Tectonic Fracturing of Coals and Host Rocks in the Leninsky District of Kuzbass

Yu. N. Popov

(Presented by Professor K. V. Radugin)

The study of tectonic fracturing in the coal-bearing sequence of the Leninsky district makes it possible to form a definite judgment about the character of fracturing in the belt of deposits of the Kolchugino series of Kuzbass, assigned by A. A. Belitsky and E. M. Pakhom [4] to the “subzone of gentle folding...” of the “...Prisalaïr zone of linear folds and ruptures.”

The tectonics of the Leninsky district is represented by a group of large, parallel, northwest-trending brachysynclines. The anticlines separating them proved to be disrupted by regional thrust faults with displacement amplitudes of 1000–1500 meters. This determined the block character of the structure of the district.

I. I. Ammosov and I. V. Eremin [1] drew attention to the endogenous (normal-cutting, according to A. A. Belitsky [2]) fracturing of the coals of the district. Our observations of the fracturing of coals and host rocks, carried out in mine workings of the mines of the Leninskugol trust, in the faces of open-pit coal mines, and in the core of a number of exploratory boreholes (Fig. 1), covered a complex of structural and lithological conditions typical for the district. More than 80 point diagrams of elements of fracture occurrence, constructed on a Wulff net, were used for the analysis. They characterize the orientation of about 20,000 studied surfaces. The degree of expression of the systems was determined by establishing the number of fracture poles falling on an equal-area elementary surface of the sphere, with subsequent processing in isolines of equal relative density. The identification of fracture systems could often be checked visually at the observation point. Attention was also paid to the morphology, frequency, and character of fracture filling.

Tectonic fracturing is represented in the district by fractures of two genetic groups—normal-cutting and oblique-cutting, according to A. A. Belitsky [2, 3].

Normal-cutting fracturing is widely developed within all the studied areas of the district. It is represented mainly by two well-expressed, mutually perpendicular systems of fractures normal to the bed; some of these have a northwest–southeast dip, while the dip azimuths of the others are grouped in the northeastern and southwestern rhumbs. Our observations showed that the fractures of the first system are longitudinal, and those of the second are transverse to the axes of the main folds of the district.

The character of the spatial relationships of normal-cutting fractures with bedding is not always revealed when analyzing only the ele-

...elements of occurrence of the bed and the fractures. Longitudinal normal-cutting fractures prove to be longitudinal, and transverse ones transverse, relative to the strike of the bed on the limb of a fold, usually when its axis is horizontal. If, however, the axis of the fold is inclined to the horizon at some angle, then the resulting change in the elements of occurrence of the bed and of the normal-cutting fractures will lead to the strike of the latter becoming diagonal not only to the strike of the bed, but also to the axis of the fold. This is readily verified by simple constructions on a Wulff net. We investigated the character of the change in the elements of occurrence of the bed and of normal-cutting fractures on the limb of a fold with dip angle \(\alpha\), when the fold axis is rotated in a vertical plane by an angle \(\gamma\) from the horizon. In this case the strike of transverse fractures remains the same, while their dip angle becomes equal to \(90^\circ-\gamma\). The strike of the bed changes by an angle \(\Delta \omega\), which is determined as

\[ \Delta \omega = \operatorname{arc tg}\left(\frac{\sin \gamma}{\operatorname{tg}\alpha}\right). \tag{1} \]

The strike of longitudinal normal-cutting fractures \(\omega_{\mathrm{pr}}\) changes in the opposite direction by the value

\[ \Delta \omega_{\mathrm{pr}} = \operatorname{arc tg}\left(\frac{\sin \gamma}{\operatorname{tg}\alpha}\right). \tag{2} \]

The dip angles of the bed \(\alpha'\) and of the longitudinal fractures \(\alpha'_{\mathrm{pr}}\) are determined, respectively, as

\[ \alpha' = \operatorname{arc cos}(\cos \alpha \cdot \cos \gamma), \tag{3} \]

\[ \alpha'_{\mathrm{pr}} = \operatorname{arc cos}(\sin \alpha \cdot \cos \gamma). \tag{4} \]

If the observer looks along the fold axis in the direction of its plunge, then the angle \(\Delta \omega\) is added to the azimuth of the strike of the bed on the fold limb whose dip is directed to the right, and is subtracted from the strike azimuth of the limb dipping to the left. The strike of longitudinal normal-cutting fractures changes in the opposite direction. The character of the interrelations of the quantities considered can be judged from the graphs given in Fig. 2.

