ON THE FORMATION OF SILICEOUS SHALES IN ANCIENT STRATA OF THE SOUTHWESTERN SPURS OF THE KUZNETSK ALATAU

L. V. PESHEKHONOV

(Presented by the Interdepartmental Seminar)

Among the ancient rocks participating in the geological structure of the southwestern spurs of the Kuznetsk Alatau, a special place belongs to siliceous shales, which have long attracted the attention of many researchers. Serious attention was first directed to them by A. N. Churakov, who wrote at the time: “I foresee that these rocks will play an enormous role in the recognition of the stratigraphy and even in the study of the tectonics of the Siberian Proterozoic over very large areas” [6]. Without doubting the sedimentary origin of the siliceous shales, A. N. Churakov regarded them as deposits of silica formed organically in cold waters, the low temperature of which was caused by the onset of glaciation.

In recent years, the organogenic origin of siliceous shales has begun to be supported by finds of siliceous microorganisms, which, to some extent, shook the existing assumption among a number of researchers concerning the metasomatic origin of siliceous shales. Both proponents of a sedimentary-organogenic origin and proponents of a metasomatic origin of siliceous shales substantiated their assumptions on the basis of the characteristic association they observed between siliceous shales and carbonate rocks. It was emphasized that siliceous shales are usually confined to the upper parts of sections of carbonate rocks of the Late Proterozoic (Yenisei suite, series).

At the same time, the characteristic association of silica with volcanogenic formations has long been known. N. S. Shatsky, considering the question of volcanogenic-sedimentary manganese-bearing formations, gave a good summary of the distribution of silica connected with volcanogenic formations [7]. He distinguished the following formations of siliceous deposits, conditioned by basic volcanism in eugeosynclinal zones: jasper, siliceous-shale, and distal siliceous.

In studying the stratigraphic position and material composition of the rocks of the spilite-diabase formation of the Cambrian-Proterozoic, widely developed in the vicinity of the settlements of Kamzas, Talon, and Taimet, the author observed a close spatial connection between volcanogenic formations and siliceous shales. The siliceous shales usually either occur at the base of a stratum of volcanogenic rocks, or replace lava flows along the strike, often alternating with them (Fig. 1).

Within the area described, siliceous shales are usually black, very dense rocks, often displaying a regular fine lamination. The black varieties of siliceous shales, in areas only slightly removed from volcanogenic formations, alternate with interbeds of brownish-gray and lilac-gray siliceous shales, with gray, light-gray calcareous-clayey and siliceous-clayey shales. In all types of siliceous shales, the coloration depends on the quantitative proportions in them of limonite, chlorite, and carbonaceous matter. The form of regular beds with a sharp rectilinear edge, separating siliceous shales that differ in color, indicates their undoubtedly sedimentary formation. The close spatial connection noted above between the siliceous shales and the volcanogenic formations of the spilite-diabase formation of the area described also testifies to the presence of a genetic connection between these rocks, different in material composition and formation. Analyzing the factual material on the relationship between siliceous shales and volcanogenic rocks, we are inclined to regard the very large amount of silica that went into the formation of the siliceous shales as a product of submarine volcanic activity and of post-volcanic phenomena connected with it.

Fig. 1. Diagram of the relationship of the principal rock types of the spilite-diabase formation and siliceous shales of the Kamzas area. 1— alluvium; 2—carbonate-shale deposits of the Adiak Formation of the Lower Cambrian; 3—granites, granodiorites; 4—diorites, quartz diorites; 5–9—deposits of the spilite-diabase formation of Cambrian–Proterozoic age: 5—siliceous shales, rare lenses of limestones; 6—limestones with algae and archaeocyaths; 7—albitized diabases and diabase porphyrites; 8—albitized basalt porphyrites and spilites; 9—tuffs, tuff breccias, lava breccias; 10—dolomites of the West Siberian Formation of the Late Precambrian; 11—rupture faults; 12—elements of bedding; 13—a) archaeocyaths, b) algae.

The close connection between the siliceous and volcanogenic products indicates the precipitation of silica directly near volcanic centers and permits the complex of siliceous rocks to be regarded as an analogue of the siliceous-shale formation distinguished by N. S. Shatsky [7]. On the basis of the predominant position of siliceous shales at the base of the sections of the formation described, it may be assumed that the process of silica removal was most intense during the initial stages of submarine volcanism.

At present it is difficult to say by what route silica was supplied to the marine basin. It should only be supposed that, in the case of ...

