Pelecypods of the Jurassic and Cretaceous Deposits of the Kuznetsk and Chulym-Yenisei Basins

I. V. Lebedev

Introduction

The continental Mesozoic deposits of Western Siberia contain numerous remains of plants, lamellibranch mollusks, phyllopods, crustaceans, fishes, insects; remains of dinosaurs, etc., have also been found. This organic world of the Mesozoic has been studied very unevenly. Whereas an extensive literature is devoted to the study of the Mesozoic flora, the fauna has scarcely been studied. All the literature on the Mesozoic fauna of Western Siberia is limited to descriptions of several species of lamellibranch mollusks, phyllopods, and insects from the Kuznetsk and Chulym-Yenisei basins. Yet even this very limited literature shows that the fauna of the continental Mesozoic deposits is an important means for correlating sections even in remote regions. Thus, on the basis of the insect and fish fauna, the Jurassic of the Irkutsk and Kuznetsk basins is well correlated; on the basis of the pelecypod fauna, the Triassic of the Kuznetsk and Tunguska basins, the Lower Cretaceous of Western Siberia and Western Europe, etc., are correlated.

Within the Mesozoic fauna of Western Siberia, pelecypods are the most widespread; they are very often encountered in core obtained during drilling in coal deposits, and for this reason they have been studied better. At present seven works are devoted to this fauna. However, the Mesozoic pelecypod fauna is not always well preserved; consequently, new collections often lead to additions to, and revisions of, earlier descriptions.

The author of the present article, thanks to the assistance of the Krasnoyarsk party of the trust “Vostsibuglegeologiya,” the Siberian Branches of the Academy of Sciences and VNIGRI, over a number of years had the opportunity to collect and study the Mesozoic pelecypod fauna; as a result, a considerable body of material has accumulated at his disposal, making it possible to supplement and correct certain previously completed works. This article gives a characterization of the generic composition of this fauna, illustrated by photographs of the most widespread and most typical species.

The continental Mesozoic of Western Siberia has a complex composition. It includes Triassic, Jurassic, and Cretaceous deposits. The Triassic deposits, known under the name of the Maltsevo Formation, occur in the Central district of Kuzbass. They consist of sedimentary and tuffaceous rocks of Lower Triassic and, possibly in part, Middle Triassic age.

Fig. 1. Diagram of the tectonic subdivision of the Karaganda depression.
Compiled by A. A. Bulynnikova, 1955.

1—boundary of the Karaganda depression along the base of the Akkuduk Formation; 2—principal tectonic faults; 3—axes of second-order anticlinal structures; 4—axes of second-order synclinal structures; 5—axes of fourth-order synclinal structures; 6—axis of the Del’del’–Dolina uplift zone (third-order structure); 7—zone of second-order anticlinal structures; 8—zones of third-order synclinal structures; fourth-order anticlinal structures: 1—Del’del’, 2—Dolina, 3—Dubovka.

Fig. 2. Structural map of the Karaganda Basin. Compiled by A. A. Bulynnikova, 1955.

1 — structure contours drawn along the floor of coal seam \(K_{12}\); 2 — structure contours drawn along the floor of coal seam \(D_{6}\); 3 — structure contours drawn conditionally along the floor of coal seam \(K_{12}\); 4 — structure contours drawn conditionally along the floor of coal seam \(D_{6}\); 5 — floor of the Ashlyarik suite; 6 — floor of the Akkuduk suite; 7 — tectonic faults.

termination of the Kokshetau massif and north of the boundary of the Ekibastuz trough. Its western and eastern boundaries are not yet clear. It should be said, however, that the indicated intermontane downwarp apparently was not homogeneous. Already in the Devonian period, centers of more intensive downwarping had appeared within it; in subsequent periods these determined the distribution of the intermontane depressions that exist at present in Central Kazakhstan.

As a result of more intensive downwarping of the future depressions, the areas surrounding them lagged behind them in their downward movement, thereby creating natural thresholds and subsequently forming shoals, in places islands, from which clastic material was supplied. Facies analysis enabled K. L. Adrianova to speak of the existence of an island zone separating the Karaganda and Tengiz depressions at the end of the Devonian period and in the Tournaisian.

