GROUND PENETRATING RADAR ANALYSIS OF MORAINE FACIES DEVELOPMENT AND GLACIOTECTONIC DEFORMATION FROM 2001 TO 2003, MATANUSKA GLACIER, ALASKA

Systematic study using ground-penetrating radar (GPR) on an active moraine complex over a three-year period provides in situ images of the development of melt-out till and glaciotectonic deformation. A grid of GPR profiles was repeatedly collected on the actively developing ice-cored end moraine of Matanuska Glacier, Alaska, in July 2001, August and October 2002, and July 2003. These 4-D GPR data (3-D, through time) show the development of two different glaciological phenomena that have been hypothesized but rarely observed while forming. First, the formation of melt-out till--generated during the melting of debris-rich basal ice--is traced as a distinct georadar facies just above the buried-ice/sediment contact. The basal ice at Matanuska Glacier (roughly 25% sediment by volume) is covered by 1-5 meters of Type II debris flows generated from surrounding elevated till deposits; thus, the development of the melt-out till is not visible on the surface (and this is generally the case for similar melt-out till deposits at other glaciers). Second, an ongoing re-advance of Matanuska Glacier beginning late Spring 2002 generated folding and faulting of till deposits (“glaciotectonic deformation”) in the region where earlier GPR data was collected. Analysis of subsequent data track the deformation and reveal a mechanism of glaciotectonic deformation in which the re-advancing active ice impacts stagnated buried ice in the ice-cored moraine complex causing it to thrust forward and deform overlying sediments (i.e., stress causing the glaciotectonic deformation is mostly transmitted in the buried ice, not the sediments). The development of melt-out till and glaciotectonic deformation are difficult to observe in situ through time in 3-D using traditional field techniques (surface mapping, trenching, etc.). Ground penetrating radar data thus provide important additional information for analyzing these glaciological features.