Woodleigh impact structure, Australia: Shock petrography and geochemical studies

Meteoritics amp Planetary Science 38 Nr 7 1109ndash1130 (2003)Abstract available online at httpmeteoriticsorg

1109 copy Meteoritical Society 2003 Printed in USA

Woodleigh impact structure Australia Shock petrography and geochemical studies

Wolf Uwe REIMOLD1 Christian KOEBERL2 Robert M HOUGH3 4 Iain MCDONALD5 Alex BEVAN3 Kassa AMARE2 and Bevan M FRENCH6

1Impact Cratering Research Group School of Geosciences University of the Witwatersrand PO Wits 2050 Johannesburg South Africa2Department of Geological Sciences University of Vienna Althanstrasse 14 A-1090 Vienna Austria

3Department of Earth and Planetary Sciences Western Australian Museum Perth Australia4Planetary and Space Sciences Research Institute The Open University Walton Hall Milton Keynes MK7 6AA UK

5Department of Earth Sciences Cardiff University PO Box 914 Cardiff CF10 3YE UK6Department of Mineral Sciences Smithsonian Institution Washington DC 20560 USA

Corresponding author E-mail reimoldwgeoscienceswitsacza

(Received 16 January 2003 revision accepted 3 June 2003)

AbstractndashThe large complex Woodleigh structure in the Carnarvon basin of Western Australia hasrecently been added to the terrestrial impact crater record Many aspects of this structure are howeverstill uncertain This work provides a detailed petrographic assessment of a suite of representative drillcore samples from the borehole Woodleigh 1 that penetrated uplifted basement rocks of the centralpart of this structure Fundamental rock and mineral deformation data and high-precision chemicaldata including results of PGE and oxygen isotopic analysis are presented The sampled intervaldisplays likely impact-produced macrodeformation in the form of fracturing and breccia veining atthe microscopic scale Contrary to earlier reports that these breccias represent pseudotachylite(friction melt) or even shockshear-produced pseudotachylitic melt breccia cannot be confirmed dueto pervasive post-impact alteration Abundant planar deformation features (PDFs) in quartz inaddition to diaplectic glass and partial isotropization are the main shock deformation effectsobserved confirming that Woodleigh is of impact origin Over the investigated depth interval thestatistics of quartz grains with a variable number of sets of PDFs does not change significantly andthe patterns of crystallographic orientations of PDFs in randomly selected quartz grains does notindicate a change in absolute shock pressure with depth either The value of oxygen isotopes for therecognition of meteoritic contamination as proposed by earlier Woodleigh workers is criticallyassessed Neither INA nor PGE analyses of our samples support the presence of a meteoriticcomponent within this basement section as had been claimed in earlier work

INTRODUCTION

The existence of the Woodleigh impact structure in theCarnarvon basin of Western Australia (Fig 1) was firstreported and discussed by Mory et al (2000a b)Subsequently Mory et al (2001) and Iasky et al (2001)presented additional information that further supported theimpact origin elaborated on the geology and geophysics of thestructure and dealt with such issues as its size and ageHowever several aspects of this structure including its sizeand age and the presence of a meteoritic component stillremain controversial or require further substantiation Moryand coworkers (eg Mory et al 2000a b 2001 Iasky et al2001) have favored a 120 km diameter while our group is ofthe opinion that the existing geophysical data and sizerelationships are more consistent with a ~60 km diameter

structure (eg Reimold and Koeberl 2000) The age of250 Ma initially proposed by Mory et al (2000a) coincideswith the PermianTriassic boundary age In contrast Uysal etal (2001) presented K-Ar data on clay minerals that theyinterpreted as indicating that this impact was coincident withthe late Devonian mass extinction This interpretation isdoubtful based on the age statistics reported to date asdiscussed by Renne et al (2002) An age for the Woodleighimpact event from absolute dating of a bona fide impact-generated phase is still outstanding

Furthermore the reports of macroscopic and microscopicshock deformation reported to date (Mory et al 2000a bUysal et al 2001 Pirajno 2001) are not satisfactory (see alsoReimold and Koeberl 2000) and have left a number ofquestions unresolved Mory et al (2000a) described a core ofuplifted granitic basement of probably less than 25 km in

1110 W U Reimold et al

Fig 1 a) Location map of the Woodleigh structure in western Australia b) major regional tectonic features and location of the Woodleighstructure and positions of the drilling sites in the central part of the impact structure (drill cores Woodleigh 1 and 2A) adapted from Mory etal (2001)

a

b

Woodleigh impact structure Australia 1111

diameter They reported the presence of shock-induced planardeformation features in quartz pervasive diaplecticvitrification of feldspar and ldquopenetrative pseudotachyliteveiningrdquo in the basement rocks intersected in a drill core(Woodleigh 1 hereafter abbreviated W-1) into the upliftedbasement Mory et al (2000a) also presented selectedchemical data that they interpreted to indicate that the allegedldquopenetrativerdquo pseudotachylite vein system within the shockedgranitoid basement was strongly enriched in Al Ca Mg NiCo Cr V and S and depleted in K and Si relative to graniteoutside the veins Despite the fact that in their paper (p 123in Mory et al [2000a]) only upper limits of elementabundances are given and some data are labelled ldquosemi-quantitativerdquo these authors used these data to suggestchemical fractionation due to shock volatilization as well asthe possible presence of a meteoritic component Theseconclusions need to be investigated further

