Glaciodynamics, Deglacial Landforms and Isostatic Uplift during the last Deglaciation of Norrbotten, Sweden

Sammanfattning: The aim of this thesis was to reconstruct the glaciodynamics, deglacial landforms, isostatic uplift and to date the deglaciation. Glaciodynamics and deglacial landforms were focused on to provide a process and depositional model for De Geer moraine and Niemisel moraine and to reveal their internal and spatial relationship, based on detailed sedimentological and structural investigations. The isostatic uplift and deglacial chronology was reconstructed from age measurements on the initial organic production in two lakes above the highest shoreline and isolation of lake basins at different altitude below the highest shoreline. The results show that the deglaciation of Norrbotten occurred earlier than previously thought, at 10 500 cal. yr BP, which implies that the deglaciation after the Younger Dryas re-advance, from the Skövde-Billingen and Salpausselkä moraines, was more rapid than previously thought. Within c. 1 000 years the ice sheet retreated c. 600 km, or in the order of 600 m/yr given an even recessional rate. This rapid ice sheet retreat is supported by the glaciodynamic conditions, i.e. the deforming bed. As the margins of the late-glacial Scandinavian ice sheet became wet-based, ice-flow velocities increased resulting in thinning of the ice. As the ice margin was subaqueous, situated in the Gulf of Bothnia, calving was enhanced promoting the rapid deglaciation. The De Geer moraine ridges was formed due to subglacial sediment advection to the ice margin during temporary halts in grounding-line retreat, forming gradually thickening sediment wedges. The proximal part of the moraines were built up in submarginal position through continuous stacked sequences of deforming bed diamictons, intercalated with glaciofluvial canal-infill sediments, whereas the distal parts were built up from the grounding line by prograding sediment gravity-flow deposits, distally interfingering with glaciolacustrine sediments. The Niemisel moraine ridges was formed due to subglacial folding/thrust stacking of pre-deposited sediments contemporaneously with lee-side cavity deposition, forming vertically and distally prograding moraine ridges transverse to ice-flow. The proximal part of the moraines were built up by subglacial folding and thrust stacking sequences of pre-ridge formation sediments, whereas the distal parts were built up by glaciofluvially and gravityflow deposited sediment in lee-side cavities.