GEOPHYSICAL STUDIES OF THE NORTHERN PART OF THE ANZHERO-SUDZHENSK DISTRICT OF KUZBASS AND ADJACENT TERRITORIES

Z. A. Myshko, L. I. Ivanchura

(Presented by Professor D. S. Mikov)

The Anzhero-Sudzhensk district is one of the oldest coal districts of the Kuznetsk Basin. Owing to the complexity of its geological structure and the widespread development of thick sequences of Quaternary and Tertiary deposits, its exploration required a large volume of geological and geophysical investigations. The results of geophysical studies of this district have not yet been covered in the literature, although they have yielded valuable information for the study of the district’s structure.

During the years from 1932 to 1962, a complex of geophysical work was carried out within the territory of the district: magnetic prospecting (airborne and ground), gravity prospecting (pendulum and gravimetric), seismic prospecting (by reflected- and refracted-wave methods), and electrical prospecting (by methods of vertical electrical sounding and electrical profiling).

Seismic prospecting work covered mainly the central part of the district described. On the basis of seismic-investigation materials, a map of boundary velocities and a hypsometric map of the relief of the pre-Mesozoic basement were compiled.

Electrical prospecting work was performed on separate small areas of the district in the form of areal surveys and scattered electrical-prospecting traverses. The results of these investigations are shown on plans of $\rho_k$ graphs, geoelectrical sections, and maps of isolines of depth to the high-resistivity horizon.

The materials of the geophysical investigations conducted in the district were at the time interpreted and presented in the corresponding reports, chiefly for individual areas; here, however, we consider the accumulated factual material for the district as a whole, comparing the physical fields observed by various methods in order to clarify more fully their relationship with the geological structure.

Orographically, the Anzhero-Sudzhensk district is a plain, in places rather strongly dissected by a system of small rivers and ravines; absolute elevations range from 167.8 to 258 m.

The geological structure of the investigated district includes rocks of the Precambrian, Cambrian, Devonian, Carboniferous, Permian, Jurassic, and Cretaceous, covered by a thick—20 to 60 m—mantle of Tertiary and Quaternary sediments; only on the bedrock slopes of small rivers does the cover decrease to 2–3 m.

The oldest deposits crop out at the surface in the eastern and southeastern parts of the area along the Zol, Kitat, and Yaya rivers. They are represented by crystalline schists, marbles, microquartzites, Precambrian effusives, and Cambrian limestones, breccias, tuff-breccias, porphyrites, and conglomerates. The Cambrian and Precambrian deposits as a whole are traceable to a depth of 2000 m.

The Devonian (Middle and Upper) has a regional distribution in the area and is composed of effusives, limestones, siltstones, sandstones, and argillaceous schists. Effusives (diabases, tuffs, porphyrites) have been encountered north of the town of Taiga, near the village of Omutnaya, and in the central part of the area, where they cut through Devonian limestones and have a sheet and dike character. The thickness of the Devonian sequence is about 900–1200 m.

The Carboniferous is represented by all three divisions. Deposits of the Lower Carboniferous—limestones, sandstones, and schists—make up the eastern limb of the Anzhero-Sudzhensk syncline and the floor of the Kaigat graben. The thickness of the sequence is about 700 m. Resting on the rocks of the Lower Carboniferous, above a basal conglomerate, is the Ostrog Formation \((C^3_1)\), consisting of sandy-clayey deposits with thin coal interbeds. The thickness of the formation is about 400 m.

The sediments of the coal-bearing sequence, assigned to the Balakhon Series and determined by age as Middle Carboniferous–Lower Permian, conformably overlie the deposits of the Ostrog Formation. The coal-bearing sequence is developed within the eastern limb of the Anzhero-Sudzhensk syncline and has been traced northward as far as the village of Shcherbinovka. The coal-bearing deposits are represented by a series of sandy-clayey rocks with coal seams occurring among them and by small beds of coaly argillites. The thickness of the coal-bearing sequence in the vicinity of the village of Shcherbinovka is about 1000 m.

