SUBSURFACE 3-D GEOPHYSICAL MODELLING OF CHARITY SHOAL STRUCTURE, LAKE ONTARIO

The Charity Shoal Structure (CSS) is a ~1.2 km diameter circular bedrock shoal in the lakebed of eastern Lake Ontario. The structure was identified in bathymetric mapping in 1990 and interpreted as a possible Ordovician-age impact crater. Detailed lake-based seismic and magnetic surveys were conducted at Charity Shoal to better resolve the structure subsurface geology and origin. 3-D forward geophysical models were constructed to evaluate three possible origins: 1) a simple impact crater, 2) a maar-diatreme with a reversed remanence magnetization, 3) a zoned volcanic intrusive with induction magnetization only.

Seismic profiling revealed up to 35 m of Quaternary sediments overlying Middle Ordovician (Trenton Group) carbonate bedrock. Seismic data and multibeam bathymetry revealed a high-relief bedrock surface, abundant evidence for normal faulting and complex 3-dimensional folding of Ordovician strata. The Ordovician carbonate bedrock surface underlies the central basin at a depth of about 30 m below the lake bed and the bedrock rim shows a much more complex geometry than a previously proposed ring anticline structure. Magnetic total field mapping recorded a > 1200 nT anomaly across the structure with a well-defined central low and increased magnetic intensity (~600 nT) over the rim. Gravity profiles indicate a maximum -1.5 mGal Bouguer gravity anomaly over the basin centre.

3-D forward geophysical modelling of a simple impact crater determined a modelled depth to basement of 400-450 m and a crater rim diameter of ~ 1 to 1.2 km. The maar-diatreme model yielded a maximum depth of basement of ~ 500 m with a diatreme diameter of ~ 200 m. A third zoned intrusive model yielded a volcanic pipe ~1000 m in diameter with a narrower 100-200 m outer high susceptibility ( k= 0.02 SI) pipe and an inner zone with a diameter of up to 800 m (k = 0.05 SI). The geophysical results are most consistent with a simple impact in the Proterozoic basement, but a pre-Ordovician volcanic intrusive body cannot be ruled out. Complex, low-amplitude 3-dimensional folding is a regional feature of the Trenton Group and is unrelated to the deep basement structure at Charity Shoal. The folding most likely records the syndepositional deformation of carbonate platform sediments on a tectonically active (Taconic) continental margin.