ИСТИНА

At the Russian Plain, a set of uplands in the marginal zone of Mid-Pleistocene glaciations are traditionally thought to have dominant glacial origin of landscape structure. The entire postglacial sedimentary sequence of interfluve areas is interpreted as mantle loams – relatively thin (2-3 m) cover sometimes correlated with loesses of the southerly regions. While still debatable most agree on its subaerial origin under periglacial conditions involving combination of eolian, cryogenic and shallow pool deposition. However, recent individual investigations show widespread much thicker (up to 8 m) silts to sandy and clayey loams even at the highest interfluves of several uplands. Those infilling depressions of initial glacial and glaciofluvial topography usually have brownish-bluish grey color and contain organic-rich lenses, which frequency increases downwards. Those covering hilltops have lower thickness (<3 m) but are found almost throughout the entire area. Complicated by various cryo- and pedofeatures and partially destroyed, these sequences are not always recognized in cores and cross-sections. To obtain theia reliable stratigraphy, we applied a hierarchical multiscale approach including detailed macromorphological description of sediments followed by comprehensive analyses of grain size, chemical, organic, and spore-pollen contents, microstructures and paleopedofeatures using scanning electron microscopy and energy dispersive spectrometry, and numerical dating of organic-rich lenses. Revealed microlamination highlighted by color and granulometric variability, general lithological homogeneity and specific structural features allow considering both addressed sedimentary units of lacustrine origin. Stratigraphic (incl. overlying paleopedofeatures of texturally differentiated soils) and geomorphological positions suggest that on hilltops short-lived residual lakes formed during the final stages of Moscow ice cover degradation (MIS6). The accumulation in depressions started later - during the Mikulino (MIS5e) or Early Valdai (MIS5d-MIS4) and, in places, continued till the Late Valdai (MIS2) when was largely substituted by the intensified mass-wasting. Several asynchronous local water table drops and erosion episodes in the Valdai caused substantial reworking of deposits in periglacial environments and local stratigraphic hiatuses. Thus, for the peripheral areas of uplands in the Mid-Pleistocene glaciation marginal zone, we propose a long-term discontinual postglacial transformation mostly by lacustrine type of exogenesis, which led to a significant flattening of its initially higher-amplitude glacial topography at least during the last glacial-interglacial macrocycle. Nonetheless, resuming the extents of lake basins either causes of their origination is still problematic. High regional base level rises in large proglacial lake systems are occasionally proposed in light of a potential Late Pleistocene ice-damming of the Upper Volga and glacioisostatic effects. However, the existence of such extensive waterbodies is supported neither by the required water sources nor by the regional topography. As for the local agents of widespread distribution of multiple small lake basins, high activity of zoogenic (during interglacials) and slope (during periglacial and transition periods) processes can be suggested.