The sedimentary and structural evolution of a recent push moraine complex: Holmstrombreen, Spitsbergen

The glacier Holmstrombreen, in Spitsbergen, surged into the ice contact scarp of a proglacial outwash sequence at some time during its Neoglacial maximum. The outwash sediments were pushed along a decollement to produce a moraine in which deformation extended for 1.5 km beyond the furthest extent of the glacier front. The style of folding and faulting and the nature of the pre-, syn- and post-tectonic sedimentary sequence across the whole push moraine is described from a continuously exposed section of the push moraine which extends from its proximal to distal extremities. The precise extent of incremental compressive shortening of the pushed sediments, of some 900 m, is established. The depth to the underlying decollement is inferred to be an average of about 30 m, indicating that stresses and movement were transmitted through a thin nappe with an aspect ratio of about 1 in 30. It is suggested that this nappe was frozen and that an artesian water pressure head of 60 m immediately beneath it reduced friction along its base to a very low value. It is calculated that a glacially generated force of about 1.5 x 10(7) kN was responsible for pushing the sediment nappe. The nature of the glacially controlled groundwater flow system rather than the magnitude of longitudinal forces generated by the glacier is the principle determinant of large-scale push moraine characteristics. The changing ice topography produced by the pushing event during the surge and in the post-surge decay had a major influence on the evolution of the meltwater drainage system and the style of fluvial sedimentation. The structure, sedimentary architecture and evolution of the whole glacitectonic/fluvial complex can best be understood by considering the impact of the surge on a complete outwash system. It is suggested that the setting and processes which produced the Holmstrombreen push moraine could account for many broad, multi-ridge and fold push moraines which formed proglacially, and that the thickness of original sediments above the decollement can be a guide to the thickness of contemporary permafrost. (C) 1999 Elsevier Science Ltd. All right reserved.