ABSTRACT: Origin of Joints in the Paleozoic Rocks of South-Central Ontario
Dragan Andjelkovic and Alexander R. Cruden , Department of Geology, University of Toronto at Mississauga, Erindale Campus, Mississauga, Canada
The distributions, orientations and intensities of fractures in the Grenvillian basement and flat lying Paleozoic cover rocks of south central Ontario have been documented by a combination of field studies and interpretation of satellite and geophysical data.
The research has established three regional joint set in both, the Precambrian basement and Paleozoic cover. These are: NNE\NE, ESE\SE and ENE-trending joint sets.
The NNE to NE-trending set is parallel to the structural grain in the Precambrian basement. This relationship suggests that formation of NNE\NE fractures in the Paleozoic strata of south central Ontario was "inherited" in some form from structures in the underlying Precambrian basement. However, field exposures exhibit very few examples where fractures in Precambrian can be observed to propagate upward into overlying sedimentary rocks implying that inheritance mechanism was not basement fracture reactivation. Rather, field observations of joint patterns formed above basement highs indicate that the NNE\NE-trending joint set was formed by differential compaction above structurally controlled elongated topographic features within the Paleozoic\Precambrian unconformity surface during the Ordovician age.
ESE\SE-trending joint set was probably formed during Jurassic breakup of the Atlantic whereas ENE-trending joint set is thought to be neotectonic in age.
This work indicates that compaction over structurally controlled basement topography may be an important mechanism in formation of joints in many sedimentary basins.
Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada