Jurassic of the Central District of Kuzbass

I. V. Lebedev

Introduction

Within the productive deposits of the Kuznetsk Basin, alongside the deposits of the coal-bearing Paleozoic series, coal-bearing deposits of Jurassic age are widely distributed. Knowledge of these distinctive sediments, while the sediments of other series have been studied fairly completely, has at present lagged considerably behind. All our information on the Jurassic deposits is limited to descriptions of bank outcrops, which have led to contradictory conclusions. Meanwhile, the problems of raw material supply for the developing coal-distillation industry urgently require the study of the latter both from the standpoint of their coal-bearing character in general and of the content of sapropelic coals in them in particular.

Along with this, it is also necessary to determine the mining-engineering conditions for the exploitation of Jurassic coals enclosed among rocks that are, to a considerable degree, loose and saturated with large reserves of underground water. The latter circumstance, which has raised the problem of supplying the industrial centers of Kuzbass with water from Jurassic deposits, also requires the study of the general structure of the areas of distribution of Jurassic deposits and the identification of the most water-bearing horizons, and consequently the stratigraphic and facies subdivision of the Jurassic deposits. Finally, until recently the character of the contact surface between the Jurassic and the underlying Paleozoic rocks has remained unclear. The exploitation of deposits of Paleozoic coals beneath Jurassic deposits makes it necessary to study the Jurassic deposits from this point of view as well.

All this taken together served as the basis for the trust “Kuzbassuglerazvedka,” already during the years of the Patriotic War, to undertake a comprehensive study of the Jurassic deposits, first of all the Jurassic deposits of the Central District, as the one closest to the industrial centers.

The present article is a summary of materials on the stratigraphy and tectonics of the Jurassic of the Central District, obtained as a result of work carried out by the author on assignment from the trust “Kuzbassuglerazvedka.”

Methods of Investigation

In the geological investigations of the district, in order to subdivide the Jurassic deposits, a unified method was adopted, consisting of careful observation of changes in the granulometric composition of the rocks both in the vertical and in the horizontal directions, as well as observation of the character of the contacts between beds and of textural features. Investigations in this direction showed that in stratigraphic sections there is a definite regular sequence in the change of granulometric composition from bottom to top: from sandstone beds (more rarely—conglomerate) to aleurolites, then to argillites and coals, after which the grain size in the rocks sharply increases and sandstones again appear—

sandstones, and then a successive alternation is observed up to the coals. As a result of such succession and regular change of rocks, all stratigraphic sections prove to consist of separate asymmetric units of the composition sandstone—siltstone—argillite—coal, sandstone—siltstone, etc. For such asymmetric units the term “cycle” became established. However, recently A. M. Kuzmin has proposed a new, more successful term—“rhythmic member.” A rhythmic member, corresponding to one complete period of geotectonic oscillation, proves to form a body occupying a definite volume—a “rhythmic lens,” which has considerable extent. As a result of such a structure of the sediments, detailed stratigraphic subdivision practically amounts to distinguishing rhythmic members and tracing rhythmic lenses.

The following criteria were adopted for distinguishing rhythmic members:

1. Breaks in the normal course of change in the granulometric composition of the rocks, when the wave-like curve showing this change is interrupted by an abrupt jump toward an increase in grain size.

2. The presence of multi-tiered and coarse cross-bedding with alternating large lenses of rocks, between which occur chains or individual inclusions of pebbles and fragments of tree trunks, i.e., the presence of so-called basal bedding in the lower components of the rhythmic members.

3. The presence of massive or finely and horizontally bedded rocks in the upper components of the rhythmic members.

The practical significance of the analysis of rhythms in coal-bearing sediments proved to be extremely important. In studying the stratigraphic succession of beds, one often encounters gaps in outcrops, the latter usually being confined to the easily weathered upper members of rhythmic units—coals and argillites. In coal-bearing deposits, where there is a gap between siltstones below and sandstones above, the presence of a coal bed at the place of the gap, and even its approximate position, can be predicted without error. Further, since each member of a rhythmic unit is characterized by a distinctive type of bedding, it becomes possible, from small outcrops, to determine the stratigraphic position (with respect to the given rhythmic member) of each component of the rhythmic member. For example, “basal,” multi-tiered cross-bedding in sandstones always indicates proximity to the footwall of a rhythmic member and proximity of a coal seam in the substratum of this component.

Furthermore, rhythm analysis is also important for correlating separate sections. Movements of the Earth’s crust that caused the manifestation of rhythmic lenses could not but have had different amplitudes of oscillation for different parts of the latter. Hence, there are facies differences in synchronous sediments within the limits of one and the same area. At the same time, in the general case, sediments similar in composition and thickness should be arranged parallel to isocatabases and isoanabases, and hence the possibility arises of facies persistence and comparative ease of correlation along their extent. Conversely, normal to the extent, in synchronous deposits, rapid facies variability is observed, which considerably complicates the tying together of sections; but in this case the rhythmic members themselves are an important “correlative” feature, i.e., their number, not the thickness and character of the sediments, completely loses practical significance. The importance of the latter circumstance is especially great for deposits in which synchronous channel and swamp facies can be distinguished. Sections of coeval facies of these deposits, elongated along the shoreline, are correlated quite simply, whereas across the strike

it is difficult to correlate two sections compiled at insignificant distances.

But it turns out that, while rhythmic members for correlating sections may find broad application in the study of strata formed as a result of sharp, intermittent oscillatory movements, they cannot always be applied to the study of strata formed during smooth epeirogenic movements in peneplained areas, or to the study of typical river deposits. In the first case, owing to the slight difference in levels between the area of denudation and that of sedimentation, the marginal components of the rhythmic members differ little. Here the rhythmic member, although deposited over large areas, is thin and has only weakly expressed individual features. Such are, in the Central District, the sediments of the Mal’tsevo suite underlying the Jurassic.

