Morphology and genesis of the Yaselda glacial valleys in Belarus

Abstract

Geological mapping revealed the oldest Yaselda (Don (on the territory of Russia), Sanian 1 (on the territory of Poland)) generation of valleys in the Quaternary strata. Valleys are widespread in marginal and inner zones of the Yaselda glaciation area. They are located in the pre-Yaselda lowlands, in the areas of deformable deposits that lie above the more hard rocks protrusion and in fault systems. The distribution of glacial valleys is depends on the radial-sectoral structure and dynamics of the Yaselda glacier, which predetermined both the common and regional features of the valley regularity and morphology. The common features are the location in lobed sectors, occurrence in heterochronous ice marginal complexes, the predominance of polygenetic valleys. The regional features include isolated position, submeridional and meridional orientation, smaller size and weak expression of valleys in the maximum phase relief, the predominance ofvalleys of glacial exaration and extrusion and tunnel valleys. Valleys of the younger (Novogrudok) phase are located in the marginal complexes of the Middle Neman, Naliboky and Middle Berezina ice lobes and tongues. Valleys are characterised by connection with the belt of marginal highlands, sublatitudinal large-festoon ordering on the area of the lobes and a fan structure in the tongues, large size and maximum depths, clear expression in relief. Here, mainly polygenetic basins and trough valleys, extrusion valleys and subglacial tunnel valleys have been established. Basins arose simultaneously with the advance of outlet glaciers and are the result of exaration and glaciotectonic overthrusts of substrate rocks in the territory of the Belarusian anteclise and lowlands enveloping from the north. Their formation became possible at the regressive stage, when the readvances of the ice lobes led to the extrusion of a large amount of bed material and its accumulation in the basin borders. Trough valleys arose due to the wedging of outlet glaciers into paleovalleys or selective linear erosion along zones of structural weakening. In areas of rock bed, they were formed by plucking and abrasion, and in areas of soft substrate – by plucking, removal of thrust blocks. Numerous extrusion valleys appeared due to the extrusion in the Meso-Cenozoic deposits from under the glacial tongues into the marginal zones during the glacial retreat phase. Tunnel valleys originated as a result of canal erosion of sediments during rapid outbursts of water from subglacial lakes established for the glacier reduction phase. Most of the large valleys are of polygenetic origin.