Materials for the Parallelization of the Jurassic Deposits of Siberia

I. V. LEBEDEV

On the territory of Siberia, sedimentary strata of Jurassic age, which make up the Irkutsk, Kansk, Chulym-Yenisei, and partly the Kuznetsk basins, are widely distributed. In recent years the sediments of these basins have been studied sufficiently fully, and there is already a possibility for subdividing and correlating the sediments of the individual basins. In the present note the author proposes a first scheme for the parallelization of the Jurassic deposits of Siberia.

Jurassic Deposits of the Kuznetsk Basin

The sedimentary strata of Jurassic age in the Kuznetsk Basin have been preserved in the form of three large, isolated areas that represented parts of a formerly single basin: in the axial part of the Kuznetsk Depression—the Central district; in the southeastern part of the basin—the Tutuyas-Podobas district; and in the Inya Gulf. In addition, a number of small patches of Jurassic deposits occur in the southwestern and eastern parts of the basin.

The Jurassic of the Central district of Kuzbass has been studied best. The general stratigraphic scheme of the district is presented in the following form (from below): 1) the Barashevo strata, with thick seams, chiefly of humic banded coals, containing durain and clarain with interbeds of vitrain and lenses of fusain. Thickness more than 325 m; 2) the Sartakovo strata, containing characteristic humic-liptobiolithic platy semibright coals, in which small 1–2 mm lenses of bright coal are usually present. Thickness 640 m; 3) the Chusovitino strata, containing platy coals in which, along with semibright coals, there are thick interbeds of fusain. The strata lie transgressively. Thickness more than 170 m; 4) the Tersyuk suite, composed of lenses of gravelly-sandy-aleurolitic composition. In places a conglomerate is observed at the base of the suite. It lies on an eroded surface of older suites. Thickness more than 170 m.

The Tutuyas-Podobas district has been studied less well. Judging from the data of route investigations, two facies of different age and isolated in space can be distinguished here—a sandy non-coal-bearing facies and a coal-bearing conglomeratic facies. In coastal outcrops near the village of Abasheva and along the Tutuyas River, above the mouth of the Tunzas-su stream, there is a uniform sandy facies containing only thin coal partings. In the lower reaches of the Tutuyas River, in the Podobas basin, and in coastal outcrops of the Tom River, on the contrary, a conglomeratic facies is represented, and in its composition here one can distinguish up to four rhythmic packets, beginning with thick conglomerates and crowned by sandy-aleurolitic beds, on which coal seams lie.

On the basis of the fossil-flora assemblage and the type of coals, the conglomeratic facies can correspond only to the Sartakovo strata of the Central district. As for the sandy facies, stratigraphically it is older, but,

apparently, does not go beyond the Middle Jurassic, since the assemblage of fossil flora and the coals of this facies differ little from those of the overlying sequence.

The total thickness of the sediments is considered to be 650–750 m. The Jurassic deposits of other districts of Kuzbass have not been studied at all, and it is difficult to judge their stratigraphic position. It can only be noted that Jurassic flora has been found in Inskoi Bay, while the loose sequence preserved in the Shestakovo-Semenushkino syncline apparently has a younger age. It is represented by red-colored conglomerates lying on red-colored rocks; and since in Siberia young red-colored rocks are known only from the Lower Cretaceous and from Lower Tertiary deposits, the age of the entire sequence cannot be considered Jurassic.

Jurassic of the Chulym-Yenisei Basin

The stratigraphy of the Jurassic deposits of this enormous basin, which in area far exceeds Kuzbass, has been worked out less fully than in Kuzbass; and in the stratigraphic schemes the subdivisions into formations, horizons, sequences, etc., have neither a complete lithological-petrographic basis nor, still less, a paleontological basis, although in some cases they are based on complete sections.

The most complete stratigraphic columns have been compiled for the Achinsk, Balakhta, and Krasnoyarsk districts. In the Achinsk district, M. K. Korovin, L. N. Zhukov, and others distinguish, from below: 1) the Selek Formation with coal seams—124 m; 2) a coal-barren formation composed of sandstones—147 m; 3) the Sobolevo Formation with coal seams, including sapropelic interbeds—226 m.

The upper coal-barren horizon of the Sobolevo Formation, composed of quartz-micaceous sands, has recently been separated by Ananyev into the Kemchug Formation, whose thickness is considered to be 100–140 m.

