THE PRE-LATE WISCONSINAN SEDIMENT RECORD OF THE SOUTHERN PART OF SIMCOE COUNTY, SOUTH-CENTRAL ONTARIO, CANADA

Recent deep drilling in support of government funded 3-D investigations for groundwater management has resulted in an improved understanding of the sediment record pre-dating the Late Wisconsinan advance of ice into the southern part of Simcoe County, an area of approximately 1500 km². Late Wisconsinan Newmarket Till, deposited by southwest-flowing Simcoe lobe ice, is underlain by a thick sequence of glaciolacustrine sediments deposited in a large basin bordered to the south and north by the advancing Ontario and Georgian Bay/Simcoe ice lobes, respectively. AMS dating constrains this glaciolacustrine event to between about 39.8 and <29 ka BP. The glaciolacustrine deposits consist primarily of rhythmically laminated silts and clays (> 1795 couplets counted) interrupted by as many as 3 cycles of sand deposition which may extend laterally in the subsurface for more than 10 kms. The glaciolacustrine sequence, which is correlated to the Thorncliffe Formation as defined in the Scarborough bluffs in Toronto, blankets a regional unconformity surface dated at 39.8 to >54.7 ka BP. Organic-bearing alluvial and lacustrine deposits containing macrofossils and pollen characteristic of newly deglaciated landscapes are frequently intercepted in borings along this unconformity. The opening of low-level outlets in response to significant ice retreat resulted in the observed subaerial conditions at this time within the region. These deposits rest on an older fine-textured till, the upper surface of which is often weathered, and whose outer limit may extend across the southern part of the county. This Georgian Bay lobe drift sequence may be equivalent to the Sunnybrook Drift of the Lake Ontario basin and of Early Wisconsinan age. An older, stony, coarse-textured till interbedded with stratified sands, some organic-bearing, underlies this unit and rests on an incised bedrock surface. The ages of these deposits are not known. AMS dating of wood recovered from these older stratified deposits often yield ages beyond the limits of radiocarbon dating (>50 ka BP). Improved understanding of the regional distribution and character of the subsurface units will be achieved as part of the 3-D modelling process. This information will assist hydrogeologists as they strive to better understand the groundwater flow system of the region.