In the second case, rhythmic members as such often do not form, since in this case the composition of the rocks in the stratigraphic sequence is not always the result of successive, continuous sediment accumulation, but often represents the result of a change in the direction of the current, subsequent erosion, etc. The appearance of a pool and an oxbow lake may cause the deposition of a lens of peat or clay on sandy soil, and a new change in current direction may cause the deposition of a lens of clay on peat and of sand on clay; as a result, “inverted” rhythmic members of the composition coal–clay–sand may be obtained. Of course, in the area of the main current, normal rhythmic members should form in this case as well, but because of the constant shifting of the stream, analysis of rhythms in such sediments becomes practically impossible. It should be noted that in individual beds of such “inverted” rhythmic members, the grain size, as usual, decreases from bottom to top, thereby proving normal, and not “inverted,” bedding. From what has been said it follows that in “inverted” rhythmic members each bed may sometimes represent a rhythmic member, but correlation of such rhythmic members is practically impossible. Such rhythmic members are not rare in the Upper Jurassic deposits of the district.

Analysis of the rhythms of sediment accumulation was always accompanied by careful bed-by-bed collection of paleontological material and by the taking of samples for microscopic study and analyses. Proceeding from the considerations that the individual character of a rhythmic member is manifested above the basal horizons, in which reworked minerals of the underlying rhythmic members may still occur, and that in sandstones features characterizing the source area, the degree of transport, and the conditions of sedimentation are better preserved, samples of medium-grained sandstone were taken for microscopic studies.

Stratigraphy

The Jurassic deposits of the Central District, on the basis of paleontological and lithological features, are subdivided into two suites—the conglomerate suite and the Tersyuk suite.

Conglomerate Suite

The conglomerate suite rests on formations of an ancient crust of kaolin weathering. On the right bank of the Tom’ River, 2 km upstream from Polyakovskii Kamen’, it was established that the conglomerates of the upper part of the Mal’tsevo suite, which have a dirty-green color, gradually become lighter upward and, after 10–11 m, acquire a white color. In the cement of the conglomerates, owing to the destruction of feldspar, a white clayey substance appears.

In places within the conglomerate there are irregular concretions with a siliceous-clayey cement. Upon these kaolinized rocks, light-yellowish siltstones of the conglomerate suite with coal beds lie with sharp unconformity. Traces of a weathering crust lying between the deposits of the conglomerate and Mal’tsevo suites occur in the basin of the N. Ters’ River, near the village of Komlevo, and then in the upper reaches of the Kyrgai River, near the village of Teplaya. In the western part of the district, beyond the “Melafirovaya Horseshoe” mountains, Jurassic deposits are underlain by deposits of Permian age, but remnants of an ancient weathering crust are also observed there. Near the village of Sartakovo they are represented by bleached argillites of the Kolchugino suite, and in the system of the Un’ga River by quartzite-like sandstones.

The conglomerate suite is composed of sandstones, siltstones, argillites, conglomerates, and coal beds.

Sandstones make up more than 75% of the total composition of the suite’s rocks. They are light gray, slightly yellowish in color; in composition they are arkosic, containing 30–40% orthoclase and 60–70% quartz¹. The quartz grains are large, weakly or poorly rounded. The feldspars—orthoclase and isolated grains of microcline—have been preserved as tabular crystals and are fresh. Fragments of siltstones, sometimes coaly, are present in appreciable quantity. Microquartzites, siliceous-clayey, and sericite schists are often encountered among them. In rhythmic bundles the thickness of sandy components ranges from 20 to 100 m, and only in the case of complicated rhythms, when sandy horizons are repeated several times, is their thickness limited to meters.

In outcrops of the sandy components of rhythmic bundles, a complex form of bedding is always conspicuous. First of all, parallel bedding can be observed, caused by the appearance of 1–2 cm interlayers of clayey material. Such bedding accounts for the appearance of separate sandstone beds within the overall thick mass of one and the same sandy component of a rhythmic bundle. The bedding planes in this case are irregular and coarsely wavy. The appearance of such interlayers of clayey rocks is probably connected with seasonal conditions, but it is possible that they are the result of sharp, local, short-term interruptions in deposition or of submarine erosion. Parallel bedding is often observed in outcrops, caused by the appearance of chains of closely spaced pebbles of medium and large size. The pebbles are usually well rounded and consist of quartzites and metamorphosed sandstones.

Within the sandstone beds, multistoried cross-bedding is observed, with each underlying series being truncated by the overlying series. Between the series there are often chains of pebbles or thin interlayers of gravelite. In the series, the cross-beds consist of interlayers 3–5 cm thick, the bedding of which is caused by the appearance of clayey or coaly material, or by an alternation of yellowish and grayer sandstones of the same composition. The thickness of the series is up to 0.5 m. The small cross-beds within the series are usually directed to one side and dip at an angle of 5–15°, more rarely up to 30°. There are cases when, within one series, the direction of dip of the cross-beds in a stratigraphic section changes in jumps by 10–30° over a distance of 0.5 m. In a group of series, no definite spatial orientation is observed.

In the lower part of the sandy components of rhythmic bundles, lenses are often encountered that are formed of mixed coarse-grained sands, small but large pebbles, and numerous fragments of plant stems. The latter are almost always destroyed and, on the surface of the sandstones—

are represented by voids. The diameter of the stems is rarely more than 10–15 cm. The dimensions of the lenses are 2–5 m × 1–2 m, rarely somewhat larger.

Siltstones make up 15% of the total rock composition. In comparison with sandstones they have a darker shade, due mainly to the presence of humic matter. In their lower parts they are often sandy and alternate with thin interbeds and lenses of sandstones. They always show a characteristic fine, angular jointing and fine oblique bedding, caused either by the appearance of more light-colored varieties or by clayey-micaceous material. In all siltstone, especially clayey-siltstone horizons, fossil flora is very abundantly represented in the form of impressions and, in rare cases, phytoleims of leaves and inclusions of charred wood, small branches, and stems.

Argillites have a very limited distribution and usually occur only in the floor of coal seams and in siltstones, with which in this case they form indistinct transitions. They are usually dark, and often, if they occur in the floor of a seam, coaly. Bedding is not observed in them, but in some cases they are filled with plant remains and split easily into flat slabs. Plant remains in them are preserved as charred substances in which the structure of the tissues can readily be determined.

