DETAILED STRATIGRAPHY ACROSS THE IMPACT LAYER AT MASSIGNANO, ITALY

Based on the discovery of impact craters and impact ejecta recovered from marine sediment, at least two closely spaced giant impacts occurred in the late Eocene. Previous work documented the presence of an Ir anomaly, shocked quartz, and Ni-rich spinel found in the Global Stratotype Section for the Eocene-Oligocene boundary at Massignano (Italy) indicating one or more closely spaced impacts. Due to different sampling techniques and intervals, it is still questionable whether these impact evidences at Massignano document one or more impacts. We looked at samples taken at 1 cm intervals across the impact layer(s) in order to address this problem. The samples were treated overnight with 10% HCl, and then wet-sieved through a 250 mm sieve. The less than 250 mm size fraction was disaggregated using ultrasonics and wet-sieved into 125 ~ 250 mm, 63 ~ 125 mm and 45 ~ 63 mm. Diagenetically altered pancake spherules (presumably clinopyroxene-bearing spherules) in the >250 mm size fractions were recovered and counted. Grains in the 63 ~ 125 mm size fraction were mounted in index oil on petrographic slides and shocked grains (grains containing one or more sets of planar features) were identified and counted with a petrographic microscope. Our preliminary data indicate that the peak abundance of the flattened spherules and the peak abundance of the shocked grains occur within 1 cm of each other. The peak abundance of flattened (>250 mm) spherules is seven per gram and the peak abundance of shocked grains in the 63 ~ 125 mm is about five per gram of sediment. There is a well-defined mica layer at 5.58 ~ 5.59 m. Below this mica layer, flattened spherules and shocked grains are rare. Most of the shocked grains are quartz, but we have also documented shocked feldspars and shocked rock fragments. The shocked grains may help identify the target rock. Further studies are in progress.