LITHOLOGICAL CHARACTERISTICS AND FACIES VARIABILITY OF THE JURASSIC DEPOSITS OF THE RYBINSK DEPRESSION

V. L. Kokunov

(Presented by Professor A. V. Aksarin)

Jurassic coal-bearing deposits cover the Paleozoic formations of the Rybinsk Depression almost as a continuous mantle and occupy about 70% of its territory.

The study of the coal-bearing Jurassic deposits of the Rybinsk Depression, known since the time of P. S. Pallas (1788), E. O. Hofmann (1844), I. D. Chersky (1888), and others, has been carried out by various organizations up to the present day, since the Jurassic contains seams of brown coal of very great thickness (N. F. Ryabokon, K. L. Kokhanchik, 1962).

The thickest coal seam—the Borodino seam (up to 45 m)—lies at a depth of 15–35 m from the ground surface and is an extremely convenient object for low-cost open-pit mining.

The most detailed lithological description of the rocks composing the Jurassic section and of their mineralogical composition was made by L. I. Bocharnikova (1946), and more recently by A. A. Semerikov (1961) and Yu. P. Kazansky (1962).

Our studies of the rocks of the Jurassic coal-bearing sequence under the microscope, in thin sections and in immersion, confirmed the mineralogical complexes distinguished by the authors mentioned, characteristic of the various Jurassic horizons; at the same time, it must be noted that mineralogical analysis of clayey rocks, which we carried out using an electron microscope in combination with staining of the rocks with MG, is being performed for this region for the first time.

In the territory of the Rybinsk Depression, which in Jurassic time was a piedmont basin (Kokunov, 1961) and which A. A. Semerikov (1961) assigned to the intermontane external trough of the Siberian Platform, Jurassic deposits with conglomerates at the base rest on the eroded surface of Paleozoic rocks and are represented by the Lower and Middle divisions of the Jurassic.

The basis for subdividing the sediments of the Jurassic system into divisions was mainly the identifications of vegetative plant remains made by A. V. Aksarin (1957), and the identifications of spores and pollen made by N. S. Sakhanova (1957). Moreover, N. S. Sakhanova-Grigoryeva made an attempt to distinguish guiding complexes of spores and pollen even for individual suites, but the complexes distinguished for the suites possess common forms and differ only slightly in the percentage content of one or another species of spores or pollen, which, in our opinion, is not a sufficiently substantiated position, since the influence of an element of chance is not excluded.

At the same time, the distinctive geological structure of the depression (the presence of two flat troughs separated by the Uyar swell, Middle Jurassic—

rocks, in which they are distributed as separate patches) makes it difficult to carry out a sufficiently reliable correlation of these deposits; this is also greatly favored by the sharp facies variability of the coal-bearing rocks. On the appended lithologic-facies map of the rocks of the roof of the Borodino coal seam (Fig. 1), covering a comparatively short interval of time, facies differing in the dynamics of the depositional environment stand out sharply. In particular, processing of grain-size-analysis data for sandstones and the morphological types of bedding recorded in them make it possible to distinguish within them typically channel facies, floodplain facies, and even eolian facies; analysis of the morphological types of bedding in siltstones and argillites indicates their lacustrine origin. It should not be forgotten that, in constructing this map, only a one-meter-thick interval of rocks occurring in the roof of the Borodino seam was used.

Naturally, in developing a stratigraphic scheme for the Jurassic deposits of the Rybinsk depression, A. V. Aksarin, as early as 1956, had

Fig. 1. Lithologic-facies map of the roof of the Borodino coal seam. Legend: 1—channel facies, 2—lacustrine facies, 3—swamp facies, 4—burned rocks, 5—floodplain facies, 6—eolian facies.

the cyclicity of the structure of the coal-bearing sequence was used; its peculiarity consists in the fact that the Jurassic section is made up of successively repeated sets of rocks, from conglomerates or sandstones, then siltstones, and up to coal seams. This scheme was approved as a unified one at an interdepartmental conference in Leningrad in 1956, but the questions of identifying the coal seams within it were not resolved, and until recently it was difficult even to judge whether the Moshchnyi coal seam, developed in the Balai trough, corresponds to the Borodino seam, widespread in the Borodino trough, to say nothing of low-thickness coal seams.

All this compelled us to analyze thoroughly all the material at our disposal on the character of changes in the thicknesses and facies of rocks; on the character of changes in the thicknesses and petrographic composition of coal seams; on the character of changes in the mineralogical and granulometric compositions of the rocks of the coal-bearing sequence, including electron-microscopic study of clays and argillites, from the standpoint of the tectono-stratigraphic method (L. L. Khalfin, 1959).

The character of changes in the thicknesses of coal-bearing deposits and coal seams makes it possible to distinguish at least three zones in the coal-bearing field: a zone of maximum Jurassic thicknesses with numerous thin coal seams, confined to the southern margin of the depression (the Sayano-Partizansk synclinal zone); a zone of maximum Jurassic thicknesses with numerous coal seams, among which there are seams of very great thickness, confined to the northern margin of the depression (the Borodino synclinal zone); and, occupying an intermediate position between them, a zone of reduced thicknesses of coal-bearing deposits with a reduced number of coal seams (the Balai synclinal zone).

