On the Prospects for the Search for Quaternary Chemogenic Sedimentary Deposits in Khakassia

G. A. Ivankin

(Presented by the scientific seminar of the Geological Prospecting Faculty of TPI)

The purpose of the present article is an attempt to draw attention to buried chemogenic lacustrine sediments of Quaternary age on the territory of Khakassia, and to show the possible practical value of these sediments.

A distinctive feature of the landscape of Khakassia is the wide distribution of deflationary landforms, in particular blowout basins. The processes of deflation, which until quite recently played the leading role in transforming the structure of the surface of Khakassia, have by now lost their former relief-forming role, which is explained by a significant humidification of the climate. The typical desert landscape, relics of which can be seen in many places, has been replaced by a steppe landscape.

Many deflationary basins have by the present time proved to be partly filled with lacustrine, bog, proluvial, and alluvial sediments. Accumulative plains have formed in them. In some basins, however, there are now lake water bodies. The process of their lacustrine aggradation began recently.

The dimensions of the accumulative plains that formed in deflationary basins, and their relief position, give grounds to believe that many of them formed in the place of closed lakes, and that the basins were autonomous drainage areas.

The process of salinization of the waters of closed lakes in so arid a climate, which prevailed in the second half of Quaternary time in Khakassia, is quite natural. Therefore, the assumption that buried chemogenic sediments are present, and that their volumes should be sufficiently large, is entirely admissible.

Let us give several justifications for this assumption.

The waters of many modern closed lakes of Khakassia are saline and bitter-saline. In them the deposition of salts periodically takes place (Shira, Shunet, Bele, and others); for example, in Lake Shira, the salt composition of whose waters is given in Table 1, with a decrease in water temperature there is a considerable reduction in salt content. In May 1958 the salinity of the waters of this lake was 6.5 t/l, while in August of the same year it reached 21.9 t/l; that is, in May the salt content amounted to less than 1/3 of the amount of dissolved salts in August. If the total amount of dissolved salts in the waters of Lake Shira in summer is determined as \(6.6 \cdot 10^6\) t, then in winter it probably does not exceed \(2—2.5 \cdot 10^6\) t. At the same time, sulfates and ...

Table 1

Composition and content of salts in the water of Lake Shira
(data obtained from V. V. Bykova)

carbonates, since the solubility of sodium chloride is almost independent of water temperature.

A large part of the salts that precipitate in winter passes back into solution in summer, but some portion of them is conserved on the bottom. The conservation of salts is facilitated by their being covered with clayey-silty material, which enters the lake in large quantities during periods of heavy rains. It is quite probable that the regular decrease in the salinity of the waters of the lake (Table 1) is explained by the increased conservation of the precipitated salts. In recent decades, the steppe lands of Khakassia have been brought under grain crops; considerable areas in the vicinity of Lake Shira have been plowed up, which could not but affect the extent of sheet wash of soils and the entry of fine clastic material into the lake.

Lake Shira has a comparatively small water-surface area; at the same time, evaporation of water from its surface fully compensates for the inflow of water into the lake. Moreover, a lowering of the water level in the lake is noticeable, i.e., a reduction in the volume of water. The latter also occurs as a result of the filling of the basin with detrital material. During the existence of Lake Shira, which is hardly more than 15 thousand years, the water surface in Shira has decreased by approximately half. There are no grounds for assuming the presence of any appreciable underground outflow of water from the lake, since comparison of the relative contents of sodium chloride in the waters entering the lake and in the waters of the lake itself indicates that such outflow is absent.

Many former lakes, on the sites of which accumulative plains later formed, were also closed basins, and their dimensions exceeded (sometimes many times over) those of Lake Shira. A comparison of the ratios of catchment areas to the areas of the water surfaces of these lakes leaves no doubt that precipitation of salts must have taken place in them (in the lakes), all the more so because in the past, when these lakes existed, the climate was more arid than it is now.

The rate of salinization of lake waters, and the quantity and quality of the salts accumulated in them, were determined by many factors. The most important of these were: 1) the composition of the bedrocks composing the lake’s catchment basin, the presence in them of readily soluble salts, and their qualitative composition; 2) the nature of the lake’s feeding (depending on many factors), determining the rate of water runoff and, consequently, the time of contact of the waters with the bedrocks before the water entered the lake; 3) periodic changes in climate.

Taking the first factor into account, one may forecast accumulations of particular salts in former lakes. For example, the accumulation of significant volumes of sulfate salts should be expected in lakes characterized by high sulfate content. In lakes into which waters were discharged from valleys cutting through ancient manganiferous strata, the accumulation of manganese salts is possible. For example, the valley...

The Sukhaya Erba cuts through the Cambrian manganiferous sequence. In its lower reaches there were lakes. Manganese salts could have accumulated in them. Similar assumptions may be made regarding the accumulation of phosphorus and other elements.

Many large mineral deposits, the substantial upper part of which has been denuded, may have served as a source for the accumulation of salts in the waters of former drainless lakes. For example, the accumulation of molybdenum (its salts) may have occurred in the Byurinskaya depression, located near the Erbinskaya railway station, into which a significant quantity of weathering products from the Sorsk ore field was carried.

There is no doubt that a more rigorous accounting of the geological structure and relief development of specific areas will make it possible to predict, with a greater degree of probability, searches for particular types of chemogenic sedimentary deposits of Quaternary age.