In this a definite role was played by volcanic hydrotherms [3], whose activity preceded active volcanic processes that caused the formation of lava and pyroclastic rocks. It is likely that siliceous shales are mainly chemical precipitates of colloidal silica. It is not excluded that part of the precipitated silica consists of organogenic deposits, since it is logical to assume that an increase in the silica content in the water caused, as a consequence, an abundance of siliceous microorganisms; this is confirmed by the discovery in siliceous formations of rare sponge spicules.

In addition to the facts noted above, the volcanic nature of the silica is indicated by the presence of a single spectrum of trace elements characteristic both of the effusive rocks themselves and of the siliceous shales. The trace elements were determined by the method of semiquantitative spectral analysis. According to the data of spectral analysis, it has been established that the following are characteristic of volcanogenic rocks and siliceous shales: Zr, Ti, Mn, V, Cr, Zn, Ni, Co, Ga, Ba, Sr. The similarity of the trace-element spectra of volcanogenic rocks and siliceous shales is especially clearly seen against the background of the trace-element spectrum of carbonate rocks, which are found in sections of the volcanogenic rocks of the spilite-diabase formation. The formation of carbonate rocks occurred during stages of relative waning of volcanism, and the trace elements in them are represented only by: Ti, Mn, Cu, Ba, and Sr.

Along with siliceous shales that formed in immediate proximity to volcanogenic rocks, deposits of silica should also be noted that formed in the region described at a considerable distance from the main centers of volcanism. In such cases no direct connection of the siliceous shales with volcanogenic rocks is observed, and they are quite comparable with the remote siliceous formation distinguished by N. S. Shatsky [7]. To these we assign those siliceous and siliceous-clayey shales that occur in the upper parts of sections of Late Precambrian carbonate rocks. Similar formations have been noted on the left bank of the Mrassa River above the mouth of the Kolzas River, in the phosphorite-bearing suite of the Belkino deposit, and in the area of the settlement of Ust-Kabyrza—the shale and Pyzas suites (formations) of K. V. Radugin [5].

The explanation of the origin of siliceous shales set forth above naturally leads to the supposition that desilicification of magma took place before its outpouring and final solidification in the form of lava flows, which was probably due to intracameral differentiation of the magma. The intensive removal of silica from volcanic centers as a result of desilicification of magma was first indicated by G. A. Kaleda in his characterization of Devonian siliceous rocks and the porphyries, diabases, and tuffs synchronous with them in the Tien Shan [4].

For confirmation of the idea expressed concerning the role of volcanism in the formation of siliceous rocks, studies devoted to the geochemistry of silica in modern seas are of great importance. Thus, in the article by S. V. Bruevich [2], an increase in the silica content is indicated in the waters of the Atlantic through the Indian to the Pacific Ocean—an area of modern volcanism. P. L. Bezrukov gives new data on the distribution of silica in the Sea of Okhotsk and on the confinement of its maximum contents to areas of modern volcanism [1].

Conclusions: 1. The siliceous shales of the ancient strata of the southwestern margin of the Kuznetsk Alatau are a synchronous volcanogenic formation that formed under submarine conditions.

1. The bulk of the siliceous shales consists chiefly of chemical precipitates of silica, brought out by volcanic hydrotherms before the eruption and solidification of magma of basic composition.

2. Establishing a genetic connection between siliceous and volcanogenic deposits makes it possible, over large areas, to correlate isolated sections of ancient strata represented by carbonate–siliceous and volcanogenic–siliceous deposits, whose siliceous formations may be regarded as marker horizons.

REFERENCES

1. P. L. Bezrukov. On the distribution and rate of accumulation of siliceous sediments in the Sea of Okhotsk. Dokl. AN SSSR, vol. 103, No. 3, 1955.

2. S. V. Bruevich. On the geochemistry of silicon in the sea. Izv. AN SSSR, geological series, No. 4, 1953.

3. G. S. Dzotsenidze. The influence of volcanism on the formation of sediments. Nedra Publishing House, Moscow, 1965.

4. G. A. Kaleda. Periodization of the geological history of silica. In: Problems of the Mineralogy of Sedimentary Formations, book 3–4, Lviv State University, 1956.

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

6. A. N. Churakov. The Kuznetsk Alatau. Publishing House of the Academy of Sciences of the USSR, Leningrad, 1932.

7. N. S. Shatskii. On manganese-bearing formations and the metallogeny of manganese. Izv. AN SSSR, geological series, No. 4, 1954.