ON THE STRATIGRAPHY OF THE PRECAMBRIAN IN THE SOUTHWESTERN PART OF THE KUZNETSK ALATAU

L. V. PESHEKHONOV

(Presented by Prof. A. G. Sivov)

This article presents some new materials on the stratigraphy of Precambrian deposits in one of the districts of the southwestern part of the Kuznetsk Alatau, situated within the watershed area between the Mrassu and Lebed river systems, west of the Kamzas gold-mining district.

The first information on the stratigraphy of this district belongs to A. M. Kuzmin [2]. According to his ideas, within the area described, Precambrian metamorphic formations are widespread and are subdivided into three suites: the graywacke–argillaceous-schist suite—$M_1$, the limestone suite—$M_2$, and the suite of greenstone rocks and schists—$M_3$. However, subsequent investigations by V. A. Kuznetsov and G. P. Nagorskii (1940), A. L. Dodin (1952), and V. I. Fominskii (1961) led to different interpretations of the age and sequence of formation of these deposits.

As a result of our investigations, carried out in 1964, materials were obtained that confirm A. M. Kuzmin’s views on the sequence of formation of the deposits of the district, but refine their age characterization.

In stratigraphic terms, the oldest of the Precambrian deposits of the above-mentioned district of the Kuznetsk Alatau are volcanogenic–terrigenous formations, which we combine into the Kaurchak suite. They compose large areas in the southwest and north of the district, and are also established within the field of distribution of younger carbonate sediments, forming the central parts of anticlinal structures and uplifted tectonic blocks (Fig. 1). The investigations carried out in the southwest of the district described make it possible to conclude that the lower part of the suite, up to 1500 meters thick, consists of terrigenous rocks and is unconformably overlain by conglomerates of the Atlin suite (V. I. Konsvalov, V. I. Zinoviev, WSGU, 1960) of Middle Cambrian (?) age. The upper part of the suite, up to 1000 meters thick, is composed essentially of volcanogenic formations of intermediate and basic composition.

The absence of a definite boundary at the base of the suite allows us to make only an approximate determination of the total thickness of the deposits—1500–2000 meters. Lithologically, the terrigenous formations are represented by greenish-gray graywacke sandstones and light-gray, more rarely lilac, dark to black argillaceous schists and aleuro-

lites. Sandstones are the principal component of the terrigenous formations of the suite; in grain size they range from coarse, with large rock fragments, to fine-grained. The deposits of the Kaurachak suite in the north of the area are represented by a somewhat different lithologic composition. Here, in addition to the above-mentioned greywacke sandstones and clay shales, cherry-red, black, and sealing-wax-red hematized siliceous shales are widely developed; these are probably the lowermost parts of the general section of the deposits of the Kaurachak suite. Carbonate sediments are also established among the terrigenous formations. In the southern part of the area they are represented by rare, thin, laterally discontinuous lenses of white marbled limestones up to 0.5 meter thick. Northward, the carbonate content of the suite increases owing to the appearance of rare but laterally persistent limestone beds up to 100–150 meters thick. Among the volcanogenic formations characteristic of the upper part of the Kaurachak suite, greenstone plagioclase porphyrites predominate, interbedded with hornblende, quartz-plagioclase porphyrites, tuffs, and tuffites of intermediate composition. Both the lower and upper subdivisions of the suite have everywhere undergone dynamometamorphism, expressed in intense schistosity and partial recrystallization of the primary formations. Moreover, in terrigenous rocks the schistosity for the most part conforms to the bedding and only in places immediately adjacent to porphyrite bodies does it follow their outlines. The intense dynamometamorphism is explained by the dislocation that affected the entire complex of Precambrian formations. As a result, they prove to be everywhere crumpled into steep folds of predominantly meridional strike. The western limbs of the folds dip very steeply (80–90°) to the west and southwest; the eastern limbs of the folds have a less steep dip (65–75°) to the east and northeast. The manifestation of low-temperature regional metamorphism, accompanied by the formation of chlorite-sericite, chlorite-actinolite, quartz-chlorite-actinolite, chlorite-actinolite-epidote schists and amphibolites, is directly connected with the intense dislocation.