Fig. 1. Diagram of the distribution of zones of different intensity of fracturing of coals in the Leninsky District of Kuzbass:
1 — coal seams; 2 — disjunctives; 3 — fold axes; 4 — boundaries of zones of different types of fracturing in coals; 5 — first type; 6 — second type; 7 — third type; 8 — principal deposits of the Leninsky District: I — Nikitinskoye, II — Leninskoye, III — Gramoteinskoye; 9 — mines: 1 — “Nikitinskaya-1,” 2 — named after S. M. Kirov, 3 — “Komsomolets,” 4 — “7th of November,” 5 — “Polysaevskaya-Severnaya,” 6 — “Polysaevskaya-1,” 7 — “Polysaevskaya-2,” 8 — “Polysaevskaya-3,” 9 — “Zhurinka-3,” 10 — named after E. M. Yaroslavsky.

It is probable that the predominance, noted by a number of authors, of diagonal fractures normal to the bedding in the peri- and centriclinal parts of folds, whereas on the limbs only longitudinal and transverse ones are known, should be associated with the changes in the elements of occurrence of the bed and fractures considered above [2, 3, 4, 6, 8].

Fig. 2. Character of the dependence of the bedding elements of the seam and of longitudinal normal fractures on the limb of a fold when the angle of inclination of its axis changes.

Visible curve labels in the diagrams:
\(\delta = 90^\circ\), \(\delta = 45^\circ\), \(\delta = 40^\circ\), \(\delta = 30^\circ\), \(\delta = 20^\circ\), \(\delta = 10^\circ\).

The foregoing casts doubt on the universality, widely applied in the practice of structural analysis, of the “reduction” method proposed by I. V. Kirillova [5], which is based on fixing the spatial position of a fracture in a system of rectangular coordinates whose axes are directed, respectively, along the dip and strike lines of the bed and also normal to it. The “reduction” method can be applied when the axis of the fold is horizontal, or after assigning it such a position.

Proceeding from what has been set out above, we have called the systems of shear fractures normal to the bedding longitudinal and transverse, bearing in mind their relationship to the axes of folds. The almost parallel arrangement of the axes of the principal folds in the district and the small dip angles of their limbs, with a rather gentle rise of the axes, caused abrupt changes in the strikes of the beds and a comparatively small scatter of the attitude elements of the normal shear fractures, especially the transverse ones (Fig. 3). This eliminated the need for additional constructions in analyzing the fracturing of the district.

In degree of expression, normal shear fracturing is the determining type in the district. Along these fractures and along the bedding, a predominant separation of coal from the massif occurs during mining. The shape of the separations is cubical, prismatic, columnar.

The surfaces of normal shear fractures in coals are usually even, smooth, without traces of displacement. Isometric films of pyrite-marcasite, measuring from 0.001 to 0.005 m in diameter, are sometimes present on them. The statements of I. I. Ammosov and I. V. Eremin [1] that longitudinal normal shear fractures are filled with incrustations, whereas transverse ones—with carbonate veinlets 0.001–0.0015 m thick, do not everywhere correspond to reality, occurring only in individual particular cases. Carbonate filling of fractures is more often represented not as veinlets but as thin films and rough coatings. The character of fracture mineralization cannot serve as a criterion for distinguishing one system of normal shear fractures from another.

Fig. 3. Graph of the orientation of dip azimuths of transverse normal shear fractures

The homogeneous petrographic composition and insignificant changes in the stages of coal metamorphism reduce to a minimum the influence of these factors on the frequency of normal shear fractures in different parts of the district. Tectonic factors become decisive. Thus, if the frequency of fractures affecting the entire seam or its individual benches fluctuates on average within 7–10 per meter, then in places where plicative—

of the structure with additional folds and disjunctives, it increases to 50–100.

Normal-cutting fracturing in the enclosing rocks is usually expressed quite well. The orientation of the fractures here is the same as in the coals. The fracture surfaces in the rocks are usually even; they cannot be called smooth, and traces of displacement along them are usually absent. In thick sandstone beds, calcite veinlets 0.5–0.01 m thick, more rarely up to 0.03 m, are formed along fractures of normal-cutting systems. As a rule, normal-cutting fractures do not cut the boundaries of beds with different lithological composition.

The frequency of fractures in rocks is inversely dependent on the grain size of the rocks and the thickness of their beds. The gaping of fractures is the greater, the lower their frequency. In this connection, thick (up to 50 m) sandstone members proved to be broken by normal-cutting fractures with a frequency of 1.5–2 per meter and with considerable gaping. The fracturing of thin interbeds of siltstones and argillites is more frequent, and almost no gaping was recorded here. A. A. Belitskii [2, 3], along with longitudinal and transverse systems of normal-cutting fractures, distinguishes two systems of diagonal normal-cutting fractures, which, in his opinion, like the fractures considered earlier, owe their origin to tensile forces, but, unlike the former, which are fractures of separation, they formed during shearing. In the Leninsky District, fractures resembling diagonal normal-cutting fractures in orientation were noted in the study of fracturing in core at the Nikitinsky deposit and in the field of the Komsomolets mine. In the other areas studied, this system is not sufficiently clearly expressed and often coincides with diagonal steeply dipping oblique-cutting fractures.

