Tectonic Structure of the Western Part of the Prokopyevsk District of Kuzbass

P. K. Kulikov

(Presented by Professor Doctor A. A. Belitsky)

As a result of a prolonged and complex process of tectonic dislocations at the junction of the Salair and the Kuznetsk Basin, a structural zone was formed consisting of a series of scales. The latter are bounded by major longitudinal faults dipping to the southwest at an angle of 60–70°, of the type of conformable and unconformable thrust faults.

Within the coal-bearing deposits of Kuzbass, five such scales are distinguished: the Prityrgan, Chertinskaya, Belovskaya, Leninsk-Kuznetsk, and Meret scales. Each of these scales is, as it were, an independent structural element and is characterized by a more or less extensive complex of folds and faults of lower orders. The intensity of the internal structures of each scale varies; moreover, an increase in intensity is manifested very clearly in movement from east to west, from the central part of the basin toward the Salair.

The Prityrgan scale, directly adjoining the Salair, is one of the most strongly dislocated. As a result of the bending of the axis of the principal synclinal fold of this scale in the vertical plane, several brachyfolds appear here, among which the Prokopyevsk–Kiselevsk brachysyncline stands out, corresponding to the deposit of the same name.

On the limbs of this fold there is a whole series of smaller, rather tight folds, elongated in a northwestern direction, and a comparatively dense network of large and small tectonic faults. In terms of the intensity and complexity of its tectonic structure, the Prokopyevsk–Kiselevsk deposit stands out among all the deposits of Kuzbass.

The Prokopyevsk district encompasses the southern half of the Prokopyevsk–Kiselevsk brachysyncline and, like the entire deposit, has a very complex structure, owing to which the tectonic factor has the greatest influence on the exploration and exploitation of the coal seams. It is therefore not surprising that the Prokopyevsk district has long attracted the attention of geologist-tectonists. Here, for example, such major geologists as Academician M. A. Usov and Professors V. I. Yavorsky, A. A. Belitsky, and I. A. Molchanov worked for a considerable time; they did a great deal to understand the structure of this district.

However, owing to certain circumstances, the area of the district was studied unevenly, and the least studied proved to be the western portion.

part of the district, where the productive deposits make up the I, II, and partly the III synclines (Fig. 1). The present work is devoted to a description of the structure of this part of the district; it uses materials from the Prokopyevsk GRP, the mine geological service, and the author’s personal observations, made over the course of three years while serving as chief geologist of Mine No. 11, “Maneikha.”

Fig. 1. Geological map of the western part of the Prokopyevsk district of Kuzbass

Principal Structures

Folds

I syncline. The I syncline, developed along the entire length of the district, has a comparatively simple structure. It is a linearly elongated fold with an axis plunging in a northwestern direction; the closures of the coal seams in plan have sharp, beak-shaped outlines, owing to which the seam outcrops are arranged almost parallel to one another.

Within Mine No. 11, near the closure of the I syncline, an additional fold appears on its eastern limb. Farther south this fold, developing more and more, assumes independent significance,

and the main syncline gradually diminishes and then dies out completely. Thus, a displacement of the axis of syncline I takes place, but this displacement is local in character and has almost no effect on the form of the entire fold. Syncline I is asymmetric. It is interesting to note that the asymmetry of the syncline in the southern and northern parts of the district is different: in the south the axial plane dips westward at an angle of 80–85°, and therefore the western limb of the syncline is the steep one; in the northern part of the district the reverse picture is observed: the axial plane dips eastward at an angle of 85–87°, and the eastern limb is the steeper one. In the central part of the district the fold is symmetrical. Thus, when the structure of the syncline in different parts of the district is compared, the impression is created that the fold has undergone torsion.

Fig. 2. Axial sections of folds

The axis of syncline I, as already noted, plunges gently northward. Against the background of this plunge, several smaller flexures of the axis are detected (Fig. 2), in connection with which, in the area of the Zenkovskie Uklony mine, a very gentle plunge of the fold to the south is indicated, while within mine No. 11 there is a broad anticlinal uplift, the northern limb of which lies at an angle of 18–20°, and the southern at an angle of 2–3°. In the field of mine No. 8, one more, less significant, anticlinal uplift of the axis is noted, as a result of which the Gorely seam between mines No. 11 and No. 8 forms a narrow, strongly elongated brachysyncline.