Stratigraphically above the deposits of the Kaurachak suite lies a thick dolomite sequence. It occupies the entire central part of the area described, forming a belt of meridional strike up to 2000 m wide in the south and up to 5000 m in the north. Among the dolomites, light-gray, gray, dark-gray fine- and medium-crystalline varieties are noted. Oolitic, brecciated, and silicified varieties of dolomites occur more rarely, but everywhere. A characteristic feature of the dolomite sequence is the increased calcareousness of its base. This is expressed everywhere in the presence of beds of limestones and calcareous dolomites up to 200 m thick. It should be noted that the number and thickness of the limestone beds increase from south to north. A similar feature is also noted in the characterization of the deposits of the Kaurachak suite. The relationship of the dolomite sequence with the underlying deposits of the Kaurachak suite is established in the southwestern part of the area described. Here the site of the presumed contact was exposed by trench No. 12, made by the Talon prospecting-and-mapping party in 1964 (Figs. 1, 2). As can be seen in the sketch of the trench presented, between the terrigenous deposits of the Kaurachak suite and the dolomites there is a transitional unit up to 10 m thick. In the composition of the transitional unit a gradual replacement of terrigenous sediments by dolomites is established, expressed in an alternation of interbeds of green clay shales 0.5–1.0 m thick with interbeds of dolomites up to 1 m thick. The entire com-

Section along A–B

Fig. 1. Schematic geological map of the watershed area of the Mrassu and Lebed’ rivers:
1. Alluvial deposits of river valleys. 2. Conglomerates of the Atlin Formation \((Cm_2)\). 3. Sakharovo limestones \((Cm_1^3)\). 4. Limestones and marls of the Adiak Formation \((Cm_1^2)\). 5. Limestones with algae and archaeocyaths of the P’yankovo Formation \((Cm_1^1)\). 6. Diabases, diabase, and basaltic porphyrites of the P’yankovo Formation \((Cm_1^1)\). 7. Dolomites of the Zapanibirsk Formation \((Prz_3^2)\). 8. Metamorphosed volcanic formations of the Kaurak Formation \((Prz_3^2)\). 9. Metamorphosed terrigenous deposits of the Kaurak Formation \((Prz_3^1)\). 10. Diorites, plagiogranites \((Cm_{2-3})\). 11. Zones of fault dislocations. 12. Intrusive and stratigraphically conformable boundaries. 13. Stratigraphically unconformable boundaries. 14. Elements of bedding attitude. 15. Localities where paleontological remains were collected: a) algae, b) archaeocyaths.

complex of rocks of the transitional member dips steeply eastward (80–85°) beneath the dolomite sequence. Similar relationships of these two different lithological varieties are observed in a number of other places, which makes it possible to conclude that the dolomite sequence conformably overlies the deposits of the Kaurchak Formation.

The most complete section of the dolomite sequence is exposed in the south of the area. Here, throughout its extent from west to east, the dolomites dip homoclinally eastward at a steep angle of dip, varying within 65–85°. A characteristic feature of the dolomite sequence is the increasing importance, from south to north, of additional second-order dislocation, which complicates the monoclinal occurrence of the sequence. From south to north there increases not only the number but also the size of folds of a higher order, characterized by northwestern, northeastern, and even latitudinal strikes. Directly connected with this factor is the increase in the width of the belt of dolomite formations in the north of the area to 5000 m. Therefore, to determine the total thickness of the sequence, a section in the southern part of the dolomite belt was chosen (Fig. 1), where the thickness of the dolomite formations is determined as 2000 m.

Determination of the age of the dolomite sequence is still possible only provisionally—on the basis of algal complexes of various groups. According to determinations by V. A. Shipitsin (Tomsk Polytechnic Institute), catagraphs represented by the groups Vesicularites, Vermiculites, Glebosites, and Nubecularites are widely developed in the dolomites. Among them the following have been identified: Vesicularites forma indet., Vesicularites cf. concretus Z. Zhur., Vesicularites cf. flexuosus Reitl., Vesicularites cf. lobatus Reitl., Vermiculites angularis Reitl., Nubecularites cf. uniformis Z. Zhur., Glebosites forma indet., Glebosites gentilis Z. Zhur.

Fig. 2. Sketch of the northwestern wall and floor of trench No. 12.
1. Soil-vegetation layer. 2. Dolomites of the transitional horizon. 3. Green clayey shales of the transitional horizon. 4. Graywacke sandstones of the Kaurchak Formation.

Recently a monograph by Z. A. Zhuravleva [1] has appeared, in which the author attempts to use oncolites and catagraphs for the biostratigraphic subdivision of Precambrian deposits of Siberia. Comparing the above list of catagraphs with the complexes of cata-

...graphs identified in the work of Z. A. Zhuravleva, it is difficult to arrive at an unambiguous conclusion about the age of formation of the dolomites, since among the catagraphs from the dolomite sequence of our area there occur, in equal numbers, catagraphs assigned by Z. A. Zhuravleva to the Middle Riphean, Upper Riphean, and Vendian complexes. This can be explained by the imperfection of the proposed scheme for subdividing the Precambrian according to oncolites and catagraphs. Therefore we provisionally assign the dolomite sequence and, in accordance with it, the underlying Kaurchak Formation to the Upper Proterozoic deposits.

