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Studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts. This results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters. In subaerial impacts, the targets are commonly hard rocks, frequently of igneous and/or metamorphic origin, whereas in submarine impacts, the targets are mostly unconsolidated or poorly lithified sediments, or sedimentary rocks, with high volumes of pore water. Such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area. Impacts in shallow-water marine (neritic) environments produced craters with low or absent rims and wide and shallow brims, as characterize by both the Montagnais (on the Scotian shelf), the Mjolnir (in the Barents Sea), 45 and 40 km in diameter, respectively, and the Chesapeake Bay (90 km in diameter). Lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity, as the water in the crater is vaporized. During this process, resurge gullies can be cut across the rim, while mass- and debris-flows, turbidites, and other gravity deposits are produced as results of tsunami and crater-wall and central high collapse, during and after the crater excavation stage. Such deposits are found both within and outside the crater structure. The only difference between gravity deposits triggered by an impact or other rare events, such as earthquakes, is the admixture of various melt particles and possible enrichments in iridium in the former. Impacts near the shelf edge may cause partial collapse of the continental margin as shown by the Montagnais and Chicxulub impacts. Some of the gravity and debris flows generated by margin collapse may be channelized, with final deposits up to several hundred meters thick, extending for hundreds of kilometers from the impact site. Other impact features such as shatter cones, tektites, spherules and Ni-spinels, shocked quartz, isotropication, and partial melts, are common to both submarine and subaerial impacts. Theoretical calculations of the destructive forces of mega-tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence. A dearth of gigantic tsunami evidence for the Chicxulub impact outside of the Gulf of Mexico, where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep North Atlantic basin should have been >1 m/s, could be the result of the mitigating effect of the bathymetry of surrounding area, causing wave diffraction and interference. Calculated maximum horizontal orbital velocity near the seafloor at the shelf for the Montagnais impact is 22 m/s for a 200-m-high wave, 5.5 m/s for a 50-m-high wave at 500 km, decreasing to 0.5 m/s at 1000 km, strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record. However, lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows. |
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