Large-scale Bedrock Displacement by Cirque Glaciers

Abstract

Three moraine complexes containing large rafts of fractured-bedrock formed by small cirque glaciers occur in Reindalen, Svalbard. Glaciotectonic landforms cored by rock rafts have been described from mid-latitude ice sheets of Pleistocene age in the past, but we believe this to be the first time large-scale bedrock rafts have been reported from moraines formed by small cirque glaciers. These moraine complexes consist of an imbricate stack of thrust slabs composed of fractured-bedrock and diamicton. Although the exact mechanism are unclear, we suggest that these moraines form by the displacement of subhorizontal strata within the cirque floor to form an initial stack of rock-cored thrust slabs, which subsequently confine the cirque glacier. Subsequent glacier advances, against this “barrier,” establishes a strong compressive regime within the glacier snout, which leads to erosion and moraine development. Estimates of the thickness of the rock rafts are used to derive erosion rates, which are an order of magnitude greater than those previously reported for arctic regions.