Abbott, D.H., Rodriguez, L.E. and Breger, D., 2011, December. Distal Deposits from a Cometary Impact Event. In AGU Fall Meeting Abstracts (Vol. 1, p. 1753).
The Gulf of Carpentaria contains an impact ejecta layer that is circa 1500 BP in age. We have found the following components of the layer in five cores: 1) quench textured impact spherules composed of magnetite and hematite, 2) white chlorinated hydrocarbons, 3) high-Si, low- Fe, low-K glass, 4) vesiculated quartz, 6) native Fe, 7) CrFeNiCl spherules, 8) native Fe-Ni spherules and 9) fragments of high-Mg and high-Ca pyroxene. The latter three materials are possible impactor fragments. The former are candidates for impact ejecta. Some of the magnetite spherules occur in rocks bearing hematite- and silica-replaced marine microfossils. The chlorinated hydrocarbons contain quench textured magnetite spherules and have twice as much Cl as polyvinyl chloride. A small fraction of the magnetite spherules are ablated and have elongated tektite-like shapes. These materials occur in layers with stratigraphic thicknesses between 6 and 20 cm. Each layer has a strong peak in magnetic susceptibility that coincides with the maximum concentration of impact ejecta. We use the high magnetic susceptibility of hematite to model a minimum thickness of the impact ejecta layer before it was reworked by bioturbation. We find that the impact ejecta layer was originally at least 10s to 100s of micrometers thick. These thicknesses, along with the typical 100-1000 micrometer diameters of the larger ejecta fragments are consistent with a distal source crater. Using an online modeling program (http://impact.ese.ic.ac.uk/) we have ruled out tsunami transport for the ejecta layer. This is consistent with the lack of graded bedding of the layer. The grain size distribution of the ejecta layer is most consistent with a thin layer deposited from the air and reworked by bioturbation. We can match our modeled initial layer thicknesses and ejecta sizes by a cometary impactor that broke up and produced a 12 km crater at the location of our 12 km wide Tabban crater candidate. Our five cores contain a distal impact ejecta layer; possibly derived from the Tabban crater candidate between 540 and 712 km away. Now that we have constrained the thickness and mode of deposition of the impact ejecta layer, we have the tools to date it with more precision in the near future.