FACIES CONDITIONS OF SEDIMENTATION OF JURASSIC DEPOSITS IN THE WESTERN KANSK–ACHINSK BASIN

A. N. Kondakov

(Presented by Prof. A. M. Kuzmin)

This paper gives a brief facies characterization of the deposits of the Middle and Upper Jurassic in the area of the Arga Range of the Kansk–Achinsk coal basin, illustrated by lithologic-facies schemes.

As source material, use was made of personal observations; geological documentation from 400 wells of mechanical core drilling, provided by geologists G. G. Pozdnyakov and V. V. Kosarev; and also the research materials of I. G. Indyukov, Ya. G. Kats, K. L. Kokhanchik, I. K. Kuzmin, A. A. Mossakovsky, G. G. Pozdnyakov, E. Z. Savchenko, and N. G. Shubina. It should be noted that the available geological materials illuminate the section chiefly from the standpoint of the production requirements of coal organizations and suffer from the poor quality of the bed-by-bed descriptions, on the basis of which, by the sum of features, only facies complexes can be distinguished.

In accordance with the unified scheme (1960), the Jurassic deposits of the area are subdivided into the Makarovo ($J_1$), Itat ($J_2$), and Tyazhino ($J_3$) formations. Within the Itat Formation, I. V. Lebedev (1956) distinguished a lower, coal-free subformation and an upper, coal-bearing one. The age of the formations has not been rigorously substantiated paleontologically, and although the Lower Jurassic age of the Makarovo Formation does not cause particular disagreement, there is as yet no single opinion in determining the age of the overlying strata. Thus, for example, according to spore-pollen determinations, V. A. Khakhlov, A. V. Aksarin, and others consider that the entire coal-bearing sequence belongs to the upper part of the Lower and the lower part of the Middle Jurassic, whereas, on the basis of finds of crustaceans from the basal parts of the Itat Formation, N. I. Novozhilova assigns the deposits of the formation to the upper part of the Middle Jurassic and the lower part of the Upper Jurassic. The Tyazhino Formation is characterized by the least rigorous paleontological substantiation and by an uncertain scope; in pure form it is practically impossible to distinguish it in the area.

The existing disagreements require that, before presenting the factual material, the positions in correlating both the sections as a whole and individual coal seams be stated. The identification of the coal-free subformation presents no difficulties (Fig. 1). The roof part of the Itat Formation, above the Itat (Berezovsky) coal seam, is characterized by a petrographic composition of deposits transitional to the rocks of the Tyazhino Formation; and, naturally, the boundary between the Itat and Tyazhino formations, drawn on the basis of indistinct differences in petrographic composition, has an artificial character. The deposits of the Tyazhino Formation and of the roof part of the Itat Formation, close in facies and geochemical-

[[unclear: word ending in “-logical”]] respect, are combined into the supra-Itat member (Fig. 1), representing undifferentiated deposits of the Middle and Upper Jurassic.

In the practice of geological exploration work, local stratigraphic schemes have been developed for individual structural units; these make it possible to correlate coal seams satisfactorily, but the correlation of individual seams for structurally separated parts of the territory is often contradictory and poorly substantiated. The most controversial question is the age correlation of the Itat, Berezovka, and Moshchnyi seams (Fig. 1).

The appearance of any coal seam corresponds to an epoch of minimal influx of clastic material. Considering that the wood-peat mass undergoes sharp compaction during coalification, reaching as much as tenfold, it may be assumed that the subsidence gradient during the pe-

Labels in Fig. 1: Bogotol trough; Itat syncline; Berezovka trough; Altat syncline; Nazarovo trough; Itat Formation ($J_2$); supra-Itat member ($J_2$–$J_3$); Upper Subformation; sandy member; Berezovka Subformation; Itat seam; “Provodnik” seam; Berezovka seam; Berezovka seam; Moshchnyi seam; V seam; 0, 50, 100, 150 m.

Fig. 1. Scheme for correlating sections of the Middle and Upper Jurassic: 1 — coals, 2 — carbonaceous rocks, 3 — clays, argillites, 4 — aleurolites, siltstones, 5 — sands, sandstones.

riod of formation of a sufficiently thick, persistent coal seam is close to the subsidence gradient of the territory during the period of accumulation of coarse-clastic rocks and, at the same time, takes place under conditions of complete peneplanation with significant removal of elevated areas of the territory. And if rhythmicity, expressed in the successive alternation of thin sandy and clayey-carbonaceous sediments, is easily explained by exogenous causes through the migration of river channels, then the appearance of thick and, as a rule, convergent seams, or of a single giant seam such as the Itat or Berezovka seams, can be explained only by tectonic causes. Hence the possibility of correlating the seams becomes apparent.

