Geophysical anomalies and quartz microstructures, Eastern Warburton Basin, North-east South Australia: Tectonic or impact shock metamorphic origin?
Introduction
Geophysical and drilling exploration of the Australian continent and off shore continental shelves have led to the discovery of large circular structures some of which contain evidence of shock metamorphism, including Woodleigh (120 km-diameter; ~ 359 ± 4 Ma; Glikson et al., 2005a, Glikson et al., 2005b, Iasky et al., 2001, Mory et al., 2000, Reimold et al., 2003, Uysal et al., 2001, Uysal et al., 2002) and Talundilly (90 km-diameter; ~ 112–115 Ma; Gorter and Glikson, 2012, Longley, 1989). Possible impact structures include Gnargoo (D = 75 km; Iasky and Glikson, 2005) and Mount Ashmore (> 50 km-diameter; end-Eocene; Glikson et al., 2010). Here we consider the significance of magnetic, gravity and seismic tomography anomalies associated with the Eastern Warburton Basin, buried under the south-western Cooper Basin (Fig. 1), including the identification of planar and sub-planar microstructures in quartz grains in late Carboniferous granites and early Palaeozoic sediments and volcanic rocks in this terrain.
The Warburton Basin extends over an area ~ 400,000 km2 in north-eastern South Australia (Fig. 1). It consists of a > 4.5 km thick early Cambrian to early mid-Ordovician sequence, comprising a basal suite of felsic volcanic rocks overlain by late Cambrian carbonates and Ordovician pelagic to shelf clastic sediments (Gatehouse et al., 1995, Gravestock and Gatehouse, 1995, Radke, 2009, Roberts et al., 1990, Sun, 1997, Sun, 1998, Sun et al., 1994). Devonian sediments are mostly missing except in the north-west. The Eastern Warburton Basin underwent deformation in the late Devonian (part of the Alice Springs Orogeny) as well as deformation associated with intrusion of mid to late Carboniferous granitoids (Big Lake Granite Suite; 323 ± 5 Ma to 298 ± 4 Ma; Gatehouse et al., 1995). Emplacement of the granites was followed by rapid uplift by 4–5 km at ~ 298–295 Ma (Gravestock and Jensen-Schmidt, 1998), leading to deep erosion associated with end-Carboniferous glaciation, which accounts for a major unconformity and a lacuna of missing mid-Ordovician to upper Carboniferous strata (Veevers, 2009). The Cooper Basin above the Eastern Warburton Basin comprises a ~ 2 km-thick sequence of late Carboniferous and Permian glacial deposits (Merimelia and Tirrawarra formations) deformed along NE–SW tectonic ridges (Gidgealpa–Merrimelia–Yanapurra, Innamincka, Nappacoongee, Murteree, Dunoon ridges) separated by paleo-depressions (Patchawarra, Nappamerri, Allunga and Tenappera troughs) (Fig. 2). The Cooper Basin, which hosts major geothermal, oil and gas deposits (Chopra, 2003, PIRSA, 2010a, PIRSA, 2010b, PIRSA, 2010c, PIRSA, 2010d, Wyborn et al., 2004), is overlain by a 3 km-thick Mesozoic sequence of the Eromanga Basin.
Planar and sub-planar elements in quartz (Qz/PE)1 grains in granites, sediments and volcanic rocks, identified in drill core samples (Appendix I), extend over an area ~ 220 × 195 km-large (between Walkillie-1 in the north, Cutapirrie-1 in the south, Kalladeina-1, and Jennet-1 in the west and Tickalara-1 in Queensland in the east) (Fig. 1, Fig. 2; Appendix I). The nature and significance of these microstructures will be considered in relation to magnetic, gravity, seismic and tomographic data from the Eastern Warburton Basin and underlying crust.
