Earth-Science Reviews 125 (2013) 114–122

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Geophysical and structural criteria for the identification ofburied impact structures, with reference to Australia☆

Andrew Glikson a,b,⁎, I. Tonguç Uysal b

a Planetary Science Institute, Australian National University, Australiab Queensland Geothermal Energy Centre of Excellence', The University of Queensland, Australia

☆ In honor of Robert S. Dietz, 1914–1995.⁎ Corresponding author. Planetary Science Institute, A

Australia. Tel./fax: +61 2 6296 3853.E-mail addresses: Andrew.glikson@anu.edu.au (A. G

(I.T. Uysal).

0012-8252/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.earscirev.2013.07.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 May 2012Accepted 4 July 2013Available online 16 July 2013

Keywords:AustraliaAsteroidImpactBuriedPlanar deformation features

The discovery of large asteroid impact structures, likely and possible impact structures, onshore and offshore theAustralian continent (Woodleigh [120 km; ~360 Ma], Gnargoo [75 km; Lower Permian — upper Cretaceous],Tookoonooka [55–65 km; ~125 Ma], Talundilly [~84 km; ~125 Ma], Mount Ashmore [N100 km; end-Eocene]and Warburton twin structures [>400km; pre-end Carboniferous]) requires re-examination of the diagnosticcriteria used for their identification. Bouguer anomalies of established impact structures (Chicxulub [170 km;64.98 ± 0.05 Ma], Woodleigh impact structure and Gnargoo probable impact structure display a uniquestructural architecture where pre-impact structural ridges are intersected and truncated by the outer ring ofthe circular structure. Seismic reflection data outline circular central uplift domes, basement plugs and rimsynclines. Sharp circular seismic tomography anomalies indicate low velocity columns under both theWoodleigh impact structure and Warburton probable impact, hinting at deep crustal fracturing. Deformed,curved and clouded intra-crystalline planar deformation features in quartz (Qz/PDFs), displaying Miller indices({10–11}, {10–12}, {10–13}) diagnostic of shock metamorphism, abound around exposed establishedimpact structures (Vredefort [298 km; 2023 ± 4 Ma], Sudbury [~250 km; 1850 ± 3 Ma], Charlevoix [54 km;342 ± 15 Ma], Manicouagan [100 km; 214 ± 1 Ma]), Tookoonooka and Talundilly). Deformed Qz/PDFs allowrecognition of shock metamorphism in buried impact structures, where original Qz/PDFs were bent,recrystallized and/or clouded during formation of the central uplift and hydrothermal activity triggered by theimpact. Planar deformation in quartz can also occur in explosive pyroclastic units but are limited to Boehmlamella (Brazil twins) with single lamella sets {0001}. It has been suggested that a class of microstructures inquartz, referred to as metamorphic deformation lamella (Qz/MDL), occur in endogenic tectonic–metamorphicterrains. However, no type locality has been established for Qz/MDL of non-impact origin.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1142. Identification of buried impact structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

2.1. Geophysical criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1152.2. Microstructural criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

3. Criteria for recognition of Australian impact structures, probable and possible impact structures . . . . . . . . . . . . . . . . . . . . . . 1174. Significance of the Australian impact record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1205. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

ustralian National University,

likson), T.uysal@uq.edu.au

rights reserved.

1. Introduction

The discovery by Robert Dietz (1914–1995) of large asteroidimpact structures, originally referred to as ‘astroblemes’, includingVredefort (298 km; 2023 ± 4 Ma) (Dietz, 1961) and Sudbury

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(250 km; 1850 ± 3 Ma) (Dietz, 1964), heralded a new era in thestudy of the asteroid impact history of Earth. Since that time theadvent of geophysical exploration and drilling has uncovered anumber of large buried impact structures identified by large circulargravity andmagnetic anomalies and confirmed by diagnostic hallmarksof shock metamorphism. Such discoveries included ChesapeakeBay (90 km; 35.5 ± 0.3 Ma) (Poag, 1996), Chicxulub (170 km;64.98 ± 0.05 Ma) (EID — Earth Impact Database, 2011) and Manson(35 km; 73.8 ± 0.3 Ma) (Koeberl and Anderson, 1996). In AustraliaWoodleigh (Carnarvon Basin, Western Australia — 120 km; ~360 Ma)(Glikson et al., 2005a,b; Uysal et al., 2005), Tookoonooka (EromangaBasin, Queensland — 55–65 km; ~125 Ma) (Gorter et al., 1989; Gostinand Therriault, 1997) and Talundilly (Eromanga Basin, Queensland —

84 km, 125 Ma) (Longley, 1989; Gorter and Glikson, 2012) impactstructures were documented. In addition a number of probable1 impactstructures, including Gnargoo (north Carnarvon Basin,Western Austra-lia; Iasky et al., 2001; Iasky and Glikson, 2005), Mount Ashmore (TimorSea; Glikson et al., 2010), and Warburton (northeast South Australia)(Glikson and Uysal, 2010; Glikson et al., 2013) remain to be confirmed.

