MICROFOSSIL MELTING BY THE EWING IMPACT

The Ewing Crater located in the abyssal equatorial Pacific struck the earth at 7-11 Ma in the early late Miocene. Cores located in the ejecta blanket of the Ewing crater contain tektites, impact spherules, and platy magnetite as well as microfossils. We first examined ocean cores to locate the possible zones associated with the Ewing Impact using visual cues such as sharp dark laminated layers. We measured the magnetic susceptibilities of the cores in close proximity to the crater site, believing that those having abnormally high susceptibilities were possibly impact layers. Samples were sieved into 150 and 63-micron fractions. Using a binocular light microscope, the samples were examined for evidence of an impact namely tektites, spherules, and platy magnetite. Particles were then examined under the scanning electron microscope. Various sponge spicule microfossils have been found that appear to be intermediate between tektites and sponges. The distal portions of these microfossil aggregates have the charactaristic tear-dropped ablated surface texture of tektites. Central zones of the particles are smooth with one half being more ablated than the other. Structures located on the less ablated sides of the fossil appear vascular where broken off. The usual sponge symmetrical or geometric shape was distorted and the individual sponge spicules bent backward. This suggests that tektites are not always formed from completely melted terrestrial substances ejected into the atmosphere under high temperatures and pressures. Partially melted siliceous and calcareous particles are also formed.