In the modern structural plan, the Karaganda depression is a typical syncline elongated in a latitudinal direction. The boundary of the depression is conventionally drawn along the surface exposures of the tops of undivided deposits of the Lower and Middle Devonian. The greatest length of the depression within these boundaries is about 120 km, and its greatest width is up to 70 km. Along the base of the Akkuduk suite, the length of the depression is 100 km, and its greatest width is 50 km.

The depression is bounded on all sides by exposures of Silurian and Lower–Middle Devonian rocks. The northern flank of the depression is characterized by the simplest structure. It has a curved configuration in plan, since it is complicated by two projections—the Saran and Maikuduk uplifts. The angles of dip of the coal-bearing strata on the northern limb are 20–40°. The disturbance of the rocks of the northern flank, compared with other parts of the Karaganda depression, is slight. The tectonic ruptures—reverse faults and normal faults—here have chiefly NW, more rarely NE, strike.

On the west the depression is cut off by a meridional tectonic reverse fault, the plane of which dips steeply to the west. The amplitude of the reverse fault varies from 3000 m in the south to 6000 m in the north. Along this reverse fault, rocks of the Lower and Middle Devonian are in contact with deposits of various suites of the coal-bearing strata.

In the east the depression does not close completely, and its natural continuation is a narrow belt of coal-bearing deposits along the Ashi-Su River, traceable for a distance of about 200 km.

In the south the basin is bounded by a zone of tectonic disturbances associated with the Dzhalair overthrust and fixed along the contact of Lower–Middle Devonian and coal-bearing deposits.

The magnitude of the vertical displacement of rocks along the Dzhalair overthrust cannot be determined precisely, but in any case it is measured in several thousands of meters. In the south of the basin the rocks are broken by a large number of faults with various amplitudes and are gathered into steep folds. The strike of the folds and faults is, as a rule, latitudinal, parallel to the Dzhalair overthrust.

The depression is filled with a thick sequence of Upper Devonian and Carboniferous deposits. The total thickness of the coal-bearing strata is more than 4000 m. Below is a scheme of the stratigraphy of the Karaganda basin.

The maximum thickness of the sedimentary sequence—more than 6000 m—is observed in the northwestern part of the basin, in the area of the Tentek trough, where a complete section of coal-bearing deposits has been preserved, crowned by the Shakhan suite.

Within the Karaganda depression, five second-order structures are distinguished: the Churubai-Nura, Karaganda, and Verkhesokur synclines,

troughs and the Dubovsko-Saranskoye and Maikuduk-Akzhar uplifts separating them.

The Churubai-Nurinskaya trough, in tectonic terms, is a complex syncline elongated in a NNW direction. It is situated in the western part of the Karaganda Basin. The length of the trough is about 60 km, and its width through the settlement of Dolinskoye is about 25 km. The NW part of the trough is the deepest; for this reason, a complete section of the coal-bearing sequence has been preserved here, ending with the youngest Paleozoic deposits, the Tentekskaya and Shakhanskaya formations.

Characteristic is the almost complete absence of a Mesozoic cover; Jurassic conglomerates have been preserved only in the west of the trough, where they compose the Konur-Tyube hill. The southern limb of the trough has a complex tectonic structure, caused by the Dzhalair thrust. Here the deposits of the coal-bearing formations and the underlying limestone-shale sequence are crumpled into steep folds of latitudinal strike and are broken by a system of likewise latitudinally oriented faults.

In the west the Churubai-Nurinskaya trough is truncated by the Tentek fault, which has already been discussed in the description of the structure of the entire Karaganda depression. In the southern part of the trough this fault is accompanied by a whole series of smaller, often adjoining the main fault, reverse faults, which break the coal-bearing deposits into a complex system of blocks. Toward the NW the trough narrows and closes somewhat north of the Nura River.

On the northern, quiet limb, secondary folding is almost undeveloped, and tectonic ruptures are noted only in small numbers.

Within the Churubai-Nura trough three third-order structures are distinguished: 1) the Deldel-Dolinka zone of uplifts; 2) the Shakhan trough; 3) the Sassyk-Kul–Dzhaimin trough.

The Deldel-Dolinka zone of uplifts extends in a latitudinal direction from the Del-Del uplift in the west to Dubovskoye in the east. This zone divides the Churubai-Nura trough into two parts—northern and southern. In the east the zone of uplifts gradually merges with the NE flank of the Churubai-Nura trough. The structure of the western part of this structure is unclear because of the absence of data on the structure of the western limb of the Deldel uplift.

