DETERMINING THE 3-D GLACIAL SEDIMENTOLOGY AND HYDROSTRATIGRAPHY OF THE SOUTHERN PART OF THE COUNTY OF SIMCOE, SOUTHERN ONTARIO

Increasing urban expansion and agricultural growth are placing growing stresses on existing groundwater reserves hosted within Quaternary sediments in southern Ontario. Preserving the quality of groundwater resources requires a detailed knowledge of the three-dimensional distribution of subsurface geologic units. In the South Simcoe region of Ontario this is made possible through integration of data from fully-cored boreholes and sediment outcrops.

Analysis of 56 outcrop exposures in cutbanks along the Nottawasaga River and 6 fully-cored boreholes within the former Lake Algonquin plain have led to the identification of six lithofacies associations (FA1–6). The stratigraphy is floored by the late Wisconsin Newmarket Till (FA1) which is locally overlain by ice-proximal debris flows (FA2). These glacial sediments are overlain by glaciolacustrine silt rhythmites (FA3) that pass upwards into deltaic sand (FA4) and channelized fluviodeltaic sand and gravel (FA5). Lying above the fluvial deposits are widespread sand and silt rhythmites (FA6), which coarsen up-section toward the ground surface. Qualitative observations of groundwater discharge through these FAs at outcrop faces has yielded important data on the internal heterogeneity of subsurface units, as well as possible preferential groundwater flow pathways through both aquifer and aquitard units within the region. The sand-rich surficial layers (FAs 4-6) within the Lake Algonquin plain form an unconfined aquifer system that permits extensive infiltration of surface water. The lower aquitard units (FAs 1 and 3) form a regionally significant hydraulic barrier for surficial water sources, directing shallow groundwater laterally into the Nottawasaga River. Coarse-grained interbeds are observed within the lower aquitard units and create layers or conduits of high hydraulic conductivity that permit transport of shallow groundwater. The potential connection of coarse-grained layers could create hydraulic windows through the aquitards, allowing surficial water sources to reach more deeply buried aquifer units. Understanding the geometry and interconnectedness of these subsurface sediments is essential for planning drinking water supply for growing urban communities in the region and for the prediction of contaminant migration pathways.