On the Genesis of Precambrian Microquartzites of the Southwestern Margin of the Kuznetsk Alatau

V. A. Sivov

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

“I foresee that these rocks will play an enormous role in recognizing the stratigraphy and even in studying the tectonics of the Siberian Proterozoic over a very large area” (A. N. Churakov, 1933).

It is well known that among the oldest deposits of the Kuznetsk Alatau the most characteristic rocks are the so-called microquartzites. However, for many years the question of their genesis and their position in the section of these deposits was resolved differently by various investigators.

Thus, for example, as early as the 1930s A. N. Churakov [5], recognizing the microquartzites as marine sediments of colloidal silica containing an admixture of organic silica, considered them important marker elements of the upper parts of the Yenisei suite of the Middle Proterozoic that he distinguished, composed predominantly of carbonate rocks. At approximately the same time G. P. Bolgov and A. G. Sivov [1] expressed the opinion that microquartzites are rocks more characteristic of the Cambrian than of the Precambrian, and distinguished among them two types, namely: dark, thin-bedded microquartzites of sedimentary origin and light-colored, massive silicified rocks of metasomatic genesis.

Later M. A. Usov [4], K. V. Radugin [3, 4], and their adherents, regarding microquartzites as metasomatic formations, did not deny their Precambrian age, but noted that they occur at different stratigraphic levels of the Precambrian and are not confined to the upper parts of its Yenisei suite, as A. N. Churakov supposed.

With further study of the geological structure of the Kuznetsk Alatau and, in particular, of its southwestern regions (Gornaya Shoriya), contradictory factual material was obtained that confirmed the hypotheses mentioned above. It should be emphasized that at the present time, in connection with the discovery of bedded phosphorites in the oldest sequences of the southwestern slope of the Kuznetsk Alatau, an unambiguous resolution of the question under discussion acquires especially great importance. Indeed, the work of the geologists of the Shalym integrated geological prospecting expedition of the West Siberian Geological Administration (E. R. Frolov, V. A. Sivov, N. E. Lipin, M. S. Kostritsky) has established that the microquartzites of interest to us occur at the same (or a very close) stratigraphic level as the bedded phosphorites and, being possibly their facies analogue, represent an important prospecting criterion.

This article presents new material concerning the stratigraphic position and origin of quartzites, obtained by us as a result of two years of geological investigations accompanied by prospecting work for phosphorites in the southern part of the Kuznetsk Alatau, in the interfluve of the Kabyrza–Uzas rivers, which flow into the Mrassu River.

The geological structure of the area studied is composed chiefly of deposits of the Late Precambrian and Lower Cambrian, with a minor role played by intrusive formations, probably of Cambrian age.

Within the Late Precambrian, the Kabyrza Formation is clearly distinguished; it is composed predominantly of dark-gray bedded and massive limestones and, conformably overlying it, the West Siberian Formation, represented by algal massive, more rarely bedded, dolomites.

Stratigraphically higher, with unconformity, lie limestones containing Lower Cambrian archaeocyathids and assigned to the Usinsk Formation. As for the microquartzites, almost everywhere (the Pegem, Bazas, Synzas, Kaygazy, and Upper Uzas rivers), occurring at the tops of various members of the Late Precambrian, they are directly overlain by the Usinsk limestones of the Lower Cambrian, which rest upon the Precambrian in places with azimuthal, and in places with parallel, unconformity (Fig. 1). In fact, in the area of Mount Pegem the microquartzites are present both in distinctly bedded dark Kabyrza limestones and predominantly in massive light West Siberian dolomites, forming there a relatively persistent stratified band that rims the base of the Usinsk archaeocyathid limestones. This band, whose visible thickness ranges from 0 to 100–150 m, consists of bed-like lenses, nests, and stocks, almost always closely adjoining one another and forming a single continuous body of microquartzites. Among them, two varieties can be distinguished morphologically.

One of them, spatially confined to the Kabyrza Formation, is manifested in a fine interbedding of layers of quartzites and limestones; the contacts between them, though sharp, are sinuous, with fanciful outlines. The thickness of the microquartzite layers, characterized by fine banding, ranges from 3–5 mm to 30–50 cm and coincides with the thickness of layers of similarly banded limestones. The interbedding of limestones and microquartzites just described was mistakenly taken by some researchers to be primary bedding, which led them to the incorrect conclusion that the microquartzites were of sedimentary origin.

