No abstract

The Charity Shoal structure (CSS) is a 1.2 km diameter, bedrock-rimmed circular depression in the lakebed of northeastern Lake Ontario (Fig. 1). The CSS has been interpreted as a possible Ordovician-age simple impact crater based on multi-beam imaging of the lakebed (Holcombe et al. 2013) but the subsurface structure was not resolved. Other possible origins include a karst sinkhole, glacially erosion (e.g. kettle hole), basement structural depression or Jurassic volcanic intrusive (Suttak, 2013). Detailed magnetic and high-resolution chirp and 1.5 kHz boomer seismic (>400-line km) surveys were conducted across a 9-km 2 area to investigate the subsurface structure. Total magnetic intensity (TMI) data (Fig. 2) reveal a large (>1400 nT) magnetic anomaly centered over the crater basin and a ring-like magnetic high (40-50 nT) corresponding with the raised bedrock rim (Fig. 3). Depth to basement below the structure was estimated at 600 m using extended Euler deconvolution (Fig. 3F).