ON THE PRECAMBRIAN STRATIGRAPHY OF THE UST-KABYRZA DISTRICT OF MOUNTAIN SHORIA

L. V. PESHEKHONOV

(Presented by Professor A. G. Sivov)

Over the long history of studying the geological structure of Mountain Shoria, several schemes have been proposed for the stratigraphic subdivision of the Precambrian of this region; these differ in the terminology used, in the interpretation of the age of the subdivisions, and in their scope and sequence [2, 4, 5, 6, 7].

In the present article the author summarizes the results of his investigations in recent years on the study of the Precambrian of the Ust-Kabyrza district, within which, over a comparatively small area, the oldest rocks of Mountain Shoria—varied in material composition—are distributed. These investigations were accompanied by detailed geological mapping, study of the lithological composition of the deposits, and collection of fossils. As a result of these works, new materials were obtained that made it possible to introduce certain changes into the previously proposed stratigraphic schemes of the Precambrian of the Ust-Kabyrza district; the most important of these are given in Table 1.

In the Ust-Kabyrza district of Mountain Shoria the oldest (Proterozoic) rocks, in the author’s opinion, are siliceous, terrigenous, and carbonate rocks that participate in the structure of a complex system of anticlinal folds spatially confined to the valley of the Mrassu River (Fig. 1). As a whole, the listed rocks form a single sequence characterized by sharp variability of lithological composition both upward through the section and along strike. Nevertheless, it has been possible to subdivide it, on the basis of lithological features, into a number of members that can be traced over the greater part of the area of their distribution. The most important of these, in the section of the sequence situated on the right bank of the Mrassu River immediately above the mouth of the Unushkol Creek, are, in stratigraphic order from bottom to top, the following.

1. A member of massive, more rarely bedded limestones of dark-gray and black color, containing from 1% to 5% admixture of dolomite. In the lower part of the member, among the limestones, there are thin (from 3 mm to 2 cm) black siliceous interbeds and lenses, as well as siliceous bodies of ellipsoidal and spherical form which, according to V. A. Shipitsin’s determination, are problematic formations of the Jusenia type. In the upper part of the member of these limestones (Sample 347 in Fig. 1), the author collected specimens with the algae Neclandia concentrica Walk., some of which were transferred to the Paleontological Institute. These fossils, as well as the problematic formations Pseudoconus

Table 1

Stratigraphic schemes of the Precambrian deposits of the Ust-Kabyrza district of Mountain Shoria, compiled by various authors

Fig. 1. Structural-geological map of the Ust-Kabyrza district.

1. Recent alluvial deposits of river valleys. 2. Variegated terrigenous deposits of the Kabyrza graben. Middle Devonian. 3. Limestones and shales with Epiphyton. Lower Cambrian. 4. Siliceous-terrigenous deposits of the Pyzas (shale) suite. Lower Cambrian? 5. Oncolitic and massive limestones of the Belka suite. Upper Proterozoic. 6. Stromatolitic dolomites of the Zapadno-Kabyrza suite. Upper Proterozoic. 7, 8. Siliceous-limestone deposits of the Kabyrza suite (7 — siliceous shales, 8 — limestones). Middle Proterozoic. 9—11. Siliceous-terrigenous-carbonate deposits of the Unushkol suite (9 — “unusual horizons,” 10 — siliceous and siliceous-clayey shales, sandstones, and limestones, 11 — limestones with Newlandia). Middle Proterozoic. 12. Plagiogranites and granites of the Middle–Upper Cambrian. 13. Intrusive bodies of the gabbro-syenite complex (Middle–Upper Devonian, after S. S. Il’enko). 14. Contacts of unconformable bedding. 15. Faults. 16. Algal sampling points.

convexus Krasn. were discovered here by K. V. Radugin. The thickness of the member is not less than 300 m.

2. A member of thin-bedded (from 0.5 cm to 2 cm) greenish-gray siltstones and sandstones, rhythmically interbedded with grayish-white limestones, the thickness of whose interbeds does not exceed 4 cm. At one time these rocks were distinguished by K. V. Radugin as an “unusual horizon,” owing to their characteristic variegation in color, lithologic composition, and low degree of metamorphism [5]. Without dwelling on the details of the analysis of the lithologic composition, we note that 1.5–2 m above the base of the member, burgundy-red sandstones, siltstones, and argillites up to 15 m thick stand out especially; their coloration is due to the presence of dispersed hematite in the cement. The burgundy-red beds, oka-

are not used, have limited distribution, since during their formation the sedimentation basin abounded in shoals. This is indicated by conglomerates 0.7–1 m thick with fragments of rocks of the “unusual horizon,” occurring in place of the claret-red beds in the section of the unit described, situated on the right side of the Mrassu River 1500 m below the mouth of the Kabyrza River. The thickness of the “unusual horizon” is 70 m.

1. A member of interbedded black platy limestones, clay shales, marls, and greenish-gray graywacke sandstones. The thickness of the interbeds of the listed rock types is 20 cm–1.5 m. The total thickness of the member is not less than 180 m.