A rock of argillite composition, but possessing fine parallel bedding, was encountered on the left bank of the Tom River, 3.3 km below the village of Cherny Etap. The occurrence of bedding in it is connected with the content of thin, greasy-to-the-touch clayey substances. The color of the rock along the strike varies from dark gray to greenish-yellow-gray. On the bedding planes of this rock there are impressions of plants, insects, and fish, remarkable in their degree of preservation and diversity.

Conglomerates in the form of separate beds occur only at two points: on the bank of the Tom, above the Shakhtovaya River, and at Polyakovsky Kamen. At the first locality there is a conglomerate bed 2 m thick, and at the second—up to 10 m. The pebbles are usually well rounded and consist of quartz, metamorphosed sandstones, and more rarely of medium-grained granite and chalcedony. Also characteristic is the presence of small rounded pebbles of quartz albitophyre. In individual cases there occur angular fragments of dark-gray, weakly lilac-colored Devonian porphyrites. The cementing material of the conglomerate is represented by coarse-grained sandstone of ordinary composition.

The coals of the conglomerate formation are distinguished by considerable diversity. Minerals of the heavy fractions of the clastic rocks of the conglomerate formation, studied from the sandstones of the Sartakovo sequence, are distinguished by abundant apatite content and a noticeable quantity of garnets.

In addition to clastic rocks and coals, the conglomerate formation contains siderites, the thickness of which reaches 0.95 m, but the form of their occurrence has not been established.

The rocks of the conglomerate formation form clear and persistent rhythmic packets. A peculiar, regular, complicated rhythmicity is often observed. Each rhythmic packet begins with sandstones several tens of meters thick and ends with a coal seam. Above the latter follows a series of small, thin, 2–5 m, more rarely up to 10 m, rhythmic packets of siltstone–coal composition or, more rarely, sandstone–coal composition. As a result, the coal seams prove to be concentrated in separate horizons, and the general stratigraphic section proves to consist of a series of thick sandy units and less thick coal-bearing horizons, although as a whole the entire assemblage of beds from the sandstones below to the upper coal seam in this case represents one

a complex rhythmic member corresponding to one full period of geotectonic movement.

The conglomerate formation is subdivided into three sequences (from bottom to top): the Barashevskaya, Sartakovskaya, and Chusovitinskaya.

The Barashevskaya sequence, established only in the western part of the district, lies on the eroded surface of the Kolchuginskaya formation. At the base of the sequence there is a basal conglomerate, sometimes replaced by coarse-grained sandstone. Above it alternate cross-bedded lenses of arkosic sandstones, dense argillites, and coal beds, of which two attain considerable thickness. The coals of the beds are black, lenticular-banded; the banding is due to the alternation of lenses of dull and bright coal with lenses of fusain up to 1 cm thick.

A good section of the lower part of the Barashevskaya sequence is present near the village of Sartakovo. Here, above the mouth of the Barashevsky ravine, the contact between the conglomerate and Kolchuginskaya formations is exposed.

To the west and east of the Sartakovo deposit, deposits of the Barashevskaya sequence have been established in the Litvinovsky area, in the basin of the Karalda River, and in the lower reaches of the Unga.

In the Litvinovsky area, located 17 km west of the Sartakovo deposit, the thickness of the Barashevskaya sequence exceeds 325 m, and the total number of rhythmic members exceeds 6. The contact of the Barashevskaya sequence with the underlying Kolchuginskaya formation has not been exposed here.

In the system of the Karalda River, the Barashevskaya sequence is represented by sandy-siltstone deposits corresponding to the very lowest part of the section. The thickness has not been established.

The Sartakovskaya sequence is the most widespread member of the conglomerate formation. Its deposits are known over the entire area of the district, beginning on the right bank of the Tom River and ending in the extreme northwestern corner.

The most characteristic component of the Sartakovskaya sequence is uniform platy coals of clarain composition. Fusain, characteristic of the Barashevskaya and overlying Chusovitinskaya sequences, is never encountered here in quantities noticeable to the naked eye. In addition, sapropelites have been found in this sequence.

On the right bank of the Tom River, above Polyakovsky Kamen, stratigraphically above the contact of the Maltsevskaya and conglomerate formations, eight large rhythmic members of this sequence have been exposed by ditches; their thickness ranges from 50 to 170 m. It is characteristic that the lowermost rhythmic member begins with a siltstone part, and already 0.6 m from the contact a thin coal bed appears in it.

On the northern segment of the Pritom section, in the area between the village of Lyagusheya and the Nizhny Kamzas stream, there are seven large rhythmic members, also with numerous coal beds.

The sections obtained in the northern and southern segments are correlated rather clearly. They form the limbs of a single latitudinal syncline. There is reason to believe that the thick sandstones of the upper rhythmic member exposed near the village of Cherny Etap will correspond to the sandstones exposed below the Upper Yachmenyukha stream. In this case it should be expected that on the northern limb of the syncline, stratigraphically below the Mitinsky bed, which is correlated with the Ananyevsky bed, there will be a group of beds corresponding to the beds from the Shmalgauzensky to the Tonky, while on the southern limb the coal beds corresponding to the Shakhtovy beds were eroded even before the deposition of their roof. The total thickness of the Sartakovskaya sequence along the Tom River is 600 m.

At the Sartakovo deposit, this sequence includes deposits with a total thickness of about 350 m. The Sartakovo section is characterized by two

features: the complete absence of coarse-clastic rocks and the confinement of coal seams to the upper part of the section.

In the Litvinovo sector, the work of Z. D. Zavistovskaya established a section analogous to the section of the Sartakovo deposit.

In the Chusovitino deposit, situated 18 km west of the Litvinovo sector, the deposits of the Sartakovo member contain 13 rhythmic packets with numerous coal seams. It is possible that the very lowest rhythmic packets and coal seams, which fell within the zone of disjunction, have not been accounted for here. The larger number of rhythmic packets, as compared with other deposits, is also reflected in the thickness of the sediments, which at the Chusovitino deposit reaches 640 m.