Jurassic deposits—sandstones, pebble beds, conglomerates, siltstones, and argillites with seams and interbeds of brown coal—fill the Ulanovka depression and attain a thickness of 450–550 m.

Cretaceous deposits are developed in the central and eastern parts of the area and are represented by sandstones, siltstones, sands, and kaolin clays. The thickness of the Cretaceous sediments is not less than 200 m.

Tertiary and Quaternary deposits—pebble beds, quartz sands, sandy loams, and loesslike loams—blanket the bedrock of the area everywhere and, to a certain extent, smooth the relief of the area.

Tectonically, the area under study is a combination of three structures of different ages: in the east of the area—the spurs of the Kuznetsk Alatau; in the west—the Tom–Kolyvan folded arc; and between them, compressed structures of the northern end of the Kuzbass.

The oldest structures are those of the Kuznetsk Alatau, which enter the area from the southeast in the form of two branches—an eastern branch (the Yaya–Ishim uplift) and a western branch (the Yaya uplift), composed of Cambrian and Precambrian rocks.

The Yaya–Ishim uplift is an anticlinal fold of northwestern strike, the western limb of which enters the area described. This limb is complicated by minor folding and disjunctive dislocations. In its axial part this anticline is cut by gabbro intrusions. Northward, the Yaya–Ishim uplift plunges beneath the Meso-Cenozoic deposits.

The Yaya uplift is also an anticlinal fold, cut by dikes of diorites, gabbro, diabases, and porphyrites. In the north, the Yaya uplift is truncated by a tectonic dislocation.

Between these uplifts extends the Zoloto–Kitat graben, filled with Cambrian rocks. The western limb of this syncli-

of the synclinal structure is complicated by plicative and disjunctive dislocations.

The ancient structures, formed in the Salair and Caledonian folding periods, during the Variscan folding cycle stood out in the form of “horsts-wedges,” as a result of which uplifts of ancient rocks (the Yaya and Yaya-Ishim uplifts) and intermontane depressions (the Kaigat and Anzhero-Sudzhensk depressions) arose.

The Kaigat graben is part of a large synclinal fold of north-northwestern strike and is bounded on the northwest and east by large disjunctive faults. To the north the graben is overlain by Meso-Cenozoic deposits.

The Anzhero-Sudzhensk syncline is represented in its northern part by only one eastern limb. Its western limb is buried beneath the Tomsk overthrust, owing to which Devonian rocks have been thrust over the Balakhon Series. The amplitude of the overthrust reaches several thousand meters. The fault plane dips to the SSW at an angle of 40–60°. In the near-contact zone with the overthrust, the rocks of the Balakhon Series are overturned. The strike of the Anzhero-Sudzhensk syncline is north-northwest, with the axis plunging in the same direction.

The Devonian rocks of the overthrust zone form a large anticlinal fold of northwestern strike. In its core there occurs an effusive-sedimentary sequence, and on the limbs a limestone-shale sequence. The eastern limb of the fold is complicated by disjunctive faults.

Jurassic deposits lie almost horizontally on Paleozoic sequences. Undisturbed bedding is characteristic of Cretaceous, Tertiary, and Quaternary deposits, which indicates the absence of major folding movements in Meso-Cenozoic time.

The physical properties of rocks within the territory under consideration have been studied insufficiently completely. The following may be noted here:

1. Cambrian and Precambrian rocks are characterized by high values of density (2.7–2.8 g/cm³), electrical resistivity (10³–10⁴ ohm m), velocity of propagation of elastic waves (more than 5000 m/sec), and weak magnetic properties.

An exception is formed by magmatic formations of basic composition, possessing magnetic susceptibility on the order of 1000–500 · 10⁻⁶ CGS, more rarely above 5000 · 10⁻⁶ CGS.

Thus, intensive gravitational and magnetic anomalies, horizons of high electrical resistivity, and high velocity of propagation of elastic waves may be associated with the crystalline basement, composed of Cambrian and Precambrian formations.

