GEOLOGICAL AND TECTONIC PREREQUISITES FOR THE FORMATION OF COAL SEAMS OF THE TATAUROVSKOYE AND CHERNOVSKOYE DEPOSITS AND THE PROCESSES OF TRANSFORMATION OF PLANT MATTER INTO COAL

O. A. DULZON, V. L. KOKUNOV

(Presented by Professor A. V. Aksarin)

The Chernovskoye and Tataurovskoye brown-coal deposits are located in Central Transbaikalia: the Chernovskoye deposit is 20 km southwest of the city of Chita, and the Tataurovskoye deposit is 65 km southwest of the city of Chita, on the territory of the Uletovsky District of Chita Oblast.

The Chernovskoye and Tataurovskoye coal deposits are situated in the Chita–Ingoda depression, which is the largest depression of Central Transbaikalia and is bounded by the Yablonovy Range in the northwest and the Chersky Range in the southeast [9]. The Chita–Ingoda intermontane depression has a northeastern strike and is filled with Meso-Cenozoic continental sedimentary deposits.

The area of the Chernovskoye and Tataurovskoye coal deposits is composed of continental coal-bearing deposits which, according to the regional stratigraphic scheme for subdividing the continental Mesozoic formations of Transbaikalia proposed by G. G. Martinson, are divided into three formations: the Bukachachinskaya—\(I_2\), the Ulanganginskaya—\(I_3—Cr_1\), and the Turgino-Vitimskaya—\(Cr_1\) [3].

The Upper Jurassic age of the Ulanganginskaya Formation is substantiated by a complex of plant remains identified for the Chernovskoye deposit by V. A. Khakhlov [3], A. N. Krishtofovich [6], V. D. Prinada and N. N. Srebrodolskaya [3], and for the Tataurovskoye deposit by A. V. Aksarin.

A detailed petrographic description of the coals of the Chernovskoye deposit, based on exploration boreholes, was carried out by Yu. A. Zhemchuzhnikov as early as 1941.

The authors of the present work selected coal samples from operating coal cuts, mines, and exploration boreholes at the Chernovskoye coal cut and at the Tataurovskoye deposit. In all, 240 transparent thin sections and polished sections were prepared. The detailed petrographic description of the material composition of the coals of the Chernovskoye and Tataurovskoye deposits was carried out by the authors using the terminology established by GOST 9414-60 [2].

In the structure of seams III and VI of the Chernovskoye brown-coal deposit, microcomponents of five groups take part: vitrinite, semivitrinite, fusinite, liptinite, and mineral impurities. Seams III

and VI are composed of brown coal; in material and petrographic composition they are fairly uniform and consist mainly of the clarain type of coal and, in part, durain-clarain and clarain-durain types. The description of the coal types is given from the bottom upward through the section.

Seam III. The clarain type of coal is deposited under conditions of a stagnant forest swamp with a high groundwater level and is the predominant type of coal. The content of clarain coal in the seam amounts to 94.35% of the seam thickness.

The durain-clarain type of coal is representative of the facies of a forest peat swamp with weak groundwater flow. This type of coal accounts for 5.65% of the seam thickness.

Seam VI. It is composed of two coal benches. In each bench there is the clarain type of coal and the clarain-durain type. Usually each bench throughout the entire extent of seam VI of the Chernovskoye deposit ends with the clarain-durain type of coal. The distribution of both coal types through the seam thickness is almost uniform.

Seams I, II, and III of the Tataurovskoye deposit are composed only of representatives of the humite class. The Tataurovo coals include the following petrogenetic types:

clarain coal, as a representative of a stagnant peat forest swamp, characterized by a low groundwater level. Clarain coals make up from 33 to 65.4% of the coal seams, with the maximum of clarain coals observed in seam III and the minimum in seam II;

durain-clarain type of coal, as a representative of the facies of forest peat swamps with weak water flow and likewise characterized by a low level of groundwater standing. This type of coal makes up from 23 to 66.7% of the coal seams, with their maximum observed in seam II and the minimum in seam III;

clarain-durain coal, as a representative of the facies of forest peat swamps with weak water flow and likewise characterized by a low level of groundwater standing. This type of coal occurs in seams I and III, making up from 7.7 to 9.1% of the coal seams;

durain type of coal is distinguished as a representative of the facies of swampy forests and, to a lesser degree, flowing forest swamps. This type of coal is also noted only in seams I and III and has limited distribution, making up from 3.9 to 9.0% of the coal mass of the seams.

The enormous body of factual material from the study of the geological structure of coal deposits of the Soviet Union has been generalized in the works of Academician P. I. Stepanov [10], Yu. A. Zhemchuzhnikov [6], and G. F. Krasheninnikov [7]. More recently, in the fundamental works of Academician N. M. Strakhov [11], I. N. Gorsky [4], and other researchers, a common view has been set forth that the general tectonic setting that developed in the humid-climate region is the foremost factor, in its significance, in coal accumulation.

