A POSSIBLE SOURCE CRATER FOR THE ELTANIN IMPACT LAYER

An impact event occurred at 2.15±0.5 Ma in the Bellingshausen Sea. It littered the oceanic floor with asteroidal debris. This debris is found within the Eltanin Impact Layer. Although the impact layer was known, the crater had yet to be discovered. We have found a possible source crater at 53.7S,90.1W under 5000 meters of water. The crater is 132±5km in diameter, much larger than the previously proposed size of 24 to 80 km. We attribute the larger size to the horizontal flattening of the asteroid during its transit through the water column. We verified the existence of the crater by examining single channel seismic records for the ejecta blanket and the denudation that accompany the formation of an impact crater. We found that discontinuous layers of varying slopes and acoustic strengths characterized the ejecta blanket. Some parts of the ejecta blanket are acoustically transparent and show a total thickness of around 600 meters. Other portions of the ejecta blanket are acoustically more opaque, typically in the regions near the crater rim. In the region surrounding the area of continuous ejecta blanket, we find areas that are completely devoid of sediment cover. This denudation lasts up to 500 km away from the edge of the ejecta blanket. Unlike normal patterns of abyssal sedimentation, the areas that are most extensively denuded are in the deepest water. Areas of abyssal hills that are border on the edge of the ejecta blanket show one of two patterns of sedimentation. In some areas, the sediment cover beyond the edge of the ejecta blanket is discontinuous, with local concentrations of sediment interspersed with completely denuded areas. In other areas, the sediment cover and internal reflectors are continuous, with continuous, consistent internal layering. We propose that these sedimentation patterns are the result of the megatsunami generated as the impact produced a 5-km deep hole in the water column. In the long term, we plan to study Pliocene age cores located within the ejecta blanket and further away to further verify the existence of this crater.