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Characterization and Significance of Shocked Quartz from the Woodleigh Impact Structure, Western Australia

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Characterization and Significance of Shocked Quartz from the Woodleigh Impact Structure, Western Australia Meteoritics & Planetary Science 38, Nr 9, 1341–1350 (2003) Abstract available online at http://meteoritics.org Characterization and significance of shocked quartz from the Woodleigh impact structure, Western Australia R. M. HOUGH, 1* M. R. LEE, 2 and A. W. R. BEVAN 1 1Department of Earth and Planetary Sciences, Western Australian Museum, Francis Street, Perth, Australia 2Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK *Corresponding author. E-mail: [email protected] (Received 16 January 2003; revision accepted 11 September 2003) Abstract–We re-examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi-ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre-Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh-1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh-1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory-etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies. INTRODUCTION called Woodleigh-1, that was drilled directly through the center of the structure (Mory et al. 2000a). Using light The Woodleigh structure (26°03 ¢19.3¢¢ S and microscopy (no U-stage), Mory et al. (2000a) reported planar 114°39¢56.3¢¢ E), located on Woodleigh Station near Shark deformation features (PDFs) in quartz and feldspars, and Bay in Western Australia (Fig. 1), has been proposed by Mory subsequently in zircons (Mory et al. 2000b; 2001). The et al. (2000a) and Uysal et al. (2001) to be a buried, 120 km in Proterozoic basement rocks encountered throughout the core diameter, multi-ring impact structure. However, this were also reported to contain extensive glass-bearing interpretation and the suggestion that the impact event may pseudotachylite veining of the S-type (Mory et al. 2000a), have been associated with an extinction event has been although these features have not been characterized in detail seriously questioned (Reimold and Koeberl 2000; Renne et al. (e.g., see Reimold and Koeberl 2000). Mafic enclaves were 2002). The buried circular feature is characterized by a also reported (Mory et al. 2001) and interpreted to have been Bouguer gravity anomaly ~60 km in diameter, and the mixed with granitoids by the impact. structure is comparable to other buried complex impact The age was originally reported to be the earliest Jurassic structures exhibiting a central feature surrounded by 2 annular and, therefore, coincides with the Jurassic-Triassic boundary positive gravity anomalies. An overall diameter of 120 km (Iasky and Mory 1999). Subsequently, workers reported that has been inferred from a subtle arcuate feature east of the apatite fission track dating pointed toward a possible obvious gravity anomaly, as an apparent incursion into the Permian-Triassic boundary age but that a lack of Triassic regional N-S trending Wandagee-Ajana ridge, and by subtle fossils in the crater fill suggested a late Triassic age (Mory et drainage patterns in the region (Mory et al. 2000a; Iasky et al. al. 2000a). Uysal et al. (2001, 2002) report a late Devonian 2001; Uysal et al. 2001). The Geological Survey of Western age (359 ± 4 Ma) from dating of authigenic, coarse fraction Australia record 2001/6 compiled by Mory et al. (2001) is pure illite clay minerals, interpreted by them to be a more circumspect, suggesting 120 km only as a possible hydrothermal product. This method and the corresponding diameter for Woodleigh. age for the impact has been questioned (Renne et al. 2002) but An impact origin for the Woodleigh structure was reiterated by Uysal et al. (2002, 2003). originally proposed following work on a cored borehole, Although the impact age is poorly constrained and many 1341 © Meteoritical Society, 2003. Printed in USA. 1342 R. M. Hough et al. microscopy (SEM), and transmission electron microscopy (TEM) to confirm the presence and nature of shock deformation features (Gratz et al. 1996; Montanari and Koeberl 2000). These are complimentary to the U-stage studies of Koeberl et al. (2001) and Reimold et al. (2003). SAMPLING AND EXPERIMENTAL TECHNIQUES W-1 had been drilled by Layton and Associates in 1981 (see Iasky et al. 2001), but no samples were preserved. The hole was subsequently re-entered and cored from a depth of 190.5 m to 333.1 m into the center of the central uplift (Mory et al. 2001). A second core, W-2A, was drilled into the first annular gravity trough. Both W-1 and