ON THE STRATIGRAPHY OF THE PRECAMBRIAN CARBONATE COMPLEX OF GORNAYA SHORIA

V. A. Sivov

(Presented by Prof., Dr. K. V. Radugin)

At present, most geologists [1, 3] studying the Precambrian carbonate complex of Gornaya Shoria distinguish three formations within it (from bottom to top): the Kabyrza, West Siberian, and Belkinskaya formations. In doing so, the principal criterion for distinguishing these formations is considered to be their lithological composition. Thus, the Kabyrza Formation includes all predominantly limestone Precambrian units; the West Siberian Formation, predominantly dolomitic units; and the Belkinskaya Formation, limestone–dolomite–phosphate units.

As a result of our work, and also of an analysis of the factual material of prospecting and mapping parties of the Shalym Expedition of the West Siberian Geological Administration, new data have been obtained indicating a more complex character of the section of the Precambrian carbonate complex of Gornaya Shoria. The presentation of these materials is the purpose of the present article. It should be noted that in one of the author’s papers [4] materials are given on the basis of which the unconformable occurrence of the Belkinskaya Formation on the underlying deposits and its Vendian–Cambrian age are established. In this connection, the Belkinskaya Formation is not considered here.

The continuous section of the Precambrian carbonate complex is most fully exposed along the Kichi, Purla, and Pyzas rivers. From southwest to northeast along line 1—1 (Fig. 1) it is represented as follows (from bottom to top):

1. Dark-gray, gray, rarely yellowish-gray limestones, fine-crystalline, massive, in places bedded, with rare interbeds of dark-gray fine-crystalline dolomites. The bedded varieties are confined to the lower part. The dip azimuth of the bedding varies from 15° to 30°, and the dip angles from 50 to 70°. Thickness—1900 m.

2. Dark-gray and gray dolomites, fine-crystalline, massive, less commonly bedded (dip azimuth 60—70°, dip angle 70—80°), with interbeds of dark-gray fine-crystalline limestones, concentrated in the middle part. Thickness—1500 m.

3. Dark-gray and gray fine-crystalline limestones, distinctly bedded. The dip azimuth of the bedding is 60—70°, the dip angle 60—70°. In the lower part of the member, the rocks are intensely silicified, up to the formation of secondary quartzites. Thickness—1470 m.

1. Gray dolomites, fine- and more rarely fine-crystalline, massive, with an interbed of gray fine-crystalline bedded dolomitic limestones. Thickness — 1100 m.

2. Dark-gray fine-crystalline massive limestones, rarely bedded, oolitic in the lower part. Thickness — 1300 m.

3. Gray fine-crystalline banded dolomites, owing to subparallel elongate stromatolite-like formations. Azimuth of dip of the banding 30—40°, angle — 60—80°. Thickness — 1150 m.

4. Dark-gray, more rarely gray, fine-crystalline bedded limestones, cut by veinlets of white calcite. The bedding is distinct; its attitude elements are generally persistent. The azimuth of dip varies

Fig. 1. Geological map of the Pyzas River basin: 1 — Middle Cambrian — sandstones and siltstones; 2—5 — Lower Cambrian; 2 — tuffaceous sandstones and clay shales; 3 — effusives and their tuffs; 4 — undivided carbonate complex; 5 — phosphate limestones, dolomites, and phosphorites of the Belkino Formation; 6—8 — Upper Proterozoic; 6 — dolomites; 7 — limestones; 8 — oolitic limestones; 9 — diorites; 10 — granites, granodiorites; 11 — bedded phosphorites; 12 — faults; 13 — attitude elements of bedding

within 40—60°, dip angle 60—80°. In the middle part of the unit, oolitic varieties of limestones are widely distributed. Thickness — 1300 m.

1. Gray to dark-gray fine-crystalline dolomites, banded owing to subparallel elongate stromatolite-like formations. The orientation of the banding is not persistent. Its dip azimuth changes from 220° to 10—40°, dip angle 75—80°. Thickness — 650 m.

2. Dark-gray, more rarely gray, fine-crystalline limestones, cut by veinlets of white calcite, with interbeds of gray dolomites. Dark-gray limestones are predominantly widespread. They have a fine-crystalline structure and a massive texture. Bedded and banded varieties occur rarely. Azimuth of dip

bedding varies from 40 to 75°. In the middle part of the sequence a bed of oolitic limestones with an apparent thickness of 100–120 m has been established. Thin beds of dolomites are confined to its lower parts. In the contact zones between them and the limestones, a distinct alternation of limestone and dolomite layers 1 to 2 cm thick is observed. Thickness—1500 m.

1. A sequence of dark-gray dolomites and limestones alternating with one another. The dolomites are confined to the lower and upper parts of the sequence, whereas the limestone beds tend toward its middle. The texture of the dolomites and limestones is usually massive, but locally their layered and patterned-banded structure is observed. The latter is due to the presence of small inclusions of gray and white calcite with highly irregular outlines. Locally these inclusions are arranged as subparallel-oriented bands. The attitudes of these bands coincide with the bedding and are: dip azimuth 45–60°, dip angle 65–80°. In the middle part of the sequence a dolomite bed with large (up to 20–40 cm in diameter) spherical concretions, probably of organic origin, has been established. Limestones containing rounded and ellipsoidal inclusions of anthraconite, resembling archaeosponge, are also confined to this part. Thickness—950 m.