1. Paleozoic rocks belonging to the Devonian, Carboniferous, and Permian are characterized in most cases by elevated values of density (2.4–2.7 g/cm³), electrical resistivity (100–200 ohm m, sometimes 1000–4000 ohm m), comparatively high values of the velocity of propagation of elastic waves (2700–5000 m/sec), and weak magnetism (50 · 10⁻⁶ CGS). Only effusive formations of the Devonian of basic composition occur both as weakly magnetic (100–1000) · 10⁻⁶ CGS and as magnetic (1000–5000 · 10⁻⁶ CGS).

With such a combination of parameters, gravitational and electrical anomalies from Paleozoic structures and magnetic anomalies from Devonian effusives may be expected. The small difference in elastic-wave velocities between Paleozoic sequences in this region does not permit a reliable subdivision of these sequences beneath the Meso-Cenozoic deposits.

1. Meso-Cenozoic deposits are practically nonmagnetic; they are distinguished by low density values (1.7–2.1 g/cm³), and low velo-

…by the velocities of elastic waves (effective propagation velocities of waves for Jurassic and Cretaceous sediments are 1600–2000 m/s, and average velocities for Quaternary deposits are 350–480 m/s) and by reduced electrical resistivity (20–90 ohm·m), with the exception of ferruginous pebble beds (200 ohm·m) and horizons of siliceous sandstones (1000–5000 ohm·m).

The Meso-Cenozoic deposits, which occur almost horizontally, do not exert a substantial influence on the magnitude of the gravitational and magnetic fields. Their subdivision can be carried out only by seismic and electrical prospecting.

Comparison of the geophysical materials obtained as a result of the investigations with the geological data known for the district permits the assumption that each of the geophysical methods, taken separately, illuminates particular features of the geological structure and the tectono-structural characteristics of the district.

Seismic prospecting by the reflected-wave method and by the correlation method of refracted waves in the central and northwestern parts of the district has yielded a sufficiently reliable study of the relief of the pre-Mesozoic basement. On the hypsometric map two uplifts are clearly distinguished, the Georgievka and the Barantsevo, with a small amplitude of 100–125 m; their areal dimensions are 25 × 25 and 10 × 12 km. Between them a narrow, shallow downwarp of nearly meridional strike is outlined. South of the Barantsevo uplift, the Ulanovka trough is contoured, with a subsidence amplitude of up to 550 m and areal dimensions of 25 × 20 km.

The map of boundary velocities is characterized by frequent changes of areas with different velocities. This is due both to the emergence beneath the Meso-Cenozoic deposits of Paleozoic strata of different ages, and to changes in the lithological composition of rocks within strata of the same age. The insignificant difference in boundary velocities does not make it possible to distinguish reliably Permian, Carboniferous, and Devonian deposits beneath the Meso-Cenozoic sediments. However, it may be considered that in the area of the Barantsevo uplift and the Ulanovka trough rocks of Devonian and Carboniferous age are developed; \(V_r = 4200\text{–}5000\) m/s. Only in the northeastern part of the trough can a persistent area with boundary velocities of 2700–3800 m/s be composed of Permian deposits. The former is confirmed by drilling carried out in the area of the villages of Barantsevo and Ulanovka. Borehole No. 4, drilled in the area where Permian deposits were expected at a depth of 550 m, was stopped at 440 m in Lower Jurassic sediments. The predominantly elevated boundary velocities of about 5000 m/s and more, characteristic of the area north of the village of Novorozhdestvenskoye and of the Georgievka uplift, according to drilling data, are associated with gabbro-diabases developed near the village of Novorozhdestvenskoye and with epidote-chlorite-sericite schists in the area of the village of Georgievka.

In the southern part of the district, on the basis of data from the CMP refraction method, reliable mapping of the Tomsk thrust was carried out northward from the village of Lebedyanka to the village of Nazarovka.

On the whole, for this district, seismic exploration work has mainly studied the renewed small structures laid down in the basement.

