ON THE HISTORY OF JURASSIC COAL ACCUMULATION IN THE EASTERN PART OF WESTERN SIBERIA

I. V. LEBEDEV

In the eastern part of Western Siberia, by which is meant the territory drained by the right tributaries of the Ob River and the left tributaries of the Yenisei River (Fig. 1), coals of Jurassic deposits have long been known.

This article describes certain regularities in the accumulation of Jurassic strata that have been identified in recent years.

1. Stratigraphy

Within the continental Jurassic of Western Siberia, with which coal accumulation is associated, deposits of all three of its divisions are present.

Lower Jurassic deposits occur in all areas. In the Kuznetsk Depression, Lower Jurassic deposits are known from the Central Kuznetsk syncline and the Podobass syncline. The Lower Jurassic of the Central Kuznetsk syncline is made up of the Karaldino Formation [17]. At the boundary between the Jurassic and the underlying Triassic deposits, formations of an ancient weathering crust have been encountered. Above the contact, on the Malaya Karalda River (50 km southwest of the town of Leninsk), yellowish-gray sandstones, siltstones, and argillites with imprints of Lower Jurassic plants, pelecypods, and insects have been exposed [17; 23].

Sixteen kilometers east of the town of Leninsk, higher stratigraphic horizons of the Lower Jurassic have been established. They consist predominantly of weak sandstones, siltstones, argillites, siderites, and seams of brown coal, nine of which have workable thickness.

In the Podobass syncline, Lower Jurassic deposits have long been established [18; 19; 20]. Their volume and boundary with the Middle Jurassic deposits have not been determined. In comparison with the Lower Jurassic of the Central Kuzbass syncline, in the south coarse-clastic rocks predominate—conglomerates and sandstones, in which clayey-siltstone members occur as lenses. In addition, a regular change in the composition of the rocks is observed from south to north and from west to east. On the southern margin of the Podobass syncline, near the village of Novaya, conglomerates and sandstones predominate in the Lower Jurassic deposits, occurring as lenses. The pebbles of the conglomerates reach 35 mm in diameter, but in general pebbles of medium size predominate. Farther north, clay interbeds with coal seams appear in the conglomerate-sandstone members, and on the Abasheva River and on the Tutyas River the conglomerates are replaced by a sandy sequence, which only rarely contains thin clayey members with coal seams.

A sharp facies change in the Lower Jurassic deposits is also observed to the southwest of the Podobass syncline. Coarse conglomerates on the le-

on the bank of the Kandoma River they pass into sandy-clayey deposits with coal seams. In other areas of the Kuznetsk Basin, reliably established Lower Jurassic deposits are unknown.

The Lower Jurassic deposits of the Chulym–Yenisei Basin, which make up the Makarovo Formation, rest on an uneven erosional surface of the Paleozoic; the age of the formation has been established from spore-and-pollen assemblages.

The composition of the rocks of the Makarovo Formation, like that of the Lower Jurassic of the Kuznetsk Basin, is subject to considerable variation (Fig. 2).

North of Krasnoyarsk, on the left bank of the Yenisei River, a basal conglomerate about 20 m thick occurs at the base of the Lower Jurassic deposits. Higher up, sandy-clayey rocks containing coal seams predominate within the Lower Jurassic deposits. On the right bank of the Yenisei River, in the lower reaches of the Batoy River, the thickness of the conglomeratic sequence, according to exploration data, reaches 200 m, but these data require verification.

North of Krasnoyarsk the Lower Jurassic deposits have been penetrated in the Bolshe-Murtinsky district. As in the Krasnoyarsk district, at the base of the Lower Jurassic here lies a sandy-pebbly sequence distinguished by sharply expressed facies variability, as a result of which the sections of two neighboring boreholes prove difficult to correlate. The total thickness of the sandy-pebbly member reaches 90 m. Higher up in the Lower Jurassic deposits appear siltstones and a group of closely spaced coal seams, the thickness of which reaches 2 m.

Almost entirely of conglomerates is composed the Lower Jurassic on the Bolshoi Kemchug River below the railway bridge (about 75 km west of Krasnoyarsk), with coarse boulder material occurring in the basal beds of the conglomerates; but already at the village of Bolshoi Kemchug, 7 km north of the preceding locality, the coarse conglomerates disappear and are replaced by a sandy-clayey sequence in which pebbles occur only in the form of lenses.

At the Sobolevskoye coal deposit (east of Achinsk), the Lower Jurassic deposits have been studied only in individual boreholes; moreover, the very lowest horizons of this formation are unknown, and the exposed thickness is 123.9 m. The basal beds of this formation have not been penetrated by drilling and do not appear in outcrops.

At the Itatsk lignite deposit, according to A. I. Sitnikova, the thickness of the Lower Jurassic penetrated by drilling reaches 75 m; within it there are seven coal seams, with thicknesses from 0.73 to 4.19 m. Conglomerates are absent here, but in the neighboring Bogotol lignite deposit, at the base of the Lower Jurassic, there is a meter-thick bed of basal conglomerate consisting of fine pebbles. Above the conglomerate occur gray, sometimes reddish argillites, in the lower part of which there are two discontinuous coal seams 0.4 to 1.7 m thick.

In the Mariinsk reference borehole, the Lower Jurassic deposits were penetrated at depths of 1092—1152 m. Compared with the surrounding districts, the Lower Jurassic in this borehole has a reduced thickness and is almost entirely composed of clayey-siltstone rocks; sandstones and conglomerates were not encountered in this borehole. Both features are evidently connected with the fact that the entire Lower Jurassic was deposited on the surface of an erosional remnant.

West of Mariinsk, the Lower Jurassic has been penetrated by drilling only north of Anzhero-Sudzhensk, in the Ulanovskaya (Voznesenskaya) trough. According to N. Elisafenko and Yu. P. Kazansky, it has a thickness of up to 275 m. Its composition here is dominated by pebbles, gravelites, sand, sandstones, and siltstones. Facies variability of the rocks is noticeable;

Fig. 1. Lithologic-facies map of the Lower and Middle Jurassic deposits.
1—Alluvial and swamp deposits; 2—alluvial, lacustrine, and swamp deposits; 3—deposits of non-overgrown lakes; 4—outcrops of the pre-Jurassic basement; 5—directions of denudation; 6—isopachs and thicknesses.
Areas of Jurassic distribution in Kuzbass: I—Podobas syncline; II—Central Kuznetsk syncline; III—Inya Bay.