The discrimination of structural and microstructural diagnosticfeatures of shock metamorphism from those of endogenic terrestrialorigin has been the subject of extensive debate (Carter and Friedman,1965; Carter, 1965, 1968; Carter et al., 1986; Alexopoulos et al., 1988;Lyons et al., 1993; Grieve et al., 1996; Vernooij and Langenhorst,2005; Spray and Trepmann, 2006; Ferriere et al., 2009; French andKoeberl, 2010; Hamers and Drury, 2011). Central to the recognition ofdiagnostic features of buried impact structures is their unique geophys-ical and structural characteristics, including presence of a central upliftand a rim syncline, distinct circular and radial fault patterns, micro-structural shock metamorphic effects and extraterrestrial geochemicaland isotopic signatures, as distinct from structural features associatedwith tectonic domes, salt domes or volcanic uplifts.

This paper aims at discussion of the nature of diagnostic criteria fordefinition of extraterrestrial impacts, with reference to Australian con-firmed, probable and possible buried impact structures, using thewealth of geophysical data from Australian sedimentary basins. It ishoped that the clarification of these criteria will help in further investi-gation of seismic, magnetic and gravity data from sedimentary terrainswith the aim of extending the impact database, with implications forthe effects of extraterrestrial impacts on crustal evolution.

2. Identification of buried impact structures

2.1. Geophysical criteria

Structural features suggestive of an impact origin of circular struc-tures include:

1. Intersections by the external rings (commonly but not invariablynear-to 360°) of older pre-impact structural elements, as displayedby the Chicxulub impact structure, the Woodleigh impact structureand Gnargoo probable impact structure (Fig. 1). Whereas structural-ly discordant intersections are also displayed by volcanic diatremesand by salt domes, the combination of these features, presence of acentral uplift core or central dome, and a ring syncline, is consistentwith diagnostic features which militate for an impact origin.

2. The central uplifts of impact structures are best defined by seismicreflection sections where the basement uplift is commonly associ-ated with thrust faults (Fig. 2a, b). Where the core of the structureconsists of sedimentary strata, a structural dome is outlined, whose

1 In this paper the term “probable impact structure” pertains to documented struc-tures which display a number of features consistent with those of established impactstructures but have not been to date included in the Earth Impact Database (EID:http://www.passc.net/EarthImpactDatabase/index.html). The term “possible impactstructure” pertains to structures suspected to be of impact origin but which are notknown to include diagnostic structural and shock metamorphic signatures.

coremay contain chaotically disrupted core zones displaying a loss ofseismic markers due to mega-brecciation, as in the Mount Ashmoreprobable impact structure (Fig. 2c). By contrast to thrust faultsaround and within the central core zone, ring synclines and outerrims of impact structures feature inward-dipping normal faults(Fig. 2a, b). These structural patterns represent centripetal to upwardblock movements which involve compression around the upliftedcore, extension within the ring syncline and inward collapse of thecrater rim, evident in the Woodleigh, Gnargoo and Talundilly struc-tures (Figs. 1 and 2). In addition, some impact structures and proba-ble impact structures display uplift of crystalline basement belowimpacted sediments, as in the Woodleigh impact structure (Fig. 2)and the Mount Ashmore probable impact structure (Glikson et al.,2010).

3. An intersection of the top of the dome or basement uplift by uncon-formably overlying post-impact sediments is typical of impact struc-tures, as demonstrated in the Woodleigh, Gnargoo, Mount Ashmoreand Tookoonooka structures (Fig. 2). Post-impact isostatic verticalmovements are indicated where the central uplift pierces throughthe unconformity, as in the Mount Ashmore structure (Fig. 2c).

2.2. Microstructural criteria

The distinction between Planar Deformation Features in quartz(Qz/PDFs) indicative of shock metamorphism within established im-pact structures on the one hand, and proposed Metamorphic Defor-mation Lamellae (Qz/MDL) of supposed purely endogenic origin onthe other hand, is extensively discussed in the literature (Carter andFriedman, 1965; Carter, 1965, 1968; Carter et al., 1986; Alexopouloset al., 1988; Lyons et al., 1993; Grieve et al., 1996; Vernooij andLangenhorst, 2005; Spray and Trepmann, 2006; Ferriere et al., 2009;French and Koeberl, 2010; Hamers and Drury, 2011). The followingcriteria have been suggested in this regard:

A. Qz/PDF lamellae are defined by diagnostic Miller indices correlatedwith specific shock levels, in particular the {10–11}, {10–12} and{10–13} planes (French, 1998). By contrast planar features referredto as Qz/MDL show a wide scatter, including low COAQz^PPDF angles(angle of the Optic Axis to the pole of planar deformation features)on frequency distribution plots (Lyons et al., 1993; French, 1998,Fig. 4.25).