Comparison of the magnetic and gravity maps of the territory gives grounds to note that large regional areas of negative and positive gravity fields generally correspond to magnetic fields of the same sign and characterize two folded regions of different ages: the eastern one—with positive values of \(\Delta T\) and \(\Delta g\), associated with the ancient structures of the Kuznetsk Alatau, and the western one—with negative values of the magnetic and gravity fields, associated with the structures of the Tom–Kolyvan folded zone. Within these…

fields there is observed a displacement and discrepancy of the local maxima and minima of \(\Delta g\) and \(\Delta T\), which is especially characteristic of the eastern part of the area (Figs. 1 and 2).

All the known structures in the most thoroughly studied part of the area find a fairly distinct expression on the gravity map; however, no direct relationship of magnetic anomalies with the structures is observed.

A large gravity maximum in the southeastern part of the area corresponds to the Yaya–Ishim uplift \((1, 1)^*\), composed of shallow-lying (5–40 m) Cambrian and Precambrian rocks. The magnetic field in this region is significantly differentiated, which characterizes the heterogeneity of the composition of the crystalline basement.

Areas of an increased alternating magnetic field are associated with mafic igneous formations. Areas of a quiet

Fig. 1. Schematic map of the gravity field of the northern part of the Anzhero-Sudzhensk district of Kuzbass.
1 — regional gravity maxima; 2 — regional gravity minima; 3 — area of a reduced gravity field; 4 — local gravity maxima; 5 — local gravity minima

Fig. 2. Schematic map of the magnetic field of the northern part of the Anzhero-Sudzhensk district of Kuzbass.
1 — regional magnetic maxima; 2 — regional magnetic minima; 3 — areas of a reduced magnetic field; 4 — local magnetic minima; 5 — local magnetic maxima

magnetic field correspond to nonmagnetic crystalline schists and limestones. Judging by the character of the gravity field (the presence of large

* The first numeral indicates the figure number, the second the anomaly number.

gradients of gravity) it may be asserted that the Yaya–Ishimsky salient of ancient rocks undergoes a sharp subsidence to the northwest, and possibly is cut off by a transverse disjunctive disturbance.

The western branch of the Yaya–Ishimsky salient—the Yaya salient (1, 2), apparently representing an uplifted part of the Kailinskaya anticline distinguished below and separated from it by a disjunctive disturbance—is marked on the gravitational-field map by a maximum of $\Delta g$ of small areal extent. The weak magnetic field over this structure indicates a predominantly sedimentary composition of the rocks.

The Kailinskaya anticline (1, 3) corresponds to a positive gravitational field and a normal magnetic field. This structure is composed of Devonian rocks (limestones, sandstones, shales) and plunges in a northwesterly direction beneath the Meso-Cenozoic deposits of the Ulanovka trough.

East of the Kailinskaya anticline, a belt of lowered gravitational field delineates the Kaigatsky graben (1, 4). The magnetic field in this part is calm, close to normal. Core-drilling wells here, beneath a thickness of Meso-Cenozoic deposits, have exposed rocks of the Lower Carboniferous. The thickness of the loose sequence, previously indicated from VEZ data as 200 m, has been confirmed by drilling.

In the south, the Kaigatsky graben is separated by a disjunctive disturbance from the Zoloto-Kitatskaya syncline (1, 5), composed of Cambrian rocks. On the gravity map the latter is expressed by somewhat higher values of $\Delta g$ in comparison with the Kailinskaya anticline and by an alternating-sign magnetic field in the eastern part, which is associated here with the presence of igneous rocks of basic composition.

A local gravity minimum situated south of the village of Ulanovka (1, 6) corresponds to the central part of the Ulanovka trough. The magnetic field over this area is positive, with low values of $\Delta T$. The observed partial discrepancy in the contours of the trough according to gravimetric data—namely, the failure of the $\Delta g$ isolines to close from the southwest and the flattening of the isogams toward the southeast—is explained by the fact that from the north and northeast this structure is surrounded by denser varieties of Devonian rocks.

