GROUNDING-ICEBERG DEFORMATION IN AN EARLY PREBOREAL GLACIOFLUVIAL DEPOSIT IN SE SWEDEN AND ITS REGIONAL SIGNIFICANCE

The recession of the ice sheet in central Sweden at the end of the last glaciation and the development of the Baltic Ice Lake into the Yoldia Sea, which took place during the late Younger Dryas and the early Holocene, have been discussed for many years. During a regional investigation in the glaciofluvial deposits of the Stockholm area, southeast central Sweden, an early Preboreal glaciomarine site was selected to document the typical facies change from lake to marine environment. The studied sections reveal, from top to bottom, a fan-delta deposit, a near-coast full glaciomarine deposit, and a unit of poorly sorted ice-rafted debris. The top part of the unit consisting of ice-rafted debris is characterized by soft-sediment deformations that have the same characteristics as material moved forwards by a bulldozer. These deformation structures are interpreted as due to grounding of icebergs, the lateral movement of which caused detachment of the lower part of the succession into a direction reflecting eastward movement of the icebergs. The absence of laminations above the grounded-iceberg structure and the occurrence of the ice-rafted facies indicate a nearby position of the grounding line of an ice sheet. Comparison with the characteristics of modern grounded glaciers with a rapidly calving ice fronts shows several similarities, among which the dense masses of debris. The fact that the dropstones at this site are restricted to a specific level implies that the area was not frequently reached by icebergs, but a sudden arrival of a single iceberg occurred. Combination of all the above data makes it most probable that an iceberg reached the study area when the area underwent rapid deglaciation during the early Preboreal. This event may have contributed to a regional calving of the Scandinavian ice sheet.