Woodleigh, Carnarvon Basin, Western Australia: a new 120 km diameter impact structure

https://doi.org/10.1016/S0012-821X(00)00031-5Get rights and content

Abstract

The Woodleigh multi-ring structure, buried by Cretaceous and, at its centre, Lower Jurassic lacustrine sediments, east of Hamelin Pool, Carnarvon Basin, Western Australia, is identified as an impact structure, the largest discovered to date on the Australian continent. An impact origin is indicated by: a central core of uplifted granitoid basement probably less than 25 km in diameter, which displays shock-induced planar deformation features in quartz, pervasive diaplectic vitrification of feldspar and penetrative pseudotachylite veining; and an inner ring syncline containing a ∼70 m thick thermally modified diamictite overlain by ∼380 m of Lower Jurassic lacustrine deposits. An outermost diameter of 120 km, defined by gravity, magnetic and surface drainage, indicates a ring fault that sharply intersects the NS-striking regional structure. At the centre of the basement uplift shock metamorphosed granitoid was intersected at a depth of 171 m, at least 1800 m higher than the gravity-modelled level of regional basement. Pseudotachylite vein systems within the shocked granitoid are strongly enriched in Al, Ca, Mg, Ni, Co, Cr, V and S, and depleted in K and Si, suggesting chemical fractionation attendant on shock volatilisation, enrichment by an injected and volatilised meteoritic component, and potentially of sulfide mineralisation. The impact age is constrained by overlying Lower Jurassic strata, reworked Early Permian palynomorphs in the Jurassic lacustrine section, and deformed Lower Devonian and older units. A regional thermal event identified by apatite fission track at 280–250 Ma hints at a possible Permian–Triassic boundary age for the impact, although the lack of Triassic fossils in the crater fill favours a late Triassic age.

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.

References (24)

  • J.G. Spray

    A physical basis for the frictional melting of some rock-forming minerals

    Tectonophysics

    (1992)
  • W.F. McDonough et al.

    The composition of the Earth

    Chem. Geol.

    (1995)
  • R.M. Hocking, H.T. Moores and W.J.E. Van der Graaf, Geology of the Carnarvon Basin, Western Australia, Western...
  • A.J. Mory, R.S. Nicoll and J.D. Gorter, Lower Palaeozoic correlations and maturity, Carnarvon Basin, WA, in: P. Purcell...
  • R.P. Iasky, A.J. Mory and S.S. Schevchenko, A structural interpretation of the Gascoyne Platform, Southern Carnarvon...
  • K.A.R. Ghori, Silurian-Devonian petroleum source-rock potential and thermal history, Carnarvon Basin, Western...
  • B.M. French and N.M. Short, Shock Metamorphism of Natural Materials, Mono Book Corp., Baltimore, 1968, 644...
  • D. Stoffler et al.

    Shock metamorphism of quartz in nature and experiment: 1. Basic observation and theory

    Meteoritics

    (1994)
  • Y.P. Dikov et al.

    Anomalous evaporation of nonvolatile elements from eudialyte, zircon and orthite

    Geokhymiya

    (1992)
  • F. Pirajno et al.

    Shoemaker Impact Structure (formerly Teague Ring Structure), Western Australia

    The Aust. Geol.

    (1998)
  • A.A. Levinson, Introduction to Exploration Geochemistry, Applied Publishing, Calgary, 1974, 617...
  • R.A.F. Grieve et al.

    The signature of terrestrial impacts

    Aust. Geol. Surv. Organ. J.

    (1996)
  • Cited by (53)

    • Meteorite impact craters as hotspots for mineral resources and energy fuels: A global review

      2022, Energy Geoscience
      Citation Excerpt :

      The resources range from minor to precious materials like gold, diamond, amber, to minerals with elements such as Co, Cu, Ni, Pb, Pt, U and Zn, and to ilmenite, agate, bauxite, gypsum, pyrite, mercury, iron, limestone, phosphorite, coal, hydrocarbons, salt, silica, trona and groundwater (Table 1). Several terrestrial craters contain/host natural resources, of which some are quantitatively and qualitatively viable enough to be economically exploited such as the nickel deposits at Sudbury (Mory et al., 2000; Grieve, 2005; Kenkmann, 2021). Ore deposits associated with terrestrial craters fall in one of the following three categories defined by Grieve and Masaitis (1994), and Grieve (2005): (1) progenetic, (2) syngenetic, and (3) epigenetic deposit/mineralization (terms used interchangeably).

    • A new method for dating impact events – Thermal dependency on nanoscale Pb mobility in monazite shock twins

      2021, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      A further goal is to evaluate Pb mobility associated with formation of monazite deformation twins previously proposed to form in either tectonic or impact settings in grains that experienced different thermal histories. The samples studied include a highly shocked bedrock sample from the deeply eroded Vredefort impact structure in South Africa, the largest (up to ∼ 300 km) preserved structure on Earth (Gibson and Reimold, 2005); an impact melt rock from the ∼ 40 km Araguainha structure in Brazil (Dietz and French, 1973); and a shocked bedrock sample from the eroded and buried 60 to 120 km Woodleigh impact structure in Australia (Mory et al., 2000). The monazite grains from Vredefort and Araguainha were previously investigated by Erickson et al. (2017), whereas this study is the first report of shocked monazite from the Woodleigh structure.

    • Structure and origin of Australian ring and dome features with reference to the search for asteroid impact events

      2018, Tectonophysics
      Citation Excerpt :

      Extensive seismic reflection surveys and drillings for oil and gas have delineated dome structures some of which display features consistent with internal brecciation, uplift and unconformable truncation and erosion by younger formations. A type example is the Woodleigh proven impact structure in the Carnarvon Basin, Western Australia (Iasky and Mory, 1999; Mory et al., 2000; Glikson et al., 2005a, 2005b). Gnargoo, a 75 km-diameter structure identified from seismic and gravity datasets in the northernmost Carnarvon Basin, Western Australia, displays close similarities to the Woodleigh impact structure.

    • The Geochemistry of Mass Extinction

      2013, Treatise on Geochemistry: Second Edition
    View all citing articles on Scopus
    View full text