Normal sections of Jurassic deposits of the margins of the Chulym–Yenisei Depression

Scale: 20 0 20 40 60 80

• Nazarovka trough (after S. K. Kashik)
• West
• Center

• East

• Ulanovka trough (after materials of N. P. Kazanskii and I. N. Elisafenko)

• Antolin (after Yu. P. Kazanskii and I. N. Elisafenko)

• Ryabchik

• Borisovo–Glebskaya (after B. A. Kuznetsov)

• Marshinsk (after S. M. Frusev)

• Itat (after A. N. Sutnikova)

• Bogotol (after I. G. Andakova)

• Achinsk (after A. N. Zhukov)

• Sobolevskoye deposit (after A. N. Zhukov)

• Potap 13 and Dubravov boreholes (after materials of KazNIGNI)

• Krasnoyarsk region (after K. L. Kozanchuk and others)

• Bolshie–Murta region (after materials of A. M. Dolinkova)

• Kas (after materials of Melnikov and Tryshinova)

• Podkamennaya Tunguska (after I. V. Lebedev)

Stratigraphic designations shown on the sections include:

• \(J_3\)
• \(J_2\)
• \(J_2^a\)

Coal-bed and horizon names visible on the sections include:

• Ustyug
• Isakov
• Arban
• Itchersk
• Chernoberyi
• Slozhnyi
• Ivanovskii
• Verkhnеserezhnikovyi
• Andreyanov
• New
• Kansk
• Bolshoi
• Slozhnyi
• Malenko-krepkii
• Gorodok
• Neudachnyi
• Glubinnaya

Legend

\(J_3^1\) — Makarovka suite;
\(J_2^a\) — lower subsuite of the Itat suite;
\(J_2^b\) — upper subsuite of the Itat suite.

Fig. 2.

in the NE part of the trough, where only the upper horizons of the suite are exposed, the amount of sandstones and siltstones increases; coals appear locally; to the south, pebble beds predominate.

In the Nazarovo depression, according to S. A. Katsiev, L. N. Zhukov, and others, the composition and thickness of the Lower Jurassic deposits are subject to considerable fluctuations. In the eastern part of the Nazarovo trough the thickness of this suite is 60 m, while farther west it reaches 95 m; in this direction the content of sandy material decreases and coal-bearingness sharply increases.

Somewhat farther south, in the Kibetensk trough, the number of coal seams and their thicknesses increase still more: in the western part of this trough, this suite already contains six seams, whose thicknesses reach 1.35–11.10 m. It is characteristic that coarse clastic material in the Nazarovo and Kibetensk troughs occurs very rarely, only as small interbeds at the base of the suite.

The Jurassic of the central parts of the Chulym–Yenisei depression has been insufficiently studied, as a result of which the identification here of Lower Jurassic deposits is not yet possible.

Middle Jurassic deposits, established on the basis of spore-pollen and leaf flora, are distributed much more widely than the Lower Jurassic and are known from all areas where Jurassic strata are developed. In the Central Kuznetsk syncline these deposits are distinguished under the names of the Sartakovo and Chusovitino formations of the conglomerate suite [3; 17]. The first of these, composed of sandstones, siltstones, argillites, conglomerates, and coal seams, is distributed over the entire area of the syncline. In it, facies variability is clearly noticeable: in the eastern part of the syncline, on the Nizhnyaya Ters’ River, it consists almost entirely of conglomerates, while farther west, on the left bank of the Tom’ River, conglomerates occur only as small lenses or strings of pebbles in sandstones. Conglomerate members also wedge out both to the south and to the north of the Nizhnyaya Ters’ River.

In parallel with the wedging-out of the conglomerates, the content of sandy-clayey material increases and coal seams appear, of which there are more than 20 on the left bank of the Tom’ River, but only six of them reach workable thickness (more than 70 cm). West of the Tom’ River the Sartakovo formation has been studied by drilling at many points, and a relative persistence of the rocks along the strike and their sharp facies variability across the strike have been established (Fig. 3).

The Chusovitino formation is known from the area of the Chusovitino deposit (18 km NW of the city of Leninsk); it differs from the underlying Sartakovo formation by its thick seams of fusain coals. In all, there are more than 10 coal seams in this suite, of which 5 are workable. The total thickness of the formation is 170 m.

South of the Central Kuznetsk syncline, the Middle Jurassic is exposed in the Yerunakovo syncline, where it is composed of sandy-clayey rocks and coal seams, five of which reach workable thickness. These deposits are assigned to the Middle Jurassic provisionally.

In the south of Kuzbass, in the Podobass syncline, as indicated above, the lower boundary of the Middle Jurassic deposits has been drawn provisionally. As in the Lower Jurassic deposits, conglomerates predominate here, northward being replaced by sandy rocks. The thickness of the Middle Jurassic deposits of this area is about 600 m.

In the Inya bay, Middle Jurassic deposits are exposed along the rivers Tarsma and Kourak, which enter the Inya River from the left. In the belt adjoining the Salair here, conglomerates predominate; according to V. P. Kazarinov, their pebbles reach 20–30 cm and even 50 cm. The thickness of the conglomerates has not been determined, but exceeds 440 m. To the northwest, the conglome—

rates are replaced by sandy-clayey deposits containing thin coal seams. The Middle Jurassic age of the rocks has been established from numerous remains of plants and pelecypods.

The Middle Jurassic deposits of the Chulym–Yenisei depression, assigned to the Itat Formation, in the central parts of this depression lie conformably

Legend

• Argillite
• Siltstone
• Conglomerate
• Sandstone
• Coaly argillite
• Coal

Fig. 3. Scheme of the facies variability of deposits of the Sartakovka member of the Chusovitino deposit in Kuzbass. Based on materials of V. V. Ponomarev, A. A. Makhov, and P. E. Petrov.

on the Lower Jurassic, but along the margins and near protrusions of the pre-Jurassic basement they transgressively overlap more ancient deposits.