This zone of uplifts apparently played a major role in the history of the formation of the Churubai-Nura trough, determining the sharply different structure of the areas situated to the north and to the south of it.

Within the Deldel-Dolinka zone of uplifts two fourth-order structures may be distinguished: the Deldel and Dolinka uplifts.

The Deldel uplift is situated in the western part of the zone near the Tentek overthrust. To the north of it lies the deepest part of the basin—the Tentek trough. In the east the Deldel uplift is separated from the Dolinka syncline by a small synclinal downwarp. Southeast of the uplift lies the Sassyk-Kul syncline.

The Del-Del uplift is elongated in a meridional direction. As a result of undulation of the axis, an additional brachysyncline appears in its northern part. Its size along the long axis is about 6 km, and along the short axis 3–4 km. The general plunge of the Deldel uplift is toward the north. Tectonic ruptures are absent.

The amplitude of plunge of the uplift on its northern pericline reaches 1800 m. The dips here are 10–15°. The amplitude of plunge of the eastern limb is 700–800 m. The dips on this limb are 15–20°.

The tectonic structure of the western limb is unclear, since this territory has not been drilled at all.

The Dolinka anticline is situated in the center of the Churubai-Nura trough. At the surface it is composed of rocks of the supra-Karaganda coal-bearing suite. Deposits of the Dolinka suite frame the structure from the north, east, and southwest.

The Dolinka uplift is a brachyanticline elongated in a meridional direction. It is situated between the Dolinka, Tentek, Dzhaimin, and Sassyk-Kul synclines. The dimensions of the uplift are about 10 km along the long axis and 6 km along the short axis (at the top of the Dolinka suite).

The amplitude of the structure from the highest point to the lowest mark in the adjacent Dolinka syncline is approximately 550–600 m, and relative to the Tentek syncline it reaches 1550 m.

The Dolinka structure has a complex tectonic structure and is divided by overthrusts into western and eastern parts, of which the western is thrust over the eastern part of the fold.

The eastern limb of the Dolinka uplift has a relatively simple tectonic structure. The dips of the beds here do not exceed 10–15°. Westward the structure of the uplift becomes more complicated. In the crestal part of the structure, disrupted by numerous faults, additional smaller folds appear. In this same direction the dips increase to 25–40°, reaching 75–90° near tectonic ruptures.

The largest tectonic fault is the “Northern” thrust, trending meridionally in the northern part of the structure. Its fault plane dips westward. The displacement amplitude is greatest within the area of mine field No. 46, where it reaches 500–600 m. To the north and south the amplitude of this fault decreases. In the south (in the area of Dolinskoye settlement) this fault dies out.

Shakhan trough. Within the Shakhan trough, situated north of the Deldel–Dolinka zone of uplifts, two large synclines are distinguished—the Tentek and the Dolinka—separated by the “Northern” thrust.

The Tentek syncline is the largest structure in the Churubai-Nura trough.

In the west the Tentek syncline does not close completely. Its western limb in the northern part is truncated by the Tentek thrust. In the southwest it adjoins, near the Deldel uplift, a narrow anticlinal fold of meridional strike, most of which has been destroyed by the Tenke thrust. In the east the Tentek syncline is truncated by the “Northern” thrust and adjoins the Dolinka syncline.

The axis of this structure trends northeast, deviating eastward. Its northwestern limb, which in the north acquires an almost latitudinal strike, is steep. The dip angles here increase from 45 to 75°. The southeastern limb is considerably gentler. As a rule, dip angles on this limb do not exceed 15–20°. The southeastern limb of the syncline, in the sector where it adjoins the Deldel uplift, has a more complex tectonic structure. Here an additional syncline of NNW strike is identified. The southwestern pericline has considerably steeper bedding dips, reaching 80–90°. The strike of the coal-bearing suites here changes to NNW.

The core of this structure is composed of rocks of the uppermost suite in the Karaganda basin—the Shakhan suite. Its limbs are composed of deposits of all the remaining suites of the coal-bearing sequence.

Near the Tentek thrust, in the southwestern part of the syncline, a series of smaller faults is observed. The “Northern” thrust, along which the Tentek syncline adjoins the Dolinka syncline in the east, has a meridional strike. Until recently it was believed that this fault joins the thrust traced in the south of the Churubai-Nura district and described under the name of the Churubai-Nura thrust. However, it has now been established that both faults die out in the central part of the district.