B. Qz/PDFs form multiple planar sets in shock metamorphosed rocks(Robertson et al., 1968; Stoffler and Langenhorst, 1994; Grieve et al.,1996). Planar features referred to as Qz/MDL consist mostly of onlyone set of lamellae within any one quartz grain (French andKoeberl, 2010), commonly form basal {0001} planes, representing~8–10 GPa shock pressures (French, 1998, Table 4.2), including inexplosive volcanic units such as at Toba ignimbrites (Carter et al.,1986).

C. Qz/PDF lamellae can be b1–2 μm-thick whereas planar featuresreferred to as Qz/MDL consist of segments usually 2–4 b μm thick.

D. Qz/PDFs are originally perfectly planar whereas planar featuresreferred to as Qz/MDL commonly display undulation, bending andwavy patterns.

E. Transmission Electron Microscopy (TEM) studies indicate little-deformed segments of Qz/PDFs displaying optical continuity be-tween bordering intra-crystalline segments, namely these segmentsdisplay no subgrain boundaries. By contrast planar features referredto as Qz/MDL display irregular/undulating boundaries and opticaldiscontinuities between separate subgrains, where optic orienta-tions depart by ≤5° from each other and from the orientation ofthe host quartz (Glikson et al., 2013).

F. TEM studies indicate Qz/PDFs are either amorphous or composed ofquartz with low dislocation densities. By contrast planar featuresdescribed as Qz/MDL are more commonly altered and display highdislocation densities (Goltrant et al., 1991).

Fig. 1. Structural relations between circular impact structures and probable impact structures and older structural ridges, as expressed in (A)Woodleigh impact structure (D = 120 km;Glikson et al., 2005a,b); (B) Gnargoo probable impact structure (D = 75 km; Iasky and Glikson, 2005) and (C) Chicxulub impact structure (D = 170 km; Sharpton et al., 1996). In theseexamples north–south-oriented gravity ridges are intersected by the outer ring of the circular structure, defining the overall diameter of the impact structure. Australian Journal of EarthScience, by permission.

116 A. Glikson, I.T. Uysal / Earth-Science Reviews 125 (2013) 114–122

G. Qz/PDFs in SEM–Cathode Luminescence form straight, narrow,well-defined features, whereas tectonic deformation lamellae arethicker and slightly curved (Hamers and Drury, 2011).

Vernooij and Langenhorst (2005) propose criteria for discriminationbetween lamella produced by shock metamorphism (Qz/PDFs) andtectonic deformation (Qz/MDL) in the following terms: (A) Qz/PDFs areperfectly planar instead of slightly curved; (B) Qz/PDFs form strictlyparallel, well defined, mostly rhombohedral crystallographic planes;(C) Qz/PDF spacing is usually b1 μm instead of 2–4 μm; (D) Qz/PDFs donot induce a misorientation within the crystal and do not featuresubgrain walls or dislocation bands, and (E) the density of free disloca-tions in Qz/PDFs is low compared to Qz/MDL. These criteria are basedon comparisons between experimentally deformed single quartz crystalsand Qz/PDFs from shocked rocks of the Ries crater, which underwentlittle post-impact deformation. However, the occurrence within con-firmed impact structures of microstructures in quartz which display fea-tures consistent with criteria pertaining to both Qz/PDFs and to Qz/MDLsuggests that the latter are derived by deformation of the former, as inthe following examples:

[A] Vredefort impact structure: Qz/PDFsmay be bent, clouded or dec-orated, and are dominated by basal {0001} sets (Grieve et al., 1990, Fig. 2)and to a lesser extent multiple sets (Fricke et al., 1990, Fig. 7), includingquartz grains in clasts entrained in granophyre veins (Buchanan andReimold, 2002, Fig. 5). Grieve et al., 1990 (Fig. 2) compare the Vredefortplanar features with slightly undulating planar features from Mistastinand Charlevoix impact structures. These authors conclude that the planarfeatures are “anomalous but still suggestive of an impact origin”; [B]

Charlevoix impact structure, containing bent {0001} lamellae in quartz(Trepmann and Spray, 2004, Figs. 1 and 4); [C] Manicouagan impactstructure (Robertson, 1975, Fig. 3A; Dressler, 1990, Fig. 7); [D] Impactbreccia of the Onaping Formation, Sudbury impact structure (Grieve etal., 2010, Figs. 4 and 6).

From the above criteria for distinguishing between Qz/PDFs asso-ciated with established impact structures and overprinted by ductiledeformation, planar features referred to as Qz/MDL of assumed endo-genic tectonic-metamorphic origin, are yet to be defined. Exceptionsare Boehm lamella {0001} documented from explosive volcanics, forexample ignimbrites at Toba, Sumatra (Carter et al., 1986). Doubtsremain regarding the origin of planar elements formed by processesother than impact-triggered explosion (Lyons et al., 1993). Examplespresented by these authors include: (1) Sandstones and quartzitefrom Finland and California showing either single or multiple crossingplanar elements; (2) The Tapeat Sandstone that contains grains withmultiple fine scale parallel lamella of ~4–5 μm width, with a scatter of10–55° on (COAQZ^PPDF) frequency plots.Whether these grains representendogenic tectonic features or, alternatively, are related to or derivedfrom an impacted source remains unclear.