2. A member of thin-bedded greenish-gray siltstones and sandstones, rhythmically interbedded with grayish-white limestones, the interbeds of which are 1–5 cm thick. Overall, this member of rocks has a close resemblance to the “unusual horizon.” In its typical expression it is represented in sections situated on the right side of the Mrassu River above the mouth of the Pyzas Stream and on the right side of the mouth area of the latter. The thickness of the member is 80 m.

3. A member of black massive, more rarely thin-bedded limestones, interbedded with black siliceous shales. In the samples of these limestones collected by us, V. A. Shipitsyn discovered the microphytolites Vesicularites sp. The thickness of the member is about 250 m.

4. A member of greenish-gray and dark-gray thin-bedded clayey, siltstone-clayey, and siliceous-clayey shales and limestones. According to data of the geologists of the Shalym Expedition, N. V. Russkikh and M. G. Dvoeglazova, the siliceous-clayey shales contain phosphorite concretions with a \(P_2O_5\) content of up to 25%. The thickness of the member is about 35 m.

5. A member of black massive and platy limestones, in which K. V. Radugin discovered Collenia sp. The thickness of the member in the section described is incomplete—60 m. In sections of the unit situated on the right side of the mouth area of the Pyzas Stream and on the left side of the Mrassu River 1.5 km above the mouth of the Kabyrza Stream, the thickness of this member of limestones is not less than 150 m.

6. A member of black siliceous shales up to 100 m thick. These rocks are the uppermost subdivision of the siliceous-terrigenous-limestone unit.

In tracing the distinguished subdivisions it has been established that within members 5, 6, 7, and 8 the role of carbonate rocks increases from north to south. Thus, in sections situated along the Mrassu River above the mouth of the Kabyrza Stream, the upper subdivisions of the unit described are represented chiefly by massive, less commonly platy, dark-gray and black limestones and marbles. From north to south the thickness of the deposits also changes. If the total thickness of the rocks lying above the second “unusual horizon” (member 4) in the northern sections does not exceed 500–550 m, then in the southern sections it increases to 1 km. The change in the material composition and thicknesses of the deposits from north to south is explained by differences in the facies in which the upper subdivisions of the siliceous-terrigenous-limestone unit were formed. In the northern part of the area the sedimentation basin abounded in shoals. Under conditions of frequent and repeated changes in the sign of oscillatory movements of the basin floor, a condensed type of section of rocks of variegated composition was formed, abounding in numerous obvious and hidden breaks and traces of erosion. In the southern part of the area the basin was deeper-water. Oscillatory movements of its floor were not accompanied by any substantial change of facies, owing to which limestone sediments of increased thickness accumulated here continuously.

In concluding the characterization of the siliceous-terrigenous-limestone

We note only that, at the first stages in the study of the geological structure of the Ust-Kabyrza district, a number of researchers regarded the deposits described as Cambrian [1]. After the work of K. V. Radugin [5], the opinion that they are Proterozoic in age would seem to have become established. However, even in the 1950s individual researchers assigned these rocks to the Cambrian [3]. Only detailed geological mapping carried out in recent years in connection with the discovery of phosphorites in Gornaya Shoria has confirmed K. V. Radugin’s conclusions concerning the Precambrian age of the siliceous–terrigenous–limestone sequence [6, 7]. K. V. Radugin divided this sequence into two formations (suites) of different ages: the Kabyrza Formation, of Middle Proterozoic age, and the Unushkol Formation, of Upper Proterozoic age. He included in the Kabyrza Formation the limestones exposed along the Mrassu River above the mouth of the Kabyrza River. The siliceous–terrigenous–carbonate units distributed along the Mrassu River north of the mouth of the Kabyrza River were described by him as part of the Unushkol Formation. At the same time K. V. Radugin believed that the Unushkol and Kabyrza formations form monoclinally bedded sequences plunging respectively to the ENE and WSW. In the works of other researchers [6, 7], the Unushkol and Kabyrza deposits were considered as part of a single monoclinally bedded sequence and were regarded as coeval. By detailed geological mapping, the author established that the Unushkol deposits participate in the structure of an anticlinal fold, the axis of which plunges to the southeast (Fig. 1). The northeastern limb of the anticline has normal bedding and plunges to the ENE at an angle of 55°–70°. The southwestern limb of the anticline has a steep (up to 90°) dip and even a slight overturning to the southwest. Within the area of distribution of the Kabyrza limestones, a similar anticline is recognized, with normal bedding of the eastern limb and a slightly overturned western limb. Given the relationship of the rocks of the siliceous–terrigenous–limestone sequence shown in Fig. 1, the author considers it expedient to draw the boundary between the Unushkol and Kabyrza formations at the boundary where terrigenous–carbonate rocks are replaced by carbonate rocks, namely at the top of the second “unusual horizon.” In this case the thickness of the Unushkol Formation proves to be about 650 m, approximately, since its base is nowhere exposed. The thickness of the Kabyrza Formation is 0.6 km in the northern sections and not less than 1 km in the southern sections.