An important feature of the section of the Sartakovo member of the Chusovitino deposit is the facies variability from southwest to northeast. In the southwestern part of the deposit, sandy-clayey deposits with coal seams predominate; toward the northeast the coal seams gradually pinch out, sandstones predominate in the sediments, and conglomerates appear. As a result of such rather rapid facies change, along the Chusovitino exploration line the sections of two adjacent boreholes located 2–3 km apart are difficult to correlate, whereas along strike the Chusovitino section is correlated quite clearly with the sections of the Litvinovo and Sartakovo deposits. In such a comparison it turns out that only the lower part of the section is represented at the Sartakovo and Litvinovo deposits.

The sections obtained in the western part of the district can be compared with the sections along the Tom River only by the general character of the members. Any, even preliminary, correlation of the seams does not appear possible.

The Chusovitino member is so far known only from the northwestern part of the district. A characteristic feature of this member is banded coals, whose banding is caused by the alternation of dull varieties with interlayers of fusain. In some cases the seams consist only of fusain, which has a clearly expressed wood structure. The member contains 6–7 rhythmic packets, 5–20 m thick (the lower packets have somewhat greater thickness).

The Chusovitino member has been exposed in the village of Chusovitino, along the Southern Un’ga near the village of Berdyugino, along the Northern Un’ga near the village of Skaryupino, and along the Smotrikha River. The thickness of the exposed part of the member in the village of Chusovitino is about 170 m. The total thickness of the conglomerate suite is about 1150 m.

Various hypotheses have been put forward concerning the genesis of the deposits of the conglomerate suite. It has been suggested that they represent fluvioglacial deposits. V. I. Yavorsky regards them as deposits of the foothills, which are further replaced by sandstones, while A. R. Ananyev and D. A. Vasil’ev refer them to lacustrine-swamp, river, and glacial deposits, which occur as distinct but synchronous facies. According to work of recent years, it has become clear that the deposits of the conglomerate suite show well-expressed rhythmic packets with thick sandstones at the base. The cross-bedding of the sandstones described above indicates that the member most likely has a fluvial origin. It is characteristic in this connection that the planes of cross-beds within one and the same series change their direction of dip and thereby reveal a rapid change in the direction of river-water flow. As indicated above, the sandstones contain abundant lenses consisting of chaotically arranged fragments of tree trunks, rare pebbles, and coarse-grained sand. The pebbles here show signs of redeposition. The origin of these lenses can be explained by assuming the existence of such accumulations in river “snags.” It is possible that during the epoch of formation of the sandy members the entire territory of the district was covered by a network of small rivers and swamps,

i.e., conditions of “floodplains” in the lower reaches of rivers prevailed. Under these conditions, part of the territory must have been occupied by channel deposits even in the final stages of each rhythm, i.e., at the moments of coal formation. Apparently, the northeastern part of the Sartakovskaya member of the Chusovitino deposit also belonged to such areas.

In the conglomerate suite, remains of flora and insect and fish fauna have been found. Among the fossils of the Barzas member, the following have now been identified:

1. Annulariopsis inopinata Zeiller.
2. Coniopteris hymenophylloides Brongn.
3. Sagenopteris karaldinensis n. sp.
4. Ginkgo lepida Heer.
5. Czekanowskia rigida Heer.
6. Podozamites lanceolatus L. et H.
7. Podozamites angustifolius (Eichw) Heer.
8. Samaropsis rotundata Heer.
9. Mesoleuctra gracilis Brauer, etc.

Most of these plants occur from the Rhaetian to the upper parts of the Jurassic, and some extend into the Lower Cretaceous; Mesoleuctra gracilis is known only from Ust-Balei, and Sagenopteris karaldinensis has been found for the first time. The exception is Annulariopsis inopinata, which characterizes only Rhaetian–Liassic deposits and is not encountered in younger deposits. If the general Jurassic aspect of the fossils and the presence of Annulariopsis are taken into account, it may be assumed that the age of the Barzas member is not younger, but also not older, than the Lower Jurassic.

Among the fossil organic remains of the Sartakovskaya member, plants have now been identified:

1. Neocalamites pinitoides Chachl.
2. Equisetites sokolowskii Eichw.
3. Equisetostachys sibiricus (Heer) Nath.
4. Dictyophyllum rugosum L. et H.
5. Clathropteris obovata Oishi.
6. Hausmannia sp.
7. Coniopteris hymenophylloides Brongn.
8. Raphaelia diamensis Sew.
9. Sphenopteris princeps Presl.
10. Cladophlebis haiburnensis L. et H.
11. Cladophlebis whitbiensis tenue Heer.
12. Cladophlebis argutula Heer.
13. Cladophlebis williamsonii Brongn.
14. Cladophlebis denticulata Brongn.
15. Stachypteris tomiensis n. sp.
16. Lycopodites tenerrimus Heer.
17. Pseudoctenis tomiensis n. sp.
18. Ginkgo digitata var. magnifolia Thomas.
19. Ginkgo lepida Heer.
20. Ginkgo sibirica var. grandifolia n. var.
21. Ginkgo sibirica Heer.
22. Ginkgo flabellata Heer.
23. Ginkgo concinna Heer.
24. Stenorhachys lepida (Heer) Sew.
25. Baiera longifolia Poemel.
26. Czekanowskia setacea Heer.

1. Czekanowskia rigida Heer.
2. Phoenicopsis angustifolia Heer.
3. Leptostrobus laxiflora Heer.
4. Schidolepidium gracile Heer.
5. Elatides ovalis Heer.
6. Pityophyllum Nordenskioldii (Heer) Nath.
7. Podozamites lanceolatus L. et H.
8. Ferganiella urjanchaica Neub.
9. Caracaniella tomiensis n. gen. et n. sp.
10. Samaropsis rotundata Heer.

