ON THE MATERIAL COMPOSITION AND GENESIS OF COALS OF THE KANSK-ACHINSK BASIN

V. L. Kokunov

(Presented by Professor I. V. Lebedev)

One of the research groups of the Department of Combustible Minerals of the Tomsk Polytechnic Institute is currently engaged in studying the material composition and conditions of formation of the coals of the Kansk-Achinsk brown-coal basin under a contract with the Krasnoyarsk Geological Administration.

The Kansk-Achinsk brown-coal basin occupies an enormous area in the territory of Krasnoyarsk Krai, with geological reserves of 1,200 billion tons, and constitutes a major base for the coal, power, and chemical industries of the USSR; for the industrial development of Krasnoyarsk Krai it is of exceptionally great importance. On the basis of the basin’s coals, large electric power stations are being built (the Nazarovo and Borodino state district power stations), with a capacity of 1,200–3,600 thousand kW. The colossal reserves of already explored coals suitable for open-pit mining (about 9,280 million tons) make it possible to regard the basin as a profitable base for the electric-power and chemical industries and as a base for the gasification of the cities and enterprises of Krasnoyarsk Krai; coal production by 1980 is planned to be brought up to 330 million tons per year, with a coal cost of 20 kopecks per ton [6].

The staff of the department have long been engaged in studying the geological structure of the Kansk-Achinsk basin, its stratigraphy, and coal content. As early as 1931, A. V. Aksarin carried out geological surveys in the basin, as a result of which he developed the main features of the stratigraphy of the coal-bearing deposits; on the basis of paleobotanical determinations, their age was established and the coal content of the basin was described in detail. In addition, as a result of route surveys, he discovered the Sayano-Partizanskoye hard-coal deposit, and subsequent exploration work by the Ivanovo Geological Expedition made it possible to confirm the industrial value of the deposit, whose coal reserves amount to more than 1,500 million tons. The coals can be used in the charge for producing metallurgical coke in an amount of 20–40%, which is very important for the construction of the country’s third metallurgical base in Siberia, which is being delayed because of the shortage of coking coals in the krai.

As a result of the work of recent years, in which the author has also participated since 1954, a large amount of new factual material has been collected, making it possible to refine the stratigraphy of the coal-bearing deposits—

...basin, its tectonic structure, a large amount of material was collected also on the material composition of the coals [3].

The staff of the department has already completed and submitted to the Krasnoyarsk Administration a report on the study of the material composition and genesis of the coals of the Irsha-Borodino and Nazarovo deposits and is currently processing the materials of field observations for the Aban and Berezovo deposits. The task is to work out the stratigraphy of these new districts, their coal-bearing capacity, and the correlation of sections of the coal-bearing strata.

In tectonic terms, the Kansk-Achinsk basin is situated at the junction of major tectonic structures: the Siberian Platform, the West Siberian Platform, and the Sayano-Altai folded region, and consists of a series of large negative structures, which include the Kansk-Taseevo syneclise, the Rybinsk depression, the Nazarovo depression, and the Chulym-Yenisei syneclise. These negative structures are separated from one another by gentle positive structures—swells, such as the Kansk swell and the Arga swell. The troughs and depressions, in turn, are subdivided into smaller structures—muldas, which are usually coal-bearing.

A block structure of the basement of the coal-bearing deposits is characteristic of the entire territory of the basin; we also had to study this basement [2].

The section of the coal-bearing deposits is represented by proluvial-alluvial-lacustrine-swamp formations in terms of facies affiliation and, lithologically, consists of an alternation of sandstones, siltstones, argillites, and coal seams, with occasional beds of conglomerates (in the piedmont areas).

The stratigraphic scheme for the Jurassic coal-bearing deposits is based by us on the principles of tectono-stratigraphic analysis, which makes it possible to take into account the cyclic character of sediment accumulation, lithological composition, and coal-bearing capacity. The age of the distinguished suites is determined from paleobotanical data (determinations of vegetative organs of fossil plants by A. V. Aksarin and of the spore-pollen composition by N. S. Sakhanova-Grigorieva [1, 7]).

