Abstract
Abstract The distribution of glacial landforms and deposits in the upper part of the Mogen-Buren River basin provide evidence for extended ice sheet formation along the Shapshal range in the late Pleistocene. At that time the Mongun-Taiga massif and Chikhachev range were also affected by glaciation. Advanced glaciers blocked the Khindiktig-Khol and Ak-Khol depressions and impounded significantly larger lakes. Overfilling any of the Khindiktig-Khol, Ak-Khol or probably the supraglacial lakes by melt-water caused repeated outburst floods along the Mogen-Buren River into the Achit-Nuur Lake, Mongolian Great Lakes Basin. Significant landscape transformation took place due to geological work of these processes. Specific landforms were generated, including high lake shorelines, lacustrine bars and sediments, marginal channels, indicators of fluvial activity on the floor of the basin, and fluvial transported boulders that mark the flood pathways as well as the specific vertical lithofacies profile of outwash deposits. Available absolute dates suggest some chronological bench marks of the hydrological system transformation in the region. Changing of deposition patterns in the Achit-Nuur Lake about 11,000 cal BP can be assumed from the age-depth model of the lake core ( Sun et al., 2013 ). The period of the maximal lakes filling at 2380 m a.s.l. in the upper part of the Mogen-Buren basin started no later than 14,000 cal BP, based on data from Ak-Khol Lake core ( Blyakharchuk et al., 2007 ). There are no absolute dates that identify the time of partial draining of former Ak-Khol Lake, which was dammed by glacier ice, and its prolonged preservation at 2270 m a.s.l. – the height of the moraine dam. This stage ended before 8200 cal BP. The postglacial hydrological system evolution within the studied area is characterized by formation of local lakes in the Mogen-Buren depression from about 11,600–8800 cal BP. All cataclysmic outburst floods from glacier- and moraine-dammed lakes took place before 11,600 cal BP, and significant hydrological system transformations were completed before 5100 cal BP.
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