ON THE DEVELOPMENT OF THE ITKUL AND KARYSH DEPRESSIONS IN KHAKASSIA

G. A. IVANKIN

(Presented by the scientific seminar
of the Department of General Geology)

Within the Minusinsk intermontane basin and on the eastern slopes of the Kuznetsk Alatau, aeolian landforms are widespread; they formed in the second half of the Anthropogene. At the same time, the morphology of the modern surface of Khakassia is determined to a considerable degree by deflationary hollows—oueds and blowout depressions.

The development of the latter, their evolution, deserves attention in connection with the accumulation in the depressions of products of modern and ancient weathering crusts, as well as organogenic sediments. Well-differentiated clastic, chemogenic, and organogenic sediments of lakes, which may be of practical interest, usually prove to be buried beneath a cover of proluvial, alluvial–proluvial, and bog deposits. A prediction of the presence of particular sediments within specific depressions can be made only on the basis of a clear understanding of the development of these hollow forms of relief and of the process by which they are filled with sediments.

This article briefly describes the development of the Itkul and Karysh depressions, located to the southeast of the railway station Shira (Fig. 1).

The Karysh depression has an irregular outline in plan. It is elongated in a NW direction and extends from Lake Marekul to the settlement of Chaltyz-Tag, i.e., it is about 9 km long. Its width, measured along the accumulative surface, reaches 3.5–4 km. From the east it is adjoined by the large depression of the Karasuk log, which in essence is a direct continuation of the Karysh depression.

The beginning of the development of the depression (as of other aeolian landforms) was conditioned by the onset in Khakassia of a typically arid climate. The decisive influence on the change of the climate toward aridity was exerted by desiccating mountain winds, whose role increased greatly when the axial part of the Kuznetsk Alatau Range became covered by glaciers. This event probably coincided in time with the maximum glaciation of the territory of Western Siberia, i.e., took place about 200 thousand years ago. Such an assumption will not seem wholly unfounded if one takes into account that continental glaciations exert a noticeable influence on climate and cause a lowering of the snow line. In addition, it is quite probable that the uplift of the Kuznetsk Alatau was uneven.

Fig. 1. Schematic location of the largest deflation basins in the area of Lake Itkul. I — Argistar, II — Arashtaevskaya, III — Itkul, IV — Spirinskaya, V — Karyshskaya.

Beginning in the Neogene, the Kuznetsk Alatau and the Minusinsk Depression have experienced uplift that was uneven in time and space. The rate of uplift of the axial part of the Kuznetsk Alatau was probably higher than the uplift of its eastern slopes and of the intermontane depression. The unevenness of tectonic movements of such a large block of the Earth’s crust over time found expression in the formation, in the Neogene and the first half of the Anthropogene, of tiers of erosional–denudational relief.

The Karysh Basin took shape as a result of the winnowing of products of destruction, chiefly from granitoid rocks, the complex of which is customarily distinguished under the name of the Ulen–Tuim intrusion.

The deviation of the basin’s elongation from the general northeastern direction can be explained by its inherited development. The basin probably formed as a result of the transformation of an ancient valley of the Karysh. In addition, there are grounds to suppose that, at a higher denudation level above the modern Karysh Basin, there was a synclinal fold into which Devonian strata had been crumpled. This supposition is based on the fact that in the Karasuk log the indicated structure is recorded in the modern section, and its axis is traced through the Karysh Basin.

It is difficult to take precise account of the dimensions of deflation, i.e., the volume of removed mountain masses, using this basin as an example, since “reliable remnants” of the ancient (“pre-deflation”) surface have not been preserved here. One thing is beyond doubt: a layer about 100 m thick has been blown away, so that during the formation of this basin, within the limits of the accumulative plain alone, about 3 cubic km of mountain masses were winnowed. If the calculation is made for the full volume of the basin, the indicated figure doubles. Despite the many assumptions adopted in a simple arithmetic calculation of the rate of deflation of granitoid rocks, the value obtained for it—0.5 m in 1,000 years—appears optimal to us.

The deepening of the Karysh deflation basin probably ceased shortly before the Holocene, about 20 thousand years ago, in connection with a general humidification of the climate and the beginning of aggradation of the basin.

