Did Two Bolide Fragments Strike the Gulf of Carpentaria around 1500 Years Ago?

Three widely separated piston cores from the Gulf of Carpentaria (northern Australia) contain a concentrated layer of magnetite spherules, hematitic fossiliferous rock with magnetite spherules and vitreous material of impact origin. The spherules have round and teardrop shapes, quench textures, passenger spherules, and ablation tracks. They have a bimodal size distribution with modes at 85 and 120 micrometers. The spherules formed from the melting of locally-derived hematite-rich rocks that contain quartz, barite, and fossil fragments. The vitrous fragments have irregular shapes, a highly variable composition and concoidal fracture. Some vitreous fragments have a composition that does not match that of any known volcanic glass but that is consistent with melt derived from aluminum-rich, silica-poor sediments. Estimates based on 14C dating indicate the spherule horizon was deposited between 70 and 900 C.E. There is evidence to suggest that chevron dunes on the western periphery of the Gulf (Groote Island, Van der Lin Island and Cape Arnhem), and which have been dated at 541±209 C.E. may be reworked tsunami deposits associated with the impact. Azimuths determined from the chevron dunes suggest an impact in the southeastern Gulf. Two elliptically-shaped gravity lows near Mornington Island (southeastern Gulf of Carpentaria), about 12 and 18 km in diameter, are candidate craters, and suggestive of two impacts occurring in rapid succession (e.g. an object that broke up as it approached the Earth). The bimodal size distribution of spherules is consistent with this idea. Nevertheless, further research needs to be conducted before the ejecta layer can be definitively linked to an impact or impacts in the southeastern Gulf of Carpentaria and before the two crater candidates can be considered as proven impact craters. Work is in progress on HF etching of mineral grains from within 50 km of the crater candidates and from the spherule bearing layer in the three widely separated cores. These grains will be examined with the SEM for the characteristic etching of micron thick glass layers within shock lamellae.