The Itat Formation along the margins of the Chulym–Yenisei depression is usually subdivided into a lower—barren—and an upper—coal-bearing—subformation. But these subformations have local significance and different thicknesses, since in the Middle Jurassic coal accumulation in different districts began at different times, and in some areas did not occur at all.

In the Krasnoyarsk district the lower subformation begins with a basal conglomerate, above which sandy rocks occur, containing thin beds of siltstones, argillites, and two small coal seams. The total thickness of the subformation is 117 m.

West of the city of Krasnoyarsk, the lower subformation of the Itat Formation is exposed near the settlement Pamyati 13 Bortsov, where it is composed almost exclusively of sandy rocks and pebble beds. Only 4–5 km to the NE of the settlement, i.e., away from the Kemchug Mountains, do siltstone interbeds appear in it.

At the Sobolevskoye deposit the lower subformation begins with basal pebble beds, above which occur sandy strata with interbeds of clayey rocks; the thickness reaches 197.5 m.

At the Nazarovo, Itat, and Bogotol deposits the lower subformation is also composed mainly of sandy rocks, but no basal pebble bed has been recorded here. The thickness of the subformation at the Nazarovo deposit is 130–150 m, and at Itat, 115 m. At the Itat deposit a small coal seam appears in this subformation.

In the section of the Mariinsk reference borehole, the lower subformation, penetrated at a depth of 1026–1092 m, consists mainly of conglomerates and coarse-grained sandstones, in which there are clay interbeds.

North of the railway, the lower subformation of the Itat suite has been penetrated in the village of Belogorka and in the village of Teguldet.

In the Belogorka borehole, a 150-meter sequence containing beds of aleurolite and one coal seam is assigned to this subformation. In the Teguldet borehole the lower subformation consists almost entirely of sandstones, but in its middle part there are packets of aleurolites. The total thickness of the subformation here reaches 215 m. Farther north, in the Maksimoyarsk and Kolpashevo boreholes, because of the general attenuation of coal-bearing character, the Middle Jurassic deposits are not subdivided into subformations.

The upper subformation, containing thick coal seams, is known in all areas of the Chulym–Yenisei depression. It conformably overlies the deposits of the lower subformation, but along the margins of the depression it rests on Paleozoic deposits.

The composition and thickness of the subformation likewise undergo considerable fluctuations. In the Krasnoyarsk district it is composed of sandy-clayey rocks, which contain more than 15 coal seams, of which nine have workable thickness. The total thickness of the subformation exceeds 400 m.

Forty-five km north of Krasnoyarsk, in the Minderla district, the upper subformation has a thickness of more than 485 m; moreover, compared with coeval deposits of the Krasnoyarsk district, it is more sandy. It is characteristic that the sandy horizons are concentrated in the lower part of the subformation.

In the Bolshe-Murtinsky district the upper subformation is distinguished by especially sharp facies variability, as a result of which the sections of adjacent boreholes here are correlated with difficulty, and the coal seams, of which there are more than 10, often pinch out. The thicknesses of the coal seams near the Yenisei Ridge rarely exceed one meter, but farther west their thickness increases considerably, and the Itat seam reaches 18 m.

On the Kas River the Jurassic deposits, penetrated near the mouth and near the settlement of Aleksandrovskoye, have not yet been subdivided. Probably among them there are deposits of both the Lower and the Middle Jurassic. In comparison with the Jurassic deposits of other margins of the Chulym–Yenisei depression, thick coal seams are absent here.

Farther north, Middle Jurassic deposits crop out on the left bank of the Podkamennaya Tunguska River, where their total penetrated thickness exceeds 50 m; there are three coal seams, and the thickness of one of them reaches 7 m.

In the Elogui reference borehole, coal seams were not encountered in the Jurassic deposits, although thin coal interlayers are present. It is not excluded that part of the Middle Jurassic deposits here is of marine origin.

West of Krasnoyarsk, the upper subformation was penetrated near the settlement of Pamyati 13 Bortsov. In contrast to other areas of the southern margin of the Chulym–Yenisei depression, the upper subformation here has a reduced thickness,

reaching only 140 m, and contains only two thin coal seams. Its lower half consists almost entirely of sandstones; in the upper part, aleurolites and argillites predominate. Good natural outcrops of the upper subformation occur on the right bank of the Bolshoi Kemchug River.

At the Sobolevskoye deposit the upper subformation was studied by numerous boreholes. According to L. N. Zhukov et al. [9], its total thickness reaches 270 m. Clayey rocks predominate in its composition; sandstones and sands are less abundant. The clayey members contain seams of brown coals and sapropelites. The total number of coal seams exceeds 12, of which 10 have workable thickness.

In the Bogotol and Itat districts and near the town of Achinsk, sandy rocks alternating with coal seams predominate in the upper subformation. It is characteristic that in the Itat district, in synclines, the thickness of the coal seams is considerably greater than on anticlines; moreover, sandy rocks predominate on the anticlines, whereas clayey rocks predominate in the synclines. The thickness of the Itat seam, locally exceeding 65 m in the syncline, decreases markedly on the crest of the anticline; at the same time, interbeds of rock contained within this seam on the anticline either decrease in thickness or pinch out completely (Fig. 4).

The total thickness of the upper subformation at the Bogotol deposit reaches 205 m, and at the Itat deposit, 220 m.

The section of the upper subformation in the area of the village of Borisoglebskaya, located 30 km WNW of Itat station, is close to the section of the Itat deposit, but differs in a sharp increase in the thickness of the coal seam overlying the Itat seam.

In the central part of the Nazarovo trough the upper subformation reaches 220 m, but toward the margins of the basin its thickness decreases considerably; at the same time, clayey rocks predominate in the central part and sandy rocks along the margins.

Parallel with the increase in the thickness of the subformation in the central part of the trough, a decrease in the thickness of the coal seams is observed. In comparison with the section of the Itat deposit, here in the upper part of the section there are more coal seams. In the Itat district, apparently, the seam that could be correlated with seam 1-m of the Nazarovo trough is absent.