The basin thus produced is a comparatively complex landform. It united a number of basins that were, to one degree or another, separate and isolated from one another. The bottom of the basin was enlivened by saucer-shaped depressions, ridge-like and blocky residual forms, and islands of low hills. Some of the positive landforms still rise above the surface of the accumulative plain. The basin was closed. The aggradation of the basin may be represented by the following scheme:

1. In the initial stage of aggradation, the temporary-lake stage, clastic material accumulated in the basin. It arrived in the form of mudflows from valleys opening into the basin and from its slopes. The main mass of proluvium came from the Karysh valley. Along the periphery of the basin, and chiefly at the mouth of the Karysh valley, alluvial fans formed. In the periodically forming temporary lake in the central zone of the basin, fine- and very fine-grained clastic material was winnowed and layers of clay and loam accumulated. Only during very heavy downpours did mudflows penetrate to the center of the basin.

2. In the stage of a permanent closed lake, which replaced the first owing to the general humidification of the climate, in addition to the above-mentioned types of deposits, purely lacustrine deposits accumulated, chiefly organogenic ones and those formed through coagulation of colloids.

With the passage of time the lake waters became saline; the degree of salinization could have reached the saturation limit, and therefore the accumulation of chemogenic deposits at this stage of the basin’s aggradation may be assumed. Especially significant layers of salt accumulated during arid periods, when the lake dried up completely or the volume of water in it was greatly reduced.

Comparing the ratio of the size of the former Karysh Lake to the size of the basin that fed it with the same ratio for the modern saline Lake Shira to its basin, one may state that conditions for the salinization of the waters of Karysh Lake did exist; and if one takes into account that in the past the climate was more arid than it is now, the probability of accumulation of chemogenic deposits increases.

1. A reduction in the water surface of the lake due to the advance of the accumulative plain, as well as the general humidification of the climate, led to a gradual rise in the water level in the basin. The maximum high water level in the former Karysh Lake corresponded approximately to the water level in the modern Lake Pionerskoye. This is indicated by the position of the former shoreline, which is marked by a sharp break in the surface of the low slopes of the basin, as well as by the upper boundary of the distribution of lacustrine gravels. A through-flowing lake formed.

In this third stage of aggradation, when the lake became through-flowing, alluvial, proluvial, lacustrine (organogenic), and bog deposits accumulated in the Karysh basin. The stage has not ended. In the relics of the former large body of water, in Lakes Pionerskoye, Marekul, and others, sapropel is now accumulating; in the bogs, the peat layer is growing; material is being carried into the basin by the Karysh River and by mudflows. For example, in 1964, after a two-hour downpour, a mudflow that emerged from Karasuk Log advanced into the Karysh plain for a distance of more than 1 km; over an area of 0.6 km², a layer of proluvium up to 1 m thick was deposited.

In the process of aggradation of the depression, a loose stratum up to 51 m thick accumulated in it and an accumulative plain was formed. Only relicts remained of the former large body of water, the largest of which is Lake Pionerskoye. It once formed a bay of Lake Karysh and was preserved because the ridge projecting into the depression to the SE of Lake Pionerskoye “protected” it from mudflows moving in a NW direction.

Because the main mass of clastic material, during all stages of aggradation, came from the southeast, from the Karysh valley, the structure of the loose stratum is not the same in different parts of the depression. In its southeastern part, adjoining the mouth of the Karysh valley, proluvial deposits predominate in the section of the loose stratum; in the middle part, lacustrine sediments occur below, and above they are overlain by proluvial and alluvial-proluvial deposits; in the northwestern part (in the area of Lake Marekul), lacustrine sediments overlain by bog deposits predominate in the section of the loose stratum.

Fig. 2. Schematic and inferred section of the sedimentary stratum of the Karysh depression. 1 — bedrock, 2 — alluvial-proluvial deposits, 3 — lacustrine silts and clays, 4 — salt layers (inferred), 5 — lacustrine clays, sapropels, silts, 6 — peat.