The section of the coal-bearing deposits of the Rybinsk depression is divided into the following suites: the Pereyaslovka suite, 100–180 m thick, the age of which is dated as Lower Jurassic, and three suites of the Middle Jurassic, from bottom to top: the Lower Kamala suite, 80–170 m thick; the Upper Kamala suite, 85–150 m thick; and the Borodino suite, 110–170 m thick.

The above scheme for subdividing the coal-bearing deposits of the Rybinsk depression is the most detailed in the basin; in the other depressions the section of the coal-bearing Jurassic is divided only into two suites.

In the Kansk-Taseevo syneclise these are the Voznesenka and Aban suites, and in the Nazarovo depression these are the Makarovo and Itat suites. A unified stratigraphic scheme for the entire basin has not yet been created.

Of greatest practical interest are the deposits of the Borodino, Aban, and Itat suites, since they contain coal seams of very great thickness (20–60 m) suitable for open-pit quarrying. It is therefore natural that the distribution and composition of these suites and of the coal seams enclosed in them interested us first of all.

The study of the material composition of these suites and coal seams and of the conditions of their formation is being carried out by a whole complex of modern laboratory research methods, including petrographic, granulometric, spore-pollen, paleobotanical, and electron-microscopic methods for studying both the coal itself and the enclosing rocks.

The data obtained already make it possible to draw a conclusion about the good quality of the coals of the deposits studied, and subsequently to outline rational ways of using them; they make it possible to distinguish various types of coal by material composition, relating them to petrographic types of coal, and to identify certain regularities in the conditions of coal accumulation, as well as to resolve the question of the coal-forming plants. All of the above makes it possible to influence the further direction of geological exploration work.

Without studying the material and petrographic composition of coal, it is impossible to choose a rational scheme for its use; therefore we devote considerable attention to this question.

As a result of the petrographic investigations we have carried out, it has been established that the Borodino, Nazarovo, and Berezovo thick coal seams, from the genetic point of view, are complexly built geological bodies, individual parts of which are formations differing in genesis.

First of all, the coals of the seams include both representatives of the class of humites and representatives of sapropelite-humites.

The class of humites is represented by the following petrogenetic types of coal (Table 1). It should be noted, incidentally, that by a petrogenetic type of coal we understand such a type of coal for which a number of features are characteristic, reflecting the conditions of accumulation and primary transformation of plant material. At the same time, the quantitative ratio of the various types of coal substance is taken into account: gelified, fusenized, and cutinized.

The clarain type of coal that is distinguished constitutes the predominant majority (Table 1) and is representative of the facies of stagnant high-water peat bogs. In addition to the clarain type of coal, a whole series of petrogenetic types is also distinguished: dureno-clarain, as representative of the facies of a weakly flowing peat bog; claraino-durain, as representative of the facies of a peat bog of medium flow-through character; and durain coals, as representatives of the facies of a flowing peat bog.

It should be noted that the listed types (except for the clarain type) are very slightly widespread and together constitute only 9.2% of the coal composition. Somewhat apart stands the fuseno-semifusene type of coal, which constitutes about 4% of the seam thickness and represents the facies of swampy forests with weak waterlogging.

We also distinguish separately beds of coaly argillites and ash-rich coals, usually having a lens-like structure. They are distinguished as representatives of strongly waterlogged areas of bogs.

Sapropelite-humites are represented by the following petrogenetic varieties:

1. Clarain and dureno-clarain coals with algae in the humic groundmass, as representatives of the facies of swamping lakes with a stagnant character of the waters.

2. Claraino-durain and durain coals with algae in the humic groundmass, as representatives of the facies of swamping lakes with weakly flowing waters.

The listed principal petrogenetic types of coal include a whole series of petrographic subtypes, which correspond to various subfacies characterized by specific features of the environment at the moment of coal accumulation.

The facies of a peat stagnant bog includes two subfacies, differing in the stability of the regime at the moment of accumulation of the peat bog.