Very rarely have multiple planar sets in quartz been documentedfrom in-situ locations in terrains not known to be associated with im-pact structures. French (1990), investigating impact suggestions forthe layered Bushveld Complex, South Africa (Hamilton, 1970; Elstonand Twist, 1986) analyzed planar microstructures in quartz fromquartzite inclusions in the Rooiberg Felsite of the Bushveld Complexthat display multiple sets but no peak orientations which correspondto shock metamorphism-related Miller indices. However, the Rooiberg

Fig. 2. Cross sections through Australian impact structures and probable impact structures, displaying central plugs and domes representing central uplifts (CU) intersected by uncon-formities and sections of outer ring synclines. (a) An east–west seismic-based geological section of the Woodleigh impact structure, Carnarvon Basin, Western Australia, displayinguplifted blocks of shock-metamorphosed Precambrian basement plug (pCa) defined by thrust faults and down-faulted lower Paleozoic sediments. The central uplift is surrounded byring syncline sectors associated with normal faults displaying down-plunge toward the center. The central uplift is unconformably overlain by Jurassic sediments (after Glikson et al.,2005a,b). (b) An east–west seismic-section of the Gnargoo probable impact structure, northern Carnarvon Basin, Western Australia, showing the central uplift consisting of two thrustfaulted basement blocks (CU) and down-faulted early Paleozoic sediments representing segments of the rim syncline. Note the chaotic seismic structure of the central blocks (after Iaskyand Glikson, 2005). (c) A NW–SE seismic section through the central dome of the Mount Ashmore dome structure, Timor Sea. The section features chaotic block structures at the core ofthe dome and a major unconformity truncating the top of the dome where Oligocene and Miocene strata overlie uplifted Eocene sediments (after Glikson et al., 2010). Whereas thedome itself is approximately 50 km in diameter, the overall structure is considered at least twice this size. Australian Journal of Earth Science, by permission.

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Felsite has been subject to little or no post-magmatic tectonic deforma-tion,which raises a question regarding it containingQz/MDL, nor can animpact origin be ruled out at the present state of knowledge. Thisrenders the quartz planar sets of the Bushveld Complex hardly a typelocality for Qz/MDL.

3. Criteria for recognition of Australian impact structures,probable and possible impact structures

The identification over the last 15 years of large buried impactstructures, probable and possible impact structures, in Australiansedimentary Basins, allowed by geophysical data and drilling, isbased on a range of structural and microstructural criteria, including(A) unique structural architecture and (B) shock metamorphicfeatures, including planar features in quartz grains, crush and meltzones. Established impact structures include Woodleigh ([120 km;~360 Ma]; Mory et al., 2000; Iasky et al., 2001; Glikson et al., 2005a,2005b), Tookoonooka ([55–65 km; ~125 Ma]; Gostin and Therriault,1997; Bron and Gostin, 2012) and Talundilly ([84 km; ~125 Ma];Gorter and Glikson, 2012). Probable impact structures includeGnargoo structure (Iasky and Glikson, 2005) and the Mount Ashmorestructure (Glikson et al., 2010). A possible large impact structure

is Warburton twin basins metamorphosed basin (Glikson and Uysal,2010; Glikson et al., 2013). Principal features of these structures aresummarized in Table 1 and are elaborated below.

Structural elements of the buried Woodleigh impact structure,Carnarvon Basin, Western Australia [120 km; 360 Ma] (Iasky et al.,2001;Mory et al., 2000; Glikson et al., 2005a,b; Uysal et al., 2005), over-lain by Jurassic strata, display significant analogies with the Chicxulubimpact structure (Fig. 1). Geophysical evidence includes the superposi-tion of the outer ring of the Woodleigh structure on the Ajana andWandagee gravity ridges, seismic-reflection data indicating a central~37 km-diameter basement plug, and consistency between outer nor-mal fault patterns and inner thrust faults with patterns in establishedimpact structures (Ries crater, Chesapeake Bay), as well as morpho-metric evidence, indicating an upper diameter of 120 km and alower diameter at greater depth. A sharp seismic low-velocity to-mography anomaly (Saygin and Kennett, 2011) coincides with the~120 km-diameter circular structure (Fig. 3a). Planar deformation fea-tures (PDFs) arewell developed in the granitoid central core, themajorityof planar sets being oriented parallel to {10–13} and lesser abundance to{10–12}. Some sets are parallel to the basal plane {0001} and {10–11},consistent with lower pressures about 5–10 GPa. Feldspar-hosted PDFsform reticulate vein networks displaying checkerboard-like to irregular

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and serrated patterns attributable to preferential replacement ofshock-formed PDFs and/or perthitic twin lamella by clay minerals.Shock-induced melting and diaplectic transformations are heavilymasked by pervasive alteration of the shocked gneisses tomontmorillonite-dominated clays.

