SEDIMENTOLOGICAL INVESTIGATION OF GLACIAL LAKE ALGONQUIN DEPOSITS EXPOSED ALONG THE NOTTAWASAGA RIVER, SOUTHERN ONTARIO: IMPLICATIONS FOR CHARACTERIZING SHALLOW AQUIFERS

Surficial sediment mapping in the southern part of the County of Simcoe, Ontario, has identified a complex stratigraphy of glaciolacustrine and fluvial deposits associated with proglacial lakes that existed prior to, and following Late Wisconsin ice advance. Much of the near surface stratigraphy records deposition in Glacial Lake Algonquin, an extensive water body that developed along the margin of the retreating Laurentide Ice Sheet approximately 12.5 ka BP.

Detailed sedimentological investigation of 56 outcrop exposures along riverbanks of the Nottawasaga River in southern Simcoe County as well as 4 continuously cored boreholes drilled by the Ontario Geological Survey provide valuable data on the near surface sediment stratigraphy and allows reconstruction of the glacial and deglacial history of the region. The stratigraphic succession is floored by the Newmarket Till, a regional stratigraphic marker bed that records the Late Wisconsin ice advance from the northeast. Newmarket Till was partially submerged by an early phase of Glacial Lake Algonquin which fronted the retreating ice margin. Glaciolacustrine silt and clay rhythmites associated with this lake are unconformably overlain by a coarse-grained, fossiliferous, fluvio-deltaic unit that passes upward into finer-grained sandy to silt-rich glaciolacustrine rhythmites. This succession is interpreted to record an early high stand of Glacial Lake Algonquin (Early Algonquin) followed by a low stand that resulted in delta progradation as a new lake outlet opened at Kirkfield (Kirkfield Low). East-trending, gravelly braid plain deposits formed at the mouths of re-entrant valleys along the Niagara Escarpment to the west. Subsequent uplift of the outlet at Kirkfield resulted in the transgressive sequence (Main Algonquin) before final drainage. Large wave-cut bluffs and beaches associated with this phase rim the Nottawasaga basin and record water planes between 220 and 235m asl.

Sediments exposed along the Nottawasaga River may be used to infer the characteristics of subsurface hydrostratigraphic units that govern shallow groundwater flow within the study area. The coarse-grained fluvio-deltaic unit observed throughout the southern parts of the basin forms an important shallow aquifer, discharging groundwater directly into the Nottawasaga River.