CATASTROPHIC WATER MOVEMENTS AT MARINE IMPACT EVENTS

Over geological time, more than two-thirds of the surface of the Earth has been covered by water. Consequently, most cosmic impacts have occurred within the marine realm. Having a layer of water as part of the upper target greatly influences the cratering process, as well as the final crater geology and geomorphology. At water depths greater than the projectile’s diameter, the transient cavity is concentric and a steep water-cavity wall is standing outside the rim of the seafloor crater. Development of this water cavity controls initial tsunami generation and the catastrophic resurge of seawater. Even at shallow water depths there will be development of outward-moving tsunami waves and the resurge in these instances may be prevented from entering the seafloor crater by an elevated rim.

We report here on geological observations of two marine-target craters that represent both relatively deep open sea (Lockne, Sweden), and shallow, near-shore realm (Wetumpka, Alabama). Observations at both craters are combined with laboratory experiments and numerical simulation to better visualize and understand impact-related water effects.

At Lockne (target water depth = projectile diameter), an initial, rim-wave tsunami forms from the collapse of the water cavity at the onset of resurge. Resurge deposits form the greater part of the crater infill and indicate the formation of a central water plume and subsequent “anti-resurge” from its collapse. However, the anti-resurge seems not to have been able to pass the crater rim to generate a collapse-wave tsunami, which may have greater distal consequences than the rim-wave tsunami (cf. Wünnemann et al. 2007).

At Wetumpka, however, the resurge deposits are relatively thin and fill only low-lying terrain inside and nearby the crater. These deposits have high matrix content and include materials transported to the near-shore impact site from a distant open-shelf area (i.e., glauconitic clayey sand). The seawater was expelled from a large area around the crater due to the deposition of the ejecta layer generating an outward-moving tsunami. After a relatively longer time (versus Lockne), this wave turned back to form a shallow resurge. Its passage over the exposed seafloor would have resembled the recent muddy tsunami event in northern Japan.

Ref. Wünnemann et al., 2007, MAPS 42(11), 1893-1903.