Subglacial drilling at Black Rapids Glacier, Alaska, U.S.A.: drilling method and sample descriptions

We employed a commercial wireline drill rig to investigate the subglacial conditions of Black Rapids Glacier, a well-studied surge-type glacier in the central Alaska Range. The four main goals were: to assess the capabilities of the commercial drilling industry for sampling subglacial material, to investigate the basal morphology, to determine the subglacial geology and to emplace borehole instruments. The drilling was done in an area where seasonal and secular variations in speed are large, and where seismic studies suggested the presence of a till layer. Four holes were drilled at three locations to a maximum depth of 620 m. Three holes yielded samples of basal ice and till, although recovery of the latter was generally poor. Bedrock was sampled in one or possibly two of the holes. In the area sampled, t he glacier is underlain by a till layer some 4–7 m thick, confirming the seismic interpretation. It consists of a sandy matrix at least 20–30% of which comprises larger clasts. Limited samples of the matrix indicate that near the top of the till the porosity is 40%, and t hat some of the pore water is frozen. Geologic studies suggest that the drilling area lies to the north of the Denali Fault, a major tectonic boundary followed by the glacier, and that most of the till is locally derived with transport distances of <2 km.