Progressive proglacial to subglacial deformation and syntectonic sedimentation at the margins of the Mid-Pleistocene British Ice Sheet : evidence from north Norfolk, UK

Detailed analysis and interpretation of the stratigraphy and structures developed within polydeformed sediments exposed in the coastal section between West Runton and Sheringham (eastern England) has allowed the characterisation of the glacitectonic signature associated with the advance of a major Mid-Pleistocene (Anglian) ice sheet. The sequence of pre-glacial deposits and glacial sediments laid down during earlier southerly directed ice advances (D1–D2) is deformed and disrupted by an ice advance from the west/southwest that marks a major change in ice flow dynamics in northern East Anglia. The simplest interpretation of this deformation event (D3) is in terms of a progressive proglacial to subglacial deformation model. Proglacial deformation occurred in advance of the ice margin and was dominated by thrusting. Positive topographic features which developed above the propagating thrusts controlled sediment dispersal patterns within outwash sandur, leading to the formation of small sub-basins between these morainic ridges. The accretion of the wedge-shaped, proglacial thrust moraines to the main push moraine formed at the margin of the advancing glacier may have temporarily stalled forward motion of the ice. Thrusting and large-scale folding associated with ice-marginal deformation led to the stacking of material excavated from further up-ice onto the stoss-side of these accreted thrust-related moraines that allowed the glacier to override the obstruction. Subglacial deformation was highly variable in its style and intensity ranging from heterogeneous folding and thrusting, through to more pervasive ductile shearing associated with the formation of a subglacial shear zone. The thickness and complexity of this shear zone increases up-ice, where it is characterised by a thick glacitectonic me´ lange. The pore water content of the deforming sediments controlled the pattern of deformation within the shear zone. However, an increase in the efficiency of the drainage system towards the front of the glacier would have led to a rapid thinning of this water-enhanced zone of ductile shear. Evidence from north Norfolk suggests that zones of preferential pore water flow within the glacier bed are a major controlling factor on the location of the subglacial shear zone. Consequently, the bulk of the forward motion of the glacier may not be accommodated by shearing within the sediments immediately below its base, but occur at a deeper level within the deforming bed.