deposits of the Uzunarg Ridge and the Karabuluk syncline, and assigned to the Chingin Formation, poorly preserved archaeocyaths were found [9, 4, 5]. This question requires further study; for the time being, the volume of the Chingin Formation should be determined by the two subdivisions described above (for which their former names are not retained here—the Maloshushenskaya and Kyzasskaya suites—but new geographical names are given: the Khailyk and Kara-Khin strata [9, Fig. 1]). Of course, other proposals connected with the subdivision of the Chingin Formation may also be advanced; but in any case it is an older formation than the Ezhim suite of the lower Aldan stage.

The Chingin Formation on the northern slope of the Kurtushibin Range is separated for an enormous distance from the Buiba Formation by the Urgun (Urbun) thrust (fault), and only in separate areas does the first of them unconformably overlap the latter. Thus, for example, on the right side of the upper course of the Urgun River near the mouth of its left tributary, the B. Khailyk River, the Chingin Formation, through a basal member, directly overlies the tightly folded rocks of the Buiba Formation. The thickness of the basal member, composed of breccia of sedimentary origin and consisting of small fragments of the rocks underlying it (chlorite-sericite, sericite-chlorite, and other shales), does not exceed several cm. It is overlain by siliceous-clayey shales and tuffs, which stratigraphically upward are replaced by spilitized diabases (Fig. 2). Then, according to observations by A. P. Shcheglov and V. P. Korobeinikov in the basin of the upper course of the Tikhaya River (left tributary of the Us River), the basal member of the Chingin Formation, sharply unconformably overlying the Buiba Formation (“the Amyly suite of the Middle Cambrian”), reaches considerable thickness and has a complex structure (Fig. 3). Finally, the unconformable occurrence of the Chingin Formation on deposits of the Buiba Formation has been established in the headwaters of the Khemterek River (left tributary of the Yenisei) and in the upper reaches of the Us River. In light of the circumstances set forth, it becomes evident that, contrary to the opinion of a number of researchers [5, 4, 12], the Chingin Formation lies unconformably on the Akkol Formation; moreover, in addition to what was earlier set forth on the question under discussion [9], this unconformity is typically stratigraphic.

The poor study of organic remains of the Chingin Formation of the Kurtushibin Range precludes the possibility of precisely determining its age, and here, by analogy with its distribution on the northern slope of the Western Sayan, it is established within the limits of the Late Precambrian.

On the northern slope of the Western Sayan, the volume of the Chingin Formation has also been considerably clarified. From its composition have been excluded the variegated and gray-colored, predominantly various-sized clastic deposits that take part in the structure, respectively, of the sections of the Naryssa River (9, Fig. 16), the middle course of the Chekhan River (9, Fig. 19; 10,

Fig. 3. Sketch map of the geological structure of the right slopes of the Tikhaya River near the Prokop’evskii mine. Early Precambrian: Buiba Formation (1–2): chlorite-sericite (1) and other schists (2); Late Precambrian: Chinginskaya Formation (3–7): 3—siliceous-argillaceous schists with a sandstone bed (a), 4—chlorite-sericite schists; 5—limestone, 6—tuffaceous rocks, 7—carbonaceous-siliceous schists; 8—undifferentiated Devonian deposits; 9—disjunctive fault; 10—bedding elements.

Fig. 5), the B. Kyzas River (9, figs. 20 and 21), and constituting members of the Kyzas (Narysa) sequence. It has been proved that the clastic rocks of this subdivision contain, in large quantities, pebbles, grains, and fragments of greenstone effusives of the basic series, microquartzites, albitophyres, light-colored limestones, carbonaceous-siliceous schists, and other rocks characteristic of the Chinginskaya Formation. This means that the clastic rocks of the Kyzas sequence accumulated through the erosion of rocks of the Chinginskaya Formation and, consequently, the presence of a break between the stratigraphic subdivisions under consideration should not give rise to objections. I shall add to what has been said that, throughout the entire belt of distribution of the Kyzas sequence, its clastic rocks contain...

lens-shaped limestone bodies of varying thickness, which are concentrated in the greatest quantity in the Chekhan belt (along the Karasuk River). In the latter belt, moreover, rare flows of spilitized diabases appear, and in the Narys belt—pyroxene porphyrites. In the above-mentioned limestones, remains of oncolites and, more rarely, algae have been found almost everywhere; according to Bezymyannyi Creek, a tributary of the Bolshoi Kyzas River, these proved to belong to the genus Osagia and to the species Renalcis granosus Vologd. [9, fig. 20].

Thus, by distinguishing the Kyzas unit as a special subdivision of the flyschoid-formation type, the volume of the Chinginskaya Formation on the northern slope of the Western Sayan is restricted to that assemblage of effusive–microquartzite–siliceous-shale rocks enclosing lens-shaped limestone bodies, which occurs, for example, in sections of the Malyi Shush River (stratotype of the Maloshushenskaya Formation), along the Yenisei [9, figs. 14, 15, 17], the Tabat and Nazaryk rivers, the Chekhan and other river systems. The base of the Chinginskaya Formation of the northern slope of the Western Sayan is not exposed, for it is cut off by the Maloshushensky thrust [9], along which crystalline formations of the Dzhebash Series of the Early Precambrian have been thrust over it for an enormous distance. The upper boundary of the Chinginskaya Formation is clearly established at the base of the Kyzas unit of the uppermost Late Precambrian or lowermost Cambrian, the details of which, unfortunately, at present cannot be strictly determined.