The Gnargoo structure, buried about 500 m below Cretaceousstrata, features structural elements which are remarkably identicalto those of the Woodleigh impact structure, located about 275 kmto the south (Iasky andGlikson, 2005) (Fig. 1). These similarities includea circular Bouguer anomaly, a central structurally uplifted core compris-ing a buried dome with a central uplifted plug, and a weakly definedinner 10 km-diameter circular Bouguer anomaly surrounded by abroadly circular zone ~75 km in diameter. The outer ring intersectsthe N–S Bouguer anomaly lineament of the Giralia Range. It includes a~28 km-diameter layered sedimentary dome of Ordovician to LowerPermian strata, surrounding a cone-shaped, central uplift plug of7–10 km diameter. Seismic-reflection data indicate a minimum centralstructural uplift of 1.5 km. An interpretation of the Gnargoo structure interms of a plutonic or volcanic caldera is unlikely as these featuresdisplay less regular geometry. No volcanic rocks are known in theonshore Gascoyne Platform. An interpretation of Gnargoo as a saltdome is also unlikely as no extensive evaporite units are known in theSouthern Carnarvon Basin. Morphometric estimates of the rim-to-rimdiameter based on seismic data for the central dome correspond tothe observed diameter deduced from gravity data, and fall within therange of morphometric parameters of known impact structures. Theage of Gnargoo is constrained between the deformed Lower Permiantarget rocks and unconformably overlying Lower Cretaceous strata.

The ~66 km-diameter Tookoonooka impact structure (Gorter etal., 1989; Gostin and Therriault, 1997; Bron and Gostin, 2012), buriedbelow ~900 m thick Cretaceous and Tertiary sediments, is expressedon seismic sections as a concentric arrangement of anticlines andsynclines surrounding a ~22 km-wide complex central dome associat-ed with central gravity low and magnetic high. Regional wells intersectLower Cretaceous strata (basal Wyandra Sandstone, Cadna OwieFormation) containing impact melt breccia which includes quartzgrains with PDFs corresponding to Miller indices {10–12}, {11–22},{21–31} and {11–21} confirming an impact origin for Tookoonooka

Fig. 3. Seismic tomography images of (a) Western Australia and (b) the Australian continent,with the Woodleigh impact structure (Carnarvon Basin, Western Australia) and marked low-Warburton Basin, northeastern South Australia. After Saygin and Kennett (2010, 2011). (Elsev

(Gostin and Therriault, 1997; Bron and Gostin, 2012). The clasts containaccretionary and melt components and lithic and mineral grainswhich correspond to the Tookoonooka target rock sequence, includ-ing basement. The timing of the impact event is confirmed to be theBarremian–Aptian boundary, at 125 ± 1 Ma.

The Talundilly structure (Longley, 1989; Gorter and Glikson, 2012),located 328 km northeast of Tookoonooka, is represented on seismicreflection transections as a major anomaly about 84 km in diameter,disrupting the early Cretaceous Wyandra Sandstone, located betweenthe Cadna Owie and Bulldog Shale formations. The seismic anomalouszone coincides with a prominent aeromagnetic (TMI) high centrallylocated within a near-circular seismic anomaly. The structure consistsof a raised central area, with radial faults extending from the centralhigh, an annular synform with disrupted seismic elements dippingat low angles towards the central uplift, and an outer faulted rim.Talundilly-1 well located about 30 km northwest of the structural peakintersected arenite which contains quartz grains with PDFs. The age ofthe structure, as determined from seismic correlation and sparse palynol-ogy, is estimated as ~125 Ma, coinciding with the age of Tookoonookaand a marine transgression.

TheMount Ashmore dome,west Bonaparte Basin, Timor Sea (Gliksonet al., 2010), is located below a major pre-Oligocene post-Late Eoceneunconformity and above a ~6 km-deep-seated basement high indicatedbymarked gravity andmagnetic anomalies.Whereas the lower diameterof the dome (Fig. 2c) is approximately 50 km, as the rim syncline is notdefined the overall structure is estimated as larger than 100 km in diam-eter. The core of the dome features chaotic deformation and centripetalkinematic deformation patterns (Fig. 2c). Lower Oligocene to LowerJurassic well intersections reveal micro-brecciation, comminutedand flow-textured fluidization of altered sedimentary material. Themicrobreccia is dominated by aggregates of poorly diffracting mi-crometer to tens of micrometers-scale to sub-millimeter particles,including relic sub-planar fractured quartz grains, carbonate, barite,apatite and K-feldspar. No volcanic material or evaporite sedimentswere encountered, militating against interpretations of the structurein terms of magmatic intrusion or salt diapirism models whichare also inconsistent with strong gravity and magnetic anomaliesconsistent with a basement high below the dome. Whereas an

showing a marked low-velocity anomalies (seismic group velocity b 2.4 km/s) coincidingvelocity anomalies (b2.1 km/s) coinciding with the East Warburton Basin and the Westier, with permission)