Table 1

The first subfacies is characterized by incomplete decomposition of plant material as a result of the suspension of the gelification process. This led to the formation of telinite clarains. The second subfacies is characterized by a more stable regime, which led to the complete decomposition of plant material with the formation of a homogeneous groundmass—initially a liquid, fluid substance in which, as in cement, cutinized elements are enclosed. This led to the formation of sporinite, cutinite, or mixtinite clarains with a homogeneous groundmass.

The noticeable predominance of clarains of the first subfacies (41.5%) over clarains of the second subfacies (about 30%) is striking; this characterizes the very process of coal accumulation as very complex over time.

It should be noted that the petrogenetic types we have distinguished are far from exhaustive, but are only the principal and most frequently occurring ones, since the processes of coal formation in the territory studied manifested themselves in great variety; an example of this is the accumulation of oil shales at the Irsha deposit, described by V. N. Kustova [5].

The main feature of the thick coal seams of the Kansk-Achinsk basin is their complex structure and the gradual complication of seam structure in the direction toward the area of denudation. The next feature of high-thickness coal seams is the sharp predominance in their composition of coals of the clarain type over the other coal types and the xylovitrain character of the groundmass in the coals. Coals of other types (duren, fusain-semifusain, etc.) usually occur only in the form of low-thickness isolated lenses.

Finally, another feature of the seam coals is the autochthonous character of accumulation of the original plant material, with periodic short-term enrichment of the swamp by allochthonous materials.

What plants, then, were the main suppliers of the original material during the formation of peat bogs in the Kansk-Achinsk basin in Jurassic time?

To study this question, we used data from the organographic method of studying impressions of leaves or remains of stems and trunks collected in the roofs of coal seams directly at the surface of the section of the coal matter and the enclosing rock matter, as well as impressions on bedding planes in the coals. In addition, we carried out a paleoxylological study of fragments of wood at the fusain and xylene stage from coals of the fusain-semifusain type; for this purpose samples were selected from which thin sections with radial, transverse, and tangential cuts of the wood were prepared. Finally, we made special collections of samples of coals of different types for spore-pollen analysis in order to determine the composition of spores and pollen of the coal-forming plants.

Summarizing the results of the three methods for studying the composition of the coal-forming plants, one may conclude that ginkgo plants and ferns took the greatest part in coal formation. Among the ginkgo plants, Phoenicopsis and Czekanowskia were significant, and among the ferns, Cladophlebis and Raphaelia. As for the conifers, judging by the occurrence of remains of leaves and needles in the plant community of both the Lower and Middle Jurassic, they are less well represented.

The role of horsetails in coal formation was also limited, although Equisetites in the Lower Jurassic underwent considerable development. Representatives of cycads are extremely rare among the leaf remains. They

were found only in the Ivanovskaya suite of the Sayano-Partizansky district and are represented by the genera Nilssonia and Pterophyllum. The representatives known to us of other groups of plants are very rare and, apparently, their role in coal formation was insignificant.

Consequently, during the formation of coal seams of great thickness, the Kansk-Achinsk Basin was a zone of development of conifer–fern–ginkgo forests, with conifers, cycads, and other groups of plants playing a subordinate role.

To this it must be added that, if one traces the pattern of distribution of deposits with coal seams of great thickness over the territory of the Soviet Union, it can be seen that deposits of especially thick coal seams with low ash content and a fairly good persistence of seam thickness are concentrated in a belt between 50 and 55° north latitude, without extending beyond the limits of the Angarid of Jurassic time. This belt of distribution of deposits with especially thick coal seams is included in one of P. I. Stepanov’s coal-accumulation belts [8]. Recently, Acad. N. M. Strakhov [9] has invested genetic meaning in the belts and nodes of coal accumulation empirically distinguished by P. I. Stepanov. According to N. M. Strakhov, P. I. Stepanov’s coal-accumulation belts are “ancient zones of a humid climate, on which alone coal accumulation could have been possible.” The areas, however, where coal accumulation was manifested most intensively are “areas of an active tectonic regime, maximally favorable for the accumulation of plant organic matter.”

