THE BEDROCK SURFACE OF THE WESTERN LAKE-ONTARIO REGION - EVIDENCE OF REACTIVATED BASEMENT STRUCTURES

Lower Paleozoic bedrock strata, in south-central Ontario and the adjac ent part of New York State are covered by a thick (100m+) blanket of P leistocene glacial and interglacial sediments. The form of the buried bedrock surface has been reconstructed from 70,000 waterwell boreholes that extend through the entire Pleistocene cover using GIS data proce ssing techniques. The sub-drift bedrock surface shows linear channels that connect the basins of lakes Huron, Ontario and Erie and which for m part of an ancestral mid-continent Great Lake drainage system prior to modification and infilling during successive Pleistocene glaciation s. This relict drainage system is cut across Lower Paleozoic carbonate s and clastics up to 500 m thick, but the position of several channels is aligned above terrane boundaries, faults and other deep-seated and poorly understood geophysical anomalies in underlying mid-Proterozoic Grenville basement rocks. Other channels are controlled by a dominant northwest and northeast trending regional joint system. A close relat ionship among deeply seated geophysical lineaments, basement structure s and topographic lineaments cut across thick Paleozoic cover strata s uggests a history of Phanerozoic reactivation and upward propagation o f fractures from the Precambrian basement. Several basement structures and lineaments are seismically active suggesting ongoing neotectonic activity across the 'stable' craton of south-central Ontario.