Abbott, D.H., Burckle, L., Goldin, T. and Hays, J., 2001, December. Ewing Structure: A Possible Abyssal Impact Crater. In AGU Fall Meeting Abstracts (Vol. 1, p. 04).


We discovered a possible abyssal impact crater about 150 km in diameter that we call the Ewing structure. It lies between the Clarion and Clipperton fracture zones and is about the age of the late/middle Miocene boundary, a prominent mass extinction event. The Ewing structure has a weak topographic expression, but 6 cm to over 850-cm thick layers with high magnetic susceptibilities that contain impact spherules are found on all sides of the structure. There are also apparent secondary impact craters located between one and two (crater) diameters from the rim of the Ewing structure. Inside the structure, there are two topographic lows with very large magnetic anomalies, up to 380 nT, that may be due to ponded impact melt. These large magnetic anomalies are not seafloor spreading related, as the basement formed at the Eocene equator. The impact spherules from the high susceptibility layers are up to 200 microns in diameter, consistent with a source crater that is at least 55 km in diameter. The impact spherules have very high K contents and relatively modest Si contents, consistent with an origin as impact melts derived from seamount crust. We argue that the weak topographic expression of the Ewing structure is due to two factors: the excavation of a pre-existing seamount and the excavation of a hole in the water by the impactor. Underwater impact craters have resurge gullies on their rims that are produced by the return of water into the crater. The spacing between gullies is about equal to the water depth, 3.8 km in the case of the Ewing structure. The rapid movement of the resurge waters into the Ewing structure erodes large quantities of abyssal sediments, which then resettle to the bottom. These sediment deposits fill the impact crater and form distal impact turbidites. Because turbidity currents move from shallow to deep water, the thickest impact turbidite layers are in local topographic lows. This idea explains why the thickness of impact layers is mainly a function of water depth, not distance from the crater. We are searching for shocked quartz in the impact airfall deposits.