The influence of tectonic deformation on facies variability in stratified debris-rich basal ice

Facies variability displayed within basal ice has hitherto been widely inte rpreted as a reflection of spatial variation in the processes of ice format ion, the basal boundary conditions, or a combination of both. Recent litera ture has consequently been characterised by attempts to derive a genetic cl assification of basal ice facies, applicable to different types of glacier. We present new observations made at the Russell Glacier (Greenland), which suggest that any such genetic classification must take account of post-for mational deformation occurring within basal ice. All the ice sub-facies occ urring within the stratified basal ice are explained in terms of two sets o f processes: the initial entrainment of a debris-rich basal layer; and subs equent flow-related tectonic deformation of that layer to produce distincti ve, tectonically-derived sub-facies. The evolution, appearance, and composi tion of some elements of the basal ice layer are therefore argued to be con trolled primarily by post-formational? flow-related deformation. Any useful genetic classification of basal ice facies must therefore include criteria able to distinguish between characteristics derived from the mechanism of ice formation, and those that are tectonically derived. (C) 2000 Elsevier S cience Ltd. All rights reserved.