The thickness of the coal-bearing complex and the total thickness of the coal seams depend primarily on the geotectonic regime of the period of sediment accumulation. At the same time, according to G. F. Krasheninnikov [7], the Mesozoic is characterized by a predominance of coal-bearing formations of the group of intermontane and piedmont basins developed in areas of mobile platforms, with a limnic character of coal accumulation, which resulted from the manifestation of differential block movements.

It should be noted that the belt of greatest coal content of Upper Mesozoic age within the territory of the USSR encompasses precisely the eas…

its exact regions, namely: the Kansk-Achinsk, Lena, and South Yakutian basins and the deposits of Transbaikalia.

The Tataurovskoye and Chernovskoye deposits are confined to the Chita–Ingoda intermontane depression and are characterized by a fairly considerable thickness of the coal-bearing sequence and a great thickness of the coal seams, which are currently being worked by open-pit methods.

An analysis of the distribution of Mesozoic deposits with thick coal seams suitable for open-pit mining shows that such deposits are located in the immediate vicinity of major tectonic disturbances, more often confined to the marginal parts of depressions filled with coal-bearing Mesozoic sediments. Such deposits include the Korkinskoye and Emanzhelinskoye brown-coal deposits with a unique seam thickness; these deposits are confined to the Chelyabinsk graben. As our studies have shown, the Irsha-Borodinskoye, Abanskoye, Nazarovskoye, and Berezovskoye deposits of the Kansk-Achinsk basin are also characterized by the presence of major disturbances in the marginal parts of troughs to which these deposits are confined.

The Tarbagatayskoye, Kharanorskoye, Tataurovskoye, and Chernovskoye deposits are not exceptions in the list of deposits in whose formation differential block movements of the basement played the role of favorable features of the tectonic regime of coal formation [1].

All geological and geophysical data characterizing the structure of the Chita–Ingoda depression allow one to conclude that its basement consists of several tectonic blocks confined to a major fault running along the northwestern margin of the depression and to transverse disturbances branching from it. This deep-seated fault, according to M. S. Nagibina [8], bounds the Chita–Ingoda group of troughs from the northwest, causing the accumulation of coarse-clastic conglomeratic deposits near the fault-side margin, and of sandy-clayey and coal-bearing facies in the central part and along the southern margin. The conclusions of M. S. Nagibina agree with our observations on the character of the distribution of clastic material within the Tarbagatayskoye, Kharanorskoye, and Tataurovskoye deposits.

The transverse tectonic disturbances of the Chita–Ingoda depression undoubtedly continue beneath the coal-bearing formations and have left their imprint on the change in the degree of coal-bearing capacity of the section along the Chita–Ingoda depression as a result of differential movements of individual blocks. In the change in the thickness of the coal seams, the following should be noted.

During the formation of coal seams I, II, and III, the western part of the Tataurovskoye deposit, tending toward a major disturbance and being more mobile, caused the splitting of the seams in this part. This agrees with the data of Ts. O. Ochirov et al. [9], who explain the mechanism of formation of the depressions of Transbaikalia by “subsidence of their basement along major zones of disjunctive dislocations.” This also explains the structure of seams resembling “horse tails,” directed westward. In connection with the foregoing, it should be noted that the most favorable conditions for peat accumulation were located on the gentle northwestern flanks of the Chersky Range, where they were preserved for the longest time; this makes it possible to distinguish, on the flanks of the Chersky Range, a zone of merged coal seams, whereas, beginning from the center of the depression and farther toward the Yablonovy Range, a zone of split seams should be distinguished.

In general, with regard to the tectonic prerequisites, it is necessary to note that the tectonic regime in the Chitino-Ingodinskaya depression that existed during coal accumulation possessed the following specific features.

1. The basement of the Chitino-Ingodinskaya depression is characterized by a block structure and by differential tectonic movements of the blocks along ancient tectonic sutures, the consequence of which is the inherited character of the tectonic structure of the coal-bearing deposits of the depression and the discontinuous distribution of the coal seams.

2. The most intense subsidence of the basement of the depression is recorded along its northwestern margin, which tends toward a deep-seated fault of great extent. It should be noted that not only the Chernovskoye and Tataurovskoye coal deposits are confined to this fault, but also a whole series of both known and, possibly, “blind” ones.

3. The processes of peat formation undoubtedly took place during periods of relative tectonic quiescence; otherwise peat formation would have ceased, but with a tendency toward subsidence. At a rate of subsidence corresponding to the rate of accumulation of dead plants, continuous accumulation of peat bogs took place in the swamps over a prolonged time, to which the formation of coal seams I, II, and III in the section of the Tataurovskoye deposit is due.

How long the time was during which a coal seam equal in thickness to the sum of the thicknesses of seams I, II, and III would have formed can be judged from the following data.

According to Yu. A. Zhemchuzhnikov [5, 6], a peat seam, when transformed into a coal seam, undergoes a reduction in thickness by approximately 4–5 times. Consequently, if the total thickness of seams I, II, and III is \(12.2 + 8.2 + 16.6 = 37\) meters, and, taking into account the thickness of two coal lenses, approximately 45 meters, then the thickness of the original peat bog was 225 meters (\(45\ \text{m} \times 5 = 225\ \text{m}\)).