To the southwest of the Itat deposit, the upper subformation is exposed at the “Rzhavchik” brown-coal deposit, situated on the southern margin of the Chulym–Yenisei basin, south of the village of Tisul. According to N. V. Kozlov [10], at the base of this subformation, on Cambrian limestones, lies a basal conglomerate of limestone pebbles, etc. The total thickness of the subformation is 91 m; that is, here it has a sharply reduced thickness, the reduction having occurred at the expense of the disappearance of its lower horizons. Farther north, the thickness of this sequence already exceeds 140 m.

The upper subformation of the Middle Jurassic in the area of the town of Mariinsk is distinguished by the presence of conglomerate horizons and by the complex structure of the Itat seam. The total thickness of the subformation here reaches 350 m.

Seventy kilometers southwest of Mariinsk, in the Ampalyk basin, the upper subformation has a thickness of only about 60 m, of which almost 30 m are made up of the Itat and overlying coal seams.

In the Ulanovskaya trough the upper subformation has a thickness of more than 170 m. As in the underlying Jurassic sequences of this area, the composition of its sediments changes from southwest to northeast. In the southwest, pebble beds and sands predominate, while to the northeast the amount of coarse-clastic material decreases sharply and the thicknesses of the coal seams increase.

Diagram
of changes in the thickness and structure of coal seams
in the southern part of the Chulym–Yenisei Depression

Scale: vertical, 25–0–25–50–75–100 m

North — South

Teguldet — Belogorka — Bukmyn — Itat Anticline — Itat Syncline

Visible numerical annotations include: 1.5; 0.75; 1.0; 1.0; 1.25; 2.75; 1.75; 4.0; 1.25; 1.0; 2.25; 2.0; 2.25; 3.5; 1.5; 11.00; 0.8; 1.0; 5.7; 1.5; 8.0; 1.5; 2.0; 3.5; 2.5; 3.5; 434; 0.65; 466; 373; 6.7; 382; 1.6; 1.2; 513; 0.3; 3.27; 5.62; 3.52; 0.35; 5.75; 12.43; 2.55; 2.0; 0.90; 1.30; 3.90; 0.80.

Fig. 4.

In the central parts of the Chulym–Yenisei depression, in the Belogorsk and Teguldet reference boreholes, the Middle Jurassic has a sharply increased thickness, reaching respectively 763 and 542 m.

The upper subformation in the section of the Belogorsk borehole comprises 613 m and in the Teguldet borehole 317 m; however, as indicated above, the subdivision of the coal-bearing deposits in the sections of these boreholes is generally conditional and is not substantiated by paleontological data. Characteristic of the Jurassic in these boreholes is the predominance of sandy rocks and the absence of thick coal seams. The Itat coal seam and the overlying thick seam in this direction gradually split, with the formation of numerous thin seams (Fig. 4).

In the Maksimoyarovskaya borehole, the Middle Jurassic, penetrated at depths of 2500–2170 m, is composed of sandy–clayey rocks. In the upper part of the Middle Jurassic section here there is one coal seam about 3 m thick. The base of the Middle Jurassic in this area has not been penetrated.

In the Tomsk–Kolyvan zone and on its northern continuation, Middle Jurassic deposits have been penetrated west of Tomsk, near Kolpashevo and near the village of Laryak.

In the Kolpashevo borehole, as indicated above, the Middle and Lower Jurassic have not been subdivided. Their total thickness reaches 508 m; no coal seams have been encountered in them. At Laryak only the upper horizons of the Middle Jurassic have been penetrated; they likewise contain no coal seams, but seams of coaly argillites are present.

West of Tomsk, near the village of Sechenovo, according to M. P. Nagorsky, the Jurassic deposits consist of gray and black coaly clays about 50 m thick; approximately in the middle part of their section, at a depth of 274.1 m, a coal seam 0.3 m thick was encountered.

The continental Upper Jurassic deposits in the eastern part of Western Siberia have a smaller area of distribution; at present they are known only in the central part of the Kuznetsk and the southeastern part of the Chulym–Yenisei depressions.

The Upper Jurassic deposits of the Kuznetsk depression, distinguished under the name of the Tersyukskaya formation [17], have been preserved from erosion only in the central part of the Kuznetsk basin, where their thickness exceeds 150 m.

The Tersyukskaya formation consists of sandstones, siltstones, and, very rarely, argillites and conglomerates. In the lower part of the formation there are thin coal layers up to 20–30 cm thick; in the upper part of the formation coals are absent. The color of this formation is usually greenish-gray, and in the upper horizons yellowish. The entire formation is characterized by rapid facies variability.

Paleontologically proven Upper Jurassic continental deposits in the Chulym–Yenisei depression, making up the Tyazhinskaya formation, were established only in recent years, when drilling revealed a unit lying above the coal-bearing Middle Jurassic and beneath the paleontologically characterized Lower Cretaceous Ilek formation.

The Kemchug formation, previously described by A. R. Ananyev [1] as Upper Jurassic, proved to be older. The Tyazhinskaya formation is distributed only along the southern and southeastern margins of the Chulym–Yenisei depression. Northward it passes into marine deposits.

The upper boundary of the Tyazhinskaya formation is drawn along the base of the basal sandstone of the Ilek formation, unconformably overlying the Jurassic; the lower boundary is drawn conditionally along the top of a thick coal seam lying above the Itat seam.

The Tyazhinskaya formation is composed of sands, weak sandstones, siltstones, and argillites. Coarse-clastic rocks occur in it very rarely. In the lower horizons, lying in the cores of synclines, thin layers of coal occur in this formation; on anticlines coal-

seams are absent. In the upper horizons of the formation, plant detritus is also absent.

The sands, argillites, and siltstones are thin- or finely bedded; they never form thick beds and are often interbedded with one another. Large sedimentary cycles, well expressed in the Lower and Middle Jurassic deposits, are not encountered here.

