SEISMIC AND MAGNETIC SURVEY OF THE CHARITY SHOAL SUSPECTED IMPACT STRUCTURE, EASTERN LAKE ONTARIO, CANADA

The Charity Shoal structure (CSS) is a 1.2 km diameter, bedrock-rimmed circular depression in the lakebed of northeastern Lake Ontario. The CSS has been interpreted as a possible Ordovician-age simple impact crater based on multi-beam imaging of the lakebed but details of subsurface structure were unknown. Detailed magnetic and high-resolution chirp and boomer seismic (>400-line km) surveys were conducted across a 9-km² area to investigate the basin subsurface structure. Total field and vector magnetic surveys were conducted over the same area and forward modelling conducted to obtain depth to basement estimates.

Total magnetic intensity (TMI) data reveal a large (>1400 nT) magnetic anomaly centered over the crater basin and a ring-like magnetic high (40-50 nT) corresponding with the bedrock rim. Depth to basement below the structure was estimated at ~600 m using extended Euler deconvolution. Forward 2-D models verify that the observed TMI anomaly requires a deep (>450 m) depression in Precambrian basement or a source body (i.e. diatreme) with a remanent magnetization opposing the main field. Boomer seismic profiles revealed >60 m of stratified glacial/post-glacial sediments overlying Paleozoic limestone bedrock. Four seismostratigraphic units were identified in the Quaternary/Holocene infill sediments and the Paleozoic bedrock surface mapped in seismic data. Apparent offsets in the bedrock reflector indicate the presence of high-angle faulting of the structure rim.

Magnetic and seismic results are not consistent with origins for CSS such as a shallow glacial erosional or karst feature or an early postglacial (Younger Dryas) impact event as proposed in previous work. Modeling and geophysical results are most consistent with an origin as a volcanic structure or a deeply buried (Late Proterozoic-Ordovician?) simple impact crater.