Insects:

1. Mesoleuctra gracilis Braner etc.
2. Samarura gigantea Brauer etc.
3. Pseudocossus Zemcuznicovi Mart.
   Fish: Palaeoniscinatus czekanowskii Rohon¹)

This list shows that the flora as a whole is very close to the Jurassic flora of Ust-Baley, and the Sartakovo sequence should be regarded as an analogue of the Cheremkhovo Formation. This similarity is also confirmed by the insect and fish fauna, known only from Ust-Baley. But in detail the composition of the flora also reveals certain deviations from the Ust-Baley flora. Thus, it contains the essentially Upper Jurassic fern Stachypteris, while at the very top Clathropteris obovata and Dictyophyllum rugosum appear, which give the flora, as it were, a more ancient aspect. The find of Ferganiella also deserves attention, since this genus appears at the tops of the Lower Jurassic and at the bases of the Middle Jurassic, and Ferganiella urjanchaica has been described only from the Middle Jurassic of Tuva. On the whole, the flora is most probably Middle Jurassic.

The age of the Chusovitin sequence is not characterized by paleontological data and is determined by the fact that it rests upon Middle Jurassic deposits and is overlain by Upper Jurassic sediments.

Tersyuk Formation

Under this name there is here for the first time distinguished a sequence of rocks lying upon the deposits of the conglomerate formation. A characteristic feature of the Tersyuk Formation is thin rhythmic units, within the latter rapidly pinching-out lenses, and, in addition, a change in the composition of the sequence from bottom to top.

The Tersyuk Formation is represented by sandstones, siltstones, very rarely argillites and conglomerates. At the base of the formation there are thin laminae of coal; upward, coal-bearing completely dies out. The color of the rocks is usually greenish gray. In this respect they approach the rocks of the Maltsevo Formation, but differ from them in a bluish tint.

Microscopic study of the rocks of the Tersyuk Formation shows that they differ from the rocks of the other formations in a number of features. First of all, the polymictic character of the clastic rocks is conspicuous. Fragments of chlorite schists and epidotized rocks are abundantly represented in them. The former are associated with a high content of chlorite. In the light fraction the rocks contain an abundant quantity of quartz and very little feldspar. The content of the latter reaches 5–20% and, consequently, is half as much as in the conglomerate formation.

¹) Identified by Academician L. S. Berg.

The heavy fraction of minerals of this formation amounts to about 8–12% of the total weight of the concentrate in the lower part of the formation and decreases sharply in the upper horizons. Among the minerals here, weakly rounded or almost unrounded grains of epidote, often schistose, are abundantly represented. There is also much zoisite and clinozoisite. In comparison with the rocks of the conglomerate formation, it contains less garnet and zircon; the sphene content increases, and pyroxenes and green hornblendes appear in small quantities.

The siltstones of the Tersyuk Formation do not differ macroscopically from the rocks of the same name in the conglomerate formation.

The conglomerates, with the exception of that occurring at the base of the formation and established at the Sartakovo deposit, are represented by fine, flat pebbles and only rarely form an independent horizon in the form of small lenses, often passing into coarse-grained sandstones and gravelites.

The coals of the Tersyuk Formation have a platy character and a clarain composition, i.e., they resemble the coals of the conglomerate formation, but differ from them in their small thicknesses, rarely reaching 20–40 cm.

Deposits of the Tersyuk Formation have been exposed at only three points: at the Sartakovo deposit, on the bank of the Tom River, and on the Bungarap River, 3 km northeast of the village of Alyabikha.

At the Sartakovo deposit the deposits of this formation begin with a thick conglomerate unit. On the right bank of Zmeinyi Log, which flows from the left into the Stepnoi Urop River above the village of Sartakovo, pits exposed (from bottom to top):

1) gray, dense argillite, at the top with interbeds of ochre-colored argillite—1.30 m;

2) clayey limonite, yellow in color, with spots of red limonite; in the northeast the upper part of the horizon is truncated by the overlying bed—0.35 m;

3) broken conglomerate. The pebbles consist of quartz, sandstone, and intrusions of granodioritic composition. The pebbles are rounded, large, with boulders measuring 0.40 × 0.25 × 0.15 m. The boulder material is represented chiefly by intrusive rocks and milky-white quartz. The cement of the conglomerate is clayey, brown in the upper part, passing downward into ochre-colored white clay. At 4.5 m from the surface of the conglomerate there are interbeds of yellow clayey sand. The pit exposed only the lower part of the conglomerate, 7.5 m thick. Its total thickness, judging from the gravel cropping out along the slope of the log, is not less than 30 m.

Bed 1, exposed by the pit, undoubtedly belongs to the Sartakovo unit of the conglomerate formation, since stratigraphically below it, in the roof of the Sartakovo seam, numerous impressions of plants were found. The uneven character of the contact of the conglomerate with the underlying rocks, the presence of brown ironstones, and also the sharp facies change indicate that there was a break between the deposition of the lower horizons and the conglomerate.

The conglomerate horizon in natural outcrops was encountered on the left side of the Stepnoi Urop above Zmeinyi Log, where, owing to selective weathering, it forms ridges elongated along the strike of the rocks. Southeast of Zmeinyi Log, at 1 km, it is exposed by small pits on the watershed; it is then encountered in the talus of the left high bank of the Chernovyi Urop River, 1 km above the village of Urop. It is characteristic that the occurrence of the conglomerate is clearly transgressive. Along Barashevskii Log and the Chernovyi Urop it lies closer to the outcrop of the deposits of the Kolchugino Formation than in Zmeinyi Log, and it cuts off the upper horizons of the Sartakovo unit, which explains the absence of coal seams in the latter southeast of Zmeinyi Log.

The higher horizons of the Tersiuk suite, exposed in Zmeinyi Log, are represented by sandstones, brownish-gray siltstones, and argillites. Coal seams are not known here. The same rocks are exposed on the left bank of the Chernovaia Urop River above the village of Urop. The thickness of the exposed part of the suite within the deposit is about 100 m.

On the Tom River, deposits of this suite are exposed between the village of Chernyi Etap and the Verkhnii Kamzas River, and also between the Verkhnyaya Yachmenyukha River and the Tersiuk River.

