On the Tomsk Thrust

L. I. Ivanchura

(Presented by Professor D. S. Mikov)

As is known, in 1919 M. A. Usov (1921), studying the tectonics of the Anzhero-Sudzhensk district, was the first to express the idea of the existence here of a significant disturbance of the overthrust-nappe type, which had thrust the Middle Devonian over coal-bearing deposits. In this connection the author wrote that “along both small rivers (meaning the Alchedat River and Mazalovsky Kitay) the Middle Devonian is directly succeeded by the coal-bearing sequence, both formations having a westerly dip, so that the Devonian rests upon younger deposits. Such bedding conditions are due to a thrust fault, or perhaps to that form of disjunctive dislocation which is called a nappe.”

On the basis of observations made in 1923, M. A. Usov (1924) emphasizes that the disjunctive disturbance of the thrust-nappe type established by him in the northeastern part of the Kuznetsk Basin, and now called the Tomsk nappe, “is the natural boundary of the Kuznetsk Basin on the northwestern side.”

In 1927 V. I. Yavorsky and P. I. Butov (1927), using the geological material that had accumulated on the Kuznetsk Basin, expressed some doubt concerning the Tomsk nappe distinguished by M. A. Usov along the northwestern margin of Kuzbass, interpreting this disturbance as a thrust fault. But this statement subsequently found no followers, and M. A. Usov’s concept of the nappe character of the northwestern boundary of the Kuznetsk coal basin was accepted by all investigators of Kuzbass in the early 1930s.

Gravimetric observations carried out in recent years in the northern part of the Anzhero-Sudzhensk district of Kuzbass have shown that, although the eastern boundary of the Anzhero-Sudzhensk syncline is emphasized and fixed by the course of the isolines of the gravitational field, the position of the Tomsk thrust on the map does not find a clear and distinct expression. Here a number of local negative fields are observed, the areal dimensions of which vary from \(5 \times 2\) to \(10 \times 5\) km. All of them are elongated in a north-northwesterly direction and are situated on one common axis of the same orientation. A certain departure from this is the presence of two additional field minima, of the same intensity as the first ones, but located somewhat to the west of them (Fig. 1).

All the negative fields are outlined by one common isoline, except for the Tukai gravitational minimum, located in the vicin—

near the village of Tukai. From the west, north, and east the fields are encompassed by positive isolines; moreover, in the southeastern part of the district the gravity field increases considerably more rapidly than in the other parts.

A comparison of the gravitational field with the geological map of this territory shows that the Anzher local gravity minimum, observed in the vicinity of the town of Anzhero-Sudzhensk, records the Anzher syncline proper; here the isoline corresponding to the lowest value outlines the deepest parts of the fold. The calculation carried out from the $\Delta g$ palette for two-dimensional bodies, along profile I—I (Fig. 2), taking into account an excess density of $0.1\ \text{g/cm}^3$, according to data from determination of the physical properties between the coal-bearing and the underlying Lower Carboniferous and Devonian deposits, confirms this proposition. The slight deviation, observed in the eastern part, of the calculated curve from the actual one is caused by the outcrop, established in this area, of Devonian and older rocks, whose influence was not taken into account in the calculation.

Fig. 1. Schematic map of the location of the Tomsk thrust in the northern part of the Anzhero-Sudzhensk district of the Kuzbass. 1 — area of reduced gravity field. 2 — local gravitational minima: a) Anzher, b) Andreev, c) Vorob’ev, d) Tukai.

The local minimum of the gravity field, situated southeast of Anzherskoe, which we shall call the Andreev minimum, records the central part of the Andreev brachysyncline.

Both of the noted gravity minima are located mainly within the field of development of productive deposits, whereas the Vorob’ev and Tukai gravitational minima, identified in the area of the villages of Vorob’evka and Tukai and lying on the same axis as the first two, are situated within the area of development of Devonian rocks. Verification of the Vorob’ev gravity minimum by drilling along a number of exploratory lines confirmed the presence, in the eastern part of the minimum, of strongly dislocated productive deposits containing coals completely identical to the coals of the mines of the Anzhero-Sudzhensk district; and the encountering by boreholes, in particular borehole 2291, of Devonian deposits made it possible to refine the boundary of the Tomsk thrust in this area. During verification of the Tukai gravitational minimum, all boreholes exposed a section of Middle Devonian deposits represented by limestones and marls with Givetian fauna. At the same time, in checking this minimum in the area of the village of Tukai, borehole 24 exposed deposits assigned to the Ostrog suite. In addition, the calculation we made along profile II—II (Fig. 3), situated across the strike of the Vorob’ev gravitational minimum, with selection of the body shape and with preservation of the excess density $0.1\ \text{g/cm}^3$, as in the

calculation along profile I—I, with a sufficiently good agreement between the calculated and practical curve \(\Delta g\), suggest that the area of the Vorob’ev minimum cannot be regarded as entirely unpromising for the presence of coals occurring at shallow depth. In fact, the observed characteristic features of the gravitational field in the area of the city of Anzhero-Sudzhensk and the adjoining areas make it possible to suppose that the Anzhero-Sudzhensk syncline is not bounded by the Tomsk thrust, that it continues in a north—northwestern direction,

Fig. 2. Schematic geological section along line I—I with the observed (1) and calculated (2) curves of the gravity field

Fig. 3. Schematic geological section along line II—II with the observed (1) and calculated (2) curves of the gravity field. Geological boundaries according to drilling data (3) and inferred boundaries (4)

closing somewhat north of the village of Tukai, and that the axis of the gravitational minima (Andreevsky, Anzhersky, Vorob’evsky, and Tukaisky) corresponds to its most subsided part. In this connection we are inclined to believe that the line of the Tomsk thrust either passes farther west than it is currently represented, or the thrust does not have a continuous front and is represented here by masses of separate blocks, which is confirmed by the work of K. V. Radugin and a number of other investigators. Therefore, perhaps, one should return to the opinion expressed in 1927 by V. I. Yavorsky and P. I. Butov (1927), that there exists here a series of parallel overthrusts, and that the line of the Tomsk thrust as currently fixed is nothing other than the line up to which parts of the Devonian, locally uplifted by overthrusts, reach the day surface. Only the additional drilling of exploratory boreholes within the limits of the Vorob’ev gravity minimum will help resolve this question definitively.

LITERATURE

1. M. A. Usov. Geological investigations of the Anzhero-Sudzhensk coal-bearing region. Izv. Sib. otd. geol. kom., 1, issue 6, 43—44, 1921.

2. M. A. Usov. Elements of the tectonics of the coal basin. Collection “Kuzbass,” 89—136, 1924.

3. V. I. Yavorsky, P. I. Butov. The Kuznetsk coal basin. Trudy geol. kom., new series, issue 177, 244, 1927.