W-2A drill cores are housed in the Geological Survey of Western Australia core repository in Perth. The W-1 core was laid out to enable observations of its full length and for thorough sample selection. This core solely comprises mixed crystalline basement lithologies (Fig. 2; Reimold et al. 2003 [Fig. 2]) including gneiss, micro-granite, granite and amphibolite. The complex is strongly deformed and shows evidence of granulite facies metamorphism including porphyroblasts of garnet. We sampled various intervals (Table 1) throughout the length of the core with an emphasis on identifying features that were potentially impact-related or provided potential macro-indicators of shock metamorphism. A striking feature of the core in general is that brecciation has not been so severe as to leave the core friable; it is a competent, uninterrupted Fig. 1. Location map of the Woodleigh structure in Western Australia section of core (Fig. 2), with only weathered/altered mica-rich and the main tectonic features of the region. The locations of the W- 1 and W-2A drill-holes are marked (adapted from Mory et al. [2000] zones being fragile. Samples were removed using a band saw and Iasky et al. [2001]). and further split into specimens for study while leaving the facing section back in the core. In our sample selection, we features of the Woodleigh structure remain unresolved, the noted the sampling of previous workers and attempted to discovery of even a 60 km-sized impact structure is globally collect from different horizons to ensure a greater important. The critical comments of Reimold and Koeberl understanding of Woodleigh geology (Table 1). In hand (2000) and reply by Mory et al. (2000b) highlight the fact that specimen, all lithologies show alignment of minerals, often insufficient studies have been performed to allow the use of illustrated by the biotite mica that also occurs as whisps, thus the terminology and interpretations reported in Mory et al. indicating that they have all been deformed to some degree. (2000a). The size of the structure (Mory et al. 2000a) and its The basement lithologies are typical of strongly deformed classification as multi-ringed (Mory et al. 2000a; Iasky et al. Precambrian metamorphic/igneous complexes, and the 2001; Uysal et al. 2001), combined with its age being foliation, which is present throughout the core, is inferred to postulated as synchronous with an extinction event (Mory et be the product of such regional metamorphism (e.g., Myers al. 2000a; Uysal et al. 2001, 2002, 2003), have led us to 1993). The W-2A core was also examined, and as described in follow the defined convention, as applied to K/T boundary Iasky et al. (2001), the main horizon of interest is a red- shock mineralogies, to differentiate between shock and colored conglomerate (66 m thick) containing sparse, tectonic features in the assumed target rocks. We have also rounded basement clasts (2–3 cm in size). The base of the attempted to constrain the shock facies presented by the target core is marked by a dolomitic breccia that is possibly Silurian rocks in the Woodleigh-1 (W-1) core. in age (Mory et al. 2001) and mudstone with sub-horizontal Here, we report results of a detailed study of the laminations. mineralogy and some of the structural features described from Samples (Table 1) were examined in hand specimen and the W-1 drill core samples (Mory et al. 2000a, 2000b, 2001) prepared as polished thin sections for petrographic and results from a preliminary study of the Woodleigh 2A (W- observations using conventional optical microscopy. A 2A) core (from <15 km west of W-1). Microstructures Philips XL-40 SEM operating at 30 kV, was used to observe preserved within quartz grains in W-1 were examined using some textures in the thin sections, and EDS analyses were optical microscopy, acid etching, scanning electron used for mineral identification. The SEM was also used for Characterization and significance of shocked quartz 1343 high magnification images of individual mineral grains that had been separated and etched to reveal structural characteristics following the techniques of Gratz et al. (1996). A sample of the microgranite (245.5 m) was crushed in a pestle and mortar and sieved to isolate the 250–500 micron fraction, and the white-colored mineral grains, presumably quartz and feldspar, were separated by hand-picking. Mineral etching was performed on these grains using concentrated liquid hydrofluoric acid (40%), following the method described by Gratz et al. (1996). Care was taken only to expose the grains to the acid for a maximum of 10 sec.
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