As it has been possible to establish, all the small bends of the syncline axis correspond to sectors of greater or lesser intensity of the fold. Where the folds are strongly compressed, as a rule, uplift of the axis is observed; where the fold is less compressed, a relative lowering of the axis takes place,

At the same time, in areas where there is no sharp change in the intensity of the syncline, there are no noticeable bends of its axis either.

II syncline. The II syncline, like the I syncline, can be traced for a considerable distance. Unlike the I syncline, it is more intense and more compressed. Owing to the development upon it of large transverse anticlinal uplifts, this fold is broken up into a series of brachysynclines, one of which is situated between the Zenkovskie Uklony mine and the northern boundary of the Prokopyevsk district.

The II syncline is asymmetrical, and its asymmetry is characterized by the same feature as in the I syncline: in the southern part of the district the western limb is steeper, while in the northern part the eastern limb is steeper. At the same time, in the central part of the district both limbs of the syncline dip equally steeply. Thus, in studying the form of this syncline, one likewise gains the impression that this fold is twisted.

The total length of the II syncline within the area of the Prokopyevsk district reaches 12 km. Along its entire extent the fold axis nowhere lies horizontally. The greatest plunge of the axis is observed in the field of the Ziminka mine, whence it rises rather steeply to the south and to the north. The angle of rise of the axis to the south is 10–15°; in the north this angle reaches 30–50°. Against the background of the general bend of the axis (Fig. 2), several second-order bends of the axis are noted; moreover, uplifts of the axis generally correspond to places of greatest intensity of the fold.

A comparison of the conditions under which minor bends of fold axes are manifested makes it possible to judge the nature of these bends. Since in some cases these bends are closely connected with the uneven development of folds, it is quite probable that one of the causes of the formation of the bends is the uneven distribution of stresses along the compression front. Where the strata were compressed more strongly, uplift of the axes is manifested predominantly; where the strata were compressed more weakly, synclinal sags are manifested. This supposition is all the more valid because the interrelation of minor bends of axes with the intensity of folds is also observed in other parts of the district.

Intermediate anticline. This is the name given to the anticlinal fold separating the I and II synclines. Since near the Zenkovskie Uklony mine the II syncline dies out, it is natural that the Intermediate anticline also dies out there. This fold has been affected by rupture dislocations to a greater degree than the adjacent synclines. Therefore, over most of the district it is extinguished and is clearly expressed only in the northern part of the district, where fault B—B, which cuts it, dies out. In the remaining part of the district, the I and II synclines along fault B—B have been brought into direct contact. In this connection, the structure of the Intermediate anticline can be judged only on the basis of geological constructions, which to a certain degree are provisional. Nevertheless, individual details of its structure have been studied quite well. We shall dwell on them.

The Intermediate anticline is asymmetrical along its entire extent, the asymmetry being especially well expressed in the southern part of the district and much less so in the northern part. In the south the steeper limb of the anticline is the western one, which in places is even overturned. In the northern part of the district, overturning of the anticline toward the west is indicated. The overturning becomes distinct beyond the limits of the Prokopyevsk district, where the dip angle of the western limb reaches 70–75°, whereas the eastern limb lies at an angle of 50–60°. In the axial section of the Intermediate anticline, very complex outlines of the fold axis are revealed. This complexity is due to a considerable extent to the fact that the anti-

the anticline is strongly dissected by tectonic faults, as a result of which its axis in the vertical plane shows very sharp jumps. A detailed study of the axial section of the anticline (Fig. 2) reveals two interesting features: first, the distance between beds along the axial plane of the fold increases sharply when moving from the south toward its central part, where it is more compressed, which once again confirms the correctness of the above-stated assumption concerning the connection between small vertical bends of fold axes and the unevenness of compression in different parts of folds; second, the increase in distances along the axial plane of the fold between the beds of the upper part of the section of the productive deposits occurs much more rapidly than between the beds of the lower part of the section. The latter means that the intensity of the anticline decreases with depth and that, with depth, the anticline probably gradually dies out.

The Taibinskaya anticline. This anticline is situated between synclines II and III. Its description also requires characterization of the western limb of syncline III; therefore we shall not separately describe syncline III, which only partly participates in the structure of the western part of the district.

The Taibinskaya anticline is asymmetric. In its southern part it is overturned to the east, and in its northern part—to the west. The overturning of the anticline is manifested equally clearly in both parts of the district. In the central part of the district the anticline is symmetrical.

