Helena Alexanderson

The Fiskarheden quarry, situated in NW Dalarna, central Sweden, reveals thick coarse-grained sediments of Scott type facies association representing a sandur deposited in an ice-proximal proglacial environment. Optically Stimulated Luminescence (OSL) dating of the sandur sediments suggests a pre-Last Glacial Maximum (LGM) age. Most acquired ages are pre-Saalian (>200 ka) and we regard each of these ages to represent non/poorly bleached sediment except for one small-aliquot OSL age of 98±6 ka. This age comes from the top surface of an arguably well-bleached sand bed deposited on the lee-side of a braid-bar, putting the sandur build-up into the Early Weichselian. Large-scale glaciotectonic structures show an imbricate thrust fan involving both ductile and brittle deformation. The deformation was from the WNW, which largely coincides with the formative trend of the predominating streamlined terrain and Rogen moraine tracts surrounding Fiskarheden. It is suggested that the deformation of the sandur sediments took place when the advancing glacier approached and pushed its own proglacial outwash sediment, during an ice-marginal oscillation either at the inception of one of the Early Weichselian glaciations in the area, or during a general ice retreat amid a deglacial phase. The Fiskarheden sandur deposits are covered by a subglacial traction till deposited from the NE/NNE. This direction corresponds with younger streamlined terrain flowsets cross-cutting the older NNW–SSE system and probably represents deglaciation in the area following the LGM. This study will add to the understanding of the formation and deformation of Pleistocene sandur successions and their relationship to past ice-sheet behaviour.