Diversity of deformation structures in varves of the Weichselian Pyrzyce Basin (NW Poland)

Abstract

Within the sediments of the Pyrzyce Ice-dammed Lake (Polish Lowlands), a number of soft-sediment deformation structures (SSDS) in varve series were distinguished. The unique morphology of the ice-dammed lake—the occurrence of level IV overload and the presence of dead ice in the central zone (Lake Miedwie) and the marginal reservoir—had a decisive impact on the formation and nature of sedimentary structures. Dead ice melting and loss of support could be the cause of plastic deformation. The highest intensity deformation in layered sediments of both plastic and rigid types was observed in the marginal (highest) part of the reservoir. Their formation is associated with the melting of rooted lumps of dead ice in sub-aquatic conditions. The exception is deformation resulting from desiccation under subaerial conditions. The genesis of the deformation observed in rhythmically-layered varves is associated with the last stage of functioning and disappearance of the reservoir during the recession of an ice sheet of the Pomeranian phase of the Weichselian glaciation. These studies detail the current state of knowledge on the processes and factors responsible for the formation of soft structures occurring in glaciolimnic basin.