GPR DETECTION OF MUD DIAPIRS AND PUSH-UP BENCHES AT THE BASE OF WAVE-DOMINATED DELTAS

The wave-dominated, Holocene William River delta, located along the south coast of Lake Athabasca (216m elev), northern Saskatchewan, Canada, is a lobate sand body 9 km long by 8 km wide by 15-22m thick. Periodic strong winds, currents and waves, generated in deep water (50-100m) with long fetches (50-110km) shape the delta surface morphology and determine its internal sedimentary structure. Its surface morphology is dominated by multiple (38) arcuate sets of eolian dune-capped beach ridges separated by peatlands. A wide (1km) arcuate, subaqueous shelf is covered by multiple (up to 19) offshore bars and troughs in water depths from 0.5 to 5.5m. Further offshore, a 1-3° foreset slope plunges to 17m deep. Lakeward from the toe of the foreset slope, gentle-sloped (0.0072 or 7.2m/km) bottomset beds of thin sand lenses interfinger with lacustrine silt/clay. Radar stratigraphic profiles to depths of 15-23m along depositional dip and strike indicate mound-like and bench-like structures.within the bottomsets. The steep-sided teepee-like sturctures (3m high by 4m wide) in the radar images are interpreted as mud diapirs, with vertical mud pipes or dykes (?) undetected by GPR, extending upward from the crest of the teepee. Several mud vents were observed at the top of the foreset slope. Similar structures were observed in lake cliff exposures of the late Pleistocene wave-dominated Big Creek delta on the north coast of Lake Erie, Ontario. Bench-like push-up structures up to 50m wide and 1m high were also noted in both bottom acoustic profiles and GPR profiles. These structures do not eject mud at the surface as do diapirs. We interpret both structures as having been caused by overpressuring of basal mud by rapid progradation of the sandy delta front during severe storms.