The Dolinka syncline is situated in the central part of the Churubai-Nura trough, extending from the northern margin southward to Dolinskoye settlement for a distance of almost 25 km.

This structure is an asymmetric fold. Its eastern limb is gentle (angles of 10–15°, increasing only in the north to 40–45°). The western limb is somewhat steeper. Here the dip angles average 15–25°. In the central and northern parts this limb is cut off by the “Northern” thrust. In the core of the Dolinka syncline lie rocks of the Dolinka suite; on the northern closure and on its eastern limb, progressively older coal-bearing Carboniferous suites crop out.

The fold axis has a NNW strike. Plunging of the axis is observed in the same direction. The Dolinka syncline has a considerably more complex tectonic structure than the Tentek syncline situated to the west of it. The complexity of its structure is due to the presence of large and small faults, which create a series of tectonic blocks. The strike of the faults is predominantly meridional and northwestern.

The eastern limb of this structure is characterized by a fairly simple structure. Here a regular decrease in the angles of dip of the rocks toward the center is observed. The central part of the syncline and the preserved part of the western limb have a complex tectonic structure. The deposits of the Dolinka formation in these areas are compressed into gentle folds and disrupted by a large number of faults.

The Sassyk-Kul–Dzhaimin trough is situated south of the Deldel–Dolinka zone of uplifts. In the character of its tectonics, this trough differs sharply from the Shakhan trough described above. The Sassyk-Kul–Dzhaimin trough has a very complex structure, caused by the effect of the Dzhalair overthrust.

The deposits of the coal-bearing sequence, especially in its southern part, are compressed into numerous complex folds and broken by a large number of latitudinally oriented faults. With distance from the Dzhalair overthrust, the secondary folding gradually dies out, and the number of faults also decreases. It should be noted that most of the area of the Sassyk-Kul–Dzhaimin trough, composed at the surface of deposits of the coal-free suprakaraganda formation, has not been explored at all.

In the northern part of the trough there are two small synclinal folds—the Sassyk-Kul and Dzhaimin folds.

The Sassyk-Kul (Karagoga) syncline. In the north this structure borders on the Dolinka uplift. In the east it is separated from the Dzhaimin syncline by a gentle anticlinal flexure. The western and southern limbs of the Sassyk-Kul structure gradually merge with the western and southern margins of the Churubai-Nura trough. The area of the syncline is about 40 sq. km. Its form is elongate-oval. The axis has a NNW trend. The core of the Sassyk-Kul syncline is composed of rocks of the lower part of the Dolinka formation. On the limbs, deposits of the suprakaraganda formation are developed.

The tectonic structure of the structure described is comparatively simple. The angles of dip of the rocks on the limbs do not exceed 10–20°. The southern pericline and the NE limb are complicated by additional minor folding. In comparison with the structures already characterized, the Sassyk-Kul and Dzhaimin synclines have been studied poorly.

The Dzhaimin syncline is situated south of the Dolinka syncline, being separated from it by a narrow anticlinal flexure elongated in a latitudinal direction. In its center, rocks of the lower beds of the Dolinka formation crop out. The axis trends in an ENE direction. The length of the syncline is about 3 km. The angles of dip of the rocks vary from 10 to 25°. The SE limb and the preserved part of the southwestern pericline are complicated by faults.

The Saran-Dubovka uplift is a large positive structural element of the second order and separates the Churubai-Nura trough from the Karaganda trough. It is a saddle-shaped flexure of the beds of the coal-bearing sequence with a general NW trend, which in the north is linked with the Saran projection, while its southern part forms a large positive structure of the fourth order—the Dubovka brachyanticline.

The Saran projection of the northern margin of the basin is a structural nose, composed in the crest of effusives \(D_1—D_2\), which plunge southward beneath younger deposits of the Devonian and Lower Carboniferous. The most submerged part of the uplift is formed at the surface by deposits of the Karaganda formation. Its width does not exceed 2 km. Still farther to the SSE begins the rise of the beds of the coal-bearing sequence along the hinge of the Dubovka brachyanticline as far as the southern margin of the basin, separated from the crest of the Dubovka structure only by a slight undulation of the hinge.

