Sedimentary facies and landform formation at the margins of polar glaciers

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Fitzsimons, Sean
Holding Date 11 October 2008

Although polar glaciers are often considered to be ineffective agents of erosion, the terminus area of modern polar glaciers are frequently characterised by moraines that have accumulated as debris-bearing basal ice is deposited at the ice margin. Furthermore, examination of the subsurface sediments and structures at the margins of modern polar glaciers demonstrates that proglacial areas are frequently characterised by widespread glaciotectonically deformed sediments. In this paper the sedimentary facies and landforms at modern cold-based polar glaciers are described and analysed. The approach taken to the analysis is to monitor contemporary glacial erosion and transportation processes beneath a modern ice margin and to link these directly to the sedimentology, structure and surface morphology of deposits that have accumulated in adjacent proglacial areas. The basal zone of the glaciers are characterised by the presence of blocks of frozen sediment that have been detached from the glacier bed and entrained in the basal ice. Although the blocks of sediment entrained in the ice experience negligible internal shear, they experience fracture and rotation as they are entrained. Measurements also show that sliding occurs between ice and frozen sediment, which, together with thin zones of high shear, results in small-scale erosion and entrainment of particles. Erosion and entrainment at both scales is achieved without the presence of meltwater. Beyond the glacier margin moraines consist of blocks of frozen blocks of sediment that have been deformed, transported and then stacked at the glacier edge. The patterns of deformation range from simple en echelon stacking of blocks to complex multi phase deformation that involves the formation of large tight recumbent folds, polyharmonic folds, and eyed folds together with well-developed thrust faults. Although the area beyond the belt of moraines is dominated by stream flow and aeolian processes, surface observations together with ground penetrating radar data demonstrate that the subsurface is dominated by glaciotectonic structures that have most likely formed when the glaciers were more extensive than present. Large parts of the resulting proglacial landscape are palimpsest features in which current surface processes have superficially altered the glacial landforms thereby concealing the extent and magnitude of glaciotectonic deformation.