3-D Subsurface Geophysical Modeling of the Charity Shoal Structure: A Probable Late Proterozoic-Early Paleozoic Simple Impact Structure in Eastern Lake Ontario

Abstract

The Charity Shoal structure is a circular, ∼1.2-km-diameter, bedrock-rimmed shoal in eastern Lake Ontario with a ∼20-m-deep central basin. The structure has been proposed as a possible Middle Ordovician impact crater or volcanic intrusion. We conducted marine seismic and magnetic surveys (9-km²) and 3-D geophysical modeling to better resolve the Charity Shoal subsurface geology and possible origins. Three models were evaluated: (a) a buried (>450 m) impact structure in Mesoproterozoic basement, (b) a maar-diatreme, (c) a cylindrical, zoned volcanic plug. Seismic profiles and multi-beam bathymetry revealed >30 m of Quaternary sediments overlying Middle Ordovician (Trenton Group) carbonate bedrock and complex, 3-dimensional folding and faulting of the structure rim. Magnetic surveys recorded an annular magnetic high (>600 nT) over the structure rim and a central magnetic low (∼500–600 nT) coincident with a ∼−1.7 mGal Bouguer gravity anomaly. The continuity of Trenton Group strata in seismic profiles rules out a previously proposed Mesozoic maar-diatreme intruded into Paleozoic strata. The zoned volcanic plug model reproduced the annular magnetic anomaly but was incompatible with Bouguer gravity profiles. The magnetic anomaly was best reproduced by a simple impact structure seated in Mesoproterozoic basement at 450–500 m depth with a rim-to-rim diameter of ∼1.2 km and rim height of ∼10–20 m. A 100-m wide and 50-m-deep channel in the Mesoproterozoic basement may record fluvial dissection of the southwestern rim. A buried (>450 m), simple impact crater is most compatible with all available geophysical data at Charity Shoal.