Fig. 4. Part (a). Mildly deformed Planar Deformation Features in quartz (Qz/PDFs), Warburton probable impact structure: Deformed Qz/PDFs of the Warburton probable twin im-pact structures. (a) Lamellar PDF displaying very weak undulation and overprinted by micro fractures; (b) three quartz grains displaying weak to marked undulation and bending;(c) fine scale lamellar Qz/PDFs displaying lamellar width of b1 μm and intervals of b4 μm, diagnostic of shock metamorphic PDFs; (d) deflected Qz/PDF lamella. Part (b). Deformedand clouded planar features in quartz grains of the Warburton probable impact structure. (a) Mildly diverging Qz/PDF lamella; (b) parallel Qz/PDF lamella overprinted bymicro-fracture systems; (c) heavily clouded micro-fracture intersecting systems; (d) divergent feather-like lamella in quartz. (Elsevier, with permission)

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Table 1Summary table of large Australian buried impact structures, probable and possible impact structures discussed in the present paper.

Structure Location Size(km)

Age(Ma)

Diagnostic features Reference

Woodleigh Carnarvon Basin,W.A.

120 ~360;end-Devonian

Circular gravity and magnetic anomalies, seismic central uplift and rim syncline,normal outer and thrust inner faults, Qz/PDFs, altered possible melt fractions.

Glikson et al. (2005a,2005b); Uysal et al. (2005)

Gnargoo Carnarvon Basin,W.A.

75 Post-Carboniferous Circular gravity and magnetic anomalies, normal outer and thrust inner faults,seismic central uplift and rim syncline.

Iasky and Glikson (2005)

Warburton Below theCooper Basin,NE S.A.

N200 End-Carboniferous Twin structures: major Bouguer and magnetic anomalies, sharp tomographyanomaly, Qz/PDFs.

Glikson et al. (2013)

Tookoonooka Eromanga Basin,SE Qld

56–66

~125 LowerCretaceous

Circular geophysical anomalies, melt breccia, Qz/PDFs. Gostin and Therriault (1997)and Bron and Gostin (2012)

Talundilly Eromanga Basin,SE Qld

84 ~125 LowerCretaceous

Circular geophysical anomalies, radial faults, breccia, Qz/PDFs. Gorter and Glikson (2012)

MountAshmore

West BonaparteBasin, Timor Sea

N50 End-Eocene Circular structure, central uplift, centripetal dynamics, inner coremega-brecciation and micro-brecciation, truncating top unconformity.

Glikson et al. (2010)

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impact origin cannot be proven, due to the lack of shock metamorphiceffects such as PDF, impact melt or coesite, an impact model is consis-tent with the chaotic structure of the dome-structured core, centripetalsense of deformation, micro-brecciation and fluidization of the Triassicto Eocene rocks. An analogy can be drawn between theMount Ashmorestructural dome and impact structures formed in volatile-richsediments where shock is attenuated by high volatile pressure. TheMount Ashmore dome is likely contemporaneous with a Late Eoceneimpact cluster (Popigai: D ~ 100 km, 35.7 + 0.2 Ma; Chesapeake Bay:D ~ 85 km, 35.3 + 0.1 Ma).

The EastWarburton Basin, northeast South Australia, featuresmajorgeophysical anomalies, including a magnetic high of near-200 nTcentered on a ~25 km-wide magnetic low (b100 nT), correspondingto density estimates of 2.8–2.99 g/cm3 and interpreted in terms of ahomogeneous magmatic body below 6 km depth (Glikson and Uysal,2010; Glikson et al., 2013). A distinct seismic tomographic low-velocity anomaly associated with the East Warburton Basin may reflecta thick (~9.5 km) sedimentary section, high temperatures and likelydeep fracturing (Fig. 3b). The anomaly is similar to a tomographicanomaly associated with the 120 km-diameter Woodleigh impactstructure, Western Australia (Fig. 3a). Scanning electron microscope(SEM) analyses of the intrusive Big Lake Suite granites resolvemicrobreccia veins of micron-scale particles injected into resorbedquartz grains. Planar to sub-planar microstructures in quartz grainsare documented in granite, volcanic and sedimentary cores from 55drill holes within the N30,000 km2-large East Warburton Basin. Quartzgrains may display multiple intersecting planar and sub-planar elementsand include relic lamella less than 2 μmwidewith inter-planar spacing of4–5 μm. Planar to sub-planar elements are deformed, displaying bent andwavypatterns commonly accompaniedfluid inclusions (Fig. 4). Universalstage measurements (total of 243 planar and sub-planar sets in 157quartz grains) indicate dominance of Miller indices diagnostic ofshock metamorphism (Π{10–12}, ω{10–13} and subsidiary §{11–22},{22–41}, r{10–11} and x{51–61}). Transmission Electron Microscopy(TEM) analysis displays relic narrow ≤1 μm-wide lamellae and relicnon-sub grain boundaries where crystal segments maintain opticalcontinuity, distinguishing the lamellae from tectonic and lowshock-pressure basal ‘Boehm lamellae’ {0001}. Extensive sericitealteration of feldspar suggests hydrothermal alteration to a depthof ~500 m below the top unconformity which truncates the quartzmicrostructure-bearing East Warburton Basin terrain. A completegradation occurs between relic Qz/PDFs and deformed Qz/PDFswhich occur in several large impact structures. An uplift of the BigLake Granite Suite by 4–5 km during ca 298–295 Ma, followed byerosion, suggested by the lack of upper Ordovician to Devonian stra-ta in the East Warburton Basin, indicates major tectonism in theend-Carboniferous. The definition of an impact structure in the EastWarburton Basin requires corroboration by deep crustal seismic

