Geophysical Investigation of Charity Shoal Structure: A Suspected Impact Crater in northeastern Lake Ontario

English Abstract:

The Charity Shoal structure (CSS) is a 1.4 km diameter, 20 m deep circular basin located northeastern Lake Ontario. The CSS has been interpreted as an Ordovician-age meteorite impact based on multi-beam imaging of the lakebed but details of the crater’s subsurface structure were unknown. In 2011 detailed magnetic and sub-bottom chirp seismic (>400 line km) surveys were conducted across a 9-km²  area of Charity Shoal to investigate its subsurface structure. In 2014, a boomer seismic (1-2 kHz) survey was conducted over the same area to image the basin fill.

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 basin 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 (e.g. 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. Apparent offsets in the bedrock surface may indicate the presence of high-angle faults. Magnetic and seismic results are not consistent with formation of the CSS as a shallow glacial erosional or karst feature or an early postglacial (Younger Dryas) impact event as proposed in previous work. Modelling and geophysical results are most consistent with an origin as a diatreme or pre-Paleozoic meteorite impact in the Precambrian basement.