Figure 2 presents a schematic section of the loose formations of the Karysh depression. In constructing it, the established regularities in the filling of many depressions of Khakassia were taken into account. The summation of numerous observed fragments of modern sediment accumulation in depressions that are at different stages of their evolution into a consecutive series supports the correctness of the (general) sectional scheme. In addition, in 1960, staff members of TPI, V. A. Andreev and Z. A. Myshko, measured the corresponding physical properties of the loose stratum by means of the reflected seismic-wave method and the VES method along two profiles: a transverse one, laid out from the TPI test site in the direction of the Darynskoye deposit, and a longitudinal one, laid out from Lake Marekul toward the mouth of the Karasuk log.

In the transverse profile, by means of both methods, two surfaces of abrupt change in physical properties are clearly established. The lower one is a continuation of the slopes of the depression and may be taken as the base of the loose stratum. It descends to a depth (from the surface) of 51 m. The other surface is recorded at a depth averaging about 20 m. Along this profile (over most of it), the surface layer is composed of proluvium; therefore, the surface of change in the physical properties of the loose stratum at a depth of about 20 m may be taken as the base of the proluvial layer.

In conclusion, let us note that for a long time the Karysh depression was an autonomous base level of erosion for the present-day upper reach of the Karysh River. Together with its basin, it constituted a closed system in the sense that all products of weathering of rocks that were carried down from the basin territory both by surface-

by surface and ground waters alike, settled in the depression. The accumulation of material in the depression was accompanied by its differentiation and sorting, probably to the state of practically valuable formations. Neither the structure of the sedimentary sequence nor the properties of the sediments themselves in this depression have essentially been studied; therefore at present it is difficult to make predictions about their use. One may only suppose that among the deposits of the depression there are sorted gravels, sands, clays, and salt beds; peat and sapropel are certainly present, and their reserves may be substantial.

For example, let us take the clays. About 1 cubic km of sediments accumulated in the depression. If we assume that half of them consist of clastic material, and that the aleurite–pelite fraction made up one third of the transported proluvium, then the reserves of lacustrine clays in the depression are estimated at approximately 150–200 million m³. At present nothing definite can be said about their quality, but they will probably be suitable as a construction material and for the production of building materials (for example, brick).

Peat may be used as fertilizer for fields; sapropel and peat may possibly serve as raw material for the chemical industry.

The Itkul depression is associated with the synclinal fold of the same name and formed as a result of deflation of weathering products, chiefly of the red-colored sandstones of the Upper Devonian. The dissected beds of the Bei Formation make up the slopes of the depression.

The morphology of the depression in plan and in profiles was determined by the morphology of the plicative structure (Fig. 3). Its form is saucer-like; the slopes are comparatively simple; the bottom is flat-concave; only the low ridges of dissected strata disturb the uniformity of its level surface.

Fig. 3. Itkul depression. 1 — granites (γδπ₂); 2 — porphyrites, albitophyres (D₁³); 3 — Matarak and Lunetsk formations: conglomerates, sandstones, aleurolites, marls (D₁²); 4 — olivine-augite porphyrites (D₂²—D₁³); 5 — Saragash and Bei formations: marls, limestones (D₃³); 6 — red-colored sandstones (D₃). Г — boundary of the former lake, Д — former lake bottom, Р — proluvial plain, Т — accumulative surface of the lake terrace, В — lake-shore rampart, К — valley of the Karysh River, ancient, М — the same, young, П — sinkholes, С — Sipirinskaya lacustrine-bog-proluvial plain.

This flat landform has a length of about 12 km and a width, measured by the average level of the slopes, of up to 7 km; its depth relative to the upper level of the slopes is estimated at approximately 100 m, and its volume at 10 km³.

The central part of the depression is occupied by Lake Itkul; its western shore is swampy; along the northeastern shore there extends a littoral lake rampart; along the northern and southern shores a sloping surface of a lake terrace up to 10 m high can be traced. There is no doubt that in the recent past the water level in the lake was 10 m higher than at present, and its area was twice as large.

Along the NE shore of the former lake, burial grounds left by peoples who inhabited Khakassia 2,500–3,000 years ago are arranged in an arc. Within the dried-up lake bed, over a large area, there is not a single burial ground. On this basis one may suppose that the lowering of the water level in the lake and, consequently, the reduction of its size as a result of deepening of the valley of the river Teploya, which flows out of the lake, occurred no earlier than 2.5–3 thousand years ago. If this is so, then using Lake Itkul as an example it is possible to determine with comparative accuracy the rates of various processes: lake abrasion, formation of the littoral rampart, growth of the Karysh delta, etc.

