Features of the Tectonic Structure of the Field of the Yagunovskaya Mine

L. M. Petrovsky

(Presented by the Department of Geology and Exploration of Mineral Deposits)

The work carried out by geological-exploration parties in exploring mine fields, as a rule, does not elucidate the regularities in the manifestation of small tectonic forms, which substantially affect both the direct conduct of preparatory cutting and stoping operations and the long-term planning of new mine construction.

The most complete elucidation of these forms can be obtained only in mining workings. Below are given certain features of the results of the author’s study of small tectonic forms in the most structurally complex part of the field of the Yagunovskaya mine.

The field of the Yagunovskaya mine is located in the southwest of the Kemerovo district of Kuzbass, in the left-bank part of the Tom River basin. In the regional structural plan it is situated on the western limb of the Kemerovo syncline, which predominates in the district.

The structure of the field includes deposits of the Kemerovo sub-suite \((P_1)\), comprising 9 workable coal seams.

A different structure is observed in the northern and southern parts of the field, with a noticeable complication of them in the southern direction.

The main distinguishing feature of the southern part of the field, the most complex in structure, is the presence of a large complete additional fold. At the same time, both the anticlinal and synclinal folds are distinctly asymmetric, with a noticeable dip of their axial surfaces to the west—southwest and, correspondingly, with steeper limbs having a southeastward dip (Fig. 1). The hinges of the folds gradually plunge to the south—southwest at angles of \(4—18^\circ\), which, with comparatively steep dip angles of their limbs, has led to the formation of narrow, in plan, periclinal and centroclinal closures. In plan the axial lines of the additional folds are nearly parallel and form a very small angle with the general strike of the western limb of the Kemerovo syncline. At the same time, the plunge of their axes coincides with the general plunge of the axis of the main syncline.

The form of the cross section of the closure part of the folds is characteristic. Thus, according to data from stoping operations, the anticlinal fold throughout its entire extent is characterized by a narrow and sharp closure, while the synclinal fold is characterized by a broad and blunt one. This feature is of interest in that a strictly longitudinal, steeply dipping disjunctive disturbance is confined to the sharp-closure form of the hinge part of the anticline, with the fault plane dipping to the northwest (in the direction

dip of the axial surface). In form, this disjunctive fault is complex: from a reverse fault in the southern part of the anticline to a thrust fault in the northern part.

The largest disjunctive fault here, and within the field as a whole, is tectonic rupture 5—5, with the plane of the displacer dipping to the northwest at angles from 76° in the northeast to 60° and considerably less in the southwest. Being distinctly diagonal to the strike of the limbs of the subsidiary syncline, it affects both limbs and its hinge part. Owing to this, together with the above-noted character of the flattening of the displacer surface, rupture 5—5 is the most complex in form: from a conformable reverse fault in the northeast to a nonconformable reverse fault in the southwest. Its vertical displacement amplitude averages 100 m, and increases noticeably from northeast to southwest, reaching 125 m along the Kemerovo seam in the extreme near-surface part.

Fig. 1. Structural plan of the southwestern part of the Kemerovo seam area.
1 — truncations by disjunctive faults of the hanging (a) and footwall (b) limbs of the seam; 2, 3, 4, 5 — small disjunctive faults of, respectively, the reverse-fault, overthrust, thrust-fault, and normal-fault types; 6 — axial lines of folds.

A far more widespread phenomenon here is the development of small disjunctive faults. At the same time, along with their more or less uniform distribution on each of the limbs of the subsidiary folds, in places an increased concentration of them is observed. The latter is observed, above all, on the sides and in the immediate vicinity of the seam truncations by disjunctive fault 5—5.

An important feature of the manifestation of small ruptures is the exclusively diagonal character of their orientation relative to the strike of the rocks. Thus, of all the small disjunctive faults recorded in the southern part of the field, in the Kemerovo seam, 77% proved to be diagonal, 13% longitudinal, and 10% transverse. Moreover, among the diagonal and longitudinal ones, 91% are conformable and only 9% are nonconformable. The latter, as a rule, develop in the str—

...spatial interrelationship with concordant ones and much more rarely independently.

It is also not without interest that, among all concordant disjunctives, upthrows constitute 25%, thrust faults 15%, upthrusts 25%, and backthrusts (reverse thrusts) 35%. Thus, the number of forms of ruptures in which the displacer dips more steeply than the bed (upthrows and upthrusts), in this case, proves to be as if balanced by the number of forms in which the displacer dips more gently than the bed.