In reality, however, they are metasomatic formations that arose through the selective replacement by silica of the bedded limestones of the Kabyrza Formation.

This is also confirmed by the following facts. On the same Mount Pegem, in separate outcrops exposed by trenches (K-3873), a gradual lateral change is clearly established from thin layers of microquartzites first into silicified and then into ordinary thin-bedded limestones. Moreover, along the strike of the microquartzite layers, short lenses and nests of them often appear, in which relics of limestone not replaced by silica are constantly found.

A completely analogous picture is also observed when tracing the microquartzite layers down dip. Thus, for example, in some outcrops reaching considerable height, their hypsometrically upper parts are expressed by typical microquartzites, which, when traced down dip and hypsometrically lower, gradually pass into limestones.

Another circumstance also attracts attention. At the southwestern end of trench 3873 there is a distinct interbedding of the bedded Kabyrza limestones and microquartzites. From the latter, which possess clear relict bedding coinciding with the bedding of the limestone (azimuth of dip of bedding 30–60°), thin “stringers” sometimes branch off, extending along the schistosity of the limestone (azimuth

Fig. 1. Schematic geological plan of the vicinity of Mount Perem:
1. Usinskaya Formation of Lower Cambrian limestones. Late Precambrian:
2 — accumulations of microquartzites of unclear genesis; 3 — massive microquartzites;
4 — thin-bedded microquartzites; 5 — interbeddings of thin-bedded microquartzites and limestones;
6 — West Siberian Formation of dolomites; 7 — Kabyrza Formation of limestones;
8 — orientation of bedding.

of dip 340–350°). The external appearance and material composition of the beds of microquartzites and of their “stringers” are identical; moreover, the latter, following the schistosity of the limestones and cutting their bedding at an angle of 60–70°, are characterized, just like the former, by relict banding.

As for the second variety of quartzites, it is formed mainly through metasomatic replacement by silica of massive West Siberian dolomites and is distinguished above all on the basis of its massive appearance. True, in the interfluve of the Synzas and Kaigaza rivers, where the Lower Cambrian overlaps bedded dolomites, bedding is also observed in the microquartzites associated with them, but it is clearly relict.

At the same time, these and other varieties of microquartzites most often have a gray, dark-gray, or black color and a cryptocrystalline structure. Their mineralogical composition is limited to quartz and carbonaceous matter, with rare participation of unreplaced siliceous-carbonate rock. The dark color of the microquartzite is due to the presence of the finest dust of carbonaceous matter, the coloring pigment of which is sometimes distributed uniformly in the rock described, but more often is collected in discontinuous bands that determine its layered texture. It is characteristic that accumulations of carbonate mass are confined to places where the carbonaceous matter is most highly developed. When interacting with ammonium molybdate, the massive varieties of microquartzites show a reaction for \(P_2O_5\).

As was stated above, microquartzites in the region studied are widely distributed and, being everywhere spatially confined to Precambrian carbonate rocks, are everywhere typically metasomatic formations that arose, in the opinion of M. A. Usov, as a result of hydrothermal processes connected with some magmatic activity (4). It was noted earlier that in the

Fig. 2. Schematic geological structure of the Bazas (I) and Upper Uzas (II) areas: 1. syenite-diorites and gabbro of the Uzas pluton of Cambrian age; 2—Ulutarga Formation of clastic rocks of the lower part of the Middle Cambrian; 3—Usa Formation of limestones of the Lower Cambrian. Late Precambrian; 4—phosphorites and phosphate rocks; 5—microquartzites; 6—West Siberian Formation of dolomites; 7—Kabyrza Formation of bedded limestones; 8—faults. Location of organic remains; 9—archeocyathids; 10—algae; 11—archeosolenids; 12—orientation of bedding.

territory of the Kabyrza-Uzas region, among magmatic rocks, intrusive formations are relatively widespread, forming several large and small plutons of unquestionably post-Precambrian age.