For example, the steep northern and southern slopes of the depression are now being actively eroded by the waters of Itkul. The shore is retreating landward. Having plotted the surface of the former lake bottom on the surface of the lake terrace, one can establish that over these 2,500–3,000 years the shore has retreated on average by 50 m, and in individual sections by as much as 70–80 m. Consequently, the rate of shore retreat, i.e., the rate of lake abrasion, is on average 20–30 m per 1,000 years.

The above-mentioned littoral lake rampart has a height of up to 4 m, a width of up to 1,000 m, and a length of more than 3 km. It formed, and is still forming, under the influence of lake ice. During the period when the lake is freed from its ice cover, when under the action of SW winds the ice floes from the entire lake are driven toward the NE shore, under the pressure of those behind them the front floes are pushed onto the shore and not infrequently advance over land by tens of meters. By these ice floes, like the blade of a bulldozer, the clastic material of the shoal is shifted to the NE. Part of the material is delivered to the littoral rampart in the form of fragments frozen into the ice floes. The rate of growth of the rampart is on average 30 m per 1,000 years. The rate of growth of the mouth alluvial fan and delta of the river Karysh is on average 0.4–0.5 per year.

The Itkul depression entered the stage of aggradation comparatively recently. This conclusion can be drawn on the basis of the following facts. First, on the bottom of Lake Itkul at a depth of 10–12 m there are rock outcrops of bedrock, i.e., the thickness of sediments in the central zone is comparatively small. Second, the volume of coarse clastic material accumulated within the Itkul depression proper is small. Third, the volume of deltaic deposits of the river Karysh before the lowering of the water level in the lake is commensurate with the volume of those that accumulated after the lowering of the water level in Itkul. The latter fact permits us to refine the conclusion formulated above and to consider that aggradation of the Itkul depression began approximately 6–7 thousand years ago, after the Karysh, Arashtay, Argystar, and Spirino depressions had for the most part already been filled with sediments. At least the filling of these depressions was so substantial that the reduced size of the surfaces of the water bodies caused a shift in the equilibrium between the inflow of water into the lakes of the indicated depressions and its loss through evaporation. The rise in water level caused its runoff from these depressions into the Itkul depression.

Thus, the Karysh, Arashtaev, and Argistar basins for a comparatively long time “protected” the Itkul basin from the influx of water and clastic material from the west and southwest, and the Spirinsk basin from the arrival of such material from the southeast. The southern, northern, and eastern slopes have a small area. The inflow of water and clastic material from these slopes was far from sufficient for the process of aggradation of the basin to begin. The Itkul basin was a site of active deflation; it deepened and expanded, while the Karysh and other basins at that time were being filled with sediments.

The example considered clearly illustrates the proposition that the basins of Khakassia entered the stage of aggradation at different times. As a rule, the basins adjoining the Kuznetsk Alatau entered the stage of aggradation earlier than the basins of the more central zone of the intermontane depression. This should be taken into account, along with other criteria, when predicting the composition of the sediments of particular basins.

The deposits of the Itkul basin are represented mainly by clastic sediments. They are widespread over a large area of the drained floor east and northwest of the lake, form the Karysh beach ridge and delta, and are also present on the floor of the modern lake.

East of the lake, the clastic lacustrine deposits are represented mainly by pebble beds overlying sands. The complete section of the loose strata has not been exposed.

In its southern part, the beach ridge is composed mainly of pebbles and gravel, with an admixture of sand. Northward, the size of the fragments decreases: pebble beds are replaced by gravel, and gravel by coarse-grained sands. It should be emphasized that the deposits of the beach ridge are well sorted; they are practically devoid of clayey material. For this reason they should be high-quality aggregates in the production of concrete.

In the delta of the Karysh River, sandy-clayey sediments have accumulated. They are overlain by bog deposits.

The bottom sediments of the lake in the nearshore zone, along steep shores, are represented by boulder and pebble deposits, and along shallow shores by sands. Silty material accumulates in the northeastern half of the lake at a depth of about 4 m, and in the western half at a depth of 1.2 m and more.

In the central zone there are probably clayey and, possibly, partly organogenic sediments. At the very least, the lake floor is now being colonized by algae.