reflection transects. Evidence for the origin of quartz lamellae inthe East Warburton Basin includes:

1. Agreement of measured COAQZ^PPDF angles with Miller indices,mainly ω{10–13} and Π{10–12}, indicative of shock levels higherthan 20 GPa (French, 1998; Langenhorst, 2002);

2. Observation by optical microscopy and TEM of lamellae b1–2 μm-wide with ~4–5 μm spacing;

3. The occurrence of multiple intersecting lamellae;4. TEM observation of relic non-sub-grain boundaries between planar

segments in optically coherent host quartz.

The planar elements are associated with, or overprinted by, wavyand bent wide-spaced planar features which form low angle bound-aries between subgrains of divergent low angle optic orientation.Re-deformation, recrystallization and annealing of quartz lamellae,which contain relic features corresponding to Qz/PDFs, are likely tobe associated with post shock centripetal-oriented deformationinherent in the formation of a central uplift as well as hydrothermalactivity triggered by impact (Naumov, 2002; Pirajno, 2005). TheWarburton Basin sediments and the Big Lake Suite granite haveexperienced significant multi-deformation phases and associatedhydrothermal events in the Early Permian, Permo-Triassic, Mid-Cretaceous and Tertiary (McLaren and Dunlap, 2006), accounting forthe re-deformation of Qz/PDFs.

North and west of the East Warburton Basin and separated by astructural ridge (Birdsville Track Ridge) is the West Warburton basin,marked by a distinct magnetic anomaly and by a strong low velocitytomography anomaly which represents a short seismic period of up to8.3 s, similar to the East Warburton Basin (Fig. 3b). Somewhat weakeranomalies pertain to the 12.5 s period. Planar deformation features inquartz (Qz/PDF) have also been identified in West Warburton Basincores. The total size of the shock metamorphosed terrains in northeast-ern South Australia appears to exceed 400 km in diameter.

4. Significance of the Australian impact record

The discovery of new buried impact structures, probable and possibleimpact structures, allowed by seismic reflection, seismic tomography,magnetic data, Bouguer gravity anomalies and drilling, applying structur-al andmicrostructural criteria discussed above, opens theway for expan-sion of the terrestrial impact inventories in Australia and elsewhere.Principal structural attributes of impact structures include near-360°ring structures (excepting rare deformed impact structures, such asSudbury) which intersect older structures, centripetal deformation andthrusting, circular normal faulting associated with rim syncline andouter rim, sub-radial and tangential faults and erosion of the top of thecentral uplift. The association of a major tomographic low-velocity seis-mic anomaly with the Woodleigh impact structure (Fig. 3a) and with

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the Warburton probable twin impact structures (Glikson and Uysal,2010; Glikson et al., 2013), hints at a potentially new criterion for identi-fying large buried impact structures. This criterion is yet to be tested byseismic tomographic studies of other large impact structures, such asVredefort, Sudbury, Manicouagan, Chicxulub, Popigai and ChesapeakeBay. Possible impact structures inferred from geophysical evidence butlacking evidence of shock metamorphism include the Deniliquin geo-physical anomaly, EastMurray Basin (Yeates et al., 2000), and the Bedoutgeophysical dome off the Pilbara coast (Becker et al., 2004; Glikson,2004).

Views vary regarding the diagnostic value of deformed penetra-tive planar features in quartz. As indicated above, where deformedplanar features in quartz display (A) multiple intersecting planarsets; (B) lamellar width less than ~1–2 μm spaced at ~4–5 μm; (C)correspondence with crystallographic Miller indices diagnostic ofshock pressures; (D) TEM-resolved non-subgrain optically contigu-ous lamellar boundaries, shock metamorphic origin is confirmed.Deformed Qz/PDFs, documented around established impact struc-tures (Vredefort, Sudbury, Manicouagan, Charlevoix) therefore arediagnostic of shock metamorphism, as stated in Garde and Glikson(2011): “Lamellae in large exhumed impact structures may thus retaincritical diagnostic features consistent with PDFs, with two or moreorientations being common, preserving the specific crystallographicorientations characteristic of shock metamorphism.” Examples wheredeformed Qz/PDF occur are the Maniitsoq (Garde et al., 2012) andWarburton (Glikson et al., 2013) impact structures. For these reasonswhere deformed Qz/PDFs are identified in anomalous structural settingsthey are likewise strongly suggestive of shock metamorphism.