Above the village of Chernyi Etap, in an outcrop, it is evident that, stratigraphically above the upper bed of the conglomerate suite, at 10–11 m there appear lenticularly occurring sandstones and gravelites. The entire section is characterized by instability of facies and by occurrence in the form of elongated, thin lenses. Along the strike, gravelites rapidly pass into sandstones, and the latter into siltstones; rhythmic packets often terminate after the formation of the siltstone component, before coal is formed. Coal laminae within this unit occur as very thin, short lenses, the thickness of which nowhere reaches 40 cm, and in the upper horizons the coal content dies out completely. The thickness of the suite exposed on the left bank of the Tom is about 100 m. On the right bank of the Tom River, below the Verkhnyaya Yachmenyukha River, the deposits of the Tersiuk suite have a thickness of up to 150 m.

The Tersiuk suite belongs to river deposits. The alternation of rapidly pinching-out lenses of sandstones, gravelites, and small coal laminae could have formed only under conditions of flowing waters. Analysis of the rhythmic packets of the suite shows that the latter cannot be correlated over distances of less than 100 m. In addition, “inverted” rhythmic packets are often observed, beginning with coals and ending with sandstones. Coals often occur on sandstones in the form of short lenses and contain interbeds of sandstone, i.e., they show clear signs of allochthonous origin.

In the Tersiuk suite on the Tom River, a rich fossil flora has been found, from which the following have been identified:

1. Laccopteris Dunkeri Schenk.
2. Laccopteris polypodioides Brongn (?)
3. Coniopteris hymenophylloides Brongn.
4. Gonatosorus tomiensis n. sp.
5. Sphenopteris tyrmensis Sew.
6. Angiopteridium kusnezkianum n. sp.
7. Cladophlebis haiburnensis L. et H.
8. Cladophlebis whitbensis tenue Heer
9. Cladophlebis vulgaris Chachl.
10. Cladophlebis argutula Heer.
11. Cladophlebis distans Heer.
12. Cladophlebis tomiensis n. sp.
13. Cladophlebis williamsonii Brong.
14. Williamsonia (?) kusnezkina n. sp.
15. Nilsonia comptula Heer.
16. Ginkgo digitata Brong.
17. Ginkgo sibirica Heer.
18. Ginkgo obrutschewi Sew.
19. Baiera longifolia Poemel.
20. Ginkgodium sibiricum n. sp.
21. Czekanowskia setacea Heer.
22. Phoeniopsis angustifolia Heer.
23. Schizolepis minima n. sp.
24. Sequoia smittiana Heer.
25. Pityophyllum Lindstromii Nath.

1. Pityophyllum staratschini (Heer.) Nath.
2. Podozamites lancolatus L. et H.
3. Podozamites lanceolatus var. minor Heer.
4. Podozamites Reinii Geyber.
5. Proteaphyllum sp.

This list shows that the flora of the Tersyukskaya suite has no analogues in the Jurassic floras of Siberia, but it reveals a number of elements characteristic of the Upper Jurassic and Lower Cretaceous floras of the Amur region, Transbaikalia, the lower reaches of the Lena, the Nikanskian stage of the Far East, the Sukhovskaya formation of the Irkutsk Basin, and the Upper Jurassic flora of the Tetori suite of Japan. It is characteristic that elements of Lower Cretaceous floras—Laccopteris Dunkeri, Sequoia, Proteaphyllum—are confined to the upper part of the section, where elements of Jurassic floras—ginkgoaleans and ferns—are almost absent. All this taken together compels one to regard the flora of the Tersyukskaya suite as Upper Jurassic, although the upper horizons of the latter may also belong to the lowermost Lower Cretaceous.

On the question of the development of the Jurassic flora of Kuzbass

The Jurassic deposits of the Central District constitute excellent material for tracing the development of the flora. Whereas the entire thickness of Jurassic sediments in the Chulym-Yenisei, Kansk, and Irkutsk basins does not exceed 300–600 m, the thickness of the Jurassic of the Central District reaches 1300 m. As indicated above, it comprises four (stratigraphic) thicknesses, differing in lithological composition and in the quality of the coals. Study of the fossil plants of three of the thicknesses shows that they also differ in the composition of their floras.

The Barashevskaya thickness is the oldest member of the Jurassic deposits of Siberia. In its lower, barren horizons there occur remains of Annulariopsis inopinata, and in the upper part, in the coal-bearing horizons, ginkgoaleans are present—Ginkgo lepida and Czekanowskia rigida. The question of why ginkgoaleans appear only in the upper parts, like the question of the limits of this flora, cannot at present be resolved. But it is not excluded that one of the causes was a change in climate, reflected in a change of lithological composition—from yellow, to a considerable degree ocherous rocks in the sections along the Karalda River, where the most ancient floras were found, to coal-bearing deposits.

Between the deposits of the Barashevskaya and Sartakovskaya thicknesses there is no sharp floristic boundary, and the flora of the Sartakovskaya thickness, from bottom to top, contains uniform elements with a predominance of ginkgoaleans, podozamites, and ferns, with an insignificant number of horsetails and extremely rare representatives of cycads. The difference between the Sartakovskaya and Barashevskaya floras lies not so much in generic composition as in the appearance of numerous species. If ginkgo in the Barashevskaya thickness is represented by one species, then in the Sartakovskaya it is represented by 8; the podozamites group (the genera Podozamites—Ferganiella—Caracaniella) by 1–6, etc. Apparently, the Middle Jurassic flora of Kuzbass represented a forest of ginkgo, podozamites, ferns, and a certain number of horsetails and conifers.

All this flora displays a clearly expressed moisture-loving character, while the absence of arborescent ferns most likely indicates a rather cool climate. The considerable severity of the climatic conditions during the time of formation of the conglomerate suite is also demonstrated by the complete absence of dinosaurs not only in Kuzbass, but also in other regions of Siberia from the Ob to Baikal.