Mory et al (2000a) reported that the central uplifted coreof the structure was surrounded by an inner ring synclinecontaining a 70 m thick thermally modified diamictiteoverlain by 380 m of lower Jurassic lacustrine deposits Thediameter of this possible impact structure was estimated at120 km from interpretation of regional geophysical datawhich would make Woodleigh one of the 5 largest impactstructures (after Vredefort Sudbury Chicxulub) known fromthe terrestrial impact cratering record (compare Grieve 1991Grieve et al 1995) Mory et al (2000a) also placed theMorokweng impact structure South Africa in a similar sizecategory but initial indications of a gt100ndash200 km diameter ofthis structure (eg Reimold et al 1999) have been refined toabout 70 or 80 km based on more recent studies (eg Henkelet al 2002 Reimold et al 2002)

The stratigraphic and geophysical information availablefor the Woodleigh region has recently been reviewed by Moryet al (2000b) However a large number of issues remainunresolved and all Woodleigh workers have agreed in thepublished literature that further detailed analysis is requiredThe shock deformation evidence cited to date (eg Pirajno2001) strongly suggests that Woodleigh is of impact originbut many basic data regarding the nature characterizationand distribution of deformation features are still missing Thecontroversies surrounding the size and age of this structurehave not yet been resolved and the geochemical datapresented so far are insufficient to justify firm conclusionsregarding fractionation ofmdashor even the presence ofmdashameteoritic component associated with allegedpseudotachylite in the basement of the central uplift

Here we present new observations of a Woodleigh drillcore (W-1) as well as the results of detailed petrographicstudies including a large number of measurements of theorientations of planar deformation features in quartz Wefurther present major trace element (including platinum-group element) and oxygen stable isotopic geochemical dataon a new suite of W-1 samples

GEOLOGICAL SETTING

The Woodleigh Structure is centered at 26deg03cent193centcent Sand 114deg39cent563centcent E (position of borehole Woodleigh 1) nearWoodleigh Station on the Gascoyne Platform in the southernCarnarvon basin of Western Australia (Figs 1a and 1b) TheGascoyne Platform represents a structural high betweenvarious sub-basins to the east and the Bernier Platform andEdel Terrace to the west (Mory et al 2001 Iasky and Mory1999 Hocking et al 1987 Myers 1990 1993) The platformis covered by flat-lying Cretaceous and Lower Jurassic strataand the former sub-basins (Byro and Coolcalalaya) formed aprominent Carboniferous-Permian depocenter

The sub-basins are bounded to the west by the regionalMadeline Fault and a southerly extension of the Kennedy FaultThe abundant normal faults in this region define the ridges andtroughs at the margin of the Yilgarn craton The Ajana andWandagee ridges mark the raised eastern rim of the GascoynePlatform and are prominent features to the east of theWoodleigh structure Further to the east of Woodleigh lies thenorthern limit of the Yilgarn craton with abundant W-Etrending faults of the Capricorn Orogen (Myers 1990 1993Myers and Hocking 1998) The Gascoyne Platform is coveredthinly by mostly subhorizontal Cretaceous strata thatunconformably overlie a thick succession (up to 5000 m) offaulted and folded Ordovician-Devonian strata In the centralpart of the Woodleigh Structure flat-lying Jurassic strata occur

In borehole W-1 the uplifted Proterozoic basement isoverlain by early Cretaceous and Quaternary sediments andin W-2A early Cretaceous and early Jurassic sedimentsoverlie an unnamed diamictiteparaconglomerate (containingPermian palynomorphs) that overlies an unnamed dolomiticbreccia and the Coburn Formation interpreted to be of upperSilurian age (Mory et al 2001) The area of the Woodleighstructure is believed to have suffered a regional tilt to the west

We must emphasize that the age of the Woodleighstructure is not well-constrained stratigraphically either Moryet al (2000a 2001) described drill core obtained from 3 wellsdrilled in the Woodleigh area Woodleigh 1 which penetratedthe central uplift and Woodleigh 2 and 2A which weredrilled into the first ring syncline according to theterminology of Iasky et al (2001) The only core to intersectin situ impact-deformed lithologies is the Woodleigh 1 corethat penetrated the Proterozoic basement AlthoughWoodleigh 2A intersects what are thought to be upperSilurian lithologies at the base of the well whether thesestrata represent the crater floor or younger deposits is notknown as no shock deformation has been recorded (Mory etal 2001) The unnamed paraconglomerate (Mory et al 2001)formerly named a diamictite by Mory et al (2000a)intersected in borehole W-2A is a sediment that contains inour observations very rare small (2ndash3 cm) rounded andshocked basement clasts The age of this deposit wouldprovide a minimum age for the impact event but this unit


itself is poorly dated (Mory et al 2000b) It is known tocontain clasts with Permian palynomorphs and as such thisprovides the youngest constraint on the age of this unit Thelower Jurassic Woodleigh formation that overlies thisparaconglomerate (Mory et al 2001) therefore provides aminimum age constraint

The oldest age for the structure is only truly bound by theage of the late Proterozoic (circa 835 Ma Mory et al 2000a)basement lithologies that were shock deformed by the impactIasky et al (2001) Mory et al (2001) and Iasky and Mory(1999) provided detailed descriptions of the regionalstratigraphy for the area Limited seismic data were used tointerpret impact deformation of lower to middle Devoniansediments near the Yaringa 1 well at a radius ofapproximately 30 km from the center of the structure (Mory etal 2000a Iasky et al 2001) However these data are poorlyresolved (Iasky et al 2001) and the limited access to thesestrata by a few drilling ventures only precludes their use atthis stage for identification of impact deformation and thusalso of constraining the maximum age of the impact event

