Environmental significance of bedrock jointing in Southern Ontario, Canada

Abstract

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 infills of bedrock channels form major groundwater aquifers, influence regional groundwater flows and contaminant migration to Lake Ontario, and may localize the release of thermogenic methane and radon within heavily urbanized surface environments. A quantitative comparison of the regional pattern of bedrock joints and the orientation pattern of buried bedrock channels and modern river valleys shows that all these orientation patterns are virtually coincident. Buried bedrock channels in south-central Ontario are not part of a simple antecedent drainage system but were likely “predesigned” by bedrock joint patterns that have subsequently been propagated upward into overlying Pleistocene sediments. Joints in sediments are of considerable environmental significance (for example, subsurface contaminant and gas migration in fine-grained clayey sediments) and of many origins (stress release, desiccation, etc.) but are widely assumed to be a predominantly surface-related phenomena; the existence of deeper joints has been noted by some authors but their origin is obscure. Data presented herein from south-central Ontario confirm that, in addition to surface-related joints, a second population of bedrock-related joints, reflecting the upward propagation of bedrock fractures, is present in Pleistocene sediments of south-central Ontario.