BEDROCK LANDSLIDES AND THEIR CONTRIBUTION TO DENUDATION OF A GLACIATED BASIN IN SOUTHERN ALASKA

In mountain environments slope failure is among the most significant paraglacial processes and delivery mechanisms by which supraglacial debris is transported to glaciers. We estimate a short term denudation rate due to large scale slope failure in the form of deep-seated bedrock landslides onto the surface of a small (19 km²) alpine glacier in the Chugach-St.Elias Mountains of southern Alaska. Three large landslides with a cumulative volume of 220,000 m³ have been deposited onto the ice surface in the ablation zone, where they have remained relatively intact and are being transported down-valley as supraglacial deposits. The exposed basin walls adjacent to the glacier have steep, 70-80° slopes, 700 meters of relief, and are fractured by large slope-parallel joints. Landslide scars visible on the valley wall implicate this as the sediment source area. Freeze-thaw processes acting on the overly steepened slope is the mechanism responsible for the landslides.

The volume of material in the three landslides was calculated by combining field measurements of surface profiles, area and thickness. The sediment flux rate (volume/time) for the basin is 8005 m³ a^-1 and the basin-scale erosion rate (volume/time/area) is 0.4 mm a^-1. Previous studies have quantified the sediment contribution from basin walls to be 25-40% of the overall denudation. This would imply an overall basin-scale denudation rate of 1 to 1.6 mm a^-1. Reported denudation rates for Alaskan glaciers range from 7-100 mm a^-1. Rockwall retreat rate is 4.4 mm a^-1. This rate greatly exceeds reported values for present day retreat rates, which range from 0.05 to 3.0 mm a^-1.

Large-scale bedrock landslides such as these contribute a significant amount to the overall denudation of alpine basins and represent failure of the length of the valley wall, which is a key mechanism of peak and basin relief and lowering.