A recent comprehensive Geological Survey of WesternAustralia report of available geophysical data includingdetailed gravity aeromagnetics and seismic data andinterpretations was presented by Iasky et al (2001) Whilethese authors supported the 120 km diameter interpreted byMory et al (2000a b) they provided significant and criticaldiscussion of the difficulties of crater scaling in general andpertaining to the estimation of the size of Woodleigh inparticular Indeed Iasky et al (2001) considered thepossibility that the geophysical data could support asignificantly smaller crater diameter of 70ndash80 km Mory et al(2001) are also more circumspect on the diameter and statethat it is only ldquopossibly as large as 120 kmrdquo This was furtherconsidered by Renne et al (2002) who critically discussedthe regional geophysical anomaly maps and presented severalarguments that do not support a 120 km diameter Until theseissues are resolved we cannot use structural deformation ofunits as indicators of the relative stratigraphic age for theoverall structure

THE WOODLEIGH 1 DRILL CORE

The Geological Survey of Western Australia (GSWA)Woodleigh 1 (W-1) core was obtained by drilling directlythrough the center of the central uplift of the Woodleighstructure as identified by the Bouguer gravity anomaly(Mory et al 2000a Iasky et al 2001) The core is housed inthe GSWA core library Perth Western Australia where itwas viewed and sampled with the permission of the directorof the GSWA (approval S31459) The purpose of visiting theWoodleigh 1 core was to test the hypothesis of an impactorigin for the structure (Mory et al 2000b)

The initial observations of this drill core had revealedbasement samples displaying ldquomultiple sets of PDFs locally

curvedrdquo (Mory et al 2000a p 120) The core was drillednear-vertically into crystalline basement of late Proterozoicage and the entire recovered section from 1905 m to a finaldrilling depth of 3331 m (Mory et al 2001) comprisescrystalline basement Mory et al (2001) noted that theoverlying material was lost when materials were notpreserved from the initial drilling of the W-1 site but that theyhad comprised early Jurassic sediments to a depth of 171 mbefore intersecting basement The uppermost section of thecrystalline basement is also absent but was described as moreor less weathered granite No units that could representimpact melt melt-bearing breccia or Bunte Breccia-typematerial have been located or documented from above thecrystalline basement We were able to obtain representativesamples of the W-1 drill core from depths between 1945 and3267 m (for drill core stratigraphy and sample locations seeFig 2)

The core we examined is competent and intactthroughout most of its length Rare friable mica-rich bandsappear heavily chloritized and therefore altered Thematerial is a mixed (migmatitic) crystalline basement (Moryet al 2001) with quartz- and biotite-rich gneisses garnet-bearing gneisses microgranites granites amphibolites andamphibolitic or amphibole-biotite rich gneisses Thelithologies show a complex deformation history and are likelyof mixed protolith origins Unravelling the magmaticmetamorphic and deformation history of this basement is notpossible from this core alone Regionally a number oforogenic events has been documented (eg Myers 19901993) A sub-horizontal foliation is evident throughout mostof the core even in microgranites and suggests significantpost-emplacement deformation that is unlikely to be related tothe impact event Vertical displacements and fracturing arerare but do occur and appear to be more prevalent in theuppermost sections (nearer to 190ndash195 m)

MACROSCOPIC SAMPLE DETAILS PETROGRAPHIC DESCRIPTION AND PDFS

For this study 11 samples that are representative of thedifferent lithological types exhibited by drill core W-1 weretaken from various depths throughout the length of corethrough crystalline basement Samples were selected cut andslabbed with the assistance of Dr Arthur Mory (GSWAofficer) The position of the samples in this granitoid core aremarked on Fig 2 Detailed petrographic descriptions for allthese samples are provided in the AppendixPhotomicrographs of characteristic features are shown inFigs 3 and 4

Fracturing and Brecciation

Generally the samples are coherent brecciation andopen fracturing are only rarely observed However likely


Fig 2 Stratigraphic column of the Woodleigh 1 core also showing depths of sampling for this study Some typical rock fabrics are alsoshown (insets)


Fig 3 Photomicrographs of characteristic planar deformation features in quartz and in a microcline grain as typically observed in all samplesfrom drill core Woodleigh 1 used in this investigation (all images taken with crossed polarizers and widths of field of view of ~1 mm a) densearray of PDFs in 2 general crystallographic orientations and a prominent (north-south trending) set of parallel but non-planar fractures in quartzof sample W-12 (3267 m depth) b) quartz grain in sample W-5 (240 m depth) with one well-developed set of PDFs Note that this grainexhibits the strong ldquotoastedrdquo texture known from samples of numerous impact structures (Short and Gould 1996) The microcline grain aboveright contains twin lamellae in a northeast-southwest direction and faintly recognizable PDFs in a WNW-ESE direction c) sample W-5 quartzgrain with 2 prominent sets of decorated PDFs The alkali feldspar grain on the right also displaysmdashthough only vaguelymdash2 sets of short andtightly spaced decorated PDFs d) sample W-5 The quartz grain in the central area shows localized toasting and 2 sets of PDFs 3 other quartzgrains in this image have 1 set of PDFs each and the grain immediately to the northeast of the central grain displays one set of near-planarshock fractures e) sample W-12 (3267 m depth) microcline crystal with several small quartz inclusions that exhibit only some shortirregularly shaped fluid inclusion trails The microcline crystal exhibits a set of PDFs trending NE-SW (arrows) and tightly-spaced and shorttwin lamellae along a NW-SE direction that are much shorter than the lamellae related to the microcline tartan-twinning These short densely-spaced twin lamellae may also be the result of shock deformation


shock-induced fracturing is noted in places and a number ofbreccia veins at the sub-mm to mm scale are present Theyare not very abundant seem to be relatively concentrated inthe more amphibolitic parts of the core and certainly do notconstitute a penetrative network as reported by earlierworkers These breccia veins sometimes occur in sub-parallelarrays at mm to several mm spacings The geometries of theseveinlets are generally irregular Macroscopically we cannotidentify whether these breccia veinlets are cataclasite or meltbreccia of some sort