In the lower part of the Tyazhin Formation the rocks have a characteristic gray color with a greenish-bluish tint. In the upper part of the formation, interbeds of argillites and clays appear that have a pinkish or lilac color, but these lilac and pink rocks do not form a persistent horizon; in some boreholes they are absent and apparently occur in the form of lenses.

The thickness of the Upper Jurassic deposits generally increases from the margins toward the center of the West Siberian Lowland. The exceptions are the Tyazhin, Itatka, and other synclines, in which it increases sharply, reaching 280 m. In the central parts of the West Siberian Lowland the Tyazhin Formation is replaced by marine deposits.

The age of the Tyazhin Formation is established mainly from its occurrence above Middle Jurassic and below Lower Cretaceous deposits. Among the paleontological remains found in it are pelecypods, phyllopods, and ostracods.

2. Conditions of Formation of the Jurassic Deposits

The conditions under which the Jurassic sequences of Siberia were formed have been considered by many investigators.

V. I. Yavorskii [26] believes that the Jurassic conglomerates of Kuzbass are piedmont formations, which, away from the mountains, are replaced by sandstones. V. A. Khakhlov [24] assigns the same deposits to fluvioglacial formations; A. R. Anan'ev and D. V. Vasil'ev [2] see in them lacustrine-bog, river, and glacial deposits occurring as separate but synchronous facies. According to Yu. A. Zhemchuzhnikov [7], the Jurassic sequence of Kuzbass has all the features of limnic origin. In the conglomerates of the Jurassic deposits of Kuzbass, Yu. A. Zhemchuzhnikov sees alluvial or, locally, proluvial-alluvial cones formed in a piedmont valley. The sandy sediments, according to his data, in a number of cases have a clearly expressed channel character, while the more fine-grained rocks may be assigned a floodplain and lacustrine origin.

According to M. K. Korovin [11], in the Chulym-Yenisei basin the deposits that are now distinguished as the Makarov Formation formed under lacustrine-river conditions; the sequences constituting the lower subformation of the Itatka Formation formed under lacustrine conditions; and the upper horizons of the Jurassic, which are united into the upper subformation of the Itatka Formation, formed under deltaic conditions.

Common to the views of all the authors listed is the assumption of the widespread development of gravels and conglomerates in the Jurassic deposits. Yu. A. Zhemchuzhnikov [8] and I. I. Gorskii [5] even believe that in Jurassic time coal formation generally took place chiefly in intermontane depressions.

The principal sections of Jurassic deposits described above show that conglomerates in them are, on the whole, of insignificant distribution, occurring as narrow bands along the margins of depressions at the foot of mountain structures; but they are often absent even in piedmont regions; in particular, there are no conglomerates on the slopes of the Arga and Solgon ridges.

Conglomerate horizons of the Jurassic deposits are most often regarded as proluvial or proluvial-alluvial formations, but among them at least three genetic types can be distinguished.

Conglomerates form a broad belt along the southern margin of the Podobas syncline of the Kuzbass and only in its northern part are replaced by sandy facies. The conglomerates, rarely reaching 50 m in thickness, are part of thick cycles of deposits and extend for considerable distances. Judging by the good rounding and good sorting of the material and by their occurrence in the form of lenses, these conglomerates were deposited in river channels. Angular, unrounded, and unsorted pebbles, characteristic of proluvial deposits, are not typical of them. Farther north the role of sandy material increases, while the number of conglomerate beds correspondingly decreases until they disappear completely.

Similar conglomerates are also widely distributed along the eastern margin of the Central Kuznetsk syncline, in the estuarine part of the Nizhnyaya Ters River, where near the Koilot Mountains the Jurassic deposits consist chiefly of conglomerates; farther downstream the latter gradually disappear, and on the left bank of the Tom River only small interbeds and strings of well-rounded pebbles are present. The conglomerates wedge out not only westward but also northward and southward from the mouth of the Nizhnyaya Ters River, indicating that they formed in the valley of a large Jurassic river where it emerged onto the plain.

Judging from the fact that the conglomerates of the Insk Gulf and the Ulanovka trough occupy separate isolated areas, they should be assigned to the same genetic type. In the Insk Gulf, apparently, proluvial formations also participate in their composition, as is indicated by large blocks of rock encountered near Salair.

To another type belong the pebble-conglomerate deposits developed on the northern slope of the Kemchug Range and along the southwestern slope of the South Yenisei Ridge.

In the deposits along the Yenisei River from the city of Krasnoyarsk to the mouth of the Kan River, and in boreholes along the Yenisei Highway, a continuous belt of conglomerates has been traced, passing westward into sandstones. In outcrops of the conglomerates here, oblique bedding, lens-shaped occurrence, and their gradual westward replacement by sandstones are clearly visible.

The conglomerates of this belt are piedmont-fan, or, according to another classification, piedmont-alluvial formations, although they are somewhat distinctive. Among them have not been recorded unstratified, poorly rounded, and unsorted deposits assignable to fanglomerates. These are typical fluvial deposits, in which lenses of conglomerates composed of rounded pebbles alternate with sandstones and coal beds.

It is characteristic that, in contrast to the Kuzbass, there are no areas here of concentration of thick and isolated pebble strata that would indicate their formation in the valley of a single large river. Although at the mouths of the Berezovaya and Kan rivers there are apparently somewhat more pebble beds than in other areas, overall this piedmont-fan belt represents an alluvial formation of many small rivers that flowed down from the South Yenisei Ridge.

The peculiarity of these piedmont-fan or piedmont-alluvial formations, therefore, consists in the fact that they are small-river formations. This character of the sediments indicates both an undeveloped hydrographic network and slow subsidence of the Chulym-Yenisei depression.

To the third type belong conglomerates occurring far from mountain structures. These are conglomerates occurring as small lenses in thickness among sandy-clayey coal-bearing rocks. They are composed of fine pebbles and gravel, occupy longitudinal depressions in coeval strata (Fig. 3), and represent typical deposits of those parts of river valleys that were distant from the source area.

