ON THE QUESTION OF CRITERIA FOR IDENTIFYING ANCIENT TYPES OF BOGS IN LIMNIC COAL-BEARING BASINS

O. A. DULZON, V. L. KOKUNOV

(Presented by Professor A. V. Aksarin)

The Irsha-Borodino and Nazarovo deposits under study are located in the Rybinsk and Nazarovo depressions. Administratively, both coal deposits are situated in the southern part of Krasnoyarsk Krai.

In the course of our work we used a method for identifying facies-genetic types of coal-bearing deposits, based on a thorough study of the morphology of bedding and the granulometric composition of sandy-silty rocks, as well as on coal-petrographic investigations.

The coal-bearing deposits of the Irsha-Borodino and Nazarovo deposits are Jurassic in age and are represented by continental deposits. The alluvial and lacustrine-bog facies groups are most widely distributed among them. The proluvial-deluvial facies group occurs considerably more rarely.

Each facies group has characteristic dynamics of the sedimentation environment, which in turn are subdivided into a whole series of macrofacies, distinguished on the basis of lithological features and the processing of quantitative granulometric composition data by the method of moments according to L. B. Rukhin (1957).

The alluvial facies group includes channel facies and floodplain facies with their mutual transitions.

The lacustrine-bog facies unites the facies of nearshore lake deposits and the facies of deep-water deposits. The bog subgroup of facies includes: the facies of a peat stagnant bog, a peat flow-through bog, the facies of waterlogged forests, and the facies of lakes undergoing bogging.

V. L. Kokunov constructed lithologic-facies maps of the floor and roof of the Borodino coal seam.

The constructed lithologic-facies map of the floor of the Borodino coal seam shows the wide distribution of siltstones with lake-type bedding over the area of distribution of the Borodino seam (Fig. 1). Argillites and carbonaceous siltstones have a very limited distribution in the floor of the seam. The map shows sandy rocks situated across the territory predominantly in the form of sinuous, long and narrow bands resembling meanders of a branching river system. For a more substantiated judgment on the nature of such sandy formations, we carried out a granulometric

analysis of sandy rocks by the sieve method. The results of processing the granulometric-analysis data were plotted on L. B. Rukhin’s dynamic diagram (1957) in order to determine the genesis of these formations. According to this processing, the overwhelming majority of the sandy rocks proved to be channel deposits.

Thus, a clear picture emerges in plan of an ancient system of channels and riverbeds that connected extensive lakes, which often had the most varied shoreline configurations.

Analysis of the distribution of lithofacies over the study area makes it possible to assert that formation of the Borodino seam took place in the Borodino and Balai troughs. Moreover, this extensive basin—

km 0 2.5 5 7.5 10 km

Legend

Fig. 1. Lithological-facies map of the floor of the Borodino seam. Compiled by V. L. Kokunov

—extended as far as the Kan River in the southeast and continued farther to the south of the depression. This sedimentation basin was an entire system of shallow semithrough-flow and through-flow lakes, whose winding low-lying shores were covered with luxuriant Jurassic vegetation.

Coal accumulation in Borodino time is characterized by an initially significant quantity of flowing waters, which caused the accumulation, in the lower part of the seam of the first member, of coal of the dyrene type. This was followed by a more intensive influx of water, which deposited a member of aleurolites, the thickness of which increases to the southeast from the coal open-pit mine, thereby indicating the direction of transport of the terrigenous material.

The petrogenetic coal members that we have distinguished within the Borodino seam make it possible to combine them into six complete rhythms of coal accumulation. Each of the rhythms begins with the deposition of coals of dyrene character, or coals enriched with algae and allochthonous fusene, and ends with the deposition of coals of the clarene type, or of the fusene–semifusene type.

Thus, the process of coal accumulation during formation of the Borodino seam was not a reflection of a smooth rise in the erosion base, but was stepwise in character over time; however, the amplitude of such jumps did not lead to the accumulation of terrigenous mine—

Conventional symbols

• River
• Lake
• Swamp
• Sandy facies
• Aleuritic facies
• Facies of a peat bog
• Boulders
• Observation points

Fig. 2. Lithologic-facies map of the roof of the Borodino seam. Compiled by V. L. Kokunov.

material. Only the petrographic types of coals changed.

The character of the subsequent sharp rise in the base level of erosion, which brought coal accumulation to an end, is well reflected on the lithologic-facies maps of the roof of the Borodino seam (Fig. 2). Judging from the lithologic-facies map, the sharp rise in the base level of erosion that led to the covering of the Borodino seam by the roof was a consequence of rejuvenation of the relief in the areas from which material was supplied, as well as of the development of a well-defined river system that fed extensive shallow-water lakes.

Analysis of the granulometric composition of sandy rocks selected from drill cores from the roof of the seam, as well as the study of bedding types in the rocks in the roof of the Borodino seam, showed the near-channel and channel character of these deposits. It should be noted here that comparison of the widths of channels in the floor and roof of the Borodino seam indicates a very substantial rejuvenation of the river system at the time when coal accumulation ended. The revived river system was evidently the principal route by which clastic material was delivered to the territory of the depression, where it was then additionally reworked and redistributed.