Microscopic examination reveals that at least some ofthese veinlets are composed of cataclastic material Theycontain fractured quartz and feldspar grains Some of thiscataclastic material appears isotropic and may be diaplecticglass but we must note that no presence of a melt (fused)component could be confirmed in these breccia zones Thediaplectic glass does not constitute a melt phase but fine-grained diaplectic material could be mistaken for being at themicroscopic scale unresolvable aphanitic material

A second type of breccia forms very thin veinlets thatmay be up to 100 mm wide The vein fillings are extremelyfine-grained and the nature of the matrix could not beresolved with the petrographic microscope They have abrown-pink color in plane polarized light and appear isotropicin cross-polarized light In plane polarized light a zone alongthe margins of such veins is composed of relatvely coarser-grained crystals This material constitutes the most reasonablecandidate for an actual melt component in Woodleighbasement samples One can reasonably assume that thismaterial corresponds to the ldquopseudotachyliterdquo alluded to byearlier workers (Mory et al 2000a 2001) In our sample suitewhich we consider representative of the basement section ofdrill core W-1 such veinlets are rare they only occur in 2 ofour 11 samples (compare Appendix) and only in very lownumbers

Backscattered electron imaging reveals a complextexture to this latter type of vein and confirms that it has anextremely fine-grained granular texture toward the centerIndividual grains of the vein fillings are small averaging2 mm in size At the margins of the veins these relativelydistinct crystals are missing and the material appearsamorphous but may just be very fine-grained (lt1 mm) Thiscan only be reconciled with our optical microscopicobservations when we assume that the actual veinlet is thinnerthan the outwards coarsening zones that are opticallydescribed The outer zones of relatively coarsened grain sizecould be the result of fine-grained recrystallization probablyfollowing annealing of material directly adjacent to a veinEnergy-dispersive X-ray emission analysis (EDX) of veinfillings shows that many tiny crystals contain Fe and Tiwhich is suggestive of the presence of titanium-bearingmagnetites This could of course also be a reason for whythese vein fillings appear isotropic at the optical microscopicscale The overall finest-grained granular texture is

interpreted here to represent alteration assemblages probablyin the form of clay mineral growth Alternatively it may be adevitrification product similar to the growth of somemagnetite in devitrified glass from Ries Crater suevites(Engelhardt et al 1995)

The timing of emplacement of veins of the 2 typesdescribed above appears to be complex as cross-cuttingrelationships are observed between the 2 types but we havenot been able so far to determine unequivocally whichveinlet type predates the other We have not observed anyevidence of flow within any of these veins or the apparentschlieren that would suggest flow

Previous workers (Mory et al 2000a b 2001 Uysal et al2001) referred to the presence of abundant pseudotachylite inWoodleigh basement samples We cannot state with certaintythat the narrow aphanitic veinlets observed by us representpseudotachylite (ie friction melt cf discussion in Reimold[1995 1998]) or alteration product after pseudotachylite Ifthey originally constituted a melt phase they may equallywell have been shock melt Mory et al (2000b) stated thatldquo PDFs pseudotachylite veins and breccia provideindisputable evidence of its [Woodleigh] impact originrdquoHowever neither the possible (altered) melt veins nor thecataclasites observed by us constitute definite evidence forthe presence of an impact structure (nor does the presence ofpseudotachylite equal friction melt per se) Only shockmetamorphic effects such as PDFs (cf below) can bequoted as bona fide impact evidence

Nevertheless some of the narrow aphanitic veinlets inWoodleigh samples W-8 and W-13 observed by us closelyresemble so-called ldquoshock veinsrdquo described from manygenerally strongly shocked (H6 L6) chondritic meteorites(eg Jackalsfontein Queenrsquos Mercy and others egBuchanan et al 2002) The true nature of the so-called shockveins in meteorites is also still debated do these narrowveinlets rarely wider than a cm in a few cases but sometimesforming up to dm-wide networks of finest veinlets form byfriction or by shock melting or even a combination of these 2processes (eg Reimold 1998 Langenhorst and Poirier 2000Kenkmann et al 2000 Langenhorst et al 2002) Furtherdetailed micro-analysis is required to ascertain the true natureof these veinlets in Woodleigh samplesmdashwhether theirmatrices indeed exclusively comprise alteration productsand whether they were originally some kind of melt breccia

Millimeter-wide fracture zones often at high angles tothe subhorizontal foliation of the gneisses do occur but arenot pervasive Many samples display local alteration effectsNarrow veinlets (generally lt15 mm wide and of irregularshape) filled with either quartz or carbonate cut across mostsamples but comprise lt1 vol in all the samples examinedQuartz veins observed are also generally lt1ndash2 mm wide andhave always been found to be shock deformed thus clearlypredating the impact event In our samples no evidence existsfor strong hydrothermal alteration as described from other