Speaking of the character of the distribution of one or another form of small disjunctives, it may be noted that, with the exception of their increased confinement to zones where parts of the bed are cut off by complex diagonal rupture 5—5 and to the hinge parts of additional folds, apparently there is no other regularity in this respect. As a rule, small ruptures of all forms occur on both limbs of the additional folds and on both sides of rupture 5—5. It must be stated that the forms of small ruptures do not repeat the form of the large rupture in whose sides they are localized. The impression is created that the entire variety of forms of small disjunctives in this case is predetermined solely by the direction of bedding (91% concordant forms).

It is important to note that, in contrast to other mine fields of the district, in the field of the Yagunovskaya Mine, backthrusts are widely developed (35%). If one takes into account that a backthrust is a disjunctive with gaping between the relatively displaced parts of the bed, it becomes clear what enormous importance tectonostructural analysis acquires here in connection with the search for the displaced part of the bed.

As observations in mine workings and in stoped-out spaces show, all beds without exception are, to one degree or another, affected by a number of differently oriented systems of fractures, and many, moreover, are characterized by the development of slabby structure, microfolding, up to complete grinding. At the same time, the degree of dislocation often proves to be different not only in different parts of the field, but also in adjacent beds, and even within one and the same coal bed.

In order to establish regularities in the relationship between small disjunctives and fracturing in the southern part of the field, the latter was studied under various structural conditions.

As a result of processing mass measurements of fracturing at 2 observation points in the Vladimir Lower bed, in the most quiet structural conditions here, on the northeastern limb of the anticline, the following results were obtained. On both circular diagrams (Fig. 2), conjugate diagonal (18° ∠ 66°, 215° ∠ 65°) and vertical transverse (25° ∠ 90°), and, less clearly, longitudinal (305° ∠ 44°) normal-fracture systems are recorded fairly distinctly. In addition, on diagram No. 1 a number of oblique-shearing systems of fractures are revealed, both concordant (42° ∠ 45°) and discordant. The more complete identification of fracture systems on diagram No. 1 should evidently be explained, in this case, by the relatively more open character of the corresponding fractures under near-surface conditions (Fig. 2a).

In comparatively more complex structural conditions, mass measurements of fractures were made in the Volkovsky bed in the near-axial zone on the northeastern and southwestern limbs of the syncline. From the diagrams (Figs. 3a, b) it is evident that, on both limbs of the syncline, generally identical fracture systems are developed. One may also state an analogy of the fracture systems distinguished under these structural conditions

Fig. 2. Fracture diagrams for the Vladimir seam on the northeastern limb of the anticline:
a) at the upper level (210 measurements; 0.5—1—2—3%);
b) at the lower level (300 measurements; 1—2—3—4%).

a)

b)

c)

Diagrams of fractures in coal in zones cut by fault 5—5: footwall (a, c) and hanging-wall (b) blocks of the Volkovsky seam (area of borehole No. 370), 200 measurements. 0.5—1—2—3—4%.

Fig. 4. Diagrams of fractures in the coal of the Kemerovo seam on the northeastern limb of the syncline, near the truncation by a disjunctive disturbance: a) footwall (230 measurements; 0.5—1—2—3%); b) hanging wall (100 measurements; 1—2—3%).

with the corresponding systems of fractures identified above for the Vladimirovsky seam.

In the most complex structural conditions, observations of fracturing were carried out in the Kemerovsky and Volkovsky seams. The observation points were selected so as, on the one hand, to study the nature of the manifestation of fracturing in the immediate vicinity of the seam exposures by the complex diagonal fault 5—5 and, on the other hand, to reveal the features of the manifestation of fracture systems in its hanging and footwall sides. According to the data of two diagrams for the Kemerovsky seam, it seems possible to note that in the footwall (Fig. 4a) and hanging-wall (Fig. 4b) sides of fault 5—5, apparently identical systems of fractures are developing.

Under somewhat different conditions, at a greater distance from the exposure of the hanging wall of fault 5—5 (already in the form of a discordant reverse fault), a mass measurement of fractures was made in the Volkovsky seam. From the diagram (Fig. 3c) it is evident that here, too, diagonal oblique-cutting systems of fractures predominate, both concordant and discordant.

The data obtained from processing observations of fracturing in the latter case are also of interest in another respect: with a distinct expression of the maxima of the poles of conjugate discordant diagonal systems ($335^\circ \angle 65^\circ$; $256^\circ \angle 57^\circ$) of oblique-cutting fractures, an asymmetry of their arrangement on the circular diagram is observed relative to the continuation of the dip line of the seam and, consequently, in general relative to the elements of its structure. Exactly the same pattern is observed on the other diagrams as well, and not only for oblique-cutting systems but also for systems of diagonal normal-cutting fractures. Moreover, this asymmetry on all diagrams proves to be of the same order: the maxima of the corresponding fracture systems, as a rule, are displaced to the left from the continuation of the dip line of the seam for any of the fold limbs, if the diagram is oriented with the dip of the seam away from oneself.

