Mass flow deposits in a Quaternary succession near Ottawa, Canada: diagnostic criteria for subaqueous outwash

Stratigraphic relationships indicate that late Quaternary outwash near Ottawa, Canada, was deposited close to the ice front and below wave base in the Champlain Sea and/or an earlier ice-dammed lake. Distinct sand and coarse gravel facies are present, the latter with clast long axes parallel to flow, indicating deposition from a high energy current with high clast concentration.The sand facies contain channels up to 10 m deep and 40 m wide, filled with essentially massive sand. In some cases the base and basal fill are contorted into ball-and-pillow structures, and the channels contain dish structures and scattered pebbles. Similar features occur in the channeled sands of deep sea submarine fan valleys, and are thought to indicate rapid deposition by a mass flow mechanism, with high sediment concentration and low turbulence. In the present case, deformation at the channel bases probably resulted from liquefaction due to rapid sediment loading; dewatering gave rise to dish structures at higher levels. Possible mechanisms for initiating mass flows include shock waves generated by iceberg calving, and the effects of rapid changes in water level and salinity as the Champlain Sea invaded the area.