AGE, ORIGIN AND INFILL OF REENTRANT VALLEYS ALONG THE NIAGARA ESCARPMENT IN ONTARIO, CANADA

The Niagara Escarpment is about 700 km long, extending from Niagara in the south to Manitoulin Island in Lake Huron in the north, and is the result of the differential erosion of southwest dipping late Ordovician shales and resistant Silurian sandstones and dolostones. The latter form a prominent caprock along the length of the escarpment. Large V-shaped reentrant valleys resembling enlarged gorges occur along the escarpment and record local accelerated retreat in response to water flowing over the edge of the escarpment such as occurs at Niagara Falls. This process of accelerated headward retreat acts in combination with active spring sapping and collapse of the cliffed sidewalls of the valleys where jointed dolostones rest on shales and results in a wide V-shaped valley platform. Valleys are structurally controlled and have also been widened by repeated glaciation but the relative importance of glacial erosion and non-glacial processes is unclear and subject to debate. Valleys are oriented northeast-southwest, consistent with regional joint directions in bedrock, and there is a clear association of the larger valleys, now drowned by Lake Huron, with the strike of deep seated basement structures. These structures are evident on aeromagnetic data that identify terrane boundaries in underlying Proterozoic rocks of the Grenville Province. More than 100 km of ‘chirp’ high resolution sub-bottom seismic profiling, 500 km of sidescan sonar data and 250 km of ship-based magnetic data have allowed the infill stratigraphy of two of the most prominent valleys, whose mouths have been drowned by Lake Huron, to be established (Colpoys Bay and Owen Sound). This poster will illustrate these data and comment on the relative importance of neotectonic structures in guiding glacial and fluvial erosion of such valleys.