It is sufficient to say that in the lower reaches of the Uzas River the Uzas syenite-diorite pluton cuts Precambrian rocks, including metasomatic quartzites, as well as undoubtedly Lower Cambrian deposits (Fig. 2).

Consequently, the intrusive activity of the region was not accompanied by silicification of the Precambrian carbonate deposits; rather, it, po-

apparently, was generated by exogenic processes. This question is independent and requires special discussion, since here it is resolved only as a first approximation to reality. It may be considered proven that long before the accumulation of the Lower Cambrian deposits, the predominantly carbonate rocks of the Late Precambrian of the southwestern part of the Kuznetsk Alatau had been folded, schistosed, brought out from below sea level, and then, under conditions of a continental regime, subjected to intense silicification. It apparently extended over considerable areas, but at the same time is unlikely to have penetrated to a depth of more than 150 m from the surface of the Precambrian land. In any case, in the modern denudation section of the southwestern slope of the Kuznetsk Alatau, the concentration of metasomatic microquartzites among the rocks of the Late Precambrian decreases sharply inward from the base of the carbonate Lower Cambrian (Fig. 1).

The time of formation of the microquartzites under consideration, which apparently represent residual-metasomatic formations of an ancient weathering crust, is established as being within the limits of the end of the Precambrian or the beginning of the Cambrian.

As evidence of this we cite the following facts. On the left bank of the Mrassy River, 1.5 km south of its left tributary, the Kolzas River, a narrow belt of argillaceous-siliceous schists of the Ust-Kolzas Formation extends; at their base lies a sedimentary breccia filled with fragments of West Siberian dolomites, with only a modest participation of microquartzites. On Mount Petem, immediately adjoining the base of archaeocyathid limestones of the Usinsk Formation, there occurs a peculiar breccia consisting entirely of coarse blocks of microquartzites. They closely adjoin one another, sometimes exceed a meter in cross section, and have brownish-yellow, light-yellow, and light-brown coloration caused by the presence of iron oxides.

The aforementioned “breccia” merges so closely with the underlying bedrock outcrops of microquartzites that its lower boundary can be established only conditionally. The origin of the described “breccia” is not entirely clear. Possibly it is an eluvial formation that arose through the disintegration of quartzites under the conditions of the continental regime of the Late Precambrian and was later buried beneath the level of the Lower Cambrian Usinsk sea transgressing onto the land. Its deposits, documented in the geological record as archaeocyathid limestones of the Usinsk Formation, formed considerably later than the deposits of the Ust-Kolzas Formation, of end-Precambrian or beginning-Cambrian age. Thus, in light of the circumstances set forth, it becomes obvious that the microquartzites of the southwestern margin of the Kuznetsk Alatau—and, possibly, of its entire territory—are metasomatic formations that arose as a result of exogenic silicification of Late Precambrian rocks, completed by the beginning of Ust-Kolzas time at the end of the Precambrian or the beginning of the Cambrian.

True, in the geological literature cases are noted of the presence of interbeds of microquartzites also among carbonate rocks of the Lower Cambrian (for example, in the Karchitsk limestones), but their genesis is unclear and, moreover, they have a limited distribution.

In conclusion it is necessary to emphasize that A. N. Churakov’s prediction of the great role of microquartzites in solving problems of Precambrian stratigraphy and tectonics, at least for the Kuznetsk Alatau, has been confirmed by the practice of subsequent geological investigations in its territory.

References

1. G. I. Bolgov, A. G. Sivov. New data on the stratigraphy of the ancient Paleozoic of the southeastern slope of the Kuznetsk Alatau. Problems of Soviet Geology, No. 9, 1933.

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

3. K. V. Radugin. Geological sketch of the Chemal sheet of the Gorny Altai. Publishing House of the Academy of Sciences of the USSR, “Gorny Altai,” 1941.

4. M. A. Usov. Phases and cycles of tectogenesis of the West Siberian Territory. Publishing House of the ZSTG, Tomsk, 1936.

5. A. N. Churakov. The Kuznetsk Alatau. The history of its geological development and its geochemical epochs. Publishing House of the Academy of Sciences of the USSR, 1932.