Bedrock jointing and geomorphology in southwestern Ontario, Canada: an example of tectonic predesign

In southwestern Ontario, Canada, Palaeozoic sedimentary strata are covered by Pleistocene glacial sediments. The topography of the buried bedrock surface has been mapped over an area of some 45,000 km2 using new computerised depth-to-bedrock data from more than 200,000 waterwells. This information shows that arching of the underlying Proterozoic basement (Canadian Shield) and differential erosion of carbonate and shale strata are first-order, regional controls on bedrock geomorphology. These data also define a network of buried bedrock channels. Quantitative analysis of the trends of these channels reveals a statistical correspondence between channel orientation and regional bedrock joint sets formed in response to the mid-continent stress field. The orientations of bedrock channels and joints correspond regionally with mean trends at 153° and 164°, respectively. In addition, two dominant orientations of postglacial river valleys and bedrock joints correspond with mean trends at 104° vs 104° and 54° vs 52°, respectively. Our findings show that the erosional channelled topography cut on bedrock together with the postglacial drainage pattern cut into younger glacial sediments, are related to the trend of bedrock joints and consequently, can be described as ‘tectonically predesigned’. In addition, this study yields new insights into the form of the ‘preglacial’ (Tertiary?) drainage system in southern Ontario prior to Pleistocene glaciation. Regardless of the precise age of this preglacial system and degree of subsequent glacial modification, an important factor in its geomorphic development has been the mid-continent stress field.