GEOMORPHOLOGY AND RADAR STRATIGRAPHY OF THE WAVE-INFLUENCED WILLIAM RIVER DELTA, LAKE ATHABASCA, CANADA

William River Delta, located in northern Saskatchewan, is a Holocene, 6-km-long (proximal-to-distal), wave-influenced, lobate-shaped sand wedge, prograding into Lake Athabasca from the south shore. Geomorphology is dominated by multiple (34) arcuate sets of eolian dune-capped beach ridges separated by peatlands. A sandy braided river system (300 wide, 0.71 m/km slope) bisects the delta with a major and minor distributary feeding mouth bar fans at the delta front. A wide (1 km) offshore zone is formed of sand offshore bars with water depths of 0.5 to 4.0 m. Further offshore a 1-3° foreset slope plunges to 14 m deep. The sand-dominated delta progrades over a clay -silt mud base, which causes occasional diapenic mud dikes to protrude into the distal offshore sand. The delta morphology is shaped by periodic strong winds, waves and long shore currents coupled with deep lake water and long fetches, up to 50-100 m by 100 km, respectively. The William Delta, although smaller in size, is remarkably similar to other studied wave dominated deltas. Radar stratigraphic profiles to depths of 15 m along depositional dip indicate periodic episodes of severe storm activity. Notably, stratigraphy between 5 and 14 m shows large-scale (9 m thick by 250 m long) lakeward included lenses of sand, bounded by prominent continuous reflections. Nested linearly within a larger lens are often 3 to 6 smaller-scale lenses (6 m thick by 30 m long). Large- and small-scale lensoid radar structures are interpreted as having been deposited and eroded by severe storm events. Internal steeper reflections within smaller lenses are caused by progradational pulses possibly during storms. The upper boundary, a less steep but prominent continuous reflection, may represent the storm climax that eroded a gentle lakeward-sloping surface into the underlying steeper rapidly prograded strata.