THAW MODIFICATION OF FROST-FISSURE WEDGES, RICHARDS ISLAND, PLEISTOCENE MACKENZIE DELTA, WESTERN ARCTIC CANADA

Thaw modification is the general process whereby frost-fissure wedges are modified during thaw, and by which frost-fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice -wedge pseudomorphs, composite-wedge pseudomorphs and deformed sand we dges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collaps e, subsidence, refreezing, loading, buoyancy, spreading, folding and s hearing. Thaw modification is believed to result in selective preserva tion of pseudomorphs and wedges. Sand wedges are more likely to be pre served than are ice-wedge pseudomorphs or composite-wedge pseudomorphs , because only those sand wedges that penetrate massive ice or icy sed iments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sens itive), their pseudomorphs appear to be selectively preserved in ice-p oor, coarse-grained sediments (thaw-stable).