2. Dark-gray, fine-crystalline, massive, locally bedded and patterned-banded limestones with rare interbeds of dark-gray dolomites. The dip azimuth of the bedding is 60–65°, the dip angle 70–75°. In a northwestern direction this sequence is rapidly, but gradually, replaced by dark-gray patterned dolomites. Thickness—150 m. (For all the sequences distinguished, their apparent thickness is given.)

The contact parts of the limestone and dolomite sequences of the described section were studied between members 11–10, 10–9, 6–5, 2–1. In all the indicated cases a close alternation of limestones and dolomites was established, indicating the continuity of the sedimentation process. The remaining contacts between the sequences remained unexposed. It is not excluded that in a number of places they will prove to be disjunctive, which, in our opinion, accounts for the considerable increase in thickness of the section presented.

The upper part of the described section, corresponding to members 9, 10, 11 and including its higher fragments, was studied along strike to the south, along line II—II (Fig. 1), where it is exposed in numerous mine workings and rocky outcrops of the Mrassu River. Its section from southwest to northeast is as follows (from bottom to top):

1. Dark-gray, more rarely gray, fine-crystalline, massive, locally bedded limestones. Thickness—1000 m.

2. Gray, fine-crystalline dolomites, massive, locally patterned and patterned-banded. Thickness—1600 m.

3. Light-gray, more rarely gray and dark-gray, banded, fine-crystalline limestones with interbeds of dark-gray fine-crystalline massive and patterned dolomites. In the lower part of the sequence there is a bed, 100–150 m thick, of greenish-gray siliceous shales. Thickness—2200 m.

4. Gray, fine-crystalline dolomites, massive and banded, with rare interbeds of dark-gray fine-crystalline limestones. Thickness—1100 m.

5. Dark-gray, more rarely gray, fine-crystalline limestones, massive and bedded. The dip azimuth of the bedding varies from 40 to 90°, the dip angle from 60 to 80°. In the middle part of the member a bed of gray oolitic limestones has been established, traced for a distance of 10–12 km. Thickness—3100 m.

1. Gray dolomites, more rarely dark-gray to black, finely and microcrystalline, with rare interbeds and lenses of dark-gray finely and microcrystalline limestones, usually composed of anthraconite. Thickness—2200 m.

2. Dark-gray and gray finely and microcrystalline limestones, thin-bedded, massive, mottled, and oolitic. Thickness—1360 m. (Stratotype of the Kabyrza Formation according to K. V. Radugin.) The oolitic limestones are confined mainly to the lower part of the unit, where they form a member about 500 m thick. In the upper part of the unit, archaeocyathid varieties of limestones are widely distributed.

3. Dark-gray and gray finely and fine-grained dolomites and dolomitic limestones, massive, in places thick-bedded, owing to subparallel-oriented stromatolite-like formations of Algostroma (stratotype of the West Siberian Formation according to K. V. Radugin). Siliceous varieties of dolomites are confined to the upper part of the unit; stratigraphically higher they are succeeded by a thin (10 m) interbed of weakly phosphatic limestones. Thickness—660 m. At the contact of the West Siberian dolomites with the Kabyrza limestones, a completely gradual lithological transition and parallelism of bedding elements are observed, indicating their conformable occurrence (the thickness for members 1–7 is apparent).

As indicated above, all the limestone and dolomite units of the Precambrian of Gornaya Shoria are at present combined, respectively, into the Kabyrza and West Siberian formations. However, from the characterization given, it is evident that, along with their similarity, a whole series of differences is established in the texture and color of the limestone, as well as the dolomite, units among themselves. Indeed, in some cases they are predominantly massive, in others—bedded, and in still others—banded. These differences, together with bedding elements indicating a persistent monoclinal dip of the rocks of the carbonate complex to the northeast, provide grounds for assuming that the distinguished limestone, as well as dolomite, units are of different ages relative to one another. Such a conclusion is reliably confirmed by the following. When all Precambrian limestones are combined into the Kabyrza Formation, and the dolomites into the West Siberian Formation, the construction of unnaturally complex arc-shaped folded structures (Fig. 1) is inevitable, the axes of which change their strike by 90 degrees and more.

From the foregoing it follows that the composition of the Precambrian carbonate complex of Gornaya Shoria is not limited only to the Kabyrza and West Siberian formations. Below them lies a thick series of carbonate sediments, composed of limestone and dolomite units that gradually, but rapidly, replace one another. Each of these units may subsequently be distinguished as an independent formation. But this must be preceded by a careful and complete study of their lithological composition and paleontological remains. In conclusion it should be noted that the conclusions of K. V. Radugin [2] on the enormous thickness of the carbonate Proterozoic of Gornaya Shoria are confirmed by our materials.

REFERENCES

1. M. K. Vinkman, V. A. Astashkin, B. G. Kraevsky. Scheme of subdivision of the Precambrian and Cambrian deposits of the Kuznetsk Alatau, Gornaya Shoria, Salair, and Gorny Altai. Tr. SNIIGTiMS, issue 29, 1964.

2. K. V. Radugin. Elements of the stratigraphy and tectonics of Gornaya Shoria. Mat. po geologii Zapadno-Sibirskogo kraya, issue 37, 1936.

3. G. I. Spandarshvili. Phosphorites of Gornaya Shoria. In: Phosphorites of Western Siberia, “Nedra,” 1965.

4. V. A. Sivov. New data on the geology and regularities of phosphate-ore occurrence in the Gorno-Shoria region. In: Phosphorite-bearing formations of Siberia, Trudy SNIIGTiMS, issue 69, 1968.