Successive subglacial depositional processes as interpreted from basal tills in the Lower Vistula valley (N Poland) |
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| Institution: | Department of Geology and Hydrogeology, Institute of Geography, Nicolaus Copernicus University, Sienkiewicza 4, PL-87-100 Toruń, Poland;Adam Mickiewicz University;University of Silesia, Poland;Adam Mickiewicz University |
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| Abstract: | Three basal-till facies from the Lower Vistula valley were examined. The lowest facies, a sandy diamicton with characteristic sand inclusions forming detached and attenuated folds, is overlain by a bedded till characterized by alternating diamictons and sorted sediment layers. The uppermost till facies is a homogeneous diamicton.The three till facies must have been formed by complex subglacial sedimentary processes during the first Late Weichselian ice advance. The lowest till facies is interpreted as a deformation till, and accumulated during the initial stage of the ice advance. The middle facies represents a stagnation phase during the initial ice advance, and was deposited during recurrent periods of subglacial melt-out followed by meltwater sedimentation. The upper till facies was deposited by direct subglacial melt-out during a stage of stagnant ice.It is suggested that bed deformation and temporarily enhanced basal sliding have been caused by ice streaming at the time of the ice-sheet advance and just before its stagnation. |
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