Fig 4 Deformation features (shock micro-deformation in several minerals from Woodleigh samples and a melt vein) in Woodleigh 1 drill coresamples a) decorated planar deformation features (PDFs) in alternating twin lamellae in a plagioclase crystal of sample W-5 (240 m depth)Width of field of view 355 mm crossed polarizers b) a single set of PDFs (arrows) in a microcline crystal in sample W-12 (3267 m depth)Width of field of view 355 mm crossed polarizers c) intensely kinkbanded relatively large biotite crystal in sample W-5 Width of field ofview 28 mm crossed polarizers d) combination of intense cleavage irregular small-scale fracturing and tight twinning in shockmetamorphosed amphibole from amphibolitic gneiss sample W-8 (2199 m depth) Width of field of view 355 mm crossed polarizers


impact structures (eg review by Naumov [2002]) althoughclay mineral growth may have taken place in at least some ofthe veinlets described above Presence of clay minerals inthese veins and not melt may also be supported by thechemical analyses offered by Mory et al (2001b) thatrecorded high volatile contents in the veins analyzed by them

Shock Microdeformation

In all our samples quartz displays extremely well-developed planar deformation features (Figs 3 and 4) Weconfirm that these features are in their majority shock(impact) diagnostic planar deformation features (PDFs)

Fig 4 Deformation features (shock micro-deformation in several minerals from Woodleigh samples and a melt vein) in Woodleigh 1 drill coresamples e) typical shock-induced cleavage and (sub)parallel fracturing in garnet of sample W-6 (200 m depth) Width of field of view 34mm plane polarized light f) aphanitic matrix veinlet in amphibolitic gneiss sample W-13 (272 m depth) The true originmdashby either frictionor shock meltingmdashis not yet certain No displacements across such veinlets have to date been noted Width of field of view 34 mm planepolarized light g) backscattered electron image of a vein in sample W-14 (1916 m depth) Bright areas are Fe-Ti rich and probably representTi-bearing magnetite surrounded by finer material thought to represent phyllosilicate minerals The darker material is comparatively enrichedin Al Si and Mg and could represent illite or montmorilloniteThe inset box refers to the magnified image shown as Fig 4h


Clearly the alleged ldquolocally curvedrdquo features reported byMory et al (2000a) do not represent PDFs All lithologiesexamined display the PDF deformation phenomenon Ourinitial optical examinations seemed to indicate that PDFsoccur more abundantly and with on average higherfrequencies of PDF sets per grain in samples from higherlevels in the core but this must be compared against the PDFstatistics presented below These samples also displayoccasional PDF formation in feldspar as well as localisotropization of felsic minerals (more pronounced in feldsparthan in quartz) Reduced birefringence is frequently noted inboth quartz and feldspar

Localized melting in samples from various depths ismanifest as small pockets with isotropic matrix andcontaining small mineral fragments Garnets exhibit well-developed cleavage as has been described in a number ofimpact structures (eg Stoumlffler 1972 1974 Dressler 1990)Biotite (Fig 4c) muscovite and chlorite where present arewell kinkbanded Amphibole from the 2199 m and 272 mlevels in the core displays distinct twinning whichpresumably has been imparted on this mineral as aconsequence of shock deformation In addition amphibolesare intensely cleaved and the combination of twinning andcleavage imparts a mosaic texture (Fig 4d) to these crystalsDespite an extensive search we did not observe any shocked

zircon crystals in our samples as reported earlier by Mory etal (2000a) and Uysal et al (2001)

In contrast to our general shock degree assessment ofentire thin sections the statistics of the number of sets ofPDFs per quartz grain in those randomly selected quartzgrains on which the crystallographic orientations of PDF setswere measured using a universal stage (Table 1a) as well asin hundreds of quartz grains statistically evaluated bypointcounting (Table 1b) do not confirm this assumption Thestatistics for samples from the entire core interval aresurprisingly similar The percentage of unshocked or weaklyshocked (undulatory extinction and minor irregularfracturing Table 1b) quartz is also rather similar We mustassume that the differences between the observed samples aremore a function of lithological differences than of shockpressure variation

Samples from greater depth do however display lesswidespread isotropization which can be interpreted asindicating somewhat reduced overall shock pressures from30ndash35 GPa at the top to 25ndash30 GPa at the bottom of thestudied interval The samples from greater depths also displaya stronger degree of brittle deformation in the form of micro-faulting of felsic minerals shear fracturing and a generallyhigher intergranular fracture density than observed in samplesfrom higher levels of the drill core Given the variety of

Table 1a Statistics of No of PDF sets in those grains for which the crystallographic orientations of PDFs shown in Fig 4 were obtained

Number of grains with (n) sets of PDFsSample Number of grains anal Number of planes anal 1 2 3 4(n)

W-7 61 130 1 51 9 0W-6 51 107 1 44 6 0W-8 31 60 4 25 2 0W-9 33 75 1 22 10 0W-3 51 104 2 45 4 0W-5 40 85 1 33 6 0W-2 63 129 2 56 5 0W-1 22 47 2 15 5 0W-4 51 104 4 42 4 1W-13 23 45 0 22 1 0W-12 42 92 0 31 11 0

Table 1b Number of sets of PDFs in quartz grans of W-1 samples Numbers in brackets are percentagesShocked quartz with n sets of PDFs Unshocked quartz

