TUNNEL CHANNELS OF CENTRAL SOUTHERN ONTARIO: CHARACTER, GENESIS AND GLACIODYNAMIC IMPLICATIONS

Central southern Ontario exhibits a regional Late Wisconsinan unconformity that truncates Paleozoic bedrock and a thick Quaternary sediment cover. This unconformity is composed of drumlins, s-forms and valleys. This study explores the character, genesis and glaciodynamic implications of these valleys.

Valleys are mapped using remote sensing, DEMs and field surveys. Buried valleys are explored by seismic reflection profiling, and outcrop and drillcore sedimentology. Bedrock valleys form an anabranched NE-SW system north and east of an area of sediment-walled valleys. Bedrock valley walls are ornamented by s-forms. Large, NE-SW-oriented, sediment-walled valleys (>40 km long, <7 km wide, <170 m deep) dissect a drumlinized terrain and are linked by shallower, anabrached valleys (< 20 km long, < 2 km wide, < 100 m deep). Large valleys are often incised through Quaternary sediment to bedrock and are partially sediment filled. South of the Oak Ridges Moraine, broad, shallow erosional corridors extend into Lake Ontario. Valleys have undulating thalwegs and are locally overdeepened. Both partially-filled and buried valleys are characterized by a suite of landforms, lithofacies and seismic facies including eskers, gravel sheets, dunes and macroforms, sandy hyperconcentrated flow deposits and sand-silt-clay rhythmites.

The valley network is interpreted to be a tunnel channel system produced and/or reutilized by subglacial meltwater during the Late Wisconsinan. The subglacial landsystem is attributed to erosion by a regional meltwater underburst that evolved into channelized flow focused along progressively larger channels; the bedrock channels were antecedent and reutilized. Sediment-walled channel erosion may have been enhanced by groundwater flow and piping at depth. Channel fills record rapid and voluminous sedimentation during waning jokulhlaup flow. As fan deposits are not observed at the southern ends of channels it is likely that channel formation was contemporaneous with an underburst event that eroded drumlins in Lake Ontario and swept away most sediment derived from channel erosion. Ice sheet thinning and flattening associated with underbursts facilitated deglaciation by regional downwasting and stagnation.