ICY DEBRIS FANS: A DYNAMIC LANDFORM IN DEGLACIATING ALPINE ENVIRONMENTS – AN EXAMPLE FROM MCCARTHY CREEK GLACIER, WRANGELL MOUNTAINS, ALASKA

Icy debris fans are poorly explored landforms dominated by mass wasting of ice and lithic material in alpine environments experiencing deglaciation. These landforms result from ice avalanches, rockfall, debris flow, and slushflow. Icy debris fans occur below ice caps or hanging glaciers along margins of valley glaciers and are important features involved in the delivery of ice and lithic material to glaciers. These landforms are common in Alaska, especially in the Wrangell-St. Elias and Chugach Mountains. Detailed studies at McCarthy Creek Glacier since 2006, including field observations, terrestrial laser scanning (TLS) and ground penetrating radar (GPR) characterize the 3-D geometry for both the surface and subsurface of three icy debris fans. In addition, time-lapse photography and ground-based mapping is documenting the nature and frequency of depositional events. Direct observations during 2013 combined with the TLS surveys estimate the geometry of individual events and fans. Ground-penetrating radar (GPR) provides subsurface data, including fan thickness, depth to underlying bedrock/glacier/talus, and internal fan architecture.

Fans exhibit different compositions in response to variations in dominant depositional processes and differ in surface morphology and subsurface architecture. Fans are characterized by lenticular reflections that are comparable in shape and size to surface ice avalanche and debris flow deposits. Such reflections become less defined at depth, consistent with the deposits becoming compacted, deformed, and transformed into ice-rich material that transitions into the glacier. Fan thickness increases down-fan and varies between fans from 20-60 m along mid-fan transects. The GPR data indicate that the average signal velocity through the fan material to be 0.155 m/ns, which is consistent with lithic material in a wet ice matrix. GPR data collected on the glacier indicate a signal velocity of 0.165 m/ns and only a few reflections. A continuous reflection at 85-110 m depth is interpreted to be bedrock at the base of the glacier.

Integrated results of the different techniques and repeated surveys in 2015 documenting fan changes over a two year period will provide a basis for estimating the annual contributions of ice and lithic material via the fans to valley glaciers.