ENVIRONMENTAL SIGNIFICANCE OF BEDROCK JOINTING IN SOUTHERN ONTARIO, CANADA

The bedrock surface of many glaciated areas is obscured by thick drift deposits. In southern Ontario, Canada, the buried bedrock surface is dissected by channels, infilled with glacial deposits as much as 150 m thick, that are part of a wider mid-continent ''preglacial'' fluvial system that predates formation of the modern Great Lake basins. The in fills of bedrock channels form major groundwater aquifers, influence r egional groundwater flows and contaminant migration to Lake Ontario, a nd may localize the release of thermogenic methane and radon within he avily urbanized surface environments. A quantitative comparison of the regional pattern of bedrock joints and the orientation pattern of bur ied bedrock channels and modern river valleys shows that all these ori entation patterns are virtually coincident. Buried bedrock channels in south-central Ontario are not part of a simple antecedent drainage sy stem but were likely ''predesigned'' by bedrock joint patterns that ha ve subsequently been propagated upward into overlying Pleistocene sedi ments. Joints in sediments are of considerable environmental significa nce (for example, subsurface contaminant and gas migration in fine-gra ined clayey sediments) and of many origins (stress release, desiccatio n, etc.) but are widely assumed to be a predominantly surface-related phenomena; the existence of deeper joints has been noted by some autho rs but their origin is obscure. Data presented herein from southcentra l Ontario confirm that, in addition to surface-related joints, a secon d population of bedrock-related joints, reflecting the upward propagat ion of bedrock fractures, is present in Pleistocene sediments of south -central Ontario.