The Dubov brachyanticline is a large fourth-order structure. Its dimensions are about 15 km in length and 6 km in width. The amplitude is more than 700 m. Its NE and SE limbs, as well as the NW pericline, are composed of deposits of the Karaganda suite. In the crest, rocks of the Ashlyarik suite crop out at the surface.

The NE limb of the structure is steep. The dip angles of the beds are 35–60°. On approaching the fold axis, the angles gradually flatten. This limb is complicated by an entire series of step-like thrusts and faults whose direction is concordant with the strike of the coal-bearing strata.

The NW pericline is complicated by disturbances of the type of step-like faults, close to latitudinal strike. The western limb is gentler and considerably less disturbed than the eastern one.

The Karaganda trough is situated in the central part of the Karaganda basin. Its dimensions along the long axis are about 30 km, and along the short axis about 18 km. The Karaganda depression is separated from the Verkhneseokur trough by the Maikuduk-Akzhar uplift. Along the Maikuduk thrust, which bounds the above-mentioned uplift from the west, passes the eastern boundary of the Karaganda trough. In its northern part the Maikuduk thrust is very clearly defined owing to the presence of an open contact between Devonian effusives and the Carboniferous sedimentary complex. In the south it proves to be covered by a thick sequence of Mesozoic deposits and has been traced by geophysicists as far as the Sokur River. The Karaganda trough is a complexly built syncline with a NE strike of the axis. The form of the trough is irregular, approaching quadrangular.

Its tectonic structure is considerably simpler than the structure of the Churubai-Nura trough.

In the studied part of the Karaganda trough, clearly expressed third- and fourth-order structures are absent. A large part of the trough is covered at the surface by Mesozoic sediments, the thickness of which increases from north to south. The trough is characterized by an asymmetric structure. Its NW limb is gentle (angles do not exceed 20°), while the southern limb is steep, with a dip angle of up to 90°.

Also steep are the eastern limb, adjoining the Maikuduk thrust, and the southwestern limb, adjoining the Dubov uplift.

The southwestern and southern limbs are disturbed by numerous thrusts.

The limbs of the Karaganda trough are composed of rocks of the Ashlyarik and Karaganda suites. In the center of the trough, beneath a thick Mesozoic cover, lie rocks of the Nadkaraganda suite, and in the southwestern, most downwarped part of the trough, beneath the Mesozoic cover, lie rocks of the lower part of the Dolinka suite.

The upper part of the Dolinka suite, as well as deposits of the Tentek and Shakhan suites, are absent in the Karaganda trough owing to a large erosional truncation of the Paleozoic deposits. The northwestern limb of the Karaganda trough is distinguished by the calmest conditions of occurrence of the coal-bearing strata. The rocks here have a persistent northeastern strike. The dip angles of the beds usually do not exceed 15–30° for the lower parts of the Karaganda suite, gradually flattening to 6–7° for its upper parts, i.e., in the direction toward the center of the trough.

In the western part of the NW limb, major tectonic disturbances are absent. In the east of the Saran area and in the Promyshlenny area, a series of thrusts appears, having predominantly NW, more rarely NE, strike.

strike. The reverse faults here are ruptures with directions of displacement that vary along strike and dip and with an inconstant amplitude.

The entire northwestern limb is complicated by small-scale folding, the axes of which have predominantly a NE direction.

The eastern limb of the trough has been poorly studied tectonically. Drilling has clarified only its northern part. Here the rocks of the coal-bearing sequence are characterized by steep (50–60°) dip angles. Their strike is parallel to the Maiduk fault. All deposits of the coal-bearing sequence adjoining this fault from the west are crumpled into steep folds and complicated by tectonic ruptures of varying amplitude. The ruptures, as a rule, have the character of reverse faults. Their direction is mainly northeastern. The most significant ruptures are the North Maiduk and South Maiduk reverse faults, adjoining the main rupture from the east.

According to seismic-exploration data, in the southwestern part of the Karaganda trough the southern half of a positive structure has been distinguished, called the Taldykuduk uplift. The absence of drilling in this area does not permit a definite conclusion to be drawn about its tectonic structure.

The tectonics of the southern limb of the Karaganda trough is very complex. The coal-bearing series and the deposits of the limestone-shale sequence are gathered into a system of folds with steep dip angles and are strongly disturbed by numerous ruptures, which often also involve the Mesozoic deposits.

The tectonic ruptures and folds have, as in the southern part of the Churubai-Nura trough, a predominantly latitudinal direction.