In the sandy rocks of the Jurassic deposits of the Kuzbass and the margins of the West Chulym depression, in most cases, multistory cross-bedding is observed, with each lower series being cut off by the overlying series. Between the series there are often strings of pebbles and thin interbeds of gravelite. In the series, the cross-beds usually dip at an angle of 5–15°, more rarely up to 30°; moreover, the direction of dip of the sandstones in stratigraphic sections often changes by 10–30° over 0.5 m. In the central part of the Kuznetsk depression, lenses can be observed in sandy rocks that are filled with chaotically arranged tree trunks up to 15–20 cm in diameter. Pebbles are sometimes found in them. These lenses are very reminiscent of logjammed reaches of present-day taiga rivers of the West Siberian Lowland.

Another type of sediment predominates in the central parts of the West Siberian Lowland. Pebble and conglomerate beds are absent here; in the sandy rocks, fine horizontal and gently undulating bedding is usually observed. Swamp facies, together with these rocks, as a rule, are not encountered. The Jurassic strata of these regions apparently belong to deposits of lakes that did not become overgrown.

The materials cited above show that the composition, structural, and textural features of the Jurassic deposits testify to their formation in river, lake, and swamp environments; in general, these are deposits of alluvial plains. Typical deltaic deposits in the Jurassic deposits of the territory under consideration have not yet been established, although they may yet be discovered. Fluvioglacial and glacial deposits are also unknown among them.

V. I. Popov [21] regards all continental Jurassic deposits of Siberia as molasses. In lithological composition, the Jurassic strata of the Kuznetsk basin and the margins of the Chulym–Yenisei depression are indeed, to some extent, similar to molasses, but the Jurassic of most of Western Siberia does not have the characteristics of a molasse formation. In general, the Jurassic rocks of Siberia formed without connection to geosynclinal zones, and their identification with molasses cannot be considered successful.

The Jurassic deposits of Western Siberia generally have a gray or green color. Red-colored formations are absent in them, with the exception of the red-colored clays of the Tyazhin Formation.

The climatic conditions that prevailed in Western Siberia during the Jurassic period are indicated by the vegetation and, in part, by the mineralogical composition of the deposits.

In the vegetation of the Siberian Jurassic, ginkgoes and conifers predominated, and together with them, in small quantities, as undergrowth, grew ferns and small horsetails. In the more southern parts of Asia, red-colored rocks appear in the Jurassic deposits; in the composition of the vegetation, cycads predominate.

These differences in lithological composition and vegetation were explained by A. N. Krishtofovich [13] as due to the more severe climatic conditions of Siberia. V. A. Vakhrameev [4] and I. I. Gorskii [5] believe that the climate of the northern province in the Jurassic was humid and moderately warm, while Yu. M. Sheinman [25] considers the Lower and Middle Jurassic flora of northern Asia to be characteristic of a temperate climate. V. D. Prinada [22] supposed that the Jurassic flora of Asia was unusually weakly differentiated, and explained this by the uniformity of climate over almost the entire globe.

However, the climatic conditions of the Jurassic in Siberia were not constant. First, it should be noted that in the composition of the Middle Jurassic moderately thermophilic flora of the Kuzbass, representatives of flora characteristic of tropical and subtropical provinces sometimes appear in fairly large numbers; but these elements in the stratigraphic section

distributed very unevenly. Thus, in the Jurassic of the central part of the Kuznetsk Basin, cycadophytes occur mainly in beds lying near the Neyburg seam [17].

An even more striking example of the confinement of plants characteristic of tropical and subtropical provinces to strictly defined stratigraphic horizons is provided by thermophilic ferns, whose numerous remains in the Kuzbass occur above the roof of the Upper Itatskii seam and are entirely absent in other horizons [16]. In the Chulym–Yenisei depression they have been found in the very lowermost Jurassic. The appearance of thermophilic flora in different horizons can be explained only by their immigration from the south in connection with changes in climatic conditions.

Some indications of changes in climatic conditions in the Middle Jurassic of Siberia are present in the lithological composition of the rocks. Yu. P. Kazanskii established, in the Middle Jurassic deposits of the Itatka district, a unit of white kaolinized sands. The origin of these rocks cannot at present be considered firmly established. They may have been redeposited from a pre-Jurassic weathering crust, but their formation in the Middle Jurassic as well is not excluded.

Some changes in climatic conditions are reflected in the mineralogical composition of the clastic rocks. According to Yu. P. Kazanskii, the lower Jurassic of the margins of the Chulym–Yenisei depression is characterized by garnet–zircon and zircon assemblages, whereas the Middle and Upper Jurassic are characterized by an epidote assemblage of heavy minerals. Such differences may depend on the rate of mechanical destruction, but chiefly on the degree of chemical weathering.

The sharp increase in epidote content in the Middle- and Upper Jurassic deposits testifies to a weakening of chemical weathering in the source areas, while the Lower Jurassic sequence, apparently, was formed mainly from redeposited products of the Triassic weathering crust.

But in the very lowermost parts of the Makarovo Formation, as indicated above, thermophilic plants are still present, pointing to a gradual change of climate and to possible deep chemical weathering during sedimentation. To this weathering is due the appearance, in the Lower Jurassic deposits, of syngenetic bed-like deposits of siderite.

3. Coal-bearing character

Coals occur in all divisions of the Jurassic system, but in the Upper Jurassic deposits there are only thin coal interbeds of no practical significance. In addition to this stratigraphic factor, reflecting the influence of climatic changes, the coal-bearing character of the Jurassic deposits depends on a number of other causes, among which the principal ones are differences in the tectonic regime of individual regions of Western Siberia.

A comparison of the composition of Jurassic deposits in individual regions shows that the Chulym–Yenisei and Kuznetsk depressions are distinguished by the greatest coal saturation, whereas within structures with a Hercynian basement (the Tomsk–Kolyvan zone and its continuation to the north) the coal-bearing character decreases—in deposits of the same age, both the number and the thickness of coal seams diminish.

This change in coal saturation is determined first of all by the character of the tectonic regime—namely, by the continuous or intermittent character of the subsidence of the basement on which the Jurassic deposits were formed.