The listed zones also differ from one another in the granulometric composition of the rocks. Thus, the Sayano-Partizansk zone is characterized by the presence of conglomerates at the base of the Jurassic and at the base of each cycle of sediment accumulation.

In the Balai synclinal zone, conglomerates occur only at the base of the Jurassic, and even then they are very fine-pebble, passing into gravelites.

In the Borodino synclinal zone there are no conglomerates in the Jurassic section.

The features of the listed zones are also well reflected in the mineralogical composition of the clayey rocks. Thus, if the clayey rocks of the Pereyaslovka Formation are characterized by a hydromica–kaolinite association of clay minerals, represented by 70–75% perfectly faceted crystals of kaolinite, with no differentiation by zones being felt, then the clayey rocks of the overlying formations are characterized by zonality of the following kind.

In the Borodino synclinal zone, the clayey rocks are characterized by a marked predominance of hydromicas, and most often a kaolinite–hydromica association of clay minerals is observed, in which minerals of the kaolinite group account for no more than 25%.

In the Balai synclinal zone, the clayey rocks are characterized by a predominance of kaolinite over hydromicas, i.e., a hydromica–kaolinite association of clay minerals.

A generalization of the results of electron-microscopic study of clayey rocks by formations showed that this type of analysis can quite well be used for purposes of stratigraphic correlation. In any case, the mineral composition of the clayey rocks of individual formations can be illustrated by the following ratio (see Table 1).

The resulting picture is well explained by the unity of the sources supplying clastic material, as is immediately confirmed by looking at the map we have constructed of the terrigenous character of the Jurassic deposits, on which lines of equal percentage content of sandy-

rocks (including conglomerates) in the Jurassic section. The gradual decrease in the sandiness of the deposits from the margins of the Rybinsk depression toward the center of the Borodino synclinal zone convincingly indicates that in Jurassic time the depression was a single closed sedimentation basin with well-defined principal directions of transport of clastic material.

The conglomerates and gravelly sandstones occurring at the bases of the suites in the Sayano-Partizansk synclinal zone contain the following mineral associations:

Table 1

the same assemblage of rocks that is developed in the framing of the depression nearby; this makes it possible, on the basis of their facies character, to classify these rocks as proluvial deposits.

In the Borodino synclinal zone, the sandy rocks, in terms of granulometric composition and the morphology of bedding types, belong to typical alluvial–lacustrine–bog deposits.

In general, in the Jurassic section of the Rybinsk depression it is necessary to distinguish three facies types of deposits: the Sayano-Partizansk proluvial–alluvial–bog type, the Borodino alluvial–lacustrine–bog type, and the Balai mixed type.

Thus, all that has been stated above makes it reasonable to distinguish three structural-facies zones in the Rybinsk depression: Sayano-Partizansk, Balai, and Borodino.

REFERENCES

1. A. V. Aksarin. Scheme of the stratigraphic subdivision of the Jurassic coal-bearing deposits of the Kansk Basin. Proceedings of the Interdepartmental Conference on the Stratigraphy of Siberia. Gostoptekhizdat, 1957.

2. L. I. Bocharnikova. On the lithology of the Irsha and Borodino deposits of the Kansk Basin. 1946.

3. K. M. Vainer, N. P. Grigor’ev. On the stratigraphy of the Jurassic deposits of the Kansk depression. Problems of the Geology of Krasnoyarsk Krai. Moscow State University Press, 1964.

4. E. Gofman. On the gold placers of Eastern Siberia. Mining Journal, part IV, books 11, 12, 1844.

5. I. I. Zadkova, Yu. P. Kazanskii. Lithological-formational analysis of the Jurassic deposits of the southern margin of the Siberian Platform. Siberian Branch, USSR Academy of Sciences, 1962, Novosibirsk.

6. V. L. Kokunov. On the facies variability of the Devonian deposits of the Rybinsk depression. Proceedings of the Tomsk Polytechnic Institute, vol. 120, Tomsk State University Press, 1961.

7. P. S. Pallas. Journey through Various Provinces of the Russian Empire. Part III, St. Petersburg, 1773–1788. Russian translation by Zuev, 1788.

8. N. F. Ryabokon’, K. L. Kokhanchik, Z. F. Leibovich. Coals of Krasnoyarsk Krai. Collection “Mineral Resources of Krasnoyarsk Krai.” Publishing House of the USSR Academy of Sciences, 1962.

9. N. S. Sakhanova. Spore-pollen assemblages of the coal-bearing deposits of the Kansk Basin. Proceedings of the Interdepartmental Conference on the Stratigraphy of Siberia. Gostoptekhizdat, 1957.

10. A. A. Semerikov. On certain features of the structure of the coal-bearing deposits of the Kansk-Achinsk Basin. Collection “Coal-bearing Formations of Certain Regions of the USSR.” Publishing House of the USSR Academy of Sciences, 1961.

11. L. L. Khalfin. On the tectono-stratigraphic trend in geology and on the principles of stratigraphy. In: “The Basic Ideas of M. A. Usov in Geology.” Publishing House of the Academy of Sciences of the Kazakh SSR, 1959.

12. I. D. Cherskii. Geological investigation of the Siberian postal route. Supplement No. 2 to volume LIX of the Proceedings of the Academy of Sciences, St. Petersburg, 1888.