252-9 Chesapeake Bay, Montagnais, and Manson Craters- Same Thing ?

Tuesday, 7 October 2008
George R. Brown Convention Center, Exhibit Hall E
Axel Wittmann, Lunar and Planetary Institute, USRA, Houston, TX, David A. Kring, Lunar and Planetary Institute, Houston, TX and Wolf U. Reimold, Museum of Natural History, Humboldt University, Berlin, Germany
The Chesapeake Bay, Manson, and Montagnais impact structures formed between Eocene and Cretaceous times in shallow 100-600 m deep waters, variably consolidated sediments with thicknesses between 1-3 km on top of crystalline basement. All three craters exhibit central uplifts with diameters between 10-12 km. The similar sizes of these central uplifts suggest that these structures resulted from similar magnitude events, although the craters collapsed to very different final diameters of 85, 36 and 45 km, respectively. Manson and Montagnais retained >50 m thick melt sheets in the vicinity of the central craters but only thin pods of impact melt occur in the central crater of Chesapeake Bay. The limited amount of impact melt in these craters may indicate encreased excavation efficiency at terrestrial impacts into shallow marine targets. However, tektite-strewn fields are only linked to the Chesapeake Bay impact structure, that is regarded as the youngest of the three. The central craters of Manson and Chesapeake Bay were buried by syn-impact resurges that deposited thick breccias and hectometer-size blocks of crystalline basement rocks, whereas the suevites that cover the central uplift of the Montagnais structure are overlain by calcareous mudstones. A comparison with the 180 km diameter Chicxulub impact structure that formed in a shallow ocean and 3-4 km thick sedimentary rocks on top of crystalline basement shows that similar to Montagnais, no deposits that indicate a catastrophic syn-impact resurge comparable to Chesapeake Bay and Manson occurred. Therefore, it appears that both Chicxulub and Montagnais were buried gradually. Further insight into shallow marine impacts could be gained from drilling the 40 km diameter Mj°lnir impact structure in the Barents Sea that has a 8 km diameter central uplift and formed in 400 m deep water and sedimentary target rocks.