CGRG Bibliography of Canadian Geomorphology
Author : Brennand, T.A.; Lian, O.B.; and Godfrey, J.
Date : 2007.
Title : Optical dating and glacial geology of Clarke bed sediments in the Lake Ontario bluffs: revealing Quaternary environmental change in the Lake Ontario basin.
Publication : CANQUA Ottawa 2007. Canadian Quaternary Association Conference, June 4-8, 2007. Carleton University, Ottawa, Ontario, Canada.
Our knowledge of Quaternary environmental change in the Lake Ontario basin has, in large part, been based on sediments exposed in the lake bluffs, excavations, and river cliffs within a thick sediment package (>100 m) overlying the Laurentian Channel in the Toronto area. 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, the Lake Ontario bluffs between Oshawa and Port Hope (> 80 km) exhibit a similar sediment package (<40 m thick). Although trace amounts of organics have been reported from one unit, this sequence has remained undated. A loose lithostratigraphic correlation has substituted for chronologic control. However, the complexity of the sedimentary sequence (rapid lateral and vertical lithologic changes, and numerous scales of unconformities) and the lithologic differences between the Oshawa-Port Hope 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 optical age estimates for the lower Clarke beds in the Oshawa-Port Hope bluffs. Our approach is based on the idea that an optical age value acquired from a sediment sample, for which the mineral grains had not all received sufficient sunlight exposure prior to burial, would fall somewhere between the true depositional age of the sediment and the average value of all the age populations inherent in the sample. The calculated optical age value would therefore be a maximum age for the sediment sample, and for the lithostratigraphic unit from which it was collected. The optical age estimate values on the lower Clarke beds range from 26 – 39 ka thus confirming deposition during MIS 3 or later, and their lithostratigraphic correlation with the lower Thorncliffe Formation in the Toronto area. 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 Oshawa-Port Hope bluffs.
Bibliography of Canadian Geomorphology