Section snippets
Paleo-structure of the Warburton Basin
Isopach maps outline the south-western part of the Cooper Basin as an elongated NE–SW depression split into at least three troughs by arcuate tectonic ridges which separate end-Carboniferous to mid-Triassic depositories of the Cooper Basin (Fig. 1, Fig. 2). These lineaments include from north to south the Birdsville Track Ridge, Gidgealpa–Merimelia–Innamincka (GMI) Ridge, Big Lake fault, Warra Ridge and the Dunoon–Murteree and Nappacoongee Ridges which separate the Cooper Basin into the
Petrology and microstructural analysis
The significance of quartz microstructures observed in the Eastern Warburton Basin and associated granites was investigated by polarising microscopy, scanning electron microscopy and transmission electron microscopy. Granitoid samples from Moomba-1, McLeod-1, Habanero-1 and Big Lake-1 wells contain penetrative lamellar sets ≤ 1–2 μm-wide spaced about ≥ 4 μm apart within sectors of quartz grains. Quartz grains in sediments and volcanic rocks from the Eastern Warburton Basin disclose similar planar
Significance of the Warburton Basin planar elements in quartz
Discriminations between Planar elements in quartz (Qz/PE), including planar features of non-genetic origin (Qz/PF), metamorphic–tectonic deformation lamellae (Qz/MDL) of suggested endogenic origin (French and Koeberl, 2010, Hamers and Drury, 2011) and planar deformation features (Qz/PDF) produced by shock metamorphism (French, 1998, French and Koeberl, 2010) are extensively discussed in the literature (Alexopoulos et al., 1988, Carter, 1965, Carter, 1968, Carter and Friedman, 1965, Fairbairn,
Hydrothermal alteration
Boucher, 1996, Boucher, 2001 analysed wireline log signatures and drill cores, reporting evidence of an altered zone up to 524 m-thick at the top of the early to mid-Palaeozoic basement underlying the Cooper Basin, interpreted alternatively in terms of weathering, hydrothermal alteration or as wireline logging anomalies. The presence of quartz microstructures within the altered granitoids and surrounding Warburton Basin sediments allows alternative views of the origin the geothermal activity,
Discussion
Major magnetic and seismic tomographic anomalies below the Eastern Warburton Basin, combined with the widespread occurrence of quartz microstructures, are amenable for alternative interpretations in terms of (1) tectonic-metamorphic origin, or (2) a potential existence of a large shock metamorphosed terrain below the Cooper Basin.
Acknowledgements
We are grateful to John Vickers and Harry Kokonnen for thin section preparation, Tony Eggleton, Peter Haines and John Fitzgerald for advice with U-stage analysis, Frank Brink and Hua Chen for help with SEM-EDS analysis, Alan Whittaker for help with image processing, Elinor Alexander, Rodney Boucher, Prame Chopra, John Gorter, Victor Gostin, Peter Haines, Peter Hough, Robert Iasky, Chris Klootwijk, Nick Lemon, Tony Meixner, Martin Norvick, Hugh O'Neill, Bruce Radke, Erdinc Saygin, John Veevers,
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2018, TectonophysicsCitation Excerpt :These structural patterns represent centripetal and upward block movements involving compression around the uplifted core or plug and inward collapse of the crater rim, evident in the Woodleigh, Gnargoo and Talundilly structures. In addition, some impact structures and probable impact structures display uplift of crystalline basement below impacted sediments, as in the Woodleigh impact structure (Iasky et al., 2001; Glikson et al., 2005a, 2005b), the Mount Ashmore probable impact structure (Glikson et al., 2010) and as magnetic highs at the centres of the Warburton impact structures (Glikson et al., 2013, 2015). However, whereas hints at an impact origin of these structures are offered by the above structural patterns, a confirmation of an impact origin can only be achieved provided shock metamorphic features are identified, including planar deformation features in minerals, shock melt textures and transformation to high density polymorphs such as coesite and stishovite.
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2016, Earth and Planetary Science LettersCitation Excerpt :In these papers the quartz microstructures were proposed to be PDFs rather than tectonic FEBs, partly because it is assumed that tectonic FEBs occur with a maximum of two sets per single grain and have a wide range of crystallographic orientations (French and Koeberl, 2010). In the case of the Warburton structure Glikson et al. (2013) also presented some TEM data to support their interpretation of shock microstructures. Microstructures that show more than two (multiple) sets of sub-planar crystallographically controlled lamellae, do not fit with either the FEB or the PDF criteria: the criteria for these microstructures assume a maximum of two sets for FEBs but true planarity for PDFs (Reimold, 1994; Reimold et al., 2014; Schmieder et al., 2015).
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2015, TectonophysicsCitation Excerpt :At the longer periods reduced wavespeeds are most likely caused by the elevated temperatures. A correlation between major low velocity tomography anomalies and the Woodleigh impact structure and Warburton East shock metamorphosed structure suggests that in these terrains deep crustal fracturing constitutes a major factor for such anomalies (Glikson et al., 2013). The total magnetic intensity (TMI) data used in the investigation of the Warburton Basin are based on an airborne magnetic survey conducted for Geoscience Australia by UTS Geophysics Pty Ltd along N–S 400 metre-spaced lines at an above-ground elevation of 60 m (Geoscience Australia, 2009).
Low-temperature intracrystalline deformation microstructures in quartz
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