In the generally uniform Middle Jurassic flora, a certain dissonance is introduced by the appearance of the fern Stachypteris, which links the Siberian floristic region with Western Europe and is rather typical of Upper Jurassic deposits; but the most striking feature seems to be the appearance of Dipteridacea—Clathropteris obovata and Dictyophyllum rugosum—these elements, regarded as index fossils of the Rhaetian–Liassic and the Oolite, in the upper parts of the Sartakovo series, while they are entirely absent from other horizons. At the same time, if one takes into account that in the Irkutsk Basin Clathropteris appears in the Sukhovka Formation, corresponding to the Tersyuk Formation, one should expect that this fern will be found in the lower parts of the Tersyuk Formation. The absence of traces of Dipteridacea throughout the entire conglomerate series, except for the upper horizons, cannot be explained by insufficient collections, since special searches were undertaken, and in the upper parts of the Sartakovo series they are represented in abundance.

A close continuity is not observed between the floras of the Sartakovo series and the Tersyuk Formation. This break is partly explained by the fact that the intermediate flora of the Chusovitino series remains unknown, but the main cause, apparently, lies in climatic factors. Although ordinary Jurassic genera predominate in the composition of the Tersyuk flora as well, the latter show significant changes. Already in the upper parts of the Sartakovo series, in the horizon with Clathropteris and Dictyophyllum, the ctenophylls become small-leaved and coriaceous, and higher, in the Tersyuk Formation, cycadophytes also appear; but these forms, too, quickly disappear, giving way to the fern Laccopteris, narrow-leaved Pityophyllum, and then sequoias, which are accompanied by very small-leaved ferns of the Cladophlebis type. Here, in the development of the Jurassic flora of Kuzbass, a definite regularity is outlined: a change toward the appearance of xerophilous and thermophilous forms.

This climatic change toward warming is reflected not only in the composition of the flora, but is also expressed in the gradual fading of coal-bearingness in the Tersyuk Formation from bottom to top. The change in climatic conditions apparently also caused a change in the generic composition of the flora, in particular the appearance of the “ancient” Dipteridacea in the high horizons of the Sartakovo series. This is all the more probable because the modern Dipteris is a tropical plant. If the flora is considered from this point of view, it will turn out that Clathropteris, and possibly Dictyophyllum as well, are not so much guides for determining age as they characterize the warm and dry climate that set in after the deposition of the Sartakovo series.

The course of development of the Mesozoic flora of Siberia after the deposition of the Tersyuk Formation is unknown, but if one takes into account that the Jurassic deposits of Kuzbass are overlain by rocks of a red-colored weathering crust, it should be considered that the cause of the disappearance of the Siberian Jurassic flora, the disintegration of which is already outlined in the upper parts of the Sartakovo series, was an epoch of warm and dry climate.

Interesting results are obtained when studying the Jurassic floras of Kuzbass in connection with changes in the physical-geographical conditions of the Jurassic period. Sedimentation in the Jurassic freshwater basins was undoubtedly connected with the time of the transgression of the Jurassic sea, which began in the upper parts of the Lower Jurassic, when deposition of the conglomerate series began in Kuzbass. The retreat of this sea ended at the end of the Jurassic—in the beginning of the Lower Cretaceous, i.e., in the epoch of deposition of the Tersyuk Formation. It must be thought that the warming of the climate toward the end of deposition of the Sartakovo series was also connected with the retreat of the Northern Sea; connected with it as well was the migration from the south of a thermophilous flora containing representatives of Dipteridacea.

The Siberian Jurassic flora, in its turn, apparently also migrated after the sea, as evidenced by coal-bearing deposits with a flora of the “Jurassic type,” lying upon Lower Cretaceous deposits in the lower reaches of the Lena and in the Arctic (Spitsbergen). Following the “Jurassic” flora, the fauna apparently also migrated without significant changes, as a result of which pelecypods close to the Liassic Ferganoconcha of Central Asia have been described from the Lower Cretaceous coal-bearing deposits of Transbaikalia by B. I. Chernyshev.

Tectonics

The Central District is a large tectonic depression known as the “Main Basin” or the “Central Kuznetsk Syncline.” The eastern half of this depression, elongated in a latitudinal direction, is known as the “Bungarap Syncline,” while the western half, whose axis has a northwestern strike, is called the “Chesnokovo Syncline.” The latter are not folds, but structural waves, and deserve to be called depressions rather than synclines.

At the junction of the differently oriented Bungarap and Chesnokovo depressions there is formed the broad Karaldin Depression, open to the north.

The Chesnokovo Depression, at the extreme northwest, is complicated by the so-called Murychak (Syromolotninsk) Anticline, which forms an embayment of the Paleozoic into the general field of Jurassic deposits, and is divided into two smaller depressions—the Plotnikovo and Skoropijka depressions.

The depressions described have steeply set flanks and a gentler floor. At the contact of the Mesozoic sediments with the Paleozoic, steep angles of dip are noted almost everywhere. On the southwestern boundary of the district, near the village of Urop, in the conglomerate suite itself, close to the contact, beds dipping at an angle of 60° have been recorded. To the northeast of this point, between the villages of Urop and Sartakovo, the contact between the conglomerate and Kolchugino suites is not exposed, but close to the contact the rocks of the Kolchugino suite here are vertical, and 150–200 m to the northeast, in the conglomerate suite, dips to the northeast at an angle of 48–50° have been noted. At the Chusovitinskoye deposit, near the contact, exploratory work established a disjunctive zone more than 300 m wide, embracing the deposits of the Kolchugino and conglomerate suites. This zone can be traced eastward as far as the village of Novogeorgievka. According to P. N. Vaskhishchev, on the northern margin of the district, on the southeastern limb of the Murychak anticline, the dips of the Paleozoic are more than 70°; and according to V. D. Fomichev, on the western limb of the same anticline, northeast of the village of Plotnikovo, a steep dip has likewise been observed over a narrow sector. Farther to the southeast, near the village of Borisovo, in the Paleozoic, close to the contact with the Jurassic, a narrow belt has also been noted with bed dips to the southwest of 60–70°; and along the Ustikha River, in the Jurassic deposits, dips to the northeast at an angle of 60° have been noted, i.e., away from the contact.

