ONE PIECE OF THE PUZZLE: UNCOVERING THE GENESIS OF LARGE VALLEYS CUT INTO QUATERNARY SEDIMENT AND PALEOZOIC BEDROCK IN SOUTH-CENTRAL ONTARIO, CANADA
Glacial sediments spanning at least the last two glacial cycles form sedimentary successions >200 m thick in some areas of central Ontario. These sediments form broad regional uplands stretching over 200 km from the Niagara Escarpment in the west to the St. Lawrence lowlands in the east. The uplands are capped by a drumlinized regional late Wisconsin till sheet (Newmarket Till) and are separated (incised?) by a network of valleys previously interpreted as tunnel channels. Recent geological investigations in Simcoe County (in the southwest and western parts of the valley network) have begun to shed light on the morphology, sedimentary infill and possible genesis of the valleys. The integration of existing geologic and geophysical data with new information from 33 continuously-cored boreholes and 40 line-km of seismic reflection surveys helps establish the relationships between regional uplands and the geometry and architecture of sediment in-filling the valleys.
The valleys in Simcoe County were formed no earlier than 28 060 ± 230 and prior to 12 805 ± 40 14C yr BP and are the largest of all the valleys in central Ontario – up to 30 km long and 18 km wide and have eroded up to 175m down into the older sediment, and, in places, Paleozoic bedrock. They may overlie lows on older till plains and/or the bedrock surface, and commonly show progressive downward erosion down-gradient but terminate abruptly or are buried by younger moraine sediments. The valleys have drumlinized flanks (and floors?), with a complex in-fill stratigraphy, floored by either Newmarket Till or coarse sand and gravel, and are partially buried by predominantly fine-grained glaciolacustrine sediment. Valley long axes commonly lie parallel to drumlins, but the upper reaches of some valleys are perched and are orthogonal to nearby drumlins.
Multiple generations of valleys likely exist within Simcoe County. The current working hypothesis is that the valleys were created by initial deformation and scouring of the substrate by glacial ice creating an irregular basal topography. Lows on the bed were eroded further by repeated moderate- to high-magnitude discharges of meltwater in comparatively small channels at the base of the ice sheet and were enhanced by ice creep and active glacial erosion/deformation during the late Wisconsin.