A compilation of global impact structures and impact ejecta/falloutlayers suggests a high rate of large impacts and impact clusters duringthe Archaean and early Proterozoic, an observation consistent withthe suggestion by Lowe and Byerly (2010) of an extension of the LateHeavy Bombardment (LHB ~3.95–3.85 Ga), as observed on the Moon(Ryder, 1990), into the Archaean. Extrapolations from the lunarpost-LHB impact incidence record to the Earth, taking the Earth/Moonsurface ratio and gravity section into account, allow a first-orderestimate of formation of craters larger than 100 km (Glikson, 2001).Based on these data, an estimate of continental impacts, as contrastedto oceanic impacts whose cratering record has been destroyed, yieldsN400 craters larger than N100 km in diameter. The 6 terrestrialcontinental impact structures of ≥100 km diameter identified todate (Vredefort, Sudbury, Chicxulub, Woodleigh, Popigai, possiblyWarburton) combined with at least 13 impact ejecta units indicativeof projectiles N10 km-large (~3482 Ma, ~3472 Ma, ~3445 Ma,~3416 Ma, ~3334 Ma, ~3257 Ma, ~3256 Ma, ~3243 Ma, ~3225 Ma,~2630 Ma, ~2557 Ma, ~2556 Ma, ~2481 Ma) (Simonson and Glass,2004; Lowe and Byerly, 2010; Simonson et al., 2010) thus total to atleast 19 large impacts and impact clusters forming 4.7% of the esti-mated impact inventory. Based on microkrystite spherule-rich im-pact fallout/ejecta layers (Glikson and Vickers, 2010; Glikson, 2013)the high-Mg composition of several of these layers suggests impactin sima-dominated crustal regions of the early Earth. The difficulty infield identification of microkrystite spherule units suggests more arelikely to be found (Glikson and Vickers, 2010). A number of largePrecambrian impact structures (cf. Otish Basin, Genest et al., 2011)await confirmation. Such advances will further increase the inventoryof impact structures, defining a major role for asteroid and comet im-pact events in the history of planet Earth.

5. Conclusions

1. Definitions of the characteristic architecture and structuralelements of buried impact structures, considered in this paper,should enhance the identification of potential impact structuresthanks to their unique geophysical signatures, seismic reflection,seismic tomography, magnetic and gravity surveys, allowing the

expansion of the global impact database and further elucidationof the asteroid impact history of Earth.

2. Deformed, bent and clouded planar deformation features in quartz(Qz/PDFs) which preserve multiple intersecting planar sets, b1–2 μm-wide lamella separated by ~4–5 μm spacing, diagnostic Millerindices and TEM-identified non-subgrain boundaries, associatedwith confirmed impact structures (Vredefort, Sudbury, Manicouagan,Charlevoix), can be used as diagnostic hallmark for shock metamor-phism. The association of deformed Qz/PDFs with the Woodleigh,Talundilly and Warburton structures is considered evidence forshock metamorphism.

3. Deformation of Qz/PDFs can occur during formation of the centraluplift of impact structures, recrystallization associated with hydro-thermal activity related to impact cratering, and/or post-impacttectonic deformation. To date no in-situ type locality has beenindicated in the literature for planar features with characteristicssimilar to Qz/PDFs produced by purely endogenic processes, re-ferred to in the literature as metamorphic deformation lamella(Qz/MDL).

4. The expansion of the asteroid impact inventory allowed by discoveryof buried impact structures, and the prospect of further discoveries,expands the asteroid impact inventory of Earth, allowing new in-sights into the effects of extraterrestrial impacts on crustal evolution.Out of 400 impacts N100 km in diameter inferred fromprojections ofpost-LHB lunar impact incidence, to date at least 19 impact struc-tures and impact ejecta units indicative of N100 km-large cratershave been identified.

Acknowledgments

We thank Vic Gostin for a review, Robert Iasky and Franco Pirajnofor collaboration in the studies of the Woodleigh and Gnargoo struc-tures, Elinor Alexander for collaboration with the Warburton project,Dariusz Jablonski for collaboration with the Mount Ashmore project,John Vickers for technical help in connection with the buried impactsprojects, Frank Brink for help with SEM/EDS analyses, John Fitz Geraldfor TEM analyses, Peter Haines and Tony Eggleton for advice regard-ing PDF analyses, Erdinc Saygin for collaboration and Anita Andrewsfor permission to reproduce figures.

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