Subsurface 3-D geophysical modelling of Skeleton Lake structure: a suspected impact crater in Muskoka, Ontario

Skeleton Lake is a suspected impact structure in central Ontario (Muskoka) Canada. The 3.5 km diameter lake has a roughly circular shoreline and a central deep basin (~ 65 m). Previous geophysical surveys conducted in the 1970's identified broad gravity and magnetic lows over the lake basin. Autochthonous breccias were also identified within Late Proterozoic basement rocks on the northwest shore but petrographic analysis did not reveal shock polymorphs or other indicators of impact origin. In 2017, bathymetric and lake-based total magnetic intensity (TMI) surveys (140 line-km) were conducted to investigate the lakebed morphology and subsurface geology of Skeleton Lake. Magnetic susceptibility measurements were also conducted on breccia samples and on Proterozoic bedrock outcrops to assist in magnetic forward modelling. 3-D geophysical models were constructed to test possible origins as a simple impact structure and a volcanic intrusive body. The TMI survey results revealed a roughly circular (~700 nT) magnetic low over the central basin, which clearly truncates high-amplitude northwest-southeast regional magnetic fabric in the Late Proterozoic basement migmatites. In the basin centre, low amplitude northwest-trending magnetic lineaments in the magnetic first vertical derivative indicate attenuated signals of a linear shear zones that outcrop on the lake shoreline. Forward modelling of a simple impact crater yielded an estimated structure depth of ~700 m and depth to structure diameter which indicates significant erosion of the structure rim. The volcanic diatreme model had a much poorer fit with the observed TMI and required multiple volcanic necks. New geophysical results indicate that Skeleton Lake is a roughly circular structure that truncates but does not completely attenuate the Proterozoic basement magnetic trends. The anomaly pattern is compatible with an origin as a simple parabolic crater which has excavated the basement to > 700 m depth and partially demagnetized the target rocks.