High-resolution seismic and resistivity profiling of a buried Quaternary subglacial valley: Northern Alberta, Canada

High-resolution seismic and resistivity profiles were acquired to assist in the delineation of a buried Quaternary subglacial valley, the existence of which was first discovered by drilling. One aspect of this valley is the glacial sediments that host substantial methane at shallow depths (similar to 50 m or less). Geophysical well logs indicate that the glacial sediments in this area generally have lower compressional wave speeds V-P <1800 m/s and greater electrical resistivity r >10 Omega.m than adjacent undisturbed Cretaceous-age rocks, which have V-P >2200 m/s and r <10 Omega.m, respectively. Seismic-refraction velocities, seismic-reflection architecture, and electrical resistivity tomography images all indicate a deep (>300 m) valley. The reflection profile reveals a complex series of events consistent with heterogeneous deposition. One major reflection dips eastward across the valley. The reflection character changes abruptly on either side of this event, with dipping clinoform reflections beneath and nearly horizontal reflections above. These may be related to changes in deposition processes beneath a retreating ice sheet, as has been suggested by earlier workers. The buried valley's basal reflections are complex and possibly reflect glaciotectonic deformation or block plucking. The near-surface methane gas is within, but at the edge of, the valley. The methane gas-saturated zone displays anomalously high resistivities both in the well logs and in resistivity tomography.