Glacial and glaciofluvial deposits in the interlobate areas of the Scandinavian ice sheet

Abstract

Rapid flow within ice streams took place in a broad marginal zone of the Scandinavian Ice Sheet during deglaciation. Ice streams and interlobate zones have been characterised by analysis of glacial flow patterns, and examination of sedimentary features of interlobate glacial and glaciofluvial deposits. Reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets. Wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin. These areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment, and sections show indications of subglacial deformation. Observations from Fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation. Glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting. Substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced. A case study made in southern Finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits. Concentration of meltwater in interlobate zones was a consequence of ice-sheet configuration. Thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage. The crevassed and strain-softened ice may also have allowed surface meltwater to penetrate down to the ice/bed interface close to the ice margin. Steep ice-velocity gradients in interlobate zones would have increased subglacial melting rates, and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment.