MONTAGNAIS IMPACT CRATER

by: Charles O’Dale

• Type: Central peak
• Age(ma): 50.5 ± 0.76^a – EOCENE
• Diameter: 45km
• Depth: 2.7km
• Location: Scotian Shelf, Canada N 42° 53’ W 64° 13’
• Shock Metamorphism:  Impact melt

^a Dating Method: K-Ar, Ar-Ar and Rb-Sr ages

Widespread erosion and mass failure from the ∼51 Ma Montagnais marine bolide impact off southwestern Nova Scotia, Canada

Mark E. Deptuck, D. Calvin Campbell

Abstract

The ∼51 Ma Montagnais impact crater on the outer Scotian Shelf is well known, but the potential effects from the impact event on the slope and rise seaward of the crater have, until now, remained poorly understood. Through detailed seismic stratigraphic correlation and ties to available wells, we define a threefold seismic stratigraphic subdivision for Upper Cretaceous to Eocene strata on the shelf and slope, calibrated to the most recent biostratigraphic results. Using this framework, we identify a number of depositional and erosional products that are temporally consistent with a late Ypresian impact event (within the limits of seismic and biostratigraphic resolution). We link a series of prominent failure scarps on the outer shelf and upper slope to a single widespread mass-transport deposit (MTD) on the lower continental slope, rise, and abyssal plain. Failed material amassed in a large debris field referred to here as the “Montagnais MTD”. It covers an area of ∼93 000 km² and travelled up to 580 km from the impact site where its distal termination onlaps the New England Seamounts, making it one of the largest known debris avalanches on Earth. We interpret these deposits, and the associated pattern of erosion landward of them, as products of widespread margin collapse caused by a combination of ground shaking and ensuing tsunamis triggered by the Montagnais impact event. This study provides insight into the potential effects of outer-shelf marine impact events immediately downslope from impact sites, and their diminished effects with increasing distance along the margin. (Published on December 2012)

The Montagnais structure represents the first complex impact crater recognized in the ocean (Jansa and Pe-Piper 1987). The crater located near the edge of the Scotian Shelf is nearly circular and it shows a central structural uplift over 1700m high. Surrounding the central uplift is an annular trough (figure below) of down-dropped basement and Jurassic/Cretaceous sediments extending out to a diameter of 45 km, interpreted to be the crater rim (Jansa 1989). Apart from backwash effects associated with the returning ocean after impact, the structure appears relatively well preserved.

The gravity signature of Montagnais (only a gravity high associated with the central uplift) most resembles those anomalies found over heavily eroded impact structures on land formed in sedimentary rocks (e.g., Upheaval Dome, Utah, USA). This unusual gravity signature of the Montagnais crater has important consequences for the location/detection of impact structures formed in offshore regions, particularly on continental shelves (Pilkington, 1995).