The largest rupture of NW strike is the reverse fault called the Taldykuduk. On the southern limb of the trough this rupture has brought into contact the effusive-clastic sequence \(D_1—D_2\) with the Kassin beds \(C_1\).

The southwestern steep limb of the Karaganda trough, on which the Dubovka and Alabas areas are situated, is characterized by a rather complex tectonic structure.

The Dubovka area is entirely covered by Mesozoic deposits lying unconformably on the Paleozoic, forming the gentle brachyanticline of the Dubovka brown-coal deposit. The coal measures here have a northwestern strike with steep dip angles (30–60°) to the NE. Numerous tectonic ruptures of reverse-fault character have a strike concordant with the strike of the coal-bearing sequence, while the normal faults are characterized predominantly by a transverse NE strike.

In the south of the limb of the structure being described lies the Alabas area, on the southern margin of which there is a sharp change from NW strike to latitudinal. This change of strike is accompanied by a whole series of small, strongly disturbed folds of latitudinal and, more rarely, NE strike. The rocks dip at an angle of 60–75°.

The central part of the Karaganda trough, especially its southwestern half, is characterized by a slight downwarping, as a result of which the rocks of the supra-Karaganda and Dolinka series lie here beneath a cover of Mesozoic deposits.

The supra-Karaganda series, bordering the deposits of the Dolinka series that occur in the center, is characterized by steep dip angles in the south and west of the trough (35–50°) and by considerably gentler angles (5–15°) in its northern part.

In the north of the central part of the trough there passes a major reverse fault of NE strike. Along this rupture the southeastern part of the trough is uplifted.

with respect to C3, and the rocks of the Karaganda suite are in tectonic contact with the deposits of the Nadkaraganda suite.

The Maikuduk–Akzhar uplift. Its structure in many respects resembles the structure of the Dubovka–Saran uplift. The Maikuduk–Akzhar uplift is likewise a zone of saddle-shaped bending of meridional strike, situated between two troughs and connected with the projection of the northern flank of the Karaganda depression.

The northern part of the uplift is formed by the Maikuduk projection, composed of an effusive–clastic sequence of the Lower and Middle Devonian, bounded on the west by the Maikuduk thrust, along which Lower–Middle Devonian effusives are in contact with deposits of the coal-bearing sequence and carbonate rocks of the Upper Devonian and Lower Carboniferous.

The width of the Maikuduk projection is about 15 km. To the south and east there is a gradual subsidence of Devonian rocks beneath Lower Carboniferous carbonate and coal-bearing deposits at an angle of 60°.

The central part of the Maikuduk–Akzhar uplift is a saddle-shaped bend, formed at the surface by deposits of the Ashlyarik suite.

The Akzhar uplift, forming the southern part of the structure described, is a projection of the southern flank of the depression, but less sharply expressed. This structure is characterized by steep dips of the strata on the western and eastern limbs.

The Upper Sokur trough is at present still very poorly studied. The coal-bearing deposits over the area of this trough are overlain by a thick sequence of Mesozoic sediments. For this reason the tectonic structure of its inner part has not yet been sufficiently clarified. The dimensions of the Upper Sokur trough along the base of the Akkuduk suite are about 40 km along the long axis and 35 km along the short one. The strike of the long axis is almost latitudinal. Its northern and southern flanks are composed of rocks of the effusive–clastic sequence of the Middle and Lower Devonian. The southeastern flank is bounded by a system of major tectonic faults, bringing rocks of the Akbastau suite of the Middle and Upper Devonian into contact with the Akkuduk suite of the Visean and deposits of the Lower Tournaisian. In the east the trough narrows sharply to 5–7 km and then continues into the synclinal structure of the Ashchi-Su River basin. In the west the boundary of the trough is formed by Devonian effusives composing the Maikuduk uplift.

According to K. I. Adrianova, the Upper Sokur trough is filled with deposits of the lower suites of the Carboniferous coal-bearing sequence (the Akkuduk and Akmerik suites), overlain by a thick mantle of Mesozoic deposits.

The character of the tectonics of the marginal parts of the Upper Sokur trough has a great similarity to that of the flanks of the Churubai–Nurinskaya trough, especially in the south. Here the Devonian effusive and terrigenous rocks are thrust over the complex of Lower Carboniferous sedimentary rocks. The latter form a system of narrow synclinal and anticlinal folds oriented in a latitudinal direction, disrupted by a large number of major and minor faults. The western flank of the Upper Sokur trough is covered by a thick sequence of Mesozoic sediments, and therefore its tectonic structure is unknown.