The stratigraphy of the Krasnoyarsk district has been studied more poorly than that of the Achinsk district. According to M. P. Nagorskii, it is represented in the following form (from below): 1) the Krasnoyarsk Formation with the following horizons: lower, Lagernyi, Korkinsky, and Badalyk. The formation contains semilustrous coals—490 m; 2) the Kubekovo Formation, lying with angular unconformity and containing seams of coals enriched in fusain—120 m; 3) the Tolstomyssk Formation, the coal-bearing character of which has not been clarified—130 m.

The total thickness of the Jurassic deposits reaches 750 m, but it is probably considerably greater.

When comparing the sections of the Achinsk and Krasnoyarsk districts, the discrepancy between the latter is striking. Judging from the few plant remains found in the Achinsk district, it may be considered that the Selek, coal-barren, and Sobolevo formations, containing a Middle Jurassic flora of the Ust-Balei type usual for the Siberian Jurassic, correspond to the Krasnoyarsk Formation.

In addition to the similarity in floral composition, these formations also show similarity in coal composition. In both the Achinsk and Krasnoyarsk districts, semilustrous coals predominate. As for the upper formations of the Krasnoyarsk district, the composition of their flora has not been studied, but it is known that peculiar ginkgoaleans predominate here, close to Ginkgo sibirica and Ginkgo digitata, although ordinary Middle Jurassic forms are also present alongside them. The coals of these formations are characterized by a high content of fusain. In the Achinsk district these formations are absent; but, on the other hand, in the Krasnoyarsk district there is apparently no coal-barren Kemchug Formation, which, according to Ananyev, lies unconformably on the older formations. The relationship of these two sequences has not been clarified, but if the Kubekovo and Tolstomyssk formations still

coal-bearing and contain Jurassic flora, whereas the Kemchug Formation is already non-coal-bearing and Jurassic flora has not been found in it; it may therefore be assumed that the latter is younger.

If all these data are taken into account, the total normal thickness of the Jurassic deposits of the basin reaches 900 m, but nevertheless, especially for the Krasnoyarsk district, it cannot be considered complete, since in the neighboring Balakhta district, according to the data of I. N. Zvonarev and I. I. Molchanov, the thickness of the Jurassic deposits reaches 1000 m, and the upper coal-free horizons, noted in the Achinsk district, are absent here. Some paleontological materials also point to a possible increase in the thickness of the Jurassic sediments to the northwest of Krasnoyarsk. It has long been known that in the Krasnoyarsk area, in coal-bearing deposits assigned by many authors to deltaic sediments, there are marly interbeds with remains of thick-shelled calcareous pelecypod shells, apparently non-freshwater. Here, as it were, a certain regular transition is outlined from coal-bearing deltaic deposits to marine facies.

The general stratigraphic scheme for the Chulym–Yenisei basin is presented in Table 1 (above):

Table 1

Kansk Basin

The stratigraphy of the Kansk basin, which is a direct continuation of the Chulym–Yenisei basin, has been clarified rather fully thanks to the work of A. V. Aksarin.

In the Rybinsk district, according to A. V. Aksarin, 9 horizons are distinguished (from below): coal-free IV, structural, coal-free III, lacustrine, coal-free II, Irsha, Borodino, coal-free I, and Sovkhozny. The total thickness of the sediments is 270 m. The coal-free horizons here are basal horizons and should be combined into a single rhythmic member with the corresponding overlying horizons; then the total number of rhythmic members will be 5. It is characteristic that in the lower rhythmic members predominantly dull coals are represented, whereas in the upper ones, beginning with the Irsha, thick interbeds of fusain occur.

The Jurassic of the Sayano-Partizansky district, subdivided into three formations (from below): Partizansky, Sayansky, and Ivanovsky, forms, in terms of the composition of the fossil flora and coals, a single whole and, judging by the type of coals, corresponds to the lower horizons of the Jurassic of the Rybinsk district. The general stratigraphic scheme of the basin is presented in Table 2.

Table 2

Comparison of the sections shows that in the Sayano-Partizansky district the lower part of the section is represented, which is absent in the Rybinsk district, and, perhaps, this partly explains the difference in the quality of the coals of these two districts.