Sample 1 setgr 2 setsgr 3 setsgr or with und ext only Total No

W-1 27 (154) 99 (565) 24 (137) 25 (143) 175W-2 54 (248) 116 (532) 7 (32) 41 (188) 218W-3 17 (73) 148 (635) 47 (202) 21 (90) 233W-4 45 (144) 189 (607) 61 (196) 17 (54) 312W5 18 (83) 131 (601) 32 (147) 37 (170) 218W-6 11 (37) 173 (588) 79 (269) 31 (105) 294W-7 44 (190) 121 (522) 12 (52) 55 (237) 232W-8 51 (323) 92 (582) 4 (25) 11 (70) 158W-9 32 (174) 106 (576) 19 (103) 27 (147) 184W-12 23 (148) 117 (755) 11 (71) 5 (32) 155W-13 53 (235) 147 (644) 9 (40) 16 (71) 225


lithologies including felsic and mafic samples of more orless strongly developed gneiss fabric and varied mineralcompositions further quantification of this decrease indeformation level with depth would require study of a largernumber of specimens

We have carried out a detailed analysis of the orientationof planar deformation features in quartz on all samplesavailable Several hundred PDF orientations (Fig 5) weredetermined by universal-stage measurement on quartz grainsthat had been selected randomly Data were then treatedaccording to the methodology described for example byStoumlffler and Langenhorst (1994) and French (1998)employing a Wulff stereographic projection and making useof the c-axis-vertical template for quartz Between 45 and 130measurements were carried out per sample On average841 of measurements per sample could be indexed withimportant crystallographic orientations (non-indexedproportions fluctuate between 95 and 217 for individualsamples) The dominant orientations measured in all 12samples are the 1013 and to a lesser degree the 1012orientations Basal orientations are consistently andsurprisingly rare These results are in agreement with ourabove conclusion that the degree of shock metamorphismdisplayed over the sampled section of drill core W-1 does notchange significantly in terms of PDF statistics Ourquantitative observation that shock metamorphism decreasessomewhat from top to bottom of this interval is not confirmedby these PDF orientation statistics either

GEOCHEMICAL ANALYSIS

Major and Trace Element Analysis

All samples available were analyzed for major and traceelement compositions by X-ray fluorescence spectrometry(XRF) at the University of the Witwatersrand in Johannesburgand by instrumental neutron activation analysis at theUniversity of Vienna Details about the analytical proceduresincluding information on instrumentation standards accuracyand precision of the data have been published by Reimold etal (1994) and Koeberl (1993) The combined results for theWoodleigh samples are listed in Table 2 The purpose for thisgeochemical work is manifold 1) to provide a database forcompositions of important basement rock types for comparisonwith compositions of breccia veinlets 2) to follow up on theclaims by Mory et al (2000a b) that some samples containedan apparent enrichment in siderophile elements as aconsequence of meteoritic contamination and 3) to investigatethe PGE abundances and patterns for Woodleigh basementrock samples to evaluate whether any siderophile enrichmentexists and could be of extraterrestrial origin as suggested byprevious workers or only represents target rock heterogeneity

The sample suite available comprises gneiss and granitesamples which are significantly varied with regard to

chemical composition Mafic to felsic compositions wereanalyzed in accordance with the mineralogical classificationof these samples Those samples characterised by relativelyhigh loss on ignition values also have the highest modalpercentages of hydrous ferromagnesian minerals (biotite andor amphibole) However these values should also be taken asan indication of alteration of feldspar as well as vein fillings(see above) and late (post-impact) fracture fill

The compositions of the gneisses and granitoids arepresented in Table 2 Silica contents vary from 536 to706 wt from mafic gneiss to granite Some of thecompositional variation also reflects the alteration of theserocks The trace element abundances are also highly variableand do not correspond to any sort of ldquostandard rockrdquocomposition as implied in the rather limited discussion ongeochemistry by Mory et al (2000) who in any casereported upper limits of abundances only

Chondrite-normalized rare earth element (REE)abundance patterns for the 11 samples are compared in Fig 6All the samples have slightly enriched light REE abundancesresulting in patterns with moderately negative slopes Only aleucocratic gneiss from the 220 m level and a granite samplefrom 3267 m deep have slightly more fractionated patternsThe 2 amphibolitic samples (2199 and 272 m deep) displayrather flat patterns while most of the other more felsicsamples are characterized by pronounced Eu anomalies inkeeping with their feldspathic nature The patterns for thefelsic gneisses are typical for felsic to intermediate crustalrocks (eg Taylor and McLennan 1985)

All these samples display very low concentrations of IrNi Co and Cr (Table 2) The iridium contents determined aregenerally below the detection limit at about 1 ppb forneutron activation analysis (also compare with the PGE datagiven below) Concentrations of the other 3 elements arerelatively low but not very distinct from average crustalcompositions The highest values are noted for theamphibolitic gneiss from 2199 m deep These data willprovide a useful baseline for potential future attempts to usechemical data for the pursuit of a meteoritic component inWoodleigh lithologies

Oxygen Isotopic Systematics

Golding et al (2001 2002) reported oxygen isotope datain rocks from the shocked granitoid from W-1 and attemptedto use these data to identify the presence of a possiblemeteoritic component This is a puzzling approach as (onaverage) more than 99 of all known impact melt rocks andbreccias from other terrestrial impact craters are of terrestrialorigin and a meteoritic component typically comprises nomore than 1 of such a rock (see review by Koeberl 1998)In addition the identification of a meteoritic component in animpact-derived melt rock is based on excess abundances ofelements (or isotopes) that are enriched in meteorites


Fig 5 PDF orientation diagrams in quartz from the Woodleigh 1 core As PDFs are abundant in all samples we present the orientation dataseparately for each sample which allows us to compare PDF development at various levels in the basement Corresponding statistics arepresented in Table 1


Table 2 Chemical composition of eleven samples from the Woodleigh 1 coreaSample WL-7 WL-6 WL-8 WL-9 WL-3 WL-5 WL-2 WL-1 WL-4 WL-13 WL-12Depth (m) 1945 200 2199 220 2243 2405 2455 2589 2643 272 3267

