GEOPHYSICAL STUDIES OF THE MONTAGNAIS IMPACT CRATER, CANADA

The 45-km diameter Montagnais impact structure, Nova Scotia, Canada, i s characterized by a positive, circular 8 mGal gravity anomaly associa ted with its central uplift. The negative gravity anomaly, which is ex pected for a complex crater of this size, is not observed within the s tructure, and magnetic data lack any well-defined, crater-related sign ature. The absence of a negative gravity anomaly implies that no low-d ensity zone generally related to fracturing and brecciation exists. Si nce Montagnais appears well preserved, this zone has not been removed by erosion. Its formation may have been impeded due to the lack of com petency in the target rocks. The crater was formed in a shallow marine environment where the lack of strength in the unconsolidated sediment s may have prevented the preservation of voids and fractures that caus e a negative gravity anomaly as observed over other impact craters. Ad ditionally, the efficient absorption of impact energy by unconsolidate d target material may have inhibited fracture/void development. Althou gh the gravity signature of impact craters formed on land is well know n, structures occurring in unconsolidated target material, such as con tinental shelf environments, constitute another signature that should also be recognized.