Both for the territory as a whole and for individual depression structures, in the deposits of the Itat Formation, ending with the Moshchnyi seam and seam I (Fig. 1), an increase in the thickness of the deposits is observed east-

...direction, which is accompanied by a noticeable coarsening of the grain size of the sedimentary material. The very weakly manifested Late Cimmerian folding could not have led to disproportions in the ratio of thicknesses, and their cause lies in differences in the relative rates of subsidence of the floors of depressions during the period of accumulation of sediments, with a higher gradient in the eastern direction. The sequence of younger Jurassic formations is characterized by an increase in thickness in the northwestern direction. This similarity is not accidental and reflects a change in the vector plan of block tectonic movements of the undational type. At the base of the sequence formed under the new tectonic conditions lies a persistent sandy member, which in this sense is analogous to the sequence of the barren subformation (Fig. 1). Thus, two macrorhythms are outlined in the structure of the Itat Formation. The first of them begins with the sandy sediments of the barren subformation, and the second with the sandy member. The similarity of tectonic conditions of formation makes it possible confidently to regard the Berezovsky and Itat beds as coeval formations.

In the crest part of the anticlinal bend separating the Nazarovo trough from the Altat syncline, the Moshchny bed was eroded by contemporaneous erosion, but on the western plunge of the bend it, separated by a sandy member from the Berezovsky bed and decreasing in thickness from 8–10 m to 2–3 m, is also traced within the Berezovsky trough. Possibly, during the formation of the sandy member the bed was partly eroded, but the almost ubiquitous development in the roof of the bed of a thin clayey interbed, connected with the bed by gradual transitions, indicates the limited, local character of the erosion. In the territory of the Itat syncline the possible analogue of the Moshchny bed is bed I (Fig. 1).

The barren subformation has been penetrated by a very sparse network of boreholes, which give only a general idea of the facies character of the section. In the east of the territory the deposits of the subformation are composed of sandy alluvial sediments, which westward are replaced by sandy-silty channel-floodplain deposits.

The coal-bearing subformation contains the principal industrial coal seams. In the generally accepted system of names for coal seams, the lower age boundary of the subformation is drawn, for the Itat area, along the floor of the third coal seam and, for the Nazarovo area, along the floor of the fifth coal seam. The upper boundary of the subformation is defined very contradictorily by different authors and in this work is taken at the floor of the sandy horizon overlying the Itat-Berezovsky seam.

By degree of coal-bearingness the subformation is subdivided into two unequal parts. The lower half of the subformation, for the Itat area (Fig. 1), is bounded by the First bed, and for the Berezovsky area by the Moshchny bed. The main part of the section is composed of sandstones ranging from fine-grained to coarse-grained, coals; siltstones are less widespread, and argillites have subordinate significance. The sediments are rather clearly differentiated and represent a regular alternation, persistent over the area and in the section, of deposits of alluvial-floodplain or lacustrine-bog complexes.

In the direction from west to east, for the lower half of the subformation, there is established both an increase in the total thickness of the deposits from 30–40 m to 150–170 m, and an increase in the thickness of the coal seams from 1–2 m to 10–18 m (in the area of the town of Nazarovo).

The upper half of the coal-bearing subformation of the Itat Formation consists of a sandy member and a thick coal seam, called in the Itat-

area by the Itatka and, in the Berezovka area, by the Berezovka. Deposits of the upper half of the subformation are most fully covered by geological work, permitting the compilation of lithologic-facies schemes.