Gneiss GneissAmph Gneiss Gneiss

Bio Gneiss Granite

Micro-granite Gneiss Gneiss

Am B Gneiss Granite

SiO2 5953 6723 4517 7024 5547 6460 6923 5355 7038 4691 7063TiO2 162 068 168 034 177 034 046 178 068 230 009Al2O3 1197 1024 1592 1124 1434 1653 1559 1407 1339 1390 1567Fe2O3 1051 580 1388 510 1163 280 261 1297 573 1435 054MnO 014 012 024 030 014 008 005 013 013 017 004MgO 401 311 759 147 523 108 111 568 172 712 006CaO 259 271 587 324 155 281 355 199 122 414 033Na2O 028 052 045 135 068 349 385 091 184 185 182K2O 389 312 278 223 534 509 154 506 362 361 946P2O5 030 056 022 014 037 084 018 044 010 039 015LOI 521 511 616 353 343 224 167 334 171 434 099

Total 10005 9920 9996 9918 9995 9990 9984 9992 10052 9908 9978

Sc 234 157 533 127 278 508 381 276 163 415 105V 285 125 517 83 260 39 45 293 109 nd lt15Cr 637 396 147 383 971 35 112 904 903 167 09Co 258 150 417 348 262 507 621 303 152 395 147Ni 24 11 50 30 41 6 8 37 28 45 7Cu lt2 13 3 156 lt2 lt2 lt2 lt2 lt2 nd lt2Zn 140 180 140 98 180 45 45 155 65 120 16Ga 15 15 11 6 15 6 3 13 10 8 3As 014 052 034 019 028 148 030 028 022 025 034Se 04 05 09 09 04 04 04 11 08 06 02Br 08 09 09 06 05 07 12 07 11 09 04Rb 285 236 160 126 390 170 581 389 230 165 321Sr 52 59 81 68 50 174 232 102 123 nd 136Y 24 32 27 31 30 42 4 27 21 nd 12Zr 110 35 95 50 155 30 175 235 180 120 20Nb 13 11 11 7 21 12 7 32 15 nd 6Sb 021 021 015 0072 011 032 027 014 011 028 0095Cs 113 399 525 451 992 221 095 17 547 293 237Ba 277 182 109 73 524 455 282 784 461 465 630La 191 781 452 192 291 238 305 193 538 193 389Ce 378 171 121 348 535 519 499 358 944 419 771Nd 203 125 913 164 278 306 209 194 423 239 489Sm 507 426 336 337 577 757 231 374 811 552 137Eu 094 041 126 073 067 105 071 057 094 184 077Gd 50 44 47 42 54 93 14 35 72 68 12Tb 073 088 094 074 083 152 015 055 096 108 016Tm 038 053 061 046 041 047 005 023 058 051 006Yb 232 339 439 334 257 246 027 147 345 311 029Lu 032 045 065 051 039 033 0038 022 055 041 0042Hf 337 112 313 132 485 065 471 727 596 435 033Ta 109 138 046 038 139 078 016 279 105 089 044W 11 126 17 83 13 24 14 103 12 15 34Ir (ppb) lt1 01 lt1 lt1 lt1 02 lt1 03 lt1 lt1 lt1Au (ppb) 03 02 06 06 03 lt2 05 05 lt2 02 lt1Th 694 288 016 628 620 206 129 428 315 311 248U 828 895 196 945 318 391 052 225 241 085 391

KU 3915 2905 11820 1966 13994 10848 24679 18741 12517 35392 20162ZrHf 326 313 304 379 320 462 372 323 302 276 606HfTa 309 081 680 347 349 083 2944 261 568 489 075ThU 084 032 008 066 195 527 2481 190 1307 366 063LaNYbN 556 156 070 388 765 654 7633 887 1054 419 906EuEu 057 029 097 059 037 038 121 048 038 092 184

aMajor element data in wt trace element data in ppm except as noted Total Fe is reported as Fe2O3 Am B = Amphibole-Biotite


compared to the terrestrial target rocks It is not clear whyGolding et al (2001) expected to be able to identify ameteoritic component using an isotopic signature for anelement that makes up most of the target and the projectileand an extraterrestrial component of which would be presentin amounts significantly less than 1 in an impact melt rocksystem Another puzzling point is why they used thisapproach on samples that if at all contain only a tiny volumepercentage of impact-generated breccia in sub-crater-floorcrystalline basement In fact no tangible evidence exists thatany impact melt injection could have been sampled byborehole W-1 Finally note that the fracture system observedin this drill core could have provided pathways for post-impact hydrothermal solutions which could also haveaffected the stable isotope systematics of these samples

Nevertheless we undertook a study using the high-precision technique routinely used to analyze O isotopes ofmeteorites (Miller et al 1999) Four subsamples of theuppermost (19 m) shocked granitoid from W-1 wereseparated for O-isotope analyses The samples were selectedfrom a sub-vertical brittle fracture in the granite and adjacentunbrecciated rock at either side of the fracture Whether thefracture was impact produced is not known but the sample is

shocked and in thin section the brittle fractures of this typecontain cataclastic granitoid and possibly an aphaniticcomponent though we are less certain of this Chips of thefractured material (sample 1) and the mixed quartz andfeldspar crystals (samples 2 3 and 4) were separated Thesamples were individually crushed and powdered and thenanalyzed using the laser fluorination technique on the systemdescribed by Franchi et al (1992) and Miller et al (1999)The samples were fused under high vacuum beforefluorination Results were compared with an internal standardof obsidian with errors for d18O of plusmn0095permil and for d17O ofplusmn0026permil (Miller et al 1999) Duplicate analyses were run forsamples 1 and 4 The results are quoted using the standardnotation (d) in per mil (permil) and relative to SMOW (StandardMean Ocean Water)