In the southern part of the area, the Anzhero-Sudzhensk synclinal structure (1, 7), filled with coal-bearing deposits of the Balakhon series, corresponds to a negative gravity field with separate local minima. The latter are probably associated with brachysynclinal folds developed within the principal structure. On the magnetic-field map the Anzhero-Sudzhensk syncline is not distinguished. In this part the magnetic field is normal, with an insignificant increase in the area of the city of Anzhero-Sudzhensk.

The Tomsk thrust, which cuts the western limb of the structure, is not marked on the gravity map; this is explained by the complex character of this disjunctive structure, which has an imbricate structure, owing to which individual thrust slices are overlain by deposits of the Balakhon series. Calculations of the $\Delta g$ anomaly over the Anzhero-Sudzhensk structure show that the western limb of the structure in this part is buried beneath the thrust, and not cut off.

Northwest of the Anzhero-Sudzhensk syncline there is distinguished a small area of negative $\Delta g$ field with a local minimum in the vicinity of the village of Tukai (1, 8). Wells drilled by the Tomsk Complex Expedition have exposed here deposits tentatively assigned to the Ostrog suite.

In the western part of the area, from the town of Taiga to the village of Omutnaya, a positive regional gravity anomaly (1, 9) with two lo—

local maxima. The magnetic field in this area is mainly negative and close to normal. The exceptions are two local magnetic maxima, noted in the area of the village of Omutnaya and north of the town of Taiga, the latter being shifted somewhat relative to the gravitational one. This regional anomaly is associated with an anticlinal structure, in the crest of which an effusive-sedimentary Devonian sequence occurs. Local gravitational and magnetic anomalies correspond to the surface outcrops of the effusives.

The northwestern part of the area under consideration is characterized by negative gravity and magnetic-field intensity fields (the Tomsk Depression, 1, 10). Against the background of these fields, local minima of $\Delta g$ and $\Delta T$ are observed in the area of the village of Georgievka. Both minima are confined to the central part of the Georgievka uplift, identified from seismic-survey data. Study of core from wells drilled in the central part of the $\Delta g$ anomaly and which penetrated dislocated metamorphic rocks showed their reduced density ($2.25\ \text{g/cm}^3$), which, it must be assumed, also caused the presence of the negative gravity anomaly.

In the area of the village of Novorozhdestvenskoe, a large gravity maximum (1, 12), gradually dying out to the northeast, and an asymmetric magnetic anomaly (2,1) have been recorded. The $\Delta T$ minimum is confined to the central part of the gravity anomaly; the maximum is shifted to the south. The magnetic anomaly is caused by a gabbro-diabase massif, emerging in the area of the $\Delta T$ minimum beneath 40–80 m of overburden (according to VES data) and plunging southward. The gravity anomaly is mainly associated with a projection of ancient rocks overlain by Devonian deposits of small thickness, and is somewhat strengthened by the influence of the gabbro-diabase massif. The Barantsev projection of Devonian rocks, because of the small amplitude of uplift, creating an anomaly of 2.4 mgal according to calculations, is not distinguished against the general background of the large gravity anomaly located here.

Between the positive gravity field of the area of the village of Omutnaya and the Novorozhdestvenskoe gravity anomaly there is an area of reduced gravitational and magnetic fields, open to the north. According to geological investigations, the Podlomskaya, otherwise Tashminskaya, syncline is distinguished here, in which Permo-Carboniferous coals have been exposed. From geophysical materials it is rather difficult to delineate this synclinal structure, but it must be assumed that small troughs with thin coal-bearing deposits occur here, and only detailed gravimetric investigations will be able to identify them.

To the northeast of the village of Novorozhdestvenskoe, a positive field is preserved on the gravity map, against the background of which the large Dubrovsky $\Delta g$ maximum (1.13), with sharp gradients along the margins, and the Ishim area of reduced field with a minimum (1.14), open to the east, stand out clearly. In the northern part there are two more insignificant $\Delta g$ maxima (1.16 and 1.17) and an area of reduced field in the vicinity of the village of Usmanka (1.15).