Irkutsk Basin

The Jurassic deposits of the Irkutsk Basin, according to the composition of the flora and coal-bearing character, are subdivided into two formations sharply differing both in coal-bearing character and in the composition of the fossil flora: the lower (Cheremkhovo) formation, which contains coal seams, and the upper (Sukhovo) formation, in the lower parts of which there are thin coal beds, while upward the coal-bearing character dies out completely. The fossil flora of the lower formation is represented by the well-known Ust-Baley complex, found in all Jurassic basins of Siberia. As for the composition of the flora of the upper formation, the latter displays a number of features consisting in the fact that here there are ferns of the family Dipteridacea and sequoias unknown from the lower formation. The thickness of the lower formation reaches 200 m; the thickness of the upper is about 100 m.

Stratigraphic Correlations

A review of the Jurassic deposits of individual basins shows that they reveal a number of common features both in the composition of the fossil flora of individual formations and in the composition of the coals.

The paleobotanical material collected from the Jurassic basins of Siberia is clearly divided into three complexes. The oldest complex, found in the lower parts of the Barashevskaya sequence of the conglomerate formation of the Kuzbass, together with forms usual for the Siberian Jurassic, contains Annulariopsis inopinata (Zeiler) Halle, known only from deposits no younger than the Lower Jurassic. The presence of this plant, together with forms usual for the Jurassic, indicates a Lower Jurassic age of the sequence. The second complex, showing a close continuity with the flora of the Barashevskaya sequence, has the characteristic Ust-Baley type, with a predominance in the flora of ginkgoaleans, ferns, podozamites, etc. This flora was first described from the Irkutsk Basin, then was found in the other basins as well, but it is especially fully represented in the Sartakovskaya sequence of the Kuzbass, where, together with plant remains, there are remains of insects and fishes, also known only from Ust-Baley.

The third complex of fossil flora is at present known from non-coal-bearing formations: the Sukhovo formation of the Irkutsk Basin and the Tersyukskaya formation of the Kuzbass. This flora is characterized by an abundant content of phenicopsids, a smaller content of ginkgoaleans than in the underlying formation, but most characteristic of it is the presence in the lower parts of representatives of ferns of the family Dipteridacea, and in the upper horizons—Se-

quoia and (in the Kuzbass) the fern Laccopteris Dunkeri Schenk, giving the flora a very young, at least Upper Jurassic, aspect.

If the deposits of the Jurassic basins are considered from the point of view of coal accumulation, it may be noted that, having begun in the Lower Jurassic epoch (in the Barashevskaya series), by the end of the Jurassic period (in the Tersyukskaya suite of the Kuzbass, in the Kemchugskaya suite of the Chulym-Yenisei basin, and in the Sukhovskaya suite of the Irkutsk basin) it had completely ceased. At the same time, the composition of the coals of the different suites and series has its own peculiarities, apparently connected with changes in geotectonic and climatic conditions. Thus, the most ancient Jurassic deposits of Siberia, constituting the Barashevskaya series of the Kuzbass, have banded coals, but in general they are close to the coals of the overlying series. Younger deposits, containing a Middle Jurassic flora of the Ust-Balei type, in all basins contain uniform, to one degree or another platy, semilustrous or semidull coals, while the overlying series, the age of which has not been established with sufficient certainty, contains seams enriched in fusain. It is characteristic that this series both in the Kuzbass (Chusovitinskaya series), in the Chulym-Yenisei basin (Kubekovskaya and Tolstomysovskaya suites), and in the Kansk basin (Irshinsky and overlying horizons) rests on the older series with a break.

On the basis of the material described above, the history of the Jurassic period in Siberia appears as follows: 1) deposition of the Barashevskaya series in the Lower Jurassic; 2) deposition of the Sartakovskaya series and contemporaneous series of other regions in the Middle Jurassic epoch; 3) a break before deposition of the Chusovitinskaya series; 4) deposition of the Chusovitinskaya series and coeval suites of other basins; 5) a break before deposition of the Tersyukskaya suite of the Kuzbass, the Kemchugskaya suite of the Chulym-Yenisei basin, and the Sukhovskaya suite of the Irkutsk basin; 6) deposition of these suites in the Upper Jurassic epoch. Judging from these data, the Jurassic deposits of Siberia are rather clearly subdivided into three formations, differing in the composition of the fossil flora, the composition of the coals, and separated by unconformity surfaces.

A correlation of the deposits of the Jurassic coal-bearing basins is shown in Table 3.

Table 3