It is quite natural to expect that, under conditions of uniform subsidence, in those parts of the sedimentation basin located closer to the source area, the processes of accumulation of plant material must have ceased somewhat earlier than in more distant areas. This most likely explains both the decrease in thickness of the Borodino seam from northeast to southwest and the splitting off of the Gusev seam from the Borodino seam in the Borodino 1–2 and Borodino—3, 4, 5 areas.

The character of the paleogeographic setting described above during the time preceding coal accumulation and immediately after its termination helps us understand the nature of the paleogeographic setting directly during coal accumulation; however, the method for distinguishing facies-genetic types must already be different here. First of all, it becomes necessary to use coal-petrographic analysis of thin transparent sections in order to determine the character of the original phytocenosis of the ancient bog. Then data from proximate analysis are required, above all the results of determining the ash content of the coal.

To reconstruct the landscape setting during the period of accumulation of the original plant material of the Borodino coal seam, as well as the type of bogs, a lithologic-facies map of the Borodino coal seam was constructed (Fig. 3). In constructing it, data from proximate analysis of seam samples from exploration boreholes were used—namely, the results of determining the ash content of the coals of the Borodino seam. It was established that their ash content never falls below 6 percent; consequently, this ash content value may be regarded as the internal ash content of the coal, the sources of which are the inorganic substances contained in the coal-forming plants themselves. Subtracting this value, we obtained on the plan zones of zero, or nearly zero, ash content, corresponding to areas of purely autochthonous coal accumulation, and zones of autochthonous-allochthonous coal accumulation. The ash-content maxima indicate the principal directions of flow of through-going mineralized waters.

What types of bogs, then, served as the site of accumulation of coals of the Borodino type?

To answer this question it is necessary to turn to data from peat science and mire science.

According to the opinion of the outstanding Soviet geographer L. S. Berg (1955), “bogs constitute a very prominent element in the landscapes of mo—

“of the temperate forest zone,” i.e., bogs undergo great development in zones where forests are widespread. At the same time, L. S. Berg points out that, by mode of origin, a distinction is made between bogs formed by the overgrowing of bodies of water (lakes and rivers) and bogs that have arisen through the paludification of dry land. Both are very widely distributed.

In the opinion of Academician V. N. Sukachev (1926), and also judging by the numerous references to his opinion in the works of leading scholars on caustobioliths, such as Yu. A. Zhemchuzhnikov, P. I. Stepanov,

Scale: km 0.25 0 0.5 0.75 1 km

Legend.

Fig. 3. Lithologic-facies map of the Borodino coal seam. Compiled by V. L. Kokunov

A. I. Ginzburg and others, lowland bogs are the most favorable for peat formation and, consequently, for coal accumulation. This proposition diverges somewhat from the latest achievements of bog science. According to Prof. S. N. Tyuremnov (1949), lowland bogs are distinguished by a high ash content of the peat (10–14%). Raised bogs, by contrast, have the lowest ash content of all bog types (from 2 to 4%). At the same time, S. N. Tyuremnov points out that Academician V. N. Sukachev’s classification was constructed on the basis of materials from the study of bogs in the European part

of the Union, while the bogs of the vast expanses of Siberia have as yet been studied inadequately.

The study of local fuel in the enormous territory of Western Siberia alone has shown that bogs occupy from 33% (Omsk Oblast) to 24% (Tomsk Oblast) of the entire land area there. According to data of the Main Administration of the Peat Fund under the Council of Ministers of the RSFSR (S. N. Tyuremnov et al., 1957), about 50% of all the presumed peat reserves of the USSR are concentrated in Western Siberia. In Western Siberia such peat deposits are known as the Vasyugan bogs, which have no equal in size and peat reserves, as well as smaller ones, such as the Imgyt bog of Tyumen Oblast or the Urpinsk bog of Omsk Oblast, whose dimensions, nevertheless, are comparable with those of deposits of thick coal seams.

A summary of all the most recent data on peat studies, compiled by the Main Administration of the Peat Fund under the Council of Ministers of the RSFSR (1951), confirmed that the genetic subdivision of bogs into lowland, raised, mixed, and transitional types reflects the factors of relief and of the original phytocenosis and is justified. At the same time, mixed and transitional deposits account for only 14% of all bogs. The remaining 86% belong to raised and lowland bogs, and their distribution is approximately equal. Each type of bog is subdivided into three subtypes: forest, forest–mire, and mire. Among the bogs of the Quaternary period, the forest–mire subtype of bogs is predominantly developed, thereby reflecting the original phytocenosis.

As the investigations of A. V. Aksarin (1959) showed, Tomsk Oblast is characterized by the presence of three types of bogs: raised bogs, transitional-type bogs, and lowland bogs. Raised bogs account for more than 60% of all bogs by number and about 53% of all bogs by area. The greatest thicknesses of peat deposits and the lowest ash content in Tomsk Oblast are characteristic precisely of raised bogs.

