Lindén, M. and Mö ller, P. 2005. Marginal formation of De Geer moraines and their implications to the dynamics of grounding-line recession.ABSTRACT: De Geer moraine ridges occur in abundance in the coastal zone of northern Sweden, preferentially in areas with proglacial water depths in excess of 150 m at deglaciation. From detailed sedimentological and structural investigations in machine-dug trenches across De Geer ridges it is concluded that the moraines 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 as 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 rapid grounding-line retreat (ca. 400 m yr À1 ) was driven by rapid calving, in turn enhanced by fast iceflow and marginal thinning of ice due to deforming bed conditions. The spatial distribution of the moraine ridges indicates stepwise retreat of the grounding line. It is suggested that this is due to slab and flake calving of the ice cliff above the waterline, forming a gradually widening subaqueous ice ledge which eventually breaks off to a new grounding line, followed by regained sediment delivery and ridge build-up.

Transverse‐to‐iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine‐dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee‐side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre‐existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity‐flow deposition in lee‐side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice‐marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up‐glacier compared to a less mobile bed down‐glacier. It is considered that the lee‐side cavities were formed as a result of ice‐bed separation on the distal slope of the thrust/fold‐created obstruction. The lee‐side cavities formed an integral part of a subglacial linked‐cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down‐flow at the receding ice‐sheet margin.

The coastal zone of Norrbotten, northern Sweden, was gradually inundated by the Ancylus Lake following the retreating ice margin and forming a highest coastline approximately 210 m above the present sea level. The succeeding shore displacement is reconstructed based on lithological investigations and radiocarbon datings of identified isolation sequences from 12 cored lake basins. The highest lake basins, along with two basins above the highest shoreline, suggest ice‐free conditions already at 10 500cal. yr BP This is at least 500 years earlier than previously thought and implies rapid ice‐sheet break‐up in the Gulf of Bothnia. The shore displacement (RSL) curve represents a forced regression of successively decreasing rate through the Holocene, from 9m/100yr to 0.8 m/100yr. During the first 1000–1200 years, the isostatic uplift is exponentially declining, followed by a constant uplift rate from c. 9500 cal. yr BP to 5500–5000 cal. yr BP. The last 5000 years seem to be characterized by a low but constant rebound rate. The development of the Ancylus Lake stage of the Baltic may also be discerned in the Norrbotten RSL curve, suggesting that the chronology of the Ancylus Lake stages may have to be revised. The Littorina transgression is also reflected by the RSL curve shape. In addition, a series of early to mid‐Holocene beach terraces were OSL‐dated to allow for comparison with the 14C‐dated shore displacement curve. Interpretations of these ages and their relation to former sea levels were clearly more problematic than the dating of the lake basin isolations.

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