There is reason to suppose that the Dubrovsky $\Delta g$ maximum is caused by a projection of ancient rocks overlain by Devonian deposits of small thickness, while the Ishim $\Delta g$ minimum is associated with a large synclinal structure, where, judging from the character of the gravitational and magnetic fields, the development of coal-bearing deposits may be expected.

The magnetic field of the central part of the sheet is mainly positive, characterized by the presence of three separate positive magnetic anomalies (2.2; 2.3; 2.4) and a large regional area of weak negative field (2.5). The strike of the positive

the anomalies is almost meridional, and that of the negative anomalies is northwestern. Magnetic anomaly 2.3 has the greatest field intensity and is caused by ultrabasic rocks of the serpentinite type, exposed by boreholes at a depth of 156 m. Associated with the area of development of these rocks are the two small $\Delta g$ maxima noted above (1.16; 1.17), localizing areas with the densest varieties of these rocks. Magnetic anomaly 2.4 is associated with amygdaloidal porphyrites interbedded with gabbro-diabases, which were encountered by boreholes near the village of Ishim at a depth of 140 m. Magnetic anomaly 2.2 is characterized by a more isometric form and, in its intensity, is somewhat weaker than the anomalous field 2.3; it may be assumed that it is caused by rocks of basic composition. The local minimum north of this anomaly may be interpreted ambiguously: it is possibly connected with this anomaly and caused by the subsidence of a massif of basic rocks to the southwest; on the other hand, the presence of the minimum may be associated with a small depression filled with deposits of nonmagnetic rocks. The latter agrees with the gravity data (minimum $\Delta g$, 1.15). The area of a calm negative magnetic field (2.5) and a slightly lowered gravity field is apparently associated with a synclinal structure composed of Devonian deposits.

A comprehensive consideration of the geophysical material for the district leads to the following conclusions:

1. By seismic exploration methods, renewed small structures embedded in the basement have been studied, and the Tomsk overthrust has been traced.

2. Magnetic prospecting has distinguished two large regions—the Tom–Kolyvan folded zone and the Kuznetsk Alatau—and has revealed and delineated massifs of igneous rocks.

3. Gravity prospecting has solved the problem of separating large structural regions, as has magnetic prospecting. Known structures have been localized and a number of new ones distinguished: the Novorozhdestvensky and Dubrovsky uplifts of ancient rocks, the Ishim synclinal structure, and the Usman depression.

4. The presence of the Gergievsky uplift on the northern continuation of the Anzhero-Sudzhensk synclinal structure limited its development in this direction.

5. The Podlomskaya synclinal structure and the Ishim depression (the area of the village of Olgovka) are the most promising with respect to the occurrence of coal here.

6. For a final solution of the question of the structural features of the district, it is necessary to carry out, along one or two profiles, deep seismic sounding and reference boreholes on the Rozhdestvensky and Dubrovsky uplifts and the Ishim syncline.

Thus, the geophysical work carried out provides grounds to believe that the structures of the Kuznetsk Alatau are not distributed continuously over the territory studied, but are developed in the form of separate uplifts (Yaisky, Yaisko-Ishimsky, Novorozhdestvensky, and Dubrovsky), and that the Anzhero-Sudzhensk syncline is not bounded by the Tomsk overthrust, but continues in a north-northwestern direction, closing somewhat north of the village of Tukay.

On the basis of an analysis of the geophysical material for the northern part

of the Anzhero-Sudzhensk district, a schematic structural map has been compiled (Fig. 3).

Fig. 3. Schematic structural map of the northern part of the Anzhero-Sudzhensk district of Kuzbass.
1 — Yaia-Ishim salient; 2 — Yaia salient; 3 — Kaila anticline; 4 — Kaitat graben; 5 — Zoloto-Kitaiskii graben; 6 — Ulanovskaya depression; 7 — Anzhero-Sudzhensk syncline; 9 — Devonian salient; 10 — Tomsk depression; 11 — Georgievskoe uplift; 12 — Novorozhdestvenskii salient; 13 — Dubrovka salient; 14 — Ishim depression; 15 — Usman depression; 16 — Mishutinskaya syncline.