On the Prospects for the Search for Quaternary Sedimentary Deposits in the South of Krasnoyarsk Krai and in Tuva

G. A. Ivankin

(Presented by the scientific seminar of the Departments of General Geology, Historical Geology, and Mineral Resources)

On the territory of Khakassia and Tuva, accumulative plains are very widespread; they arose mainly as a result of the filling, by sediments, of deflationary hollow forms of relief, chiefly deflation basins.

Vast areas within which a thick—sometimes more than 100 m—cover of loose deposits overlies more ancient formations were practically excluded from the number of areas subject to prospecting: the loose deposits themselves did not attract attention, while the search for possible buried deposits presents considerable difficulty.

Analyzing the conditions of formation of the accumulative plains and the loose deposits filling the basins of Khakassia, the author came to the conviction that the deflation basins, in the development of the relief of the indicated regions, served as local bases of denudation and autonomous basins of accumulation for products of synchronous and asynchronous (more ancient) weathering crusts. Accumulation of organogenic lacustrine and bog sediments also took place here.

The basins are located in various landscape zones and in different parts of these zones. The conditions under which they were filled with sediments were also different. This led to the accumulation in the basins of loose strata differing in their composition and in the genesis of the sediments. Various accumulative plains arose in place of the basins.

A number of plains can be distinguished, which usually represent members of an evolutionary series (Table 1). Let us emphasize that the formation of accumulative plains occurred (and is occurring) with a directed change of climate from typically arid (desert) toward ever greater humidification. However, this directed change of climate was not smooth: there was an alternation of epochs of a more arid and a more humid climate.

As can be seen from the scheme presented, the accumulative plains were formed as a result of the accumulation of lacustrine, bog, proluvial, and alluvial sediments. We shall give a brief description of the conditions of accumulation only of lacustrine deposits and attempt to determine the prospects for their use.

Among lacustrine sediments it is customary to distinguish clastic, chemogenic, and organogenic deposits.

The clastic sediments of the lake basins of Khakassia (and also of Tuva) were formed mainly from products of the weathering crust brought in by temporary and permanent watercourses. Especially

large quantities of material were carried (and are carried) into the basins during periods of heavy rains in the form of mudflows. Suspended particles settled out of the turbid waters mainly far from the shore. At the bottom of the deepest parts of the lake, thicker layers of fine-clastic sediments accumulated. The deposits of river deltas and alluvial fans of temporary streams in relatively large lakes were reworked by the waters of the basin and sorted. Coarser clastic material settled in the coastal zone, finer clastic material farther from the shore, where organogenic and, under certain conditions (for example, excessive evaporation), chemogenic sediments were mixed with it.

The high degree of reworking of clastic material in the coastal zone (its sorting) ensured the accumulation of well-sorted lacustrine pebbles, gravel, and sands, sometimes practically completely devoid of silty material. Such, for example, are the deposits located along the northeastern shore of Lake Itkul in Khakassia. It is probable that similar lacustrine deposits could be used as aggregate for high-quality concretes and for other construction purposes.

In coastal zones, lacustrine placer deposits of heavy minerals could also have formed. This should not be overlooked when studying loose deposits of accumulative plains, especially in those cases when, in the source area, there are sources of minerals for their accumulation in placers. Fine-clastic sediments of flowing and temporary lakes are represented predominantly by clays. Clay beds may reach considerable thickness, but they are always overlain by proluvium, swamp deposits, or even alluvium.

In 1962, under the direction of V. L. Khomichev, the near-mouth part of the dead valley of Tarlykhoi was drilled in the area of the Tuim mine. On the basis of an analysis of the plan of borehole locations, made on a topographic base, and of descriptions of the borehole logs, it is easy to establish that at the site of the modern accumulative plain of valley type there was formerly a deflation basin. At the base of the loose lacustrine-proluvial sequence, whose thickness reaches 50 m, lie clays, sometimes with clearly expressed stratification. The thickness of the clay layer reaches 35 m. Unfortunately, the quality of the clays has not been studied, but their suitability as a construction material or as raw material for the production of building materials (for example, brick) can hardly be doubted.

During the erosion, in the source area, of an ancient, substantially lateritic weathering crust, high-alumina sediments could have formed in lakes; and when products from oxidation zones of sulfide deposits were carried into basins, ochres could have accumulated.

In 1970, the Western Integrated Geological Exploration Expedition of KGU drilled a borehole in a dead valley in the south of the Batenevsky Range. At the base of the loose sequence, a layer of ochreous clays with a high (more than 30%) content of alumina was uncovered (oral communication of A. K. Mkrtchyan).

Very many modern lakes of Khakassia are drainless water bodies, and the waters of a considerable part of them are bitter-saline. When lakes dry up, salt deposition is observed along their shores. Salinization of the waters of a drainless lake is an inevitable process. Under conditions of a more arid climate in the past than at present, during the driest periods there undoubtedly took place an accumulation of saline sediments. One may count first of all on the discovery of deposits of sulfates, in particular gypsum and epsomite, on the following grounds.

The high gypsum content of the Beyskaya Formation is well known. In many cases, a significant part of the weathering products of the rocks of this stratum was not carried beyond the limits of the drainless basins. Naturally, the sulfate salts also could not have disappeared without a trace.

On the same basis, one may expect an accumulation in lake sediments of elevated concentrations of molybdenum, copper, phosphorus, and other elements. The upper parts of deposits exposed at the modern surface are denuded; in many cases a significant portion of the weathering products settled in the basins. If the role of plant and animal organisms in the precipitation of elements dissolved in water is taken into account, the supposition expressed will not appear so illusory.

Organogenic lake sediments, represented mainly by sapropelic silts, together with bog sediments, may find broad use both as an agronomic raw material and as a raw material for the chemical industry.

Scheme of the Formation of Accumulative Plains on the Site of Deflation Basins

Deflation basin

• Filling with water → Lake
• Filling with lacustrine sediments → Lacustrine plain
• Filling with lacustrine sediments + proluvium → Lacustrine–proluvial plain
• Filling with lacustrine sediments and eluvium → Lacustrine–alluvial plain
• Filling with lacustrine sediments, proluvium, and alluvium → Lacustrine–proluvial plain
  • Covered by proluvium → Lacustrine–bog–proluvial plain

• Covered by alluvium → Lacustrine–bog–alluvial plain

• Filling with proluvium → Proluvial plain

• Filling with lacustrine sediments, proluvium, and alluvium → Lacustrine–proluvial plain

• Swamping → Bog plain

• Filling with lacustrine sediments and overgrowing → Bog
  • Covered by proluvium and alluvium → Lacustrine–bog–proluvial–alluvial plain