The results of drilling by the Karaganda office of exploratory drilling of the “Aktyubneft” trust showed that the rocks of the Akkuduk suite and the lower part of the Ashlyarik suite, developed on the northern flank of the trough, dip southward at an angle of 25–30° and are crumpled into small folds, complicating the monoclinal bedding of the rocks of the northern flank of the trough. The largest of these is the meridionally elongated Burmoosinskaya syncline, located in the western part of the northern flank.

On the southern limb, the rocks of the Akkuduk and Ashlyarik suites occur at angles of 65–70° and are broken by a system of tectonic faults.

The tectonic structure of the central part of the Upper Sokur depression, according to geophysical data (G. V. Matveev, 1951) and general geological considerations, was depicted as extremely simple. It was assumed that in the center of the depression, beneath Mesozoic sediments up to 700 m thick, lay its most deeply downwarped part, composed beneath the Mesozoic cover of deposits of the Karaganda suite, surrounded along the periphery by rocks of the Ashlyarik suite. The amplitude of subsidence of the central part relative to the exposed margins was estimated at 2000 m. However, borehole No. 7-k, drilled somewhat south of the settlement of Numyz-Kuduk, instead of the deposits of the Karaganda suite into which, judging from G. V. Matveev’s map, it should have entered beneath the Mesozoic cover, encountered rocks of the Famennian stage—Clymenia beds—dipping at an angle of about 30°. Thus, instead of the supposed depression with an amplitude of 2000 m, there is an uplift in the central part of the Upper Sokur depression. The amplitude of the uplift relative to the depression adjoining it from the east, filled with sediments of the Ashlyarik suite, is not less than 1500 m.

Tectonics of the Mesozoic Deposits of the Karaganda Depression

Jurassic deposits, almost absent in the Churubai-Nura district, cover a large part of the Karaganda depression. They occur with a major hiatus and angular unconformity on the eroded surface of dislocated Paleozoic rocks. A rapid increase in the thickness of the Mesozoic from the margins of the depression toward the center is observed. Usually the dips of the Jurassic strata in the northern and central parts of the depression reach 8°. In the south, the angles increase to 15–20°. In Jurassic clays and sandstones exposed by quarries, brachyfolds are observed, with limb dips of up to 40–45°.

In the accumulation of Mesozoic deposits, G. L. Kushev distinguishes two cycles. The first cycle comprises the lower conglomerate-sandstone and Dubovka coal-bearing suites; the second comprises deposits of the upper conglomerate-sandstone and Mikhailovka coal-bearing suites. The sediments of the second cycle lie on the deposits of the first cycle.

Two large synclinal folds are distinguished in the Mesozoic sequence of the Karaganda depression: the Mikhailovka and Akzhar folds.

The Mikhailovka depression extends in a meridional direction from the Mikhailovka brown-coal deposit to the Kamyr-Adyr hills. All Mesozoic suites take part in the structure of the depression. The amplitude may reach 500 m.

The Akzhar depression is located WNW of the settlement of Akzhar. Its amplitude, according to geophysical data, does not exceed 300 m.

The principal large tectonic movements took place already after the formation of the coal-bearing sequence, in pre-Jurassic time, and are connected with Late Variscan tectogenesis. At that time five principal second-order structural elements were formed: the Churubai-Nura, Karaganda, and Upper Sokur depressions, separated by the Dubovka-Saran and Maikuduk-Akzhar uplifts. Apparently at the same time the Dzhalair thrust arose, as well as structures of the third and fourth orders. The Tentek reverse fault formed considerably later and complicated the already formed folded structure.

Tectonic movements continued for a long time even after the folded structure of the Karaganda basin had essentially taken shape. They led, first of all, to the fact that the deposits of the coal-bearing sequence were brought to the surface and partially

were eroded. Then, on the eroded surface of the Paleozoic, Upper Triassic and Jurassic sediments were deposited.

As a result of Cimmerian folding, the Jurassic sediments were crumpled into broad, gentle folds and broken by faults.

Subsequent tectonic movements led to the almost complete erosion of the Mesozoic deposits in the southwestern part of the basin, where these movements were most intense.

VNIGRI