INSIGHT INTO QUATERNARY ENVIRONMENTAL CHANGE IN THE LAKE ONTARIO BASIN REVEALED THROUGH NEW CHRONOLOGICAL AND GLACIOGEOLOGICAL INFORMATION

Reference sections in the Toronto area have, in large part, defined our knowledge of Quaternary environmental change in the Lake Ontario basin. From bottom to top these sediments record a lake and deltaic assemblage (Don and Scarborough formations), a lake-glacial lake assemblage (Scarborough and Thorncliffe formations), a glacial assemblage (mainly Newmarket till), and a glacial lake assemblage (glacial Lake Iroquois sediments). For many years the chronology of this sequence relied on: (i) radiocarbon dating of detrital organic material, (ii) the assumption that most diamicton units recorded a major glacial advance, and (iii) the associated counting-down method of assigning climatostratigraphic intervals. From this, the sedimentary sequence was thought to span marine oxygen isotope stages (MIS) 5e-2. More recently a thermoluminescence chronology suggests that the majority of the sedimentary sequence in the Toronto area may be younger than originally thought, spanning MIS 5a-2.

East of Toronto, sediments in the Bowmanville bluffs, contain a glacial lake assemblage, the Clarke Beds, that has been lithostratigraphically correlated to the Thorncliffe Formation; the Clarke Beds have not been dated. The complexity of the sedimentary sequences in the bluffs (rapid lateral and vertical lithologic changes, and numerous scales of unconformities) and the lithologic differences between the Bowmanville and Toronto sediments, mean that nagging doubts persist about the lithostratigraphic correlation, and these doubts hinder firm interpretations of regional environmental change in the Lake Ontario basin.

Here, we report on 13 optical age estimates for the lower Clarke Beds, sampled from the Lake Ontario bluffs between Port Hope and Newcastle. Using rudimentary optical dating experiments we show that the Clark Beds must have been deposited during MIS 3 or later (optical age estimates range from 26 – 39 ka), confirming their lithostratigraphic correlation to the Thorncliffe Formation. We interpret regional environmental change in the mid-Wisconsinan in the Lake Ontario basin based on this new chronology and the glacial geology of the Clarke beds and associated sediments in the Bowmanville bluffs.