The Taibinskaya anticline is complicated by faults A—A and M—M, owing to which a rather complex bending of its axis is observed in its longitudinal section. Along the Slozhnyi bed the axis of the anticline undergoes a considerable plunge, confined to the central part of the district. In the southern and northern parts of the district the axis of the anticline along this bed approaches the ground surface. At the same time, the axis along the Moshchnyi bed in the central part of the district does not undergo any noticeable plunge. The axis of the anticline along the Gorelyi bed, and still more along the Vnutrennii beds, undergoes not a plunge but a general uplift.

Faults

Within the area of development of the folds described, five large faults are known: B—B, C—C, D—D, A—A, and M—M. All these faults are steeply dipping reverse faults and cut the folded sequence parallel to the fold axes. There are no large transverse faults here. All the listed faults, with the exception of M—M, dip westward. The amplitude of displacement along the fault plane reaches 800–1500 m and is characterized by extreme variability. The latter is clearly shown in Fig. 3, where longitudinal vertical projections of the traces of these faults are given.

All these major faults are accompanied by a more or less dense network of apophyses. Observations show that the number of small faults located in the flanks of the large ones depends above all on which limb of the fold is intersected by the large fault and at what angle it is intersected.

Within the area of the part of the district described, the major faults are distributed unevenly: in the southern part of the district 4 faults are known, whereas in the northern part only one fault, M—M, is developed. The dying out of all the faults is confined to the central part of the district.

As is known, in this same part of the district there also occurs a change in the direction of overturning of the folds. It is quite clear that both these phenomena—the change in the direction of overturning of the folds and the dying out of the faults along the strike of the folds—are closely connected with one another. In the opinion of A. A. Belitskii, the principal cause of the different structural pattern

of the northern and southern parts of the district is the development in them of faults with different directions of displacement. The overturning of folds to the west in the northern part of the district should be regarded as a local phenomenon that arose in connection with the development here of the \(M—M\) fault, dipping to the east.

Fig. 3. Sections of seams by faults in projection onto a vertical plane.
Sections of seams: 1—in the footwall, 2—in the hanging wall of the fault.
Dip of seams: 3—to the east, 4—to the west.

Internal structures of the folded sequence

Swellings and pinch-outs

Tectonic changes in the thickness of coal seams are known as swellings and pinch-outs. Swellings and pinch-outs, as a rule, encompass a limited area of a seam, and the dimensions of this area may vary within very wide limits, from units to tens and hundreds of meters. The areas over which swellings and pinch-outs develop have irregular, most often elongated outlines (Fig. 4). They have an equally irregular form in section (Figs. 5, 6).

It is commonly assumed that swellings and pinch-outs necessarily occur together. However, this is not always the case. Most often pinch-outs are independent structural elements. In such cases they are bands of crushing of a seam, arising as a result of the simultaneous action on the seam of tensile and compressive forces, which is possible, for example, in the case of overturning of the limb of a fold. The pinch-out of a seam shown in Fig. 5 may serve as an example of precisely this kind of pinch-out. Here the decrease in thickness in the crushed part of the seam is compensated by a general increase in the area of this seam. Sometimes swellings and pinch-outs occur together in space. These combinations are already the result of local crushing of the seam and the forcing of the crushed mass of coal into areas adjacent to the pinch-out (Fig. 6). The latter probably have, in the general process of tectoni-

tectonic dislocations is much less important than independent pinches.

The development of swellings and pinches is accompanied by a fundamental change in the internal structure of the coal seam and in the properties of the coal. The coal in places of swellings and pinches is, as a rule, strongly disturbed; moreover, in individual cases, especially near the side rocks that have been pressed into the seam,

Fig. 4. Isolines of thickness of the Bezymyannyi II seam, western limb of the I syncline

the coal is a dust-like, usually moist mass. The strength of the coal decreases sharply. The luster of the coal and its banded structure, especially in places where pinches develop, disappear. In cases of considerable crushing of the coal, rock partings, if present in the seam, are abraded and disappear, mixing with the coal. Partings of hard and very

Fig. 5. Character of the pinch of the Podlutuginskii II seam, II syncline. Legend as in Fig. 6.

Fig. 6. Combination of a pinch (a) and a swelling (b) in the Bezymyannyi II seam, eastern limb of the II syncline. 1—side rocks, 2—swollen coal, 3—undisturbed coal.

hard rocks are not abraded, but are broken up and scattered through the mass of coal. Sometimes, in places of swellings, small folding is observed, arising as a result of compression of the seam from the side of the pinch. The coal in places where small folds appear also proves to be very strongly disturbed. Since, during the formation of swellings and pinches, the side rocks are also destroyed, the coal thereby becomes heavily contaminated, as a result of which there is a sharp increa—

its ash content and bulk density increase, and the contact of the coal with the enclosing rocks becomes uneven, ragged, or diffuse.