If we turn to the Jurassic of the “horseshoe,” then on its northern limb only gentle angles of dip are observed. The eastern boundary, in contrast, is outlined by a sharp disjunctive zone. Within the “horseshoe,” zones of crumpling have been established at the contact of the Jurassic and Triassic on the Tersyuk River and at the mouth of the Gromatukha River.

These fragmentary data show that the Central District, at least from the southwest and east, is outlined by a zone of major disjunctions, partly also encompassing the conglomerate suite.

Within the large depressions there is a slight, gentle undulation. On the Tom River, near Babiy Kamen, the beds dip to the northeast at 30–40°, with a dip angle of 20–30°. This direction persists as far as the Upper Yachmenyukha River, but here the dip angles of the beds gradually flatten to 5–7°. North of the Upper Yachmenyukha River the strike of the rocks gradually becomes meridional, and at the mouth of the Lower Yachmenyukha River persistent eastward dips at an angle of up to 12° are observed. West of the latter, 4–5 km from the mouth of the Ubik River and north of Ubikskii Kamen, westward dips are recorded. Thus, a broad anticlinal fold is indicated here, along the axis of which the Tom River flows. Gentle undulation has also been established in the western part of the district. Thus, in the lower reaches of the Chernovaia Urop River persistent northwestern dips at an angle of up to 20° have been recorded, marking the closure of a brachysyncline whose axis follows the watershed of the Chernovyi and Stepnoi Urops.

Traces of small waves are also present in the vicinity of the villages of Krasavino and Litvinovo.

The conglomerate suite lies at different horizons of the Maltsevo and Kolchugino suites. The tectogenesis that caused the interruption in sediment accumulation (deposits younger than the Lower Triassic are unknown) manifested itself at the end of or after the Lower Triassic and belongs to the Ancient Cimmerian phase of diastrophism. Although this diastrophism was weak in intensity, it caused prolonged subaerial denudation, which produced a pre-Jurassic kaolin weathering crust and, most importantly, established new structural trends. The age of the paleobasalts at present cannot be regarded as firmly established, but throughout the eastern half of the district they occur as sills in the rocks of the Maltsevo suite. The occurrence of basalts in deposits of the conglomerate suite has not been established. The conglomerates of Maly Karakan indicated by Yu. F. Adler, on which the basalts rest, are more likely conglomerates of the upper parts of the Maltsevo suite. If this is so, then the system of meridional and latitudinal fractures along which magma effusion took place arose during the Ancient Cimmerian phase of diastrophism, and at that time the “renegade” structure of the eastern half of the Central District, cutting the frame of the Kuznetsk Basin, was also established.

Between the deposits of the conglomerate and Tersyuk suites, judging by the presence of conglomerates at the base of the latter and chiefly by the transgressive bedding observed near the village of Sartakovo, traces are established of movements of the Earth’s crust that caused an interruption in sediment accumulation. The duration of this break is determined by the fact that during this time the Middle Jurassic flora was replaced by a younger one, and in the eastern part of the district the entire Chusovitino sequence was eroded away.

The Tersyuk suite marks the end of the main stage of sediment formation in the Central District, interrupted by Upper Cimmerian movements that definitively formed the present tectonic structure of the district. That the modern structures were created precisely by these movements is proved by the horizontal bedding of the Upper Chalk and Lower Chalk (?) deposits established in the Urop River system, whereas the older Tersyuk suite participates in the general folded structure of the district.

The nature of the disturbance of the marginal zones of the district appeared to one of the first investigators of Kuzbass, B. V. Polenov, as a ring-like overthrust [4]. According to M. A. Usov [5], the zones of twisting and complication of folds in Kuzbass are associated with zones of overthrusts emerging from the basement. The time of formation of this structure of the Central District apparently belongs to the Upper Cimmerian epoch, when major block movements were taking place in all the coal-bearing basins of Siberia.

Conclusion

The Jurassic deposits of the Central District are subdivided into the Conglomerate and Tersyuk formations. The Conglomerate Formation rests on deposits of an ancient crust of kaolin weathering. The formation contains a large number of coal seams. The entire formation was formed under conditions of sharp oscillatory movements and forms distinct rhythmic packets. The deposits of the Conglomerate Formation are divided into three strata of different ages: the Barashevo stratum of Early Jurassic age, the Sartakovo stratum of Middle Jurassic age, and the Chusovitino stratum, whose age has not been precisely established. The thickness of the formation is 1150 m.

The Tersyuk Formation lies transgressively on the deposits of the Conglomerate Formation. The age of the Tersyuk Formation is Upper Jurassic, but the upper horizons may be Lower Cretaceous. The thickness of the formation is about 150 m.

The fossil flora of the Jurassic deposits shows that it migrated from south to north, and that the moisture-loving flora in the Upper Jurassic epoch was replaced by a warm- and dry-loving flora.

The Jurassic deposits of the Central District form one large depression.

Cited Literature

1. Adler Yu. F.—Some results of geological-exploration work on the right bank of the Ini River southwest of the Leninsk District in the central part of the Kuznetsk coal basin. Materials on the Geology of the West Siberian Territory, issue 23, 1935.

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

3. Korovin M. K.—The Achinsk sapropelite district according to new data from 1933. Vestnik ZSGRT, 1933.

4. Polenov V. K.—Geological description of the southwestern part of sheet 15, row VIII, Tomsk Province, Kuznetsk sheet. Transactions of the Geological Section of the Cabinet, issue 2, 1907.

5. Usov M. A.—Geological sketch of Kuzbass. Mineral Resources of the West Siberian Territory, vol. 3, 1935.

6. Chernyshev B. I.—On some pelecypods from Transbaikalia and the Far Eastern Territory. Transactions of VIMS, issue 143, 1939.

7. Yavorsky V. I.—Some results of geological investigations in the Kuznetsk Basin in the summer of 1932. Transactions of TsNIGRI, issue 26, 1934.

1. The mineralogical composition of the rocks was determined by M. P. Nagorsky. ↩