INITIATION OF THE TACONIC FORELAND BASIN, EASTERN ONTARIO: A NOT SO PASSIVE BEEKMANTOWN PLATFORM

In eastern Ontario, the base of the Taconic foreland basin succession is usually placed at the Sauk-Tippecanoe boundary; the contact between sandstones of the Middle Ordovician Rockcliffe Formation and dolostones of the Carillon Formation that form the uppermost unit of the regional Beekmantown carbonate platform. As previously reported (Dix and Molgat, 1998), evidence for weak, locally developed, basement reactivaton occurring during early stages of Rockcliffe deposition supported evidence for far-flung influence of the new pattern of foreland basin development. Eastern Ontario is underlain by the Ottawa-Bonnechere Graben (OBG), a fault system initiated in the late Precambrian. Most previous workers had assumed that reactivation of the OBG occurred only in the Mesozoic.

Recent work on outcrop and subsurface geology of the Carillon Formation reveals that timing of local tectonic effects heralding onset of foreland basin development extends further back, well within the time of Beekmantown carbonate platform sedimentation. From the subsurface, shale marker beds across eastern Ontario identify significant differential subsidence (and local reversal) that can be best explained by fault-block movements. Locally, the Carillon Formation sits on paleokarst and differentially eroded carbonates of the Beauharnois Formation; elsewhere the boundary appears conformable. Northwest of Ottawa, a 20-m wide fault zone preserves multiple stages of soft-sediment deformation within the Carillon Formation illustrating the impact of seismicity and reactivation along a regional Precambrian fault.

Combined, these pieces of evidence suggest that there was a significant change in regional crustal stress prior to and during Carillon deposition. Timing of the Carillon Formation appears to coincide with development and migration of the peripheral bulge in the Appalachians.

The current mosaic of faults in eastern Ontario reflects a greater impact of Ordovician history than previously considered. The role of the OBG, possibly like that of the Southern Oklahoma Fault Zone, may have been a tectonic "shock absorber" for the Taconic orogen; a narrow structurally weak corridor, relative to the otherwise "stable" adjacent basement, predisposed to preferentially taking up crustal stress.