On the Structure of the Basement of the South Minusinsk Depression Based on Geophysical Studies

M. M. Lubyankova

(Presented by Professor D. S. Mikov)

The South Minusinsk Depression is one of the largest in the system of Devonian depressions of the Minusinsk intermontane downwarp. It occupies the southern part of the downwarp and is bounded on the west by the Kuznetsk Alatau, on the south by the Western Sayan, on the east by the Eastern Sayan, and on the north by the eastern branch of the Kuznetsk Alatau—the Batenyov Range. The depression is composed of volcanogenic-sedimentary and sedimentary formations of the Middle and Upper Paleozoic, from the Lower Devonian to the coal-bearing Permo-Carboniferous.

It is assumed that the basement of the depression is made up of metamorphosed, strongly dislocated Lower Cambrian formations, represented by limestones, various schists, marls, effusives, and sandstones. Within the depression the basement has not been penetrated by any exploratory borehole. The rocks composing it can be judged from the composition of the rocks of the mountain structures bordering the depression.

The structure of the basement can be judged from the data of geophysical surveys carried out in the South Minusinsk Depression in 1950–1960 by exploration organizations of Siberia: the trusts “Zapsibneftegeofizika,” “Sibneftegeofizika,” the Novosibirsk Geophysical Trust, and others.

The magnetic-field map (compiled by I. M. Zalipukhin) makes it possible, in general terms, to form an idea of the structure of the basement. From the magnitude of the magnetic-field intensity $\Delta T_a$, it may be assumed that in the eastern part of the depression, on the right bank of the Yenisei, the basement is uplifted relative to the central part. The basement is most deeply submerged in the central part of the depression, in the area between the Yenisei and the Abakan. From the increase in the magnetic field in the southwestern part, it may be assumed that the floor of the depression in its southwestern part is uplifted.

Thus, judging by the character of the magnetic field $\Delta T_a$, it may be assumed that the floor of the depression, in the southwest–northeast direction, has a graben-like form.

In the southern part of the depression, in the marginal areas, in the region south of the village of Novomikhailovka and Lake Trekhozernoye, according to seismic-survey data, it is possible to assume a monoclinal uplift of the basement to the south, in the direction of the Western Sayan. For a more concrete understanding of the structure of the Cambrian basement, the author carried out a quanti—

qualitative interpretation of the gravity-anomaly map in the Bouguer reduction.

The quantitative interpretation was based on the following assumptions:

1. In terms of the density characteristics of the rocks, a single interface is assumed. The overlying sequence is less dense and includes the volcanogenic-sedimentary cover of the depression, with an average density of \(2.61\ \text{g/cm}^3\). Below the interface lies a denser basement, with an average rock density of \(2.80\ \text{g/cm}^3\).

2. For most of the depression, the density contrast between the two density sequences was taken as \(0.19\ \text{g/cm}^3\). An exception is the southern near-margin part, where, from the sharp decrease in the gravitational field, a change in the material composition of the basement is inferred. In the southern near-margin part, adjacent to the Western Sayan, the behavior of the basement was determined from seismic sections and from separate quantitative calculations of basement depth using the magnetic-field map \(\Delta T_a\).

3. The method used for solving the inverse problem in the case of a single interface was that proposed by L. V. Sorokin. For supplementing and checking the interpretation of the gravity field, seismic sections were used, illuminating in some areas of the depression the behavior of the top of the basement.

On the basis of geophysical survey materials, the author compiled a structural scheme of the basement for most of the South Minusinsk Depression (Fig. 1).

According to the compiled scheme, the structure of the depression basement is folded. The folding of the basement is caused by virgations of the Kuznetsk Alatau. The Uibat spur in the basement of the depression branches into three arms: sublatitudinal, northeastern, and north-northeastern in strike, expressed in the volcanogenic-sedimentary cover by the Uibat–Tagar zone of uplifts, the Uibat (or Chernogorsk) swell, and the Abakan–saltworks group of uplifts. The virgations of the Uibat spur are separated by downwarped basement areas.

In the basement of the South Minusinsk Depression, the following largest structural forms are distinguished:

1. The Uibat–Tagar swell-like high—a sublatitudinal continuation of the Uibat spur. Its axis in plan has a curvilinear form and is turned with its concave side to the north. In the area of the villages of Raikovo–Krasnopolye the axis undulates, rising in the area of the Tagar uplifts.