Within the area of development of the Hercynian basement structures (the epigercynian platform in the narrow sense), subsidence during the Jurassic was

Scale: horizontal — 0.5, 0, 0.5, 1, 1.5, 2 km; vertical — 25, 0, 25, 50, 75, 100 m.

Legend:

• Coal
• Argillite with clay
• Siltstone
• Granite
• Sands and sandstones
• Pebble beds and conglomerates
• Effusive rocks

Labels in the section: \(J_2\), \(J_2^{b}\), \(J_2^{a}\), \(J_1\).

Fig. 5. Section of the Jurassic deposits of the northern limb of the Itat anticline
(after materials of B. A. Kuznetsov).

continuous, which led to the formation of Jurassic sequences practically devoid of coal (Fig. 1).

In the Chulym–Yenisey and Kuznetsk basins, with their Salair–Caledonian basement, regional movements were intermittent in character and led to a sharply expressed cyclic structure of the Jurassic sediments. These intermittent regional movements created both large cycles—formations—and cycles of a lower order, which led to the formation of a succession of rocks from conglomerates or sandstones to coal. As a rule, the greater the number of such small cycles, the greater the total thickness of the coal seams in the section. Exceptions are cycles containing the Itat seam (the Thick seam) and some other seams of the Chulym–Yenisey basin; in these cycles the thickness of the coal seams is extremely great, and in stratigraphic sections the total coal thickness depends not so much on the number of cycles as on these thick seams.

In Fig. 1 this regularity is clearly visible on the largest scale—the replacement of coal-bearing Jurassic deposits, developed along the eastern and southern margins, by coal-barren deposits toward the central areas of the West Siberian Lowland. But even the regions peripheral with respect to the entire lowland are not homogeneous in terms of tectonic regime, and within them (in particular, in the Kuznetsk and Chulym–Yenisey basins) the same regularity is observed: the cyclicity of the Jurassic deposits, as indicated above, is well manifested only along the margins of these basins, where tectonic movements created a sharp change of facies.

Beyond the zone of development of these marginal facies, toward the center of the basins, where continuous subsidence persisted without the formation of swamp facies, the coal seams gradually pinch out, cyclicity in its classical expression disappears, and, in essence, the Jurassic coal-bearing formation is replaced by a formation of lacustrine deposits containing no coals.

The pinching out of coal seams, or the reduction of their thickness as a result of rapid subsidence, sometimes also occurred on the margins of the Chulym–Yenisey basin, in particular in the Nazarovo trough, where, as indicated above, in the central parts of the trough the thicknesses of the coal seams noticeably decrease.

On the other hand, the cyclic structure of coal-bearing deposits changes sharply in areas lagging behind in subsidence and in areas of prolonged accumulation of piedmont alluvial deposits. In the first case, several cycles merge into one, and the total thickness of the coal seams is considerably reduced. Thus, in particular, the thickness of the seam lying above the Itat seam changes within the Itat anticline (Fig. 5). The greatest coal-bearing capacity is observed in areas where downwarping was compensated by the accumulation of peat bogs, as, for example, in the Itat syncline (Fig. 4).

Against the background of causes of a tectonic character, the distribution of facies within each cycle exerts a significant influence on the coal-bearing capacity of individual areas, and the coal-bearing capacity of the Jurassic deposits is to a considerable degree determined by the character of the oro-hydrographic elements at the time of coal accumulation.

In the central part of the Kuznetsk basin, a belt of predominance of channel deposits, almost devoid of coals, stands out rather clearly; beyond it lie floodplain and swamp facies with coals (Fig. 3). In areas where piedmont alluvial fans of large rivers predominated—for example, in the southern part of the Podobas syncline, on the southwestern margin of the Inya Bay, and in the lower reaches of the Lower Ter—

productive facies wedge out; in the stratigraphic sections of these areas there are only pebble and sandy facies.

This regular dependence of coal content on the character of the alluvial stream and its parts was first established by G. F. Krasheninnikov [12] for the Chelyabinsk basin, but it apparently manifests itself in all intermontane depressions.

Finally, of great importance for coal content—especially of the lower Jurassic horizons—is the character of the primary unevenness of the Paleozoic basement. The Jurassic of the territory under consideration was deposited on a peneplain surface that had been substantially reworked by pre-Jurassic erosion. As a result, beneath the Jurassic deposits in many areas there are buried erosional remnants, above which the coal-bearing sequence and coal seams wedge out.

Most seams of Jurassic coals in the Kuznetsk depression have a complex structure; on the margins of the Chulym-Yenisey depression there are more seams of simple structure, but in the cores of synclines and troughs, owing to more rapid subsidence, the seams often split (Fig. 4).

Splitting of coal seams caused by non-tectonic reasons is observed in the Ulanovo trough. In this trough, near the Kuznetsk Alatau, owing to a change in the direction of the river flow, coal accumulation in places ceased; the coal mass was probably eroded and replaced by channel and floodplain deposits, as a result of which the seams split toward the margins.

The petrographic composition of coals of the Jurassic deposits was studied by Z. V. Ergolskaya [6], A. A. Larishchev [14; 15], and A. B. Travin. According to A. A. Larishchev, among the Jurassic coals of Kuzbass there are humite, humite-liptobiolite, sapropelite, and mixed types of coal.

Humite coals, represented chiefly by banded woody-durain varieties, predominate in the lower parts of the coal-bearing deposits; in the Chusovitino sequence, according to data of I. V. Lebedev, fusain varieties of humite coals predominate. Humite-liptobiolite coals, according to A. A. Larishchev, are characterized by a predominance of dense, almost homogeneous and indistinctly striated semilustrous variegated coals of the clarain series. They consist mainly of the periderm of ginkgo and coniferous plants in the form of vitrain, xylovitrain, xylain, and cork tissues, as well as products of their fusainization. In addition, cuticular, spore, and resinous varieties of humite-liptobiolite coals occur. Humite-liptobiolite coals are found in all sequences. Sapropelic coals, in the form of individual bands, have been found only in the Sartakovo sequence in the basin of the Ubik River.

