Wed 2nd August @ noon, Rm 312.222

Abstract:

The 260 m diameter Hickman Crater is a circular topographic feature in the eastern Hamersley Range, Pilbara Craton. The partly rimmed crater lies along the contact between lower Paleoproterozoic rhyolite and overlying iron formation. A meteorite impact origin was first proposed in 2007. In 2012 a diamond-cored hole was drilled into the sediment filled centre of the crater in a collaborative project between Atlas Iron Ltd and the Geological Survey of Western Australia. The hole encountered a poorly consolidated sedimentary succession of clay, sandy clay, gravel and boulders to a depth of 48.4 m, underlain by 6.7 m of rhyolite-dominated breccia to 55.1 m, with the remainder of the hole intersecting heavily fractured but in situ rhyolite to total depth of 64.7 m. The breccia unit also contains fragments of vesicular glass and partially melted rock. Geochemical analysis of the core reveals pronounced enrichment in siderophile elements, particularly Ni, Co and PGE (up to 2381 ppm Ni, 85 ppb Ir), in the breccia compared to crater basement or overlying sediments. PGE inter-element ratios in the breccia are near chondritic and are interpreted as contamination by oxidised remnants of an iron meteorite projectile, probably a so called primitive iron belonging to chemical class IVB. No fresh meteoritic material is present. Petrographic examination of the crater basement and overlying breccia reveals abundant grain fracturing, but no uniquely impact-diagnostic structures such as planar deformation features were observed. The lack of obvious shock metamorphism and unusually high meteoritic component of the breccia may be explained by significant atmospheric deceleration of a relatively small projectile prior to impact. The crater has yet to be accurately dated radiometrically, but considering its youthful appearance a late Pleistocene age is considered likely. Drilling of Hickman Crater represents the first scientific drilling of a simple impact crater of this size (<1 km) in Australia, and one of few worldwide.

Short Bio:

Peter Haines holds BSc Honours and PhD degrees from the University of Adelaide. He has held positions with the Northern Territory Geological Survey, University of South Australia, University of Tasmania, and most recently is with the Basins and Energy Geoscience group at the Geological Survey of Western Australia.