SUBSTRATE DEFORMATION ATTRIBUTED TO OVERRIDING ICE, BERING GLACIER, ALASKA

Eastern foreland stratigraphy across which Bering Glacier has repeatedly surged in historic time is dominated by sand and gravel outwash containing only four till units. Although glacial deposits are sparse, a surface diamicton and overridden terrain show evidence of deformation. Field study and microstructural analysis indicate that overriding ice caused several different forms of subglacial deformation. Finer sediment, in which resistance to stress diminishes as pore water pressure increases, contains disturbed bedding, distorted contacts, and shears that offset and deform buried fossil trees. Microstructural analysis of diamicton and sand units show evidence of mobilization with a directional component parallel to ice movement. Depth of deformation varies with sediment type and grain size. Deformation of diamicton at the ice interface may penetrate just a few centimeters. In contrast, basal silt and clay in sand sheets interstratified with outwash sand and gravel are deformed at depths reaching 7.9 to 9.0 m beneath overlying till. Thin sections of units overridden during historic surges show readily recognizable microstructural deformation, such as fabric, partings, foliation, folds and offsets. Less conspicuous forms are rotated grains, boudinage, galaxy structure, strain shadows, and silt wisps. Collectively, these suggest ductile and brittle regimes. Deformation attributed to overriding ice is common on the Bering foreland. Although the structures within deformed substrate are clearly linked to overriding ice, they do not definitively distinguish surging flow from normal advance.