Ingmar Borgström

Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model

The evolution of ice-sheet configuration and flow pattern in Fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model, i.e. a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets. The model uses mapped flow traces and deglacial melt-water landforms, as well as relative chronologies derived from cross-cutting striae and till lineations, as input data. Flow-trace systems were classified into four types: (i) time-transgressive wet-bed deglacial fans, (ii) time-transgressive frozen-bed deglacial fans, (iii) surge fans, and (iv) synchronous non-deglacial (event) fans. Using relative chronologies and aggregation of fans into glaciologically plausible patterns, a series of ice-sheet Configurations at different time slices was erected. A chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume. Geological evidence exists for several discrete ice-sheet configurations centred over the Scandinavian mountain range during the early Weichselian. The build-up of the main Weichselian Fennoscandian ice sheet started at approximately 70 Ka, and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern Norway. This configuration had a flow pattern which is poorly reproduced by current numerical models of the Fennoscandian ice sheet. At the Last Glacial Maximum the main ice divide was located overthe Gulf of Bothnia. A major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the Scandinavian mountain chain. Widespread areas of preserved pre-late-Weichselian landscapes indicate that the ice sheet had a frozen-bed core area, which was only partly diminished in size by inward-transgressive wet-bed zones during the decay phase.

RECONSTRUCTION OF PALAEO‐ICE SHEETS: THE USE OF GEOMORPHOLOGICAL DATA

The article discusses the nature of the glacial inversion problem, which is defined as the extraction of time-slice ice-sheet flow patterns from the patchy and partly overprinted landform record present in former ice-sheet areas. A coherent inversion model for derivation of flow patterns and interior ice-sheet configuration from geomorphological data is presented. Glacial landscapes are classified according to the three criteria of internal age gradients, presence or absence of meltwater traces aligned to flow traces, and basal condition (frozen bed/thawed bed) inferred from morphology. The inversion model uses landscapes classified accordingly, spatially delineated into fans, as input data. Relative chronologies at fan intersections are used to sort fans in a relative-age stack that can be linked to stratigraphic (dating) information.

Glacial land forms indicative of a partly frozen bed

In parts of the core area of the Fennoscandian ice sheet relict periglacial surfaces occur. The boundary between periglacial and glacial landscapes is often sharp and erosional, with fluting truncating patterned ground. The periglacial surfaces are older than the last ice sheet and are interpreted to represent patches of continuous frozen-bed conditions. A specific land-form assemblage occurs at the edges of such patches. On the basis of three type localities along the eastern rim of the Scandinavian mountains, four thermal boundary land forms, characteristic of the frozen-patch environment, are defined. Stoss-side moraines and transverse till scarps, not previously described, are interpreted to have formed in detachment zones where soil frozen to the glacier overlies thawed soil. The detachment zones are located where subglacial warming raises the phase-change surface (water/ice) until it intersects the soil layer upand down-glacier from residual frozen-bed patches. The up-glacier ends of frozen-bed patches are located on topographic highs, but downglacier the location of lateral sliding boundaries is occasionally independent of topography. The identification of relict surfaces and thermal boundary forms can improve paleo-ice-sheet models by providing estimates of the extent of frozen-bed conditions.

Evidence for a relict glacial landscape in Quebec-Labrador

Abstract Two major glacial landform systems occur in Labrador. These are the radial lineation and esker swarm reflecting Late Wisconsinan decay, and the Ungava Bay lineation and esker swarm reflecting convergent northward flow. In some sectors the two landform systems give conflicting evidence regarding deglaciation pattern. We have interpreted the ice sheet dynamics in Labrador from morphological data, using a new inversion model that treats spatial patterns of deglacial meltwater landforms separately from lineation patterns. In the George River area we have found that the Ungava Bay swarm of deglacial landforms has been overprinted at a right angle by a younger regional meltwater pattern from the last deglaciation. A similar overprint also exists along the intersection line in west-central Labrador. These relations show that the previously accepted relative ages of the two landform systems (Hughes, 1964; Boulton and Clark, 1990a,b; Klassen and Thompson, 1993) have to be reversed. We interpret the Ungava Bay lineation and esker swarm to represent a 0.25 × 106km2 pre-Late Wisconsinan relict landscape, formed during the deglaciation of an older ice sheet and later preserved in a dry-based central zone of the Labrador dome.

Geomorphic Evidence for Late Glacial Ice Dynamics on Southern Baffin Island and in Outer Hudson Strait, Nunavut, Canada

We here describe glacial geomorphology that sheds light on ice-dynamic conditions during the Noble Inlet advance, a glacial event involving northward ice flow across Hudson Strait and large-magnitude meltwater drainage across Meta Incognita Peninsula at around 8.9 to 8.4 14C kyr BR Through airphoto interpretation and field inspection of key sites we mapped the glacial geomorphology of interior Meta Incognita Peninsula, the postulated terminal zone for northward expansion of ice from Quebec-Labrador during the Noble Inlet advance. A 170-km-long zone of glaciofluvial canyons, washing zones and boulder deltas was traced from Shaftesbury Inlet to Henderson Inlet. This zone reflects initial drainage across Meta Incognita Peninsula at >520 m elevation, followed by ice marginal drainage at progressively lower levels along the southern slope of the peninsula. The ice marginal outline required to explain the glaciofluvial zone is compatible with northward-trending striae previously reported from the southern coast of Meta Incognita Peninsula. A very large flux of meltwater across Meta Incognita Peninsula probably occurred because eastward supraglacial drainage on ice in Hudson Strait was temporarily impeded and steered northward by a raised ice surface level in outer Hudson Strait, induced by an enhanced outflow of ice from Ungava Bay.

The 1997 Flash Flood at Mount Fulufjället, West Central Sweden: Geomorphic and Vegetational Investigations Of Stora Göljån Valley

On 30-31 August 1997, extreme precipitation fell locally over parts of west central Sweden, causing flash floods on the eastern and southern slopes of Mount Fulufjallet. Here we report from fieldwork carried out during the first year after the event. A survey map of the Stora Goljan flash flood channel is pre-sented. The geomorphic effects are described, as well as the general status of the recolonisation of vegetation. The erosional effects of the flash floods were extensive, and included the expansion of stream channels, mass movement, and the almost complete removal of vegetation in broad strips along the water-courses. Future work is presented in a theoretical context.