Lake Vättern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice‐sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vättern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high‐resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vättern, are used here together with a unique 74‐m sediment record recently acquired by drill coring, and with onshore LiDAR‐based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying >120‐m deglacial sequence. Three distinct retreat–re‐advance episodes occurred in southern Lake Vättern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re‐advanced from north of Visingsö to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag‐and‐tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north‐central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a−1. The pronounced topography of the Vättern basin and its deep proglacial‐dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.

M., 2015: Regional deglaciation and postglacial lake development as reflected in a 74 m sedimentary record from Lake Vättern, southern Sweden.Abstract: The withdrawal of the Late Weichselian ice sheet and rapid isostatic uplift in southern Scandinavia led to the entrainment of large volumes of melt water within the proglacial Baltic Ice Lake (BIL). The eventual western outpost of BIL, Lake Vättern, has been a focal point for studying the dynamic retreat history of the Late Weichselian ice sheet in south central Sweden. This part of the deglacial history is described from an abundance of terrestrial studies, but, to date, no complimentary long sediment cores from Lake Vättern have been available. Here, we present the results from a unique, 74 m borehole in southern Lake Vättern that recovered a Late Pleistocene to Holocene sedimentary sequence. Physical and chemical analyses of the sediment and pore water, together with geophysical mapping, reveal glacial as well as postglacial imprints implying an oscillating ice sheet margin, evidence for neotectonic activity and one or more marine incursions into the lake during deglaciation. We attribute the glaciotectonic deformation of the sediments at 54 m below the lake floor to an ice readvance that likely occurred at the same time or before the advance that formed the Levene moraine (,. After this event, potential readvances were likely restricted to a more northerly position in the basin. We identify the final drainage of the BIL, but find evidence for an earlier marine incursion into the Vättern basin (,13.0 cal. ka BP), indicating water exchange between the North Atlantic and the Baltic Ice Lake during the late Alleröd.

Lake Vättern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vättern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake fl oor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vättern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infi llings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vät-tern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the signifi cance of glacio-isostatic unloading.

A 74-meter Late Pleistocene to Holocene sedimentary sequence was recovered from southern Lake Vättern in the autumn of 2012. At ,54 m below the lake floor, shear strength and high-resolution bulk density measurements suggest the presence of an unconformity in the varved proglacial clays. Incremental load consolidation tests reveal highly overconsolidated sediments below this level. Preconsolidation pressures for the underlying sediments are between 1250 and 2100 kPa, up to ,1700 kPa more than the current in-situ effective stress. The highly overconsolidated sediments indicate either substantial erosion (the removal of 215-360 m of sediment), or consolidation under a large grounded ice mass sitting up to 230 m above paleo-lake level. Glaciotectonic deformation in underlying sediments supports the interpretation of a grounded ice mass. It is likely that this horizon is either contemporaneous with or older than the Levene moraine, formed between 13.4 and 13.8 ka. In the ,30 m of overlying proglacial clays, there is no further evidence for grounded ice, indicating that any ice advance to southern Lake Vättern during the Younger Dryas would have been limited to an extremely thin ice tongue.

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