A pre-LGM sandur deposit at Fiskarheden, NW Dalarna : sedimentology and glaciotectonic deformation

Abstract: The Fiskarheden quarry, situated in NW Dalarna, central Sweden, reveal thick deposits of coarse-grained sediments of Scott type facies association; a sandur deposited in an ice-proximal proglacial environment. Preliminary OSL dating of the sandur sediments suggest that they are of a pre-Saalian age (>200 000 years). Of special interest here is, besides its pre-LGM age, the large-scale and intense glaciotectonic deformation of the sediments. Both ductile folding and brittle deformation structures suggest glacial stress from NW which coincides with the trend of surrounding streamlined terrain and to that trend perpendicular Rogen moraine. From sedimentological relations and the glaciotectonic architecture it is suggested that either (i) the deformation of the sandur sediments took place when the glacier approached and overrode its own proglacial meltwater sediment, or that (ii) the deformation is from a younger ice advance over the area, decoupled from the deposition of the sandur sediments. The Fiskarheden sandur deposits are covered by traction till deposited from NE/NNE. This direction conforms to younger cross-cutting streamlined terrain to the older NW-SE system and is suggested to represent the LGM deglaciation phase over the area.