The dolomites complete the section of the Precambrian deposits of the area described. Thus, from the stratigraphic review presented above it is clear that the Precambrian section is represented here by a shale–carbonate type and thereby differs from the substantially carbonate Precambrian section of the Siberian group of formations of K. V. Radugin [3]. According to A. K. Mkrtchyan’s collections from the dolomites of the “West Siberian” Formation along the Kaigyzy River, a tributary of the Kabyrza River, V. A. Shipitsyn identified the catagraphs Vesicularites flexuosus Reitl., Glebosites forma indet., which are characteristic of the dolomite sequence of our area and of the dolomites of the Tarzhul Formation on the eastern slope of the Kuznetsk Alatau. These facts allow us to correlate the dolomites described above with the dolomites of the Tarzhul and “West Siberian” Formation of the Kaigyzy River. Taking into account that the dolomites of the West Siberian Formation are underlain without a break by the limestones of the Kabyrza Formation, we are inclined to regard the deposits of the Kurchak Formation as a stratigraphic analogue of the Kabyrza Formation. The sharply different lithological composition of these two formations is explained by the different facies setting in which the sediments were formed. This also explains why the limestones of the Kabyrza Formation, in the direction toward the area described, are gradually replaced by terrigenous deposits of the Kaurchak Formation and, already within this area, almost completely disappear.

Stratigraphically above the Precambrian deposits, with a break, lies a thick sequence (up to 3000 m in the eastern part of the area) of effusive rocks of the Pyankovo Formation (Fig. 1), represented chiefly by diabases, diabase and basalt porphyrites, with subordinate participation of pyroclastic products and terrigenous and carbonate rocks. These formations differ from the volcanogenic rocks of the Kaurchak Formation in their more basic material composition and weak degree of metamorphism, expressed only in diagenetic alteration of the effusives. In the upper part of the Pyankovo Formation, among basalt porphyrites, there occur light-gray limestones up to 500 m thick, among which the author collected a poor assemblage of archaeocyaths and algae (Fig. 1). V. D. Alimova (Tomsk Polytechnic Institute) identified from the archaeocyaths: Archaeolynthus sp., Coscinocyathus sp., Ajacicyathus proskuriakovi Toll., Ajacicyathus sp., Tumulocyatthus sp., Archaeofungia sp.; from the algae V. A. Shipitsyn identified: Epiphyton sp., Girvanella sibirica Maslov., Girvanella sp. nov. Botominella et densata Schip., Subtifloria sp., Vologdinelleae sp.

It should be borne in mind that at the stratigraphic meeting held in 1965 in Novosibirsk it was recognized that the limestones with archaeocyaths, occurring above the effusive sequence (Kamzas area), were assigned to the Ust-Kundat horizon. Obviously, the archaeocyath limestones occurring within the effusives of the Pyankovo Formation will be older than the Ust-Kundat horizon.

Analysis of the lithological composition and organic remains of the deposits participating in the geological structure of the southwestern part of the Kuznetsk Alatau permits the following conclusions to be drawn.

1. The Precambrian of the area is represented by two strata differing in lithological composition, which formed in the course of a single continuous process of Upper Proterozoic sedimentation.

2. The shale–carbonate type of Precambrian section in our area, toward the north, gradually acquires the features of the carbonate type of section of the Siberian group of the Precambrian formation, and toward the south gradually assumes the appearance of the West Sayan effusive-shale strata.

3. The presence of archaeocyaths and algae in limestones occurring among the volcanogenic formations of the Pyankovo suite indicates the undoubtedly Lower Cambrian age of this stratum.

REFERENCES

1. Z. A. Zhuravleva. Oncolites and catagraphs of the Riphean and Lower Cambrian of Siberia and their stratigraphic significance. Tr. GIN AN SSSR, issue 14, Nauka Publishing House, Moscow, 1964.

2. A. M. Kuzmin. A brief report on geological investigations in the upper part of the basin of the Lebed River, a right tributary of the Biya River. Izv. Sib. otdeleniya Geol. kom., vol. III, issue 4, Tomsk, 1924.

3. K. V. Radugin. Elements of the stratigraphy and tectonics of Gornaya Shoria. Materials on the Geology of the West Siberian Territory, issue 37, West Siberian Geological Trust, 1936.