IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

MONTAGNAIS IMPACT CRATER

• Age(ma): 50.5 ± 0.76
• Diameter: 45km
• Depth: 2.7km
• Location: Scotian Shelf, Canada N 42° 53’ W 64° 13’
• Dating Method: K-Ar, Ar-Ar and Rb-Sr ages

Residual Bouguer gravity anomaly map of Montagnais area (RIGHT) after removing regional crustal component associated with the continental shelf edge. The image shows that the structure is marked by a gravity anomaly high of ~8 mGal with width ~11km centered on the central uplift. Contour interval = 2 mGal.

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).

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 I-94 drill project results confirmed the seismic interpretation of an impact crater. Rocks penetrated by the 1,646-m well can be subdivided into three sequences (Friedenreich 1988):

• Holocene to Eocene marine post structure sediments (540 m);
• early Eocene to Cretaceous beds (547 m) containing three basaltic melt zones. K-Ar dating of the melt zones provides an impact time of 49 - 51 Ma, which is in agreement with the early Eocene age of the overlying sediments. The lack of enrichment of the melt rocks in siderophile elements compared with basement rocks and a slight enrichment in iridium suggest that the impactor was either a stony meteorite or a cometary nucleus with an estimated diameter of 2-3km (Jansa it al. 1989), and;
• the basement of uplifted and disturbed metasedimentary Paleozoic rocks.

The Montagnais structure is located in 113 meters of water on the continental shelf 200km southeast of Nova Scotia. Shag Bay, on the south shore of Nova Scotia just west of Halifax, points approximately toward the Montagnais underwater crater, 200km in the distance.

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References

Friedenreich, O. R., Montagnais submarine meteorite impact structure, offshore Nova Scotia, Canada. Annual meeting of the American Association of Petroleum Geologists, Houston, TX, USA, 20 Mar 1988

Jansa, L. F., Pe-Piper, G., Robertson, P.B. and Freidenreich,O., Montagnais: A submarine impact structure on the Scotian shelf, eastern Canada. Geological Society of America Bulletin, v. 101, pp. 450-463. 1989.

Jansa, L. F., Pe-Piper, G., Identification of an underwater extraterrestrial impact crater. Nature, v. 327, pp. 612-614. 1987.

Pilkington, M., Jansa, L.F. and Grieve, R.A.F., Geophysical studies of the Montagnais impact crater, Canada. Meteoritical Society, pp. 446-450. 1995.

Earth Impact Database

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