GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 179-5
Presentation Time: 9:00 AM-6:30 PM


ARMOUR, Mary-Helen, BOYCE, Joseph I. and ZILKEY, David, School of Geography and Earth Sciences, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4K1, 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.
  • GSA2017mharmourcharityV4.pdf (2.9 MB)