Stratigraphically above the Kabyrza Formation lies a sequence of stromatolitic dolomites of the Zapadnosibirka Formation. The lower boundary of the formation is exposed in the stratotype section, situated on the right bank of the Mrassu River 7.5 km above the mouth of the Kabyrza River. Here the black limestones of the Kabyrza Formation, interbedding with dolomitic limestones, are gradually replaced by stromatolitic dolomites of dark-gray and light-gray color. The dolomites participate in the structure of the Olun syncline, the axis of which plunges eastward, while the southern limb is overturned to the north. The thickness of the formation in this section is not less than 1800 m. An analogous relationship of the Zapadnosibirka and Kabyrza formations is established on the left bank of the Mrassu River, 1500 m above the mouth of the Kabyrza River, with overturned bedding. In the Zapadnosibirka Formation, K. V. Radugin and P. S. Krasnopeeva found: Algastroma elbrus Rad., Kabyrsina grandiformis Krasn., Sibirephicus mrassiensis Krasn., Sibirephicus fibratum Krasn., Newlandiella schoria Krasn., Newlandiella sola Krasn. According to A. G. Poselov’s data, the dolomites contain: Osagia, Archaeospongia, Palaeomicrocystis, Sinzasophyton [4]. From our collection of dolomites, V. A. Shchipitsyna identified Vesicularites lobatus Reitl., Vesicularites sp., Glebosites sp., Glebosites cf. gentilis Reitl.

The last stratigraphic unit that is undoubtedly of Precambrian age is the Belka Formation. It is represented by dark-gray

oncolitic and light-gray massive limestones. Together with the dolomites of the Zapadnosibirskaya Formation, the deposits of the Belkinskaya Formation participate in the structure of the Aizas synclinal, located in the basin of the Aizas River (a right tributary of the Pyzas River). The nature of the contact of the Belkinskaya Formation with the Zapadnosibirskaya has not been precisely established. Apparently, the rocks of the Belkinskaya Formation lie conformably on the dolomites of the Zapadnosibirskaya Formation. This supposition is supported by the presence at the base of the former of rocks of transitional type, represented by dolomitic limestones. The thickness of the Belkinskaya Formation is 1000–1200 m.

Above the rocks described lies the Pyzasskaya Formation, which is represented by gray and greenish-gray graywacke sandstones interbedded with dark-gray argillaceous and black siliceous shales and tuffaceous sandstones. In subordinate amounts there occur thin beds of dark-gray and black limestones and calcareous sandstones. On the whole the formation is rhythmically bedded. The bedding is most often thin. The thickness of the beds is 3–10 cm. The rocks of the Pyzasskaya Formation are separated from the underlying ones by an unconformity and a hiatus. This is indicated by the fact that, in the section along the Pyzas River, its deposits lie on the oncolitic and massive limestones of the Belkinskaya Formation, while on the right side of the Mrassu River they lie on the limestones of the Kabyrzinskaya Formation (Fig. 1). To this it should be added that the fine-pebble conglomerates occurring in the lower part of the formation described contain fragments of the underlying rocks. The total thickness of the Pyzasskaya Formation is not less than 2 km. In the siliceous shales of the formation described, distributed on the right side of the Mrassu River (the shale formation according to K. V. Radugin), according to A. G. Pospelov, there occur Protospongia sp., Chancelloria sp. These data on the paleontological characterization of the formation are insufficient to speak definitively about its age. Therefore its Early Cambrian age is determined by us conditionally, from the relation of its deposits to unquestionably Precambrian formations.

In conclusion, we note that reliably Lower Cambrian deposits in the Ust-Kabyrza district were not previously known. In recent years the author has discovered, on the left side of the Kabyrza River, dark-gray and black sandy-argillaceous and carbonate-argillaceous shales and calcareous sandstones and gravelites. The latter contain fragments of light-gray limestones, in which the author found epiphytons (outcrop 324 in Fig. 1). It is possible that these deposits are the oldest formations of the Lower Cambrian division in Mountain Shoria.

REFERENCES

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2. M. K. Vinkman, A. B. Gintsinger, L. I. Egorova. Reference sections of the Lower Cambrian and Sinian of Mountain Shoria and the Gorny Altai. Soviet Geology, No. 12, 1962.

3. A. L. Dodin. Stratigraphy of the Kuznetsk Alatau and Mountain Shoria. In: Iron-ore deposits of the Altai-Sayan mountain region. Vol. 1, Book 2. Publishing House of the Academy of Sciences of the USSR, Moscow, 1959.

4. A. G. Pospelov, Yu. S. Nadler. Paleontological substantiation of the age of bedded phosphorites of Mountain Shoria. In: Phosphorites of Western Siberia. Materials on the Geology of Western Siberia, issue 65. Nedra Publishing House, Moscow, 1965.

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