At the present time, in the bogs of the central part of the RSFSR, the layer of annually dying vegetation has a thickness from 1.7 cm to 3.2 cm, which on average is about 2.5 cm. Then this average annual increment of 2.5 cm is compacted, still at the surface, owing to further decomposition and dehydration under the load of the overlying layers, to 0.6 mm, i.e., by 40 times (\(2.5\ \text{cm} : 0.6\ \text{mm} = 40\)) [6]. Thus, the thickness of the layer of initial vegetation in our case would be \(40 \times 225 = 9000\) meters. From this one can find the time required for the formation of the coal seams of the Tataurovskoye deposit: \(9000\ \text{m} : 25\ \text{mm} = 360000\) years. If one also takes into account that the normal distance between coal seams I, II, III is 91.7 meters and that the interbeds are composed mainly of sandstones of alluvial genesis, then the figure of 360 thousand years must be increased at least to 500 thousand years. Consequently, the formation of the coal seams required a time on the order of half a million years.

The calculations given above, based on observations of modern peat formation, of course cannot be transferred without corrections to the Jurassic time because of the specific nature of the coal-forming plants of that time, as well as of the external agents of the peat-formation process (atmosphere, climate). Nevertheless, they make it possible to judge approximately the magnitude of the time interval required for the formation of the seams of the Tataurovskoye deposit.

The character of the processes of transformation of plant matter into coal is determined by the manner of accumulation of organic material,

geochemical conditions of the bogs and by diagenetic processes that took place in the peat bog after it had already been covered by the roof rocks.

Observations of the soils of coal seams show that the accumulation of plant material in the peat bogs proceeded autochthonously. The floors of seams I, II, and III of the Tataurovskoye deposit are characterized by the presence of root systems of the coal-forming plants directly at the rock–coal boundary. This feature is one of the primary indications of autochthonous accumulation of plant material at the onset of coal formation. It should be noted that imprints of fossil flora are invariably present in the floors of the coal seams of the Tataurovskoye deposit.

No less important evidence indicating autochthonous accumulation of plant material is provided by microscopic studies of the coals. Such evidence includes the absence of sorting among fragments of plant material and their disorderly accumulation, as well as the frequent absence of distinct bedding in the coals.

Thus, in terms of the nature of the accumulation of the original plant materials, the coal seams of the Tataurovskoye deposit, as our observations show, are autochthonous. Nevertheless, elements of allochthony did occur during the formation of the Tataurovo coals. They consisted in the input, by running waters at times of a rise in the base level of erosion, of terrigenous material, which constitutes the main part of the ash content of the Tataurovo coals; according to technical and petrographic analyses, their ash content ranges from 4.3 to 39.7%.

The gradual intensification of the activity of running waters during the formation of the seams led to an increase in the average ash content from seam III to seam I.

The organic material of the coal-forming plants underwent a whole series of transformations in the course of the formation of the coal substance.

In the peat stage, changes in the plant material were associated with bacterial activity. Biochemical processes proceeded both through the activity of aerobic bacteria and through the activity of anaerobic bacteria, since the formation of the peat bogs took place under the conditions of a semi-dry forest bog at the boundary between the subaerial and flooded parts. Thus the biochemical processes in the Tataurovo peat bogs were, on the one hand, destructive of the organic substances of the original plants, while, on the other, they generated new organic formations in the peat bog with a higher content of condensed aromatic compounds. The predominance of fusinite–semifusinite varieties (57.5%) in the composition of the Tataurovo coals suggests that the activity of aerobic bacteria was very significant.

I. I. Ammosov [2] regards bacterial processes as a cause that disturbed the relative equilibrium established during the life of the coal-forming plants in the complex system of organic substances. The disturbance of this relative equilibrium was one of the causes of further changes in organic substances during coal formation, up to and including the brown-coal stage.

While assigning primary importance to biochemical factors in the peat stage of alteration of organic substances, it should be noted that mineral substances and peat waters also played a very substantial role at this time, producing not only oxidative but also reducing reactions that had a very strong effect on the course of transformation of the original plant material.

After the peat stage, once the peat bog had been covered by roof rocks, the phase of diagenesis begins, characterized by the predominant development of

physicochemical processes of transformation at a relatively low temperature and somewhat elevated pressure.

The coals of the Tataurovskoye deposit underwent the first and second stages of fundamental changes in organic matter and are at the brown-coal stage of metamorphism.

The most characteristic processes in the formation of the Tataurovskoye coals were the processes of gelification and fusenization, and they manifested themselves in approximately equal measure. It should be noted that the areal distribution of coal types is extremely uneven, which is connected with the different level of the peat-bog surface relative to the water level. Areas in which fusinite–semivitrinite coals predominate correspond to more elevated, forested parts of the bog, while areas in which clarain coals predominate correspond to shallow bodies of water between them, with an open water surface. In the first areas the processes of fusenization predominated; in the second, the processes of gelification. The absence of sapropelic material in the clarain varieties indicates a certain flow-through character of these bodies of water, but not so strong that the process of eluviation could develop.

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