2. The Chernogorsk swell-like high—a northeastern continuation of the Uibat spur, separating the Chernogorsk and Kalyagino troughs, whose expression in the cover may be the Bystryan uplift and the Kavkaz structural nose.

3. The Abakan saltworks high—a north-northeastern branch of the Uibat spur, separating the Chernogorsk and Kuten-Buluk troughs.

4. The Chernogorsk trough.

5. The Kalyagino trough.

6. The Kuten-Buluk trough.

7. The Kalsk–Duben syncline with the Bey trough in its most downwarped part, of sublatitudinal strike.

On the basis of an analysis of seismic sections and interpretation of the gravity-anomaly map, the author notes that the position of the Chernogorsk trough in the basement does not coincide with its position in the cover of the depression. Specifically: in the Lower Cambrian formations the trough is situated 6 km southwest of its position in the sedimentary complex of the cover.

Fig. 1.

Visible labels in the figure:

• Shibatskii spur
• Askiz
• Beya
• Chernogorsk
• Abakan
• Novo-Mikhailovka
• Western Sayan

Visible legend numerals and symbols:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

Visible contour/value labels:

• −5000
• −4000
• −3000
• −2000
• −6000
• −7000
• 6000
• 5000
• 4000
• 3000
• 2000

In the opinion of most researchers, it is assumed that the basement of the South Minusinsk Depression has a block-folded structure (N. G. Chochia-Z, I. V. Luchitskii [1]). It is assumed that the basement is divided into blocks.

On the basis of an analysis of the character of the occurrence of Devonian deposits, I. V. Luchitskii [1] distinguishes in the basement a number of blocks, stepwise subsiding from the periphery toward the center of the depression along faults of northwestern strike. According to I. V. Luchitskii, the most deeply subsided is the central block, located in the central part of the depression in the area of the Abakan–Yenisei interfluve.

From the basement-roof markers obtained by the author, it appears possible to distinguish in the basement of the South Minusinsk Depression three large blocks: the central, most deeply subsided block, and the uplifted eastern and western blocks.

The central block is elongated in a northwestern direction and borders the eastern one along the line: Abakan–Krivinskoe–Shushenskoe, i.e., along the Yenisei River. The position of the most deeply subsided block and its boundary with the eastern one coincide with the most deeply subsided block according to I. V. Luchitskii. The boundary of the central block with the western one is broken. In the central part of the depression the author traces it along the line Orositelnoe–Arshakov. North of Orositelnoe station the direction of the boundary changes to northwestern. It is possible that the undulation of the axis of the Uibat Tatar swell-like uplift in the area of the villages of Krasnopol’e–Raikovo is a consequence of the subsidence of the central basement block.

It is quite possible that the large blocks distinguished from the geophysical data in the basement may be differentiated. Apparently, there is also an Askiz fault in the basement (the author’s term, after the Askiz flexure in the cover of the depression) of sublatitudinal strike, along which displacement of the extreme southwestern block occurred, as a result of which this block proved to be the most uplifted.

Summarizing the foregoing, the following may be noted:

1. The basement of the South Minusinsk Depression has a block-folded structure and is divided into a number of blocks, stepwise subsiding from the northeast and from the southwest from the periphery toward the center of the depression along faults of northwestern strike.

2. The most deeply subsided is the central block, located in the central part of the depression and elongated in a northwestern direction.

3. The folding of the basement is caused by the vergences of the Kuznetsk Alatau.

4. In the basement structures two directions are developed: northwestern, caused, in all probability, by movements of basement blocks, and northeastern, caused by the vergences of the Kuznetsk Alatau.

5. The fact that on some seismic sections reflecting surfaces from the basement are observed, arranged regularly and sometimes making it possible to trace along them a conditional horizon, gives grounds to confirm that the Lower Cambrian basement of the South Minusinsk Depression is noncrystalline and less dislocated than has been assumed up to the present time.

REFERENCES

1. I. V. Luchitskii. Volcanism and Tectonics of the Devonian Depressions of the Minusinsk Intermontane Trough. Publishing House of the Academy of Sciences of the USSR, Siberian Branch, 1960.

2. N. G. Chochin et al. Geological structure of the Minusinsk intermontane depressions and prospects for their oil and gas potential. Proceedings of VNIGRI, issue 120, Gostoptekhizdat, 1958.