In Omsk Oblast, according to the data of B. N. Valukhov (1940), the above-mentioned types of bogs are also widespread, but the most widespread type is the lowland type. At the same time B. N. Valukhov indicates that the sizes of raised bogs usually exceed those of lowland bogs, that the ash content of their peat deposits is 3–5% lower than in lowland bogs, and that their origin, as a rule, is lacustrine.

With movement into the interior of the continent, to its more elevated parts, the raised type of bog becomes less characteristic. Thus, according to data of M. F. Evseev and I. G. Meshkov (1940), lowland–floodplain bogs are usually developed in Altai Krai. The ash content of the peat of lowland–floodplain bogs is 3–5% higher than that of the peat of lowland bogs in depressions. At the same time, Altai peat deposits are characterized by a pronounced sinuosity of their boundaries, with islands of dry areas, as well as by a fairly considerable thickness of the peat deposits. Such are the bogs: Dergach, Poperechnoe of the Zonalny District, and the Kislukhinsk bog of the Talmensky District.

Thus, there is a zonality in the distribution of bogs of one type or another on the modern surface of Western Siberia, depending on the character of the relief. This is a very important point, and it must be borne in mind when analyzing the regularities in the location of deposits with especially thick coal seams.

The process of formation of peat deposits in Jurassic time was in itself a rather complex process. Here a specific imprint was undoubtedly imposed by the composition of Jurassic plants, which cannot be compared with the composition of modern flora, by the mode of nutrition of the plants and their longevity, and also by the climate of that remote epoch. But the fact that

can confidently be extended, using the principle of actualism, to the Jurassic time—namely, the zonality, observed for the modern epoch, in the distribution of bog types on a continent depending on the character of the relief and its elevation above sea level.

Undoubtedly, in Jurassic time there also existed mountain ranges on the ancient continent (Angaraland), down whose slopes numerous rivers flowed into the ancient epicontinental seas. Undoubtedly, on the ancient continent there were also low-lying peneplainized areas on which, under favorable conditions, ancient bogs developed: lowland, raised, lowland-floodplain along the river network, as well as bogs of mixed and transitional types. They have not been preserved to our time, and we observe them already in a new form, as coal seams; therefore, reconstructing the conditions under which known coal deposits formed—specifically, reconstructing the types of bogs in which coal accumulation took place—is a practically useful task. With the correct decoding of the paleogeography of large regions, it makes it possible to indicate, in a preliminary way, the location of as yet undiscovered blind coal deposits.

Turning to the determination of the types of bogs in which the accumulation of the original material of the Borodino coal seam took place, it should be noted that the original phytocenosis, as well as the low ash content of the Borodino coals together with the character of the relief and the distribution of ash-content maxima over the area of coal accumulation, make it possible to conclude that the Borodino seam was formed in a piedmont semi-enclosed depression; moreover, the coal-accumulation basin represented an entire system of raised bogs connected with one another by bogs of the lowland-floodplain type, with a clear predominance of raised bogs. The types of bogs are excellently contoured on the lithologic-facies map of the Borodino coal seam (Fig. 3). Judging by the composition of the coal-forming plants, as well as by the character of decomposition of the plant material, reflected in the identification of the corresponding petrogenetic types of coal, we are dealing with bogs of the forest-swamp subtype.

Analysis of the map indicates that the coal-accumulation basin was a rather complex system of raised and lowland bogs situated in a semi-enclosed piedmont basin.

LITERATURE

A. V. Aksarin. Non-ore mineral resources of Tomsk Oblast. Manuscript, Tomsk, 1959.

L. S. Berg. The Nature of the USSR. Geografgiz, 1955.

B. N. Valukhov. Peat bogs of Omsk Oblast. In: Local Fuel of Western Siberia. ZSGU Publishing House, Tomsk, 1940.

M. F. Evseev and I. G. Meshkov. Peat bogs of Altai Krai. In: Local Fuel of Western Siberia. ZSGU Publishing House, Tomsk, 1940.

V. L. Kokunov. Petrographic characterization of the Borodino seam of the Irsha-Borodino deposit of the Kansk brown-coal basin. Proceedings of TPI, vol. 127, issue 1, 1964.

V. I. Popov, S. D. Makarova et al. Guide to the Determination of Sedimentary Facies Complexes and Methods of Facies-Paleogeographic Mapping. Gostoptekhizdat, 1963.

L. B. Rukhin. Granulometric analysis of loose and weakly cemented sedimentary rocks. Article in the collection Methods for the Study of Sedimentary Rocks, vol. 1, Gosgeoltekhizdat, 1957.

Summary of the Main Administration of the Peat Fund under the Council of Ministers of the RSFSR, 1951.

V. N. Sukachev. Bogs, Their Formation, Development, and Properties. Leningrad, 1926.

S. N. Tyuremnov, A. S. Olenin et al. Peat Fund of the RSFSR, 1957.