EVIDENCE FOR A LATE HOLOCENE OBLIQUE IMPACT IN THE GULF OF CARPENTARIA, AUSTRALIA

Examination of crater morphology, chevron dunes, and ejecta fan deposits suggest an oblique impact angle of less than 15° arriving from the southeast into the Gulf of Carpentaria, Australia. Located to the west of Mornington Island, Kanmare crater is slightly elliptical (10km by 8km). The crater is funnel-shaped, and both the long and the short axes are asymmetrical.

Megatsunami generated from the impact formed V-shaped dunes called chevrons [1]. Over 80 chevrons have been identified on the western coast of the Gulf, the intervening islands and on one coast 900 km away. The long axes of the chevrons strike towards the crater. The longest inferred run-ups and highest elevations above sea level occur along the same trend as the long axis of the crater, approximately N20W. The chevrons display a range in run-up from 1 to 70 meters and inland extent from 0.18 km to over 9 km depending on the relation to the long axis.

Impact ejecta from the event (melted microfossils, magnetite spherules, and glass) are found in a restricted region and imply a tightly constrained fan angle of less than 50°. Melted microfossils are found in core GC04 in the middle of the ejecta fan (over 500 km from Kanmare crater) to a depth of at least 60 cm [2]. Ejecta layers on the margins of the fans thicken towards the center, most likely due to the focusing of energy from the impact.

Tabban crater is also elliptical (6 km by 5.13 km). However, the asymmetrical crater morphology contradicts what is expected for an impact from the southeast. This asymmetry may have been caused by erosion of the northwestern wall from resurge of displaced water. Classification of Tabban as a crater, though, is supported by the bimodal distribution of magnetite spherules [3], and the presence of chevron dunes on Mornington, Bentick, and Sweers Islands whose long axes strike towards Tabban.

[1] Abbott et al. 2006 [2] Meyers et al., 2007 – this volume. [3] Martos et al. 2006.