Undoubtedly, structural elements similar to swells and pinches in coal seams also arise in the enclosing rocks, especially in argillite beds; however, they have been studied so little that we do not consider it possible to dwell on them.

Fracturing

Many geologists, including mine geologists, have studied fracturing in the Prokopyevsk district; however, there are very few published works devoted to questions of fracturing in the rocks of the district. They are by I. A. Molchanov and A. A. Belitskii (1939, 1949).

The entire variety of fracture systems is subdivided by the latter into two groups: normal-cutting and oblique-cutting fractures, each of which is characterized by its own features of manifestation and distribution. We also used this subdivision in our work.

Normal-cutting fracturing. This group includes fracture systems arranged perpendicular to the bedding and forming an angle of 90° with one another. One of these systems cuts the rocks along the strike line and is called the longitudinal normal-cutting system. The other cuts the rock mass along the dip line and is called the transverse normal-cutting system. All bedding-plane fractures may also be assigned to this same group.

All three systems of fractures, intersecting in space, break the rock mass or coal seam into more or less regular blocks, producing cubic, platy, or columnar jointing.

The dimensions of the faces of the joint blocks vary greatly, showing a close dependence on rock composition, bed thickness, and the intensity of the tectonic structure.

The dependence on rock composition is the most distinct. In coal, normal-cutting fracturing is very abundant. The distance between individual fractures here sometimes reaches 0.5–1 cm. However, in some seams the fractures are much more widely spaced. For example, in the Moshchnyi seam, whose coal is distinguished by increased strength, the distance between individual conspicuous fractures reaches 0.2–0.5 m. Argillites are also characterized by a high density of fracturing; in this respect they approach coals. In siltstones and sandstones, fracturing is developed much more weakly. Thus, in massive sandstones the distance between adjacent fractures reaches 1.0 m and more.

The dependence of fracture density on bed thickness is clearly manifested when beds of the same lithological composition but of sharply different thickness are compared. As a rule, in thick beds fracturing is less abundant than in thin ones. In rocks with increased fracturing this dependence is weakly expressed.

We noted above that normal-cutting fractures cut the rock mass either along its dip or along its strike. However, there are cases in which sharp deviations from this rule are observed. During open-pit mining of the Dvoinoi seam, we were able to trace in detail the change in the relationship between fractures and bedding. The results of these observations are presented in Fig. 7 and amount to the following: 1) the orientation of normal-cutting fractures relative to bedding is constant; 2) the orientation of these same fractures in space and relative to lines

the dip and strike of the bed, the thickness is not constant. In the hinges of folds the two systems seem to change places, as a result of which longitudinal fractures cut the bed along its dip line, and transverse fractures along its strike line.

Obliquely cutting fracturing. This group includes all fractures that form an angle of less than 90° with the bedding.

Fig. 7. Orientation of normally cutting fractures in a fold.
1—transverse system; 2—longitudinal system.

It may be considered proven that this group of fractures arises during compression of the sequence. It is therefore natural to assume that the intensity of obliquely cutting fracturing depends, to a certain degree, on the stress state of tectonic structures. Indeed, intensely dislocated areas are, as a rule, characterized by more abundant fracturing than weakly dislocated areas.

Just as clearly as in the case of normally cutting fracturing, there is a dependence of fracture density on the lithological composition of the rocks and on the thickness of the beds. The essence of this phenomenon remains the same; therefore we shall not describe it.

Obliquely cutting fracturing is characterized by varying extents of individual fractures. In this respect all fractures may be subdivided into three groups. The first group includes the most widespread fractures, which have a very small extent—from several centimeters to several tens of centimeters. The second group comprises larger, but also rarer, fractures that cut a bed or a layer. The third group includes fractures that intersect an entire series of beds and constitute a transitional link from ordinary fracturing to faults.

Fractures in space have the most varied orientations; consequently, as factual material accumulates, the points corresponding to individual fractures densely cover the entire field of the circular diagram (Fig. 8). This, in fact, creates difficulties in identifying the causes, time, and conditions of their formation. At present,

it is known only that they arise as a result of compression and are generated by the same forces as many other structural elements.