The Jurassic coals of the Chulym-Yenisey depression, according to A. A. Larishchev [15], belong to mixed sapropelite-liptobiolite-humite semibituminous varieties. In addition, at the Sobolevskoye coal deposit the coal seams of the Itatka Formation contain interbeds of sapropelites [9], and in the Maksimoyarsk borehole—humic coals.

A. B. Travin, who studied coals from the Mariinsk reference borehole, notes that among them the following varieties occur: lustrous coal with a bright resinous luster, semilustrous indistinctly banded coal of massive structure, semidull indistinctly striated coal, dull finely banded coal, sooty essentially fusain coal, etc. In the upper parts of the borehole section, lustrous and semilustrous varieties of coals predominate; lower down, the content of semilustrous varieties increases, being greatest in the Itatka seam, and in the lowermost parts, judging from individual samples, semidull and semilustrous coals are present.

In terms of the degree of metamorphism, among the coals of the Jurassic deposits there are distinguished brown coals, gas coals, and long-flame coals. A. B. Travin believes that, in the territory under consideration, according to microscopic features, the least metamorphosed are the coals from the Mariinsk reference borehole, which are at the stage of brown coals passing into long-flame coals. The coals of the central part of the Kuznetsk depression are somewhat more metamorphosed, being at the stage of gas and long-flame coals of low rank, while the most metamorphosed are the coals of the Middle Jurassic deposits from the Maksimoyarsk reference borehole, which belong to long-flame coals of a high degree of maturity. These coals are more mature than the coals of the Sayano-Partizansk district of the Kansk basin, which were considered the most metamorphosed Jurassic coals of Siberia. A. B. Travin explains such a sharp difference among coeval coals by the different depth of their occurrence at the time of maximum coalification.

CITED LITERATURE

1. Ananyev A. R. New materials on the geology of Mesozoic deposits in the Chulym-Yenisei basin. Proceedings of Tomsk State University, vol. 124, 1953.

2. Ananyev A. R. and Vasilyev D. A. Materials for the study of Jurassic deposits of the central part of Kuzbass. Proceedings of Tomsk State University, vol. 96, 1939.

3. Butov P. I. and Yavorsky V. I. Materials for the geology of the Kuznetsk coal basin (southwestern margin of the basin). Materials on General and Applied Geology, issue 48, 1922.

4. Vakhrameev V. A. Stratigraphy and fossil flora of the Cretaceous deposits of Western Kazakhstan. Regional Stratigraphy, vol. 1, 1952.

5. Gorsky I. I. Belts and nodes of coal accumulation in light of new data. Proceedings of the Coal Geology Laboratory of the Academy of Sciences of the USSR, vol. V, 1956.

6. Yegolskaya Z. I. Microscopic study of some Jurassic coals of Kuzbass. Proceedings of the All-Union Geological Prospecting Association, issue 340, 1933.

7. Zhemchuzhnikov Yu. A. Development of coal accumulation in geological history. Izvestiya of the Academy of Sciences of the USSR, geological series, no. 3, 1955.

8. Zhemchuzhnikov Yu. A. Coal-bearing strata as formations. Izvestiya of the Academy of Sciences of the USSR, geological series, no. 5, 1955.

9. Zhukov L. N., Kalmankin P. P. and Yatsuk V. I. Materials on the geology of the Chulym-Yenisei basin. Materials on the Geology of Krasnoyarsk Krai, no. 5, 1939.

10. Kozlov N. V. The Tisul deposit. In: Local Fuel of Western Siberia, 1940.

11. Korovin M. K. The Chulym-Yenisei coal-bearing basin. In: Mineral Resources of Krasnoyarsk Krai, 1938.

12. Krasheninnikov G. F. Facies changes of the coal-bearing sequence of the Chelyabinsk basin. Soviet Geology, no. 10, 1940.

13. Krishtofovich A. N. The Angara suite. Baikal Division. Proceedings of the All-Union Geological Prospecting Association, issue 326, 1933.

14. Larishchev A. A. Fossil coals from the central part of Kuzbass. Scientific Notes of Tomsk State University, vol. 4, 1946.

15. Larishchev A. A. On the stratigraphy of the Meso-Cainozoic of the West Siberian Lowland. Proceedings of Tomsk State University, vol. 132, 1954.

16. Lebedev I. V. The Jurassic of the central district of Kuzbass. Proceedings of Tomsk State University, vol. 65, issue 2, 1950.

17. Lebedev I. V. The Mesozoic of the Kuznetsk depression. Problems of the Geology of Kuzbass. Ugletekhizdat, Moscow, 1956.

18. Martynova O. M. The first find of a Jurassic insect in the Kuznetsk basin. Reports of the Academy of Sciences of the USSR, vol. 65, no. 5, 1949.

19. Neiburg M. F. On the stratigraphy and age of coal-bearing deposits of the Kuznetsk basin in Siberia. Reports of the Academy of Sciences of the USSR, series A, 14, 1929.

20. Neiburg M. F. Coal-bearing deposits of the Mesozoic of the Kuznetsk basin. Geology of the USSR, vol. XVI, 1940.

21. Popov V. I. Brief regional-lithological characterization of the Upper Ob (Cenozoic) molasse formations of Central Asia. Proceedings of the Central Asian Conference of Petroleum Geologists, 1948.

1. Prinada V. D. On the Mesozoic flora of Siberia. Materials on the Geology and Mineral Resources of Eastern Siberia, issue 19, 1944.

2. Rohdendorf B. B. Paleoentomological investigations in the USSR. Transactions of the Paleontological Institute of the Academy of Sciences of the USSR, vol. XVI, 1957.

3. Khakhlov V. A. Geological section of the productive strata in the southeastern part of Kuzbass. Proceedings of the West Siberian Geological Prospecting Trust, vol. 12, issue 4, 1932.

4. Sheinman Yu. M. Upper Paleozoic and Mesocenozoic climatic zones of Eastern Asia. Bulletin of the Moscow Society of Naturalists, Geological Section, No. 6, 1954.

5. Yavorsky V. I. Some results of geological investigations in the Kuznetsk Basin. Transactions of TsNIGRI, issue 26, 1934.

Tomsk Polytechnic Institute
named after S. M. Kirov