IMPACT EJECTA AND MEGATSUNAMI DEPOSITS FROM A HISTORICAL IMPACT INTO THE GULF OF CARPENTARIA

Chevrons are V-shaped sand dunes that resemble swash on a beach. The original evidence for a megatsunami origin of chevrons involves three observations: 1) chevrons sometimes form where there is no beach, 2) chevrons are often oriented in a different direction from the prevailing wind, and 3) chevrons contain particles too large to be transported by the wind (1). We have now assembled more compelling evidence for a megatsunami origin of two sets of chevrons in the Gulf of Carpentaria. The tsunami source was a double impact event. In layers from 9 to 250 cm thick, we find quench textured, magnetite impact spherules (2), vitreous CaCO3 and microfossils, vitreous hydrous calcium phosphate containing significant Mg and Na verified by microprobe analyses (whitlockite?), and candidates for shocked quartz. The Gulf of Carpentaria contains two impact crater candidates, the 18 km Kanmare crater and the 12 km Tabban crater. The thickness distributions of impact ejecta layers and the bimodal size distributions of the impact spherules are both indicative of two source craters rather than one. The strikes of the long axes of the chevrons fan over a 45-degree range. The chevrons striking towards Kanmare crater are up to 70 meters above sea level. The chevrons striking towards Tabban crater are up to 4 meters above sea level. In the western Gulf of Carpentaria, chevrons are absent from sandy shores shielded by offshore islands, and present on sandy shores lacking offshore islands. In the northwestern part of the Gulf of Carpentaria, at least two offshore islands contain boulder fields and eight islands exhibit soil removal to the east. Overall, these features imply a megatsunami origin for the chevrons. The age of the tsunami event is quite recent. By performing detailed stratigraphic work on the distribution of magnetic susceptibility and impact ejecta with depth in cores with previous C-14 age dates, we estimate the age of the Carpentaria impacts as 572±86 A.D. This is the same as the 1480±200 B.P. C-14 age of chevrons located on Groote Island (3). (1) Kelletat and Scheffers, 2003, Science Tsunami Hazards, v. 21, p. 174-188. (2) Martos et al., 2006. Abstr. With Program, GSA, p. 299. (3) Shulmeister et al., 1993, Geomorphology, v. 8, p. 47-64.