A considerable, if not the greatest, disturbance of the coal seams within the southern part of the field is associated with intra- and interbed displacements in them. The latter, owing mainly to the complex lithological composition of the Kemerovskaya subformation, are characterized by a distinctly selective character both in the section of the subformation and with respect to the thickness of individual coal seams. It has been noted that coal seams of complex structure (the upper and lower Vladimirovsky, Viktorovsky) are more strongly affected by intrabed displacements, while seams of relatively simple structure (Volkovsky, Kemerovsky) are affected more weakly.

In the seams of the first group, intrabed movements usually occur only in the most plastic parting, without substantially affecting the others. For example, in the lower Vladimirovsky seam such a parting is the lower (0.60 m) parting of coaly argillite, characterized by an alternation down the dip of areas of normal structure and areas with development of schistosity, microfolding, and grinding to a scaly state. Along with this, an intrabed parting (0.30 cm) of massive argillite undergoes comparatively slight deformation. The latter is expressed in the uneven development, chiefly within the part of the underlying coal packet directly adjoining it, of ledges characteristic along a number of fracture surfaces.

The interbed slipping accompanying intrabed displacements in this case is confined to the boundary of the most plastic parting of coaly argillite with the adjacent more or less rigid parting. In this case it apparently occurs selectively,

directly at the boundary with an overlying or underlying, as in the present case, relatively rigid interbed of the seam. Morphologically, interbed displacements are expressed in the development of a usually thin (1–2 m) zone of clearly expressed slickenside surfaces and slip mirrors, often accompanied by the development of viscous tectonic clay.

The manifestation of intra- and interbed displacements in the seams of the second group has its own distinctive features. Thus, the thick (up to 6 m) Volkovskii seam, at least in its upper part, is simple in structure and in the general case consists of several coal bands, directly contacting in the roof with a layer of conglomerate that is sufficiently persistent in thickness (0.15–0.35 m) and extent. The latter, called by mine operators the false roof, is replaced above by the immediate roof, represented by sandstone. In contrast to the seams of the first group, here, instead of complex cleavage and microfolding, one often observes only intense abrasion of the coal to a scaly, in places sooty, state and, comparatively more rarely, simple cleavage. Moreover, this deformation, differing in degree, is manifested respectively either in the coal band bordering the false conglomerate roof, or in one of the internal coal bands.

Alongside the noted regularity, individual phenomena, at first glance contradictory, were observed in the same Volkovskii seam. From observations in the area of the southwestern limb of the additional syncline, between exploration lines IIIa and B, situated in its near-axial zone and in the immediate vicinity of the cut of the hanging limb of the seam by the complex fault 5–5, above the very upper coal band there appears a distinctive “interbed” of carbonaceous argillite (Fig. 5).

Fig. 5. Structure of the upper layer of the Volkovskii seam in the face of longwall No. 2:
a) in the area of exploration line B and b) 120 m northeast of exploration line IIIa.
1 — coal of the upper (a) and lower (b) layers of the seam; 2 — argillite;
3 — sandstone; 4 — disjunctive lines.

It has been established that, first, the indicated “interbed” has a highly variable form both down the dip and along the strike of the seam, with its thickness changing from 0.6 to 2 m; second, this “interbed” extends only over a comparatively limited area down the dip of the seam, reaching a length of 30 m in the central section (Fig. 5b); third, a close spatial confinement of the “interbed” is noted—

layer” to the zone where fault 5—5 intersects the axial part of the plunging additional syncline; fourth, such an “interlayer” is absent in an analogous surrounding setting (in the near-fault zone and on the side of the same fault 5—5), characterized, however, by the fact that fault 5—5 cuts the limb of the syncline rather than its hinge part. The close spatial connection of this complex form of a lens-shaped interlayer with relatively small associated disjunctive disturbances developing in the hanging wall of fault 5—5 is also unquestionable. At the point of greatest swelling of the “interlayer” there develops a series of closely spaced, macroscopically well-expressed, nearly parallel slip surfaces or “viscous” joints, usually extending somewhat beyond the limits of this “interlayer” and directly displacing the coal band bounding it below (Fig. 5a). Finally, in this case one cannot fail to note the comparatively high degree of disturbance of the carbonaceous argillite itself and the locally observed detachment of coal interlayers from the upper band of the seam.

On the basis of only what has been set forth, it seems probable that the formation of such a secondary false roof should be linked with the result of processes of flow and injection of carbonaceous argillite under favorable, in this case local, tectonostructural conditions.