The 45-km diameter Montagnais impact structure, Nova Scotia, Canada, is characterized by a positive, circular 8 mGal gravity anomaly associated with its central uplift. The negative gravity anomaly, which is expected for a complex crater of this size, is not observed within the structure, and magnetic data lack any well-defined, crater-related signature. The absence of a negative gravity anomaly implies that no low-density zone generally related to fracturing and brecciation exists. Since Montagnais appears well preserved, this zone has not been removed by erosion. Its formation may have been impeded due to the lack of competency in the target rocks. The crater was formed in a shallow marine environment where the lack of strength in the unconsolidated sediments may have prevented the preservation of voids and fractures that cause a negative gravity anomaly as observed over other impact craters. Additionally, the efficient absorption of impact energy by unconsolidated target material may have inhibited fracture/void development. Although the gravity signature of impact craters formed on land is well known, structures occurring in unconsolidated target material, such as continental shelf environments, constitute another signature that should also be recognized.
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