Oblique-cutting fracturing is initiated in the process of fold-forming movements, being mainly the result of shearing stresses under compression.

I. I. Ammosov and I. V. Eremin [1], evaluating the general character of fracturing in the Leninsky District, note the presence of a considerable number of oblique-cutting fractures only near disturbances. Undoubtedly, in the district, with the exception of individual areas, oblique-cutting fracturing is not, in degree of expression, the principal type determining mining-engineering conditions. Nevertheless, the presence of oblique-cutting fractures at observation points is noted very often (see Table 1).

Thus, in mine fields of simple tectonic structure, the number of systems of oblique-cutting fractures recorded at individual observation points is generally no more than four, whereas in fields of complex structure it often reaches 5–7. It is characteristic that, under identical structural conditions, the number of systems of oblique-cutting fractures in the enclosing rocks is greater than in the coals. What has been said applies primarily to rocks of clayey and sandy-clayey varieties.

In contrast to normal-cutting fractures, the surfaces of oblique-cutting fractures are mainly wavy in character. They are characterized by traces of displacement in the form of striations and grooves of slickensiding, the orientation of which depends mainly on the spatial position of the fracture. Thus, on the surfaces of longitudinal fractures, striation along the dip predominates; on transverse and steep diagonal fractures, along the strike; on gently dipping diagonal fractures, the direction of displacement striations occupies an intermediate position between those named above. Slickensides are often observed on the surfaces of fractures in coals and rocks. They are especially widely developed in argillites and siltstones of the immediate floor and roof of coal seams and at the contacts of these rocks with sandstones—

…folds, i.e., in zones of stress concentration arising in an inhomogeneous bedded mass during the formation of folds. In places, slickensides are observed in thick sandstone beds; they are excellently expressed by calcite veinlets, which predominantly fill steep diagonal oblique fractures. In addition, slickensides along oblique fractures in rocks of various lithological composition are often present in zones affected by disjunctives.

Table 1

Character of the manifestation of oblique fracturing in the Leninsky District

*) All observation points are in rocks.

The orientation of the maxima of systems of oblique fractures is determined by the direction of the principal tangential stresses at the moment of fracture formation. Analysis of oblique fracturing in the district showed its connection with folding, expressed in the regular orientation of fractures relative to the axes of folds.

We were able to distinguish five pairs of conjugate systems of oblique fractures, oriented in a definite way with respect to the average spatial position of the axes of the principal folds of the district. Among the systems there are longitudinal, transverse, and two pairs of gentle diagonal fractures. In addition, a system of steep diagonal fractures has been distinguished, which in orientation often coincide with diagonal normal fractures. Both these and the other fractures are the result of shearing, but the diagonal normal fractures arose under tension, and the steep diagonal oblique fractures under compression. The former, according to prevailing views, are earlier. Evidently, the unity of the source of active

movements, which caused both sagging of the basin floor and the formation of folds, led to the fact that, where the bedding is gentle, both fracture systems are superimposed. On the steep limbs of folds it is sometimes possible to record fractures of both types. The fractures of these systems have no morphological differences characteristic only of a definite type. Since in the area studied small dip angles of the beds predominate, in summarizing the data on fracturing we have taken the combined fracture system as an oblique steep diagonal one, which was noted at 76% of the observation points.

Among the other systems of oblique fractures, longitudinal ones substantially predominate; they were encountered at 50% of the observation points, and transverse ones at 47%. Gently dipping diagonal oblique fractures, noted respectively at 25 and 23% of the observation points, are of subordinate importance. The latter fractures, in most cases the latest, are a reaction of the rocks already considered to the arising stresses. Some of the gently dipping diagonal fractures owe their origin to stresses arising within the bed during interbed sliding, being fractures of a local deformation pattern.

The presence of the identified fracture systems in all the studied areas of the Leninsky District, despite the substantial difference in their structural position (Table 2), indicates in the main a common plan of deformation

Table 2

Character of the occurrence of individual systems of oblique fractures in mine fields and areas of the district

of the coal-bearing sequence of the district, caused by the same active tectonic factors. The differences in the character of fracturing consist mainly in the degree of expression of fractures of one or another oblique system.

The frequency of oblique fractures in coals is considerably lower than the frequency of normal fractures. In a calm tectonic setting it lies within the range of 3–10 per meter, more rarely 1.25–2. This is, as it were, the background frequen

the frequency of oblique fractures. In the Leninsky District their frequency depends mainly on structural factors. In places where the structure is complicated by additional plicative and disjunctive forms, the frequency of oblique fractures increases by a factor of 2–2.5.

