GEOPHYSICAL SIGNATURE OF SUSPECTED AND CONFIRMED IMPACT STRUCTURES, ONTARIO, CANADA

Geophysical data are increasingly employed to identify potential impact structures and to model their subsurface structure. In this study, 2- and 3-D gravity and magnetic models were employed to investigate two suspected impact structures (Skeleton Lake, Charity Shoal) and to compare their geophysical response with the 2.4 km diameter (Late Proterozoic-Cambrian age?) Holleford impact crater. Holleford was identified in the 1960’s as a simple impact crater based the crater morphology, gravity anomaly (~2 mgal) and impact breccias recovered by drilling. Charity Shoal is a 1.2 km diameter, 30 m deep bedrock-rimmed depression located in eastern Lake Ontario. Geophysical surveys show a ring-like magnetic anomaly with a central magnetic low (> 1200 nT) and small decrease in Bouguer gravity (< 0.5 mGal) across the structure. The results of 2-D and 3-D magnetic modelling indicate either an impact structure in the Precambrian basement with an estimated crater depth of ~450 m or a volcanic source body (i.e. diatreme) with a remanent magnetization opposing the main field. The Skeleton Lake structure is a 3.6 km diameter, 65 m deep circular lake with a welI–defined magnetic anomaly low and ~3 mGal Bouguer gravity anomaly. The Holleford Crater has a ~3 mGal Bouguer gravity low but newly acquired land-based magnetic surveys show that the structure has little or no magnetic signature. The lack of a magnetic anomaly at Holleford is attributed to the low magnetic susceptibility of the Late Proterozoic target rocks. Forward modelling of the gravity data yielded a crater depth of about 550 m, indicating the structure has likely undergone significant post-impact erosion.