Comparing the tectonic structure of the territory of the basin with the structure of other deposits with thick coal seams, one should conclude that the tectonic regime that existed during coal accumulation in all these deposits possessed common specific features. These features are as follows:

1. Deposits with thick coal seams are situated in zones where the basements of platforms come to the surface, characterized by a block structure and by differential (according to G. F. Krasheninnikov [4]) tectonic movements of blocks along ancient tectonic sutures, the consequence of which is the inherited character of the tectonic structure of the coal-bearing deposits.

2. The zones where the basements of platforms (the Siberian and West Siberian) come to the surface are characterized by the presence of swell-like uplifts established during the formation of the coal-bearing deposits, separating the piedmont depressions from the foredeeps of the platform cover. The slopes of these swell-like uplifts, having a weakly expressed stepped character as a result of negative differential tectonic movements, were the most favorable place for the formation of thick coal seams (Fig. 1), which, of course, does not depend on the age of the platform basements.

3. The processes of peat formation certainly took place during periods of relative tectonic quiescence and must be connected with slow secular movements of the earth’s crust, forming enormous “waves” of conjugate uplifts and subsidences; and their differential character, in our opinion, is connected with the block structure of the basement of the coal-bearing deposits.

We associate the genesis of thick coal seams with the presence of a combination of favorable paleogeographic and tectonic prerequisites, the principal ones of which are: the location of the Kansk-Achinsk Basin in a zone of humid climate at the time of the luxuriant flourishing of the Jurassic flora. On the other hand, the above-mentioned basin is situated in a zone where the basements of platforms (Siberian-

...and West Siberian). Coal formation took place on the slopes of near-platform geoanticlinal uplifts (the Kansk and Arginsk swells), distinguished by a tectonic regime in which the processes of accumulation of the initial plant masses corresponded to the rate of subsidence of the given area, which accounts for the uninterrupted accumulation of coal over a long period of time.

The totality of observations showed that the formation of thick coal seams on the arches of geoanticlinal uplifts did not occur, as shown in the diagram (Fig. 1), but that, in principle, coal formation

DIAGRAM OF COAL ACCUMULATION

Visible labels in the diagram:

Ю-В — SE
С-В — NE
С-З — NW
Ю-В — SE

Coal-accumulation zone
1 — zone of absence of coal accumulation
2 — subzone of continuous seams
3 — subzone of split seams

Area of denudation
Base level of erosion
Uplift of the Eastern Sayan
Rybinsk Depression
Kansk Swell
Kansk-Taseev Depression
West Siberian epi-Hercynian platform
Arginsk Swell
Nazarovo Depression
Kuznetsk Ala-Tau

Fig. 1. Idealized profiles across the Kansk-Achinsk Basin during the period of seam formation, perpendicular to the strike of the main structures.

may also occur in the arched parts of geoanticlinal uplifts, in places where their axes undulate, since coal formation is controlled by the height of the erosion base level.

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. V. L. Kokunov. On the facies variability of Devonian deposits of the Rybinsk Depression. Izv. TPI, Tomsk, vol. 120, 1961.

3. V. L. Kokunov. Material composition and genesis of the coals of the Kansk-Achinsk Basin. Abstracts of reports of the First Scientific Session of Higher Educational Institutions and Research Institutes of the West Siberian Council, Tomsk, 1963.

4. G. F. Krasheninnikov. Conditions of accumulation of coal-bearing formations of the USSR. Moscow State University Press, 1957.

5. V. N. Kustova. Petrographic characterization of brown coals of the main seams of the Irsha and Borodino deposits. Ilansk, 1942.

6. N. F. Ryabokon, K. L. Kokhanchik, Z. F. Leibovich. Coals of Krasnoyarsk Territory. In: Mineral Resources of Krasnoyarsk Territory. Publishing House of the Academy of Sciences of the USSR, 1962.

7. 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.

8. P. I. Stepanov, S. I. Mironov. Geology of caustobiolith deposits. ONTI, 1937.

9. N. M. Strakhov. Fundamentals of the Theory of Lithogenesis. Publishing House of the Academy of Sciences of the USSR, 1962.