PRE-LATE WISCONSINAN EVENTS, STRATIGRAPHY, AND CHRONOLOGY IN SOUTHERN ONTARIO

Early reconstructions of the last glacial-interglacial transition of the Laurentide Ice Sheet (LIS) were based primarily on the study of shallow, near surface exposures. Recent deep drilling campaigns in southern Ontario have resulted in the acquisition of >300 continuously cored boreholes which have helped to further constrain LIS evolution over the last two glacial-interglacial cycles. The Quaternary succession is commonly floored by tills deposited during Illinoian (Marine isotope stage; MIS 6) or older glaciations. Two distinct tills are observed and they are commonly separated by glaciofluvial and/or glaciolacustrine deposits. A well-developed subaerial unconformity with soil-forming attributes such as oxidation, carbonate leaching and/or secondary carbonate deposition, and rarely, ice wedge casts, is developed in several areas of southern Ontario. The unconformity is overlain by non-glacial, organic-bearing deposits that generally record boreal conditions with rarer warm, Carolinian and cold, Arctic and sub-Arctic counterparts. Together, these record the transition from the Sangamon interglacial (MIS 5) to colder conditions with the onset of Early Wisconsinan glaciation (MIS 4). The associated sediments record LIS build-up into the Great Lakes basins and the blockage of eastward drainage to the Atlantic Ocean via the St. Lawrence River. This configuration promoted deposition of till and glaciofluvial sand in the eastern parts of the Lake Ontario basin and glaciolacustrine sediment distal to ice margins in Great Lakes basins, while soil development and non-glacial sediment deposition continued in inter-lake areas at higher elevations. Further advance of the LIS during the Middle Wisconsinan (MIS 3) is recorded by till(s) in the east and glaciolacustrine muds, sand and diamictons at increasing elevations within and surrounding the Great Lakes basins. Sediments record rising and fluctuating water levels in the Great Lakes basins resulting from blockage and opening of the Rome and other outlets. Minimum ages for the final overriding of the region by the LIS are interpreted from organic material in stratified sediments directly underlying Late Wisconsinan (MIS 2) till which are generally younger towards the southwest and range between 28.06 and 23.5 ¹⁴C ka BP.