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

The glacier Holmstrombreen, in Spitsbergen, surged into the ice contact sca rp of a proglacial outwash sequence at some time during its Neoglacial maxi mum. The outwash sediments were pushed along a decollement to produce a mor aine 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 mo raine is described from a continuously exposed section of the push moraine which extends from its proximal to distal extremities. The precise extent o f 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 trans mitted through a thin nappe with an aspect ratio of about 1 in 30. It is su ggested 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 l ow 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 l ongitudinal 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 m eltwater drainage system and the style of fluvial sedimentation. The struct ure, sedimentary architecture and evolution of the whole glacitectonic/fluv ial complex can best be understood by considering the impact of the surge o n a complete outwash system. It is suggested that the setting and processes which produced the Holmstrom breen push moraine could account for many broad, multi-ridge and fold push moraines which formed proglacially, and that the thickness of original sedi ments above the decollement can be a guide to the thickness of contemporary permafrost. (C) 1999 Elsevier Science Ltd. All right reserved.