Last ice-dammed lake in the Kuray basin, Russian Altai: New results from multidisciplinary research

Abstract

Results from geomorphological, sedimentological and geochronological analyses, together with micropaleontological and mineralogical characteristics of lacustrine deposits in five locations within the Kuray intermountain depression, southeast Altai, mountains of south Siberia, support the thesis of repeated formations of ice-dammed lakes during MIS-2 and their draining by high energy floods. Our data suggest that the timing of one of the last cataclysmic draining events in the area can be estimated by an Optically-Stimulated Luminescence (OSL) age of 19.0 ± 1.1 ka for a sandy layer at the top of the diluvial (i.e. large flood) deposit, revealed in a sedimentary sequence of the 1570 m a.s.l. strandline – one of the lowest preserved strandlines in the western part of the basin. New OSL and radiocarbon ages, augmenting previously published dates, indicate that the last lake to occupy the Kuray depression occurred around 19–16 ka with a depth of at least 170 m in the central part of the basin and to a depth of no less than 220 m near the glacier dam. Lacustrine deposits are represented by two horizons of sandy clays separated by interlayers of mixed-size sands. The mineralogical data, supported by analysis of sedimentological and micropaleontological records, indicate accumulation of a lower lacustrine horizon in a deeper reservoir. Finding of Leucocythere sp.1, Leucocythere sp.2, and Leucocythere dorsotuberosa ostracod species in lacustrine deposits characterizes these reservoirs as periglacial freshwater cold and deep lakes. The presence of well-crystallized mica and chlorite in lacustrine silts and clays from the lower lacustrine horizon indicates cold, dry conditions at the time of their formation, as well as a predominance of physical weathering of rocks within the denudation area. After an abrupt dropping of the lake level around 16 ka, determined from OSL dating, the lake never recovered its former depth. The available radiocarbon ages for organic material in subaerial deposits within the study area and the new OSL ages suggest that the last ice-dammed lake in the Kuray basin was drained between ~16.7 and 9.9 ka. The presence of this lake might explain the absence of late Paleolithic surface finds within the basin that remained generally unsuitable for human habitation until its final drying. The last outburst flood passed along the Chuya and Katun river valleys, which had been already carved by older cataclysmic floods, but did not significantly affect the topography downstream of the Kuray-Chuya intermountain depressions. We numerically simulated the draining of a palaeolake in the Kuray basin with the water level 1650 m a.s.l. (maximal depth about 220 m near the dam) with different scenarios of breaching the ice dam. In contrast to a relatively gradual breach of the ice dam due to thermal erosion, an instantaneous dam break due to structural failure can cause an outburst flood with a peak discharge of around 2 × 106 m3 s−1. The high speeds of the water flow, 1.9–5.6 m s−1, with the maximum Froude numbers of 0.06–0.22, and peak Shields values of 0.03–0.25 indicate competence to mobilize gravel. Generally, the simulated flow remained subcritical, suggesting that bedforms developed under supercritical flows, such as antidunes, could not have developed, although the development of dunes cannot be precluded. Our data also contribute to the issue of correlating the low lake strandlines in the Kuray basin with the landforms associated with cataclysmic outburst floods.