Woodleigh, Carnarvon Basin, Western Australia: a new 120 km diameter impact structure
Section snippets
Regional geological setting
We report a newly discovered basement-cored, multi-ring impact structure on the western margin of Western Australia centred at latitude 26°03′25″S, longitude 114°39′50″E, 160 km south–southeast of Carnarvon, directly east of Hamelin Pool and Shark Bay, on Woodleigh Station after which it is here named (Fig. 1). The structure is covered by flat-lying Cretaceous and Lower Jurassic strata within the Gascoyne Platform, the southernmost part of the Ordovician to Quaternary Carnarvon Basin [1], [2].
Shock metamorphic features
Cuttings from a 189 m deep well at the centre of the structure (Fig. 1), Woodleigh 1981/2 drilled in 1981 by Layton and associates, contain quartz fragments with 3–15 micron-spaced planar lamellae attributed by the company to drilling-related deformation. Our re-interpretation of these lamellae as planar deformation features (PDF) [5], [6] led to re-coring of this site (GSWA Woodleigh 1) in March 1999, as none of the original samples or thin sections had been kept. Twenty-five core samples from
Geophysical signature
The Bouguer gravity anomaly image of the Gascoyne Platform shows a central gravity peak and five annular gravity ridges and troughs (Fig. 4). The gravity data were collected on an 11 km grid by the Australian Geological Survey Organisation (AGSO, formerly Bureau of Mineral Resources or BMR), as well as 800 m apart along some tracks by exploration companies, and is now held in the AGSO national gravity database. In addition, ground gravity measurements were collected in March 1999 within 40 km
Age of the Woodleigh impact
The restriction of flat-lying, weakly indurated, Lower Jurassic lacustrine deposits of the Woodleigh Formation to the central part of the Woodleigh structure defines the younger age limit of the impact event, but only the upper part of this unit has been dated. Older age limits for the structure should be defined by the age of the youngest units affected by impact-related deformation but the paucity of core makes it difficult to determine the age of the structure this way. At present, the older
Discussion
An extra-terrestrial impact origin for Woodleigh is confirmed by the classic features of impact structures, including: an annular multi-ring structure cored by a basement uplift and rimmed by ring synclines; and by the abrupt intersection of regional structural trends by the external ring fault (Fig. 1), features diagnostic of an explosive event as distinct from tectonic processes [12], [13], [14]. Shock metamorphism is evidenced by multiple PDF in quartz, diaplectic vitrification of feldspars
Acknowledgments
A.J.M., R.P.I. and F.P. publish with the permission of the Director, Geological Survey of Western Australia. We thank R.A. Armstrong for performing the Rb–Sr isotopic analysis of biotite, S.P. Kelley for Ar–Ar and K–Ar analyses, T.P. Mernagh for the laser Raman investigation of pseudotachylites, D. Vowles and F. Brink for advice with SEM/EDS analyses, Alex Bevan and Philippe Masson for reviewing the manuscript, and Mick Clausen of Woodleigh Station for his assistance during the drilling program.
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