SEDIMENT-LANDFORM ASSEMBLAGES IN SOUTHERN SIMCOE COUNTY, SOUTHERN ONTARIO: APPLICATION TO THE SHALLOW GROUNDWATER REGIME

Southern Simcoe County is an area of projected urban development and has been the focus of a surficial geology and aquifer mapping program conducted by the Ontario Geological Survey.

The region is characterized by broad uplands separated by narrow, deeply incised valleys, with higher ground to the south (Oak Ridges Moraine) and west (Niagara Escarpment). Uplands are draped by silty sand diamicton interpreted as the late Wisconsin Newmarket Till. Continuously-cored boreholes and outcrops exposed in amphitheater-shaped erosional forms reveal a thick succession of glaciolacustrine sediments beneath the Newmarket Till. Fine-grained glaciolacustrine rhythmites and diamicton overlie the till, with localized sand and gravel deposits at higher elevations. The southern part of the study area is characterized by hummocky terrain dissected by deep channels. In this region, fine sands with minor silts, muddy diamicton and gravels overlie the Newmarket Till, forming part of the Oak Ridges interlobate moraine. To the west, close to the Niagara Escarpment, kame terrace deposits at 400 m asl are composed of coarse gravel and dissected by channels running north to south, recording early deglacial sedimentation. Isolated flat-topped features with upper surfaces between 290 and 300m asl lie near the mouths of northeast-trending re-entrant valleys along the escarpment. A borehole drilled through one such feature reveals a 40m coarsening-upward succession passing from silts into sands and gravels that record progradation of a delta into Glacial Lake Schomberg. Valleys are up to 20 km long, 3-7km wide and 100m deep. Sediment exposures through the valley fills reveal thick successions (up to 75m) of glaciolacustrine and fluvio-deltaic sediments, often lying directly on Newmarket Till and recording the evolution of a series of deglacial lakes.

Incision by glaciofluvial systems and extensive groundwater piping during lake level fluctuations have created hydraulic windows through near-surface aquitard units across the study area, forming potential pathways for surface water to infiltrate into underlying aquifers. Understanding the characteristics and spatial relationships between sediment-landform assemblages in the area is essential to evaluate potential groundwater contamination threats due to future urbanization.