The results listed in Table 3 unquestionably plot on theterrestrial fractionation line within error This is notsurprising as any shift from the terrestrial fractionation trendwould require incorporation of a substantial contributionfrom a meteoritic projectile into the sample As generally lessthan 1 meteoritic contamination has been detected in otherimpact structures that any meteoritic contamination could bedetected using the O-isotopic system is highly unlikely and

Fig 6 Chondrite-normalized rare earth element (REE) diagram of the various rocks from the Woodleigh 1 core (Amph = AmphiboleBioB = biotite)


our results confirm that this is the case with the presentWoodleigh basement samples

Platinum Group Elements

Concentrations of the platinum-group elements (PGE)and Au were determined using a modified nickel sulfide fireassay procedure followed by Te coprecipitation and analysisby ICP-MS Further details of the method includingequipment detection limits assessment of accuracy andtypical reagent blanks can be found in Koeberl et al (2000)McDonald et al (2001) and Huber et al (2001) Due to thelimited amount of sample material all but one of the sampleswere analysed as single powder aliquots so that precisioncould not be determined by duplicate analyses (cf McDonald1998) For noble metal concentrations of less than 03 ppbthe uncertainties (expressed as the coefficient of variation) arelikely to range between 30 and 70 due to the small massesof samples used The uncertainties on concentrations between03 and 10 ppb are estimated at 15ndash30 and theuncertainties on concentrations above 10 ppb are probablylt15 (compare Koeberl et al 2000 Huber et al 2001McDonald et al 2001)

In an earlier preliminary report Koeberl et al (2001)described apparently strong enrichment in Rh and Pt relativeto the other PGE in this suite of Woodleigh samples andsuggested that if this enrichment was real it might indicatethat oxide transport and fractionation of these 2 metalsoccured Subsequent analysis has shown that the apparentlyhigh Rh and Pt concentrations were caused by unusually high

Rh and Pt in a new batch of HCl that was used to digest thefinal noble metal concentrate before analysis by ICP-MS TheRh and Pt concentrations in Table 4 have been corrected (byfollow-up analysis of the HCl for PGE) for this additionalblank contribution However this is not an ideal solution andcaution should be exercised in the interpretation of these data(see below and discussion)

CI chondrite-normalized plots of the PGE data are shownin Fig 7 The patterns are generally fractionated relative tochondrite and exhibit complexity in the middle portion with adistinctive humped appearance (enrichment in Ru Rh and Ptrelative to Ir and Pd) for some samples (eg W-5 W-6 and W-8) Given the possibility that blank Rh and Pt might not havebeen fully corrected out for these samples to ascribe any majorsignificance to this feature would be premature at this stage Nosamples exists where the PGE patterns could be interpreted toindicate significant evidence of meteoritic contaminationGold is enriched relative to the other metals in all of thesamples but it is not currently possible to tell whether this isa primary lithological feature or something imposed at a laterstage for example due to hydrothermal ateration

Note that the samples with the highest Ir concentrations(WL1 and WL8) are among the most mafic in our sample suite(see Table 2) WL8 contains a vein that may contain some melt(as discussed above we cannot determine whether thesestrongly altered veins ever contained significant melt and ifthey did whether it was pseudotachylitic melt or shock melt)but the RuIr ratio in this sample is 308 which is more thantwice the chondritic ratio Ir and Ru concentrations and the PtIr and PdIr ratios in WL1 and WL8 are more similar to high-Mg lavas or volcaniclastics (Brace and Wilton 1990Greenough and Owen 1992 Zhou 1994 Reimold et al 2000)than to impact melts (cf McDonald et al 2001) Therefore thePGE in the more mafic gneisses are more likely to reflectformer ldquogreenstonerdquo llithologies in the basement than anycontribution from a meteoritic projectile Also interestinglysample WL9 which contains late sulfide mineralization in theform of pyrite contains the lowest PGE concentrations whichcan be interpreted to support the idea that this sulfide-formingevent did not mobilize or bring significant PGE into the system

Table 3 O isotopic resultsa

aResults are in per mil (permil)

Sample d18O d17O D17O

1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037

Table 4 Platinum-group element and gold concentrations (ppb) in Woodleigh samplesa

Ir Ru Rh Pt Pd Au

W-1 042 045 045 285 122 154W-2 026 024 008 129 188 066W-3 nd lt015 038 400 098 044W-4 016 lt015 070 454 225 210W-5 013 plusmn 004 119 plusmn 022 024 plusmn 004 194 plusmn 018 059 plusmn 015 127 plusmn 041W-6 009 152 045 294 048 348W-7 019 122 039 365 400 304W-8 064 197 062 356 066 453W-9 lt005 lt015 lt006 065 061 165W-12 029 043 008 129 089 061

aNotes WL5 represents the mean and standard deviation of duplicate analyses The data for all other samples are single analyses ldquondrdquo = not detected TheRh and Pt values presented are the blank-corrected data (compare text)


The only sample appearing slightly anomalous is WL12This granite shows the least evidence for shockmetamorphism and has low concentrations of Cr Co and Nibut it contains 029 ppb Ir has a broadly chondritic RuIrratio (148) and the normalized PGE pattern in this sample isthe least fractionated relative to chondrite

DISCUSSION

The most important finding from this work is that thepetrographic observations and particularly