Within the sandy member in the east of the territory (Fig. 2), sandy sediments predominate, often medium-grained and coarse-grained, sorted, with coarse cross-bedding; on this basis they have been assigned to the complex of channel facies. Westward, the sandy sediments become

Fig. 2. Lithologic-facies scheme of the sandy member of the coal-bearing subformation of the Itatka Formation. Legend: 1—2—alluvial-channel deposits: 1—coarse-grained sandstones, 2—medium-grained sandstones; 3—11—channel-floodplain deposits: 3—medium-grained sandstones with interbeds of siltstones, 4—medium-grained sandstones and siltstones, 5—fine-grained sandstones, 6—siltstones with interbeds of medium-grained sandstones, 7—fine-grained sandstones with interbeds of siltstones, 8—fine-grained sandstones and siltstones, 9—siltstones with interbeds of fine-grained sandstones, 10—coals, siltstones, fine-grained sandstones, 11—coals with interbeds of fine-grained sandstones; 12—17—floodplain-lacustrine and swamp deposits: 12—siltstones, 13—siltstones and argillites, 14—coals and siltstones, 15—argillites with coal interbeds, 16—coals with interbeds of argillites and siltstones, 17—coals; 18—21—delta-lacustrine and lacustrine deposits: 18—argillites and coarse-grained sandstones, 19—argillites and fine-grained sandstones, 20—argillites, siltstones, and fine-grained sandstones, 21—argillites; 22—presence in the section of substantial coal interbeds, 23—limit of distribution of deposits: a) reliable, b) inferred; 24—boundary of lithologic-facies zones; 25—isohypses of the top of the deposits; 26—areas of development of rocks of the Paleozoic basement, 27—geochemical sampling boreholes.

thinner, more silty; in the deposits, along with cross-bedding, horizontal bedding is established, indicating [1] the significant role of floodplain-lacustrine facies in the composition of the sedimentary sequence. In general, the areas of development of coarser sediments are elongated along the axes of subsidence of synclinal structures of the present-day structural plan.

The thickness of the member deposits, in contrast to the underlying deposits, increases from east to west and northwest from 40—50 m to 100—120 m.

The deposits of the period of formation of the Itat–Berezovka coal seam are composed of dull and semilustrous coals with thin interbeds of siltstones, argillites, and sandstones.

In the eastern half of the Berezovka trough (Fig. 3) there was an inherited river system, the deposits of which appeared in the form

Fig. 3. Lithologic-facies scheme of the deposits of the period of formation of the Itat (Berezovka) seam (for symbols see Fig. 2).

of the appearance of individual rock interbeds in the floor-adjacent and roof-adjacent parts of the seam.

The southwestern part of the Berezovka trough and the Itat syncline, apparently, represented a swampy interfluve. According to the textural criterion, in the understanding of T. N. Davydova et al. [3], the coals of the Itat syncline and the eastern half of the Berezovka trough formed mainly in flowing swamps, whereas in the southwestern part of the Berezovka trough coals accumulated under conditions of stagnant swamps predominate.

In the southwestern part of the territory, owing to vigorous subsidence, along with deposits of swamp facies, deposits of channel–floodplain facies are of substantial importance.

The thickness of the seam increases westward from 8–10 to 80 m.

The Naditat member is composed of siltstones and argillites with subordinate interbeds of sandstones and coals (Fig. 4).

At the base of the unit, as noted above, lies a sandy horizon of small thickness. The upper boundary in most cases is drawn conventionally and is taken at the base of the first sandy horizon above the last manifestations of coal-bearing deposits and the first interbeds of red-colored rocks.

In the southwestern and central parts of the Berezovka trough, sediments of a closed lacustrine basin with horizontal bedding and accumulations of freshwater fauna predominate, whereas in its eastern half and in the Itat syncline an important role is played by deposits of swamps and swampy lakes of the floodplain facies complex.

The thickness of the deposits of the member increases westward from 30–50 m to 275 m.

In general, the accumulation of predominantly clayey bog–lacustrine sediments of the Naditat member in the area studied took place under conditions of peneplanation and a gentle passive tectonic regime, favorable for the development of a complete weathering profile, in contrast to the deposits of the Itat Formation, which are characterized by a somewhat more

Fig. 4. Lithofacies scheme of the deposits of the Naditat member
(for the legend, see Fig. 2).

coarse character of the clastic material and by a predominance of alluvial sediments, caused by repeated rejuvenation of the Middle Jurassic relief.

REFERENCES

1. L. N. Botvinkina. Stratification of Sedimentary Rocks. Publishing House of the Academy of Sciences of the USSR, 1962.
2. M. P. Burtsev. The Kansk-Achinsk Coal Basin. Publishing House of the Academy of Sciences of the USSR, 1961.
3. T. N. Davydova, P. L. Goldshtein, L. S. Kaminskaya. On the genetic approach to clarifying paleogeography, using the Bureya coal basin as an example. In: Problems of Sedimentation. Gostoptekhizdat, 1960.