The organic remains of this unit have been studied far from adequately, and its stratigraphic relations with the formations of the spilite–keratophyre Lower Monok Formation, at the top of which limestones with the Kameshkovka complex of organic remains occur [9, 11], require clarification. However, irrespective of the solution of these complex problems of the stratigraphy of the Western Sayan, the Chinginskaya Formation, widespread on its northern slope, cannot be younger than Late Precambrian. This is substantiated not only by stratigraphic but also by paleontological evidence. Indeed, along the Fedorkin Creek (a tributary of the Chazyryk River), in gray limestones [9, fig. 2], and on the left slope of the Yenisei, 70 m north of the Uy River, in their light-gray, bedded varieties [9, figs. 17 and 22], lying at the top of the Chinginskaya Formation in the volume developed here, remains of stromatolites and catagraphia have recently been found. In the first locality the following have been identified among them: Sinzasophyton ussovi Pospel., Palaeomicrocystis kaizasensis Posp., P. cf. usasensis Posp., Vesicularites cf. flexuosus Reitl., Bugojulia sp., Algatactis frolovi Posp., A. cf. kabirsaensis Posp.; and in the second, Radiosus radius Z. Zhurav., Vermiculites tortuosus Reitl., Glebosites glebosites Z. Zhur., Sinzasophyton usovi Posp., Osagia sp.

The Late Precambrian age of the fossils listed is entirely obvious; moreover, their composition may prove close to that of the problematics of the Karakhem unit of the Chinginskaya Formation on the northern slope of the Kurtushibinsky Range, and especially to the upper part of the carbonate section of the Late Precambrian on the southwestern slope of the Kuznetsk Alatau.

Thus, the Chinginskaya Formation of the Late Precambrian on the northern and southern slopes of the Western Sayan, in the volume specified here, although expressed with differing completeness of the stratigraphic section, is everywhere characterized by remains of catagraphia and oncolites; it lies below the base of the Lower Cambrian and, possibly, even of the Vendian (Kyzas Formation), and on the northern slope of the Kurtushibinsky Range it unconformably overlies deposits of the Buiba Formation. In this connection, the latter, as well as the metamorphic rocks of the Akkol Formation underlying them, will probably represent links of the upper Early Precambrian. This is not contradicted by the primitive appearance of the oncolites, which are abundant...

…also the fine-grained marbles of the Akkol Formation. Its age is recognized as younger than the age of the gneisses and crystalline schists of the Dzhebash Series, which apparently corresponds to the lower parts of the Early Precambrian in the Western Sayan.

It is true that, on the basis of the potassium–argon method, the absolute age of 12 samples of crystalline schists of the Dzhebash Series has been determined within the range of 933—583—329 million years. But it is obvious only that the cited figures for absolute age reflect not the relative geological age of the crystalline (pre-gneiss) schists of the Dzhebash Series, but successive superimposed metamorphism.

A comparison of the stratigraphic sections of the Precambrian of the southwest of the Kuznetsk Alatau (Gornaya Shoria) and of the Western Sayan shows that the carbonate–effusive–clastic subdivisions of the Late Precambrian, characterized by a variety of problems in the first region, correspond completely to the Chinginsk spilite–siliceous formation of the second region. In the southwest of the Kuznetsk Alatau, the lower subdivisions of the Late Precambrian, on the basis of the presence in their deposits of catagraphs and stromatolites, correspond to the upper half of the middle part (the upper parts of the Middle Riphean) of the Upper Proterozoic. In that case, the subdivisions of the Early Precambrian of the Western Sayan will be correlative with the stratigraphic interval of the lower half of the middle part of the Upper Proterozoic (the Buibinsk Formation) and, possibly, the Middle Proterozoic (the Dzhebash Series).

The following circumstances also attract attention. In the pre-Chinginsk deposits of the Western Sayan, including the Buibinsk Formation, small and at the same time rare plutons of hyperbasites are known. They are concentrated mainly in the area of distribution of the Chinginsk Formation, where their plutons sometimes attain considerable extent, and are completely absent from deposits of the type of the Kyzasskaya sequence of the Vendian–Lower Cambrian. This means that the age of the hyperbasites of the Western Sayan and, possibly, Tuva may prove to be Late Precambrian rather than Middle Cambrian, as was previously believed. Suffice it to say that in the clastic rocks of the Sanashtykol Horizon of the uppermost Lower or lowermost Middle Cambrian of the Western Sayan and Tuva there are fragments of pebbles, grains of serpentinites, chromites, pyroxenites, and other varieties of hyperbasites.

In conclusion, we emphasize that much attention will continue to be devoted to the question of the stratigraphy of the Precambrian of the Western Sayan; but in resolving it, one cannot ignore the fact that the volume of the Buibinsk, Akkol, and Chinginsk formations exceeds the volume of suites, and that the latter and the next-to-last formations were once erroneously recognized as facies analogues of the Lower Monok and Upper Monok formations [4].

REFERENCES

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