EXOGENOUS DISLOCATIONS OF THE QUATERNARY STRATA IN THE AREA OF LAKE MAREKUL (KHAKASSIA)

G. A. IVANKIN, E. V. YAROSHINSKY

(Presented by Professor K. V. Radugin)

To the southeast of the Tuim railway siding, in the Shirinsky District of the Khakass Autonomous Oblast, lies the Karysh accumulative plain, formed in a deflation basin on the site of what was formerly a comparatively large lake body (Fig. 1). The sediments that accumulated and continue to accumulate in the basin are constantly being compacted, which leads to disturbance of the primary bedding of the layers. The modern exogenous dislocations of the Quaternary strata, both plicative and disjunctive, are most clearly manifested here in the northwestern part of the accumulative plain, in the area of Lake Marekul; some information about them is presented in this article.

The formation of the Marekul sector of the accumulative plain is connected with the accumulation in the basin of lacustrine sediments (below) and bog sediments (above). In places (at the mouths of ravines and in the eastern part), the bog sediments are overlain by a sheet of proluvium. Near Lake Marekul we discovered a layer of proluvium buried beneath bog sediments.

A remarkable feature of the surface structure of the Marekul sector is the presence, amid the bog, of areas of firm surface rising 1–2 m above the bog. They undoubtedly represent remnants indicating a higher hypsometric level of the surface of the entire sector. Such areas have different plan-view shapes. They are bounded by cracks and surrounded by narrow, elongated in plan, but in places comparatively deep, lakes (Fig. 2).

There is no doubt that the formation of lakes near such sectors is connected with the manifestation of exogenous faults, whose amplitudes reach 5 m. Observations carried out over two decades indicate periodic renewal of the grabens and a reduction in the areas of firm surface. At the same time, parts of the grabens adjoining the bog are rather quickly filled with peat, and the lake here turns into bog. Over 20 years, in one of these sectors, the scarp retreated toward the firm area by an average of 5–6 m. Here, almost every year, cracks form and blocks of peat bog subside.

When movements cease, the slopes are rapidly flattened, and the adjoining lake becomes bogged.

The manifestation of modern exogenous disjunctive dislocations, as we understand it, is connected with the compaction of bog and lacustrine-bo...

Fig. 1. Schematic plan of the Karysh accumulative plain

Fig. 2. Schematic plan and section of a site located 120 m west of Lake Marakul (1 — ancient peat layer; 2–4 — young peat layers)

…of compacted sediments overlying the uneven bottom of a deflation hollow.

The mechanism of formation of exogenous faults may be represented in the form of the following scheme. The thickness of the loose sequence above the protrusions of the bedrock floor of the hollow is much less than the thickness of the sediments in adjacent areas: \(h_1 > h_2 < h_3\) (Fig. 3, a). With the same coefficient of compaction of the sediments, the total reduction in thickness of the sedimentary sequence will be different. Let us assume that over some interval of time the thickness of the loose sequence decreased by half, i.e., the surface points \(a\), \(b\), and \(v\) move respectively to the points \(a^1\), \(b^1\), and \(v^1\) (Fig. 3, a).

In the case of sufficiently steep slopes of the buried relief, cracks will inevitably arise, along which the areas \(a\) and \(v\) will subside relative to area \(b\) (Fig. 3, b).

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Fig. 3. Diagram illustrating the formation of exogenous faults

Subsided areas are covered by water; peat bogs rapidly accumulate here, and the surface rises to the level of the water table. Only recently subsided blocks are marked by relatively deep depressions filled with water. Since, during subsequent subsidence, the blocks adjacent to the depression on the side of the hard-surface areas are displaced, and at the same time peat accumulates in the lakes, the basins acquire an asymmetrical structure (Fig. 2).

The presence of protrusions in the relief of the buried floor of the basin is confirmed by many factual data. First, many such protrusions are found on the lower slopes of the basin that fringe the accumulative plain. Second, some of the positive forms of the buried relief rise above the surface of the accumulative plain. Thus, north of Lake Marekul, amid a bog, there is a hill composed of dolomites and gabbro-diorites. Rock outcrops rising above the bog can be observed in the southeastern part of the area. Finally, in 1972 E. V. Yaroshinskii, using the VES method, established that the thickness of loose deposits beneath the hard-surface area situated south of Lake Marekul is about 10 meters, whereas the depth to the bedrock on the neighboring boggy area reaches 35–40 m.

The amplitude of the exogenous faults manifested in this area averages 4 meters, i.e., the difference in the reduction of the thickness of loose sediments with a thickness of 40 meters and 10 m is determined as 4 m.

Disjunctive exogenous dislocations of the Quaternary strata in the area of Lake Marekul are clearly expressed, but they form only where there are steep slopes in the buried relief. On gentle slopes of the buried relief, plicative exogenous dislocations appear, although their presence is more difficult to prove.

The following facts may be cited as evidence of the general subsidence of the surface of the Marekul area. A stream flows out of the Marekul bog; over the last 20 years it has incised its channel by an average of 1 m; on the bog a layer of peat up to 30 cm has accumulated, and in Lake Marekul a layer of sapropel up to 20 cm has accumulated. Thus, lowering of the drainage base and growth of the sedimentary strata from above should have led to drainage of the bog and shallowing of Lake Marekul. In fact, neither one nor the other is observed. On the contrary, the depth of Lake Marekul has increased, and the water saturation of the bog has grown. The previously mentioned hard-surface areas are direct evidence of a lowering of the level of the general surface of the Marekul area. Fluctuations of the surface level are an expression of the ratio between the rate of accretion of the sedimentary strata from above and the rate of decrease in the thickness of this stratum, occurring as a result of compaction of the sediments. According to observations in the area of Lake Marekul, the equilibrium has repeatedly shifted first in one direction, then in the other.

If, at the protrusions of the buried relief, the relative subsidences

thicknesses reach 5 meters, then above the basins, where the thickness of the loose sequence is greater, the amount of subsidence should increase. Irregularities of the buried relief are expressed in exogenous plicative structures.

From the materials presented it follows that:

1. The lakes of the Marakul group are revived lakes: some of them formed in collapses—exogenous grabens; others, in areas of intense sagging—in the troughs of exogenous synclines.

2. Compaction of sediments may lead to noticeable deformations of the layers of sedimentary sequences. This remains a poorly studied question and, as it seems to us, is insufficiently taken into account in the study of the disturbed bedding of certain ancient sedimentary sequences.