Fig. 8. Circular diagram of jointing.

Minor faults

Minor faults in the strata in the described part of the district are very diverse and numerous.

It has been calculated that, during the driving of preparatory workings, minor faults with an amplitude of \(1.0\)—\(10.0\) m occur in separate areas on average every \(15\)—\(20\) m. Still smaller faults occur even more often, but as a rule they are not recorded. Faults with an amplitude of more than \(10\) m occur relatively rarely. The average distances between them are \(50\)—\(100\) m.

Minor faults within the folded sequence are distributed extremely unevenly. In the area of development of the I and II synclines, sectors are often encountered in which minor faults are entirely absent. Such sectors include the eastern limbs of the I and II synclines in the field of Mine No. 11, the eastern limb of the I syncline within the mine field of the Zenkovo Inclines, and others. But alongside such sectors, there are often also ones where, because of the disturbed condition of the coal seams, their extraction proves impossible. An example of such disturbance is the eastern limb of the II syncline, near the M—M disturbance in the field of the Ziminka Mine.

A. A. Belitskii, analyzing the distribution of minor faults over the area of the Prokopyevsk–Kiselevsk deposit, established a number of regularities. These regularities are manifested, it is true, not so distinctly in the western part of the Prokopyevsk district as well.

Most often, sectors of increased disturbance are associated with major faults and, above all, with discordant reverse faults. Distur-

the disturbance of the rocks on the flanks of large concordant thrust faults is relatively slight. Owing to this, the eastern limb of syncline II in the field of the Ziminka mine, which is cut by the discordant thrust fault M—M, proved to be exceptionally strongly disturbed, as did the eastern limb of the Taibinsk anticline, which is likewise cut by the discordant thrust fault A—A. At the same time, the eastern limb of the second syncline, cut by the large concordant thrust fault D—D, is less disturbed, while in the fields of the Zenkovskie Uklony mine it is not disturbed at all.

A clustering of small ruptures is also observed in areas where large ruptures converge. This accounts for the increased disturbance of the field of mine No. 7, where the disturbances A—A, M—M, C—C, and D—D come together. In addition, small ruptures appear in all areas where large ruptures cut the strata at a high angle, i.e., in places where bends of beds, small folds, flexures, etc., develop.

Thus, the regularities in the distribution of small ruptures in the deposit have by now been identified fairly completely. True, small areas of increased disturbance are sometimes encountered whose origin we cannot yet understand, but this does not alter the overall picture, thanks to which the known regularities in the distribution of ruptures can be confidently used for solving certain practical problems, in particular for predicting the degree of disturbance of undeveloped areas.

The small ruptures studied in the western part of the district are not only numerous but also varied. Their variety is manifested in their geometric form, in the amplitude of displacement, in the direction of displacement, and also in the orientation of the displacement planes.

Fig. 9. Circular diagram of ruptures.

On the circular diagram (Fig. 9) are plotted the ruptures whose sketches we were able to find in the mines of this part of the district. As can be seen, this diagram resembles a diagram of jointing. This similarity suggests that jointing and ruptures are products of

of the same forces, and that the faults arise by making use of the fracture network already present by that time.

Conclusion

This article gives a very brief description of the most widespread structures of the western part of the Prokopyevsk district: folds, major and minor faults, fracturing, splits, and pinch-outs. There is no doubt that all structural elements are in a definite interrelation. Thus, for example, the displacement of rock masses, recorded in the form of folds, is directed toward the side to which the hanging walls of the major faults are displaced. Minor faults show a fairly clearly expressed dependence on major faults, as we emphasized when analyzing the features of the distribution of minor faults. Minor faults are also connected with fracturing, which probably accounts for the variety in the orientations of their displacement surfaces.

Comparing the structure of the western part of the district with the structure of the main part of the deposit, we have come to the conclusion that it possesses the same features. Consequently, the regularities identified in the main part of the district by A. A. Belitskii are also characteristic of the western part.

Literature

1. Belitskii A. A.—On the question of the mechanism of formation of cleavage fractures. Proceedings of the Mining-Geological Institute, West Siberian Branch of the Academy of Sciences of the USSR, issue 6, Novosibirsk, 1949.

2. Molchanov I. A., Belitskii A. A.—Cleavage of thick seams in the Prokopyevsk district and its influence on extraction work. Publishing House of the Tomsk Industrial Institute, vol. 60, issue 1, Tomsk, 1939.