In the enclosing rocks the frequency of oblique fractures varies within wide limits and depends, along with tectonic factors, on the thickness and lithological composition of individual beds. The nature of the combination of these factors determines the frequency of fractures. With equal bed thicknesses under identical structural conditions, the frequency of oblique fractures increases as the grain size of the rocks decreases; in this case fine-grained rocks, usually less competent, prove to be broken by a denser network of oblique fractures. At the same time, thin interbeds of sandstones enclosed between beds of clayey rocks prove to be intensely fractured. In studying fracturing from the cores of exploratory boreholes and in the faces of the Gramoteinsky coal strip mine, it was established that zones of increased fracturing not connected with tectonic factors are mainly confined to contacts between rocks of different lithological varieties, especially to beds of argillites and aleurolites at the boundary with sandstones. The thicknesses of the near-contact zones of increased fracturing in sandstones and clayey rocks are in a ratio of 1 to 3–5.

Despite differences in the degree of expression, the orientation of fractures in such zones does not change appreciably.

In the district two types of zones of increased fracturing connected with tectonic causes may be distinguished: the first comprises zones confined to disjunctives of different orders; the second comprises zones of intensive development of oblique fractures connected with additional folding. Thus, against the background of the fairly well developed fracturing of the Nikitinsky deposit, the zone of influence of the Kilchigiz thrust, up to 600 m thick and localized predominantly in the hanging wall of the fault, is represented by strongly fractured rocks, among which intervals of crushed, brecciated, and mylonitized rocks have been noted. Most fractures bear traces of intensive displacements on their surfaces. Within the zone, as in a quiet structural setting, the less competent rocks, especially at contacts with more competent ones, prove to be especially intensely fractured. Disjunctives of higher orders are also accompanied by zones of increased fracturing of rocks and coals. The thickness of such zones depends on the amplitude and orientation of the fault relative to bedding. Intensive fracturing in the walls of faults is accompanied by extensive development of movements along fracture surfaces, the result of which is striae and slip grooves, as well as slickensides, developed predominantly along fractures parallel to the fault.

Under conditions where the principal folds of the district are complicated as a result of undulation of their axes and the appearance of additional plicative forms, the frequency of oblique fractures increases and new systems appear over considerable areas more or less uniformly. This is clearly seen in the study of fracturing of coal seams in the mine fields of the Zhurinka-3 and Yaroslavsky mines; here the intensity of oblique fracturing, being higher than in mine fields of simple structure, remains approximately the same throughout the entire mine field, increasing somewhat near disjunctives.

In the enclosing rocks, where the lithological composition of beds and their thickness exert a great influence on the character of fracture manifestation,

the relationships considered also occur, although they are expressed less distinctly.

The intensity of coal fracturing is an important mining-geological factor. According to the five-point system proposed by I. I. Ammosov and I. V. Eremin [1] for classifying coal seams by the character of fracturing and the predominant character of separation from the massif, the coals of the seams of the Leninsky district may be assigned to the first three types.

Fracturing of coals of the first type is represented mainly by normal-cutting fractures. It is characteristic of the undisturbed limbs of large gentle folds, such as the Leninskaya syncline. Examples are the greater part of the field of the S. M. Kirov mine and sectors of the fields of the “Polysaevskie” mines, with the exception of zones influenced by disjunctive faults (Fig. 1).

The second type includes coal seams in which, in addition to normal-cutting fractures, there are at least three obliquely cutting systems, with the frequency of one of them being 3–10 per meter. This type of fracturing is widespread over most of the Nikitinsky and Egozovo-Krasnoyarsky deposits. It also occurs in the Leninsky deposit in sectors where the limbs of the Leninskaya syncline rise.

The third type includes coal seams broken, along with normal-cutting fractures, by no fewer than three systems of obliquely cutting fractures, with the frequency of some of them lying within the range of 10–20 per meter. This type of fracturing is characteristic of zones affected by disjunctive faults of various orders; it is also encountered in the centroclines of the Egozovo-Krasnoyarsk syncline.

The fourth type includes coals in which obliquely cutting fractures obscure the normal-cutting fracturing. Such coals are extremely rare in the district.

At the present stage of study of the fracturing of the enclosing rocks, we do not consider it possible to group the fracturing of the rocks in any way by type of expression as a function of structural conditions, since the undoubted influence of the latter is masked by the influence of factors of sedimentational and physico-mechanical heterogeneity of the coal-bearing sequence. The problem can be solved through a differentiated approach to each specific case, taking into account the degree of influence of each factor and subsequent generalizations; if necessary, the solution of these questions may become the subject of a special investigation.

REFERENCES

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