Abbott, D.H., Tester, E.W., Meyers, C.A., Breger, D. and Chivas, A.R., 2007, December. Sediment transport, mixing, and erosion by an impact generated tsunami: Gulf of Carpentaria, Australia. In AGU Fall Meeting Abstracts (Vol. 1, p. 07).
The Gulf of Carpentaria contains two impact crater candidates, the 18 km Tabban and 12 km Kanmare craters. We have identified an impact ejecta layer in cores from the Gulf of Carpentaria containing probable shocked quartz, magnetic iron oxide impact spherules with a bimodal size distribution, vitreous Ca phosphate with a few percent Na2O and MgO(whitlockite?), lithified glauconite microfossil casts with partial calcite rims, and other impact ejecta. The quartz grains have at least 3 different orientations of closely spaced linear fractures only a few micrometers apart (probable planar deformation features). As we have imaged these planar features using a scanning electron microscope, the shocked nature of the grains must be confirmed by measuring crystallographic directions on conventional thin sections. We found impact ejecta at the deepest depth of our sampling in six different cores (Table 1). The ejecta layer extends up into the top 2 cm of every core. However, the thickness of the layer in cm (Obs) is much greater than that predicted by simple air fall models(1) (Calc) of ejecta thickness as a function of kilometers from the nearest crater candidate (Dist). The concentration of impact ejecta is much too high to explain the layer thicknesses by bioturbation of a formerly thin layer of ejecta. Thus, we interpret these layers as a megatsunami deposit from the impact event that formed Tabban and Kanmare craters. As supporting evidence, core MD31 (also from the Gulf of Carpentaria) has 14C ages in the top 70 cm that do not increase uniformly with increasing depth, but instead fluctuate in a random manner(2). Although the dominant ostracod assemblage is marine, MD32 has a large percentage of reworked lacustrine fossils and broken shell in the top 38 cm(3). Many Holocene marine sequences from the deepest part of the Gulf of Carpentaria contain reworked lacustrine fossils(4). Because the Gulf of Carpentaria was a lake until around 10,400 yr B.P., the mixture of fossil types is suggestive of erosion and redeposition by a tsunami or other means.
Table 1. Impact Ejecta in Deep Sea Cores Latitude Long. Core Calc Obs Dist -16.850 139.885 VC01 232 >245 31 -16.501 139.890 VC23 052 >69 69 -15.659 138.010 BC48 7.4 >20 151 -12.313 138.979 MD32 0.3 >37 471 -10.789 138.719 MD29 0.1 >32 641 -9.8395 135.348 GC04 0.1 >61 846 References. 1. G. S. Collins, H. J. Melosh, R. A. Marcus, Meteoritics and Planetary Science (2004). 2. J. M. Reeves, A. R. Chivas, A. Garcia, P. D. Deckker, Palaeogeography, Palaeoclimatology, Palaeoecology 246, 163 (2007). 3. J. M. Reeves, A. R. Chivas, S. Holt, M. J. J. Couapel, B. G. Jones, Quaternary International , (in press). 4. P. De Deckker, Palaeogeography, Palaeoclimatology, Palaeoecology 62, 463 (1988).