DYNAMIC BEHAVIOR AND FINAL COLLAPSE OF THE FENNOSCANDIAN ICE SHEET REVEALED BY GLACIOFLUVIAL SEDIMENTS IN EAST CENTRAL SWEDEN

In an effort to gain a better understanding of the main causes and characteristics of an early Preboreal subglacial jökulhlaup flood in the Stockholm area, east central Sweden, the interplay of various factors including climate signals, paleo-seismicity, spatial distribution of glaciofluvial deposits, and bedrock topography were studied. Clastic sedimentologic analyses from 45 gravel pits were used through evaluation of each outcrop data in a regional context developed by previously established clay-varve chronology. The Fennoscandian Ice Sheet covered the study area after a rapid deglaciation at the end of the Younger Dryas cold spell (GS-1), an event characterised regionally by the rapid drainage of the Baltic Ice Lake into the sea at 11.500 B.P. The glaciofluvial deposits are spatially distributed in a narrow time span following the GS-1 event. The study shows a rapid increase in the paleo-seismicity of the area documented in fine-grained sediment successions. The unloading effect of the ice lake drainage through a rapid removal (up to 2 years) of a volume of ca. 0.5 km3 of water must have induced the local seismogenic crustal rebound activities at the time. An intensive ice-calving event, documented in some of the studied outcrops as well as in the Baltic Sea glacial varves might be the result of this increased seismicity. Further to the north underneath the ice, an established subglacial conduit system in the collapsed, retreating thin ice sheet would have then channelised the jökulhlaup flood into the newly opened Yoldia Sea. Consequently, a large volume of coarse grained material were transported further to the ice margin incepting the subglacial Stockholm and Uppsala esker systems.