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GPR investigations of subglacially deformed fossil tree horizons: Bering Glacier Alaska

Authors
Publisher
University of Birmingham
Publication Date
Keywords
  • L700 Human And Social Geography
Disciplines
  • Earth Science

Abstract

The eastern forefield of Bering Glacier encompasses a lake and island complex overridden by glacier surge advances in 1965-1967 and 1993-1995. Multiple "fossil" tree horizons (still in growth position) have been identified within outwash deposits in proglacial stratigraphic sections. Those at the near surface demonstrate deformation and shearing indirectly linked to the stresses applied by overriding glacier advance. This project aims to assess the suitability of ground-penetrating radar (GPR) as a tool for detecting continuous till and diamicton surfaces associated with the deformation layer, as well as to identify sheared and deformed trees. In 2006, a grid of six 100 m common offset GPR lines were collected on Bentwood Island using a PulseEKKO Pro 1100 system at a nominal frequency of 200 MHz. A maximum penetration depth of approximately 11 m within materials composed of diamicton, sand, gravel and some silts and clays, has been achieved. Two sub horizontal reflections have been identified at depth that can be traced across the grid. These are interpreted to represent the upper and lower boundaries of sand and gravel units, which are bounded by till and peat at the upper and lower contact, respectively. Between the prominent reflections, a number of stacked hyperbolae or point source reflections can be identified, indicating the presence of sheared trees, which are still upright within the substrate. GPR observations of the spatial continuity of deformation units on Bentwood Island coupled with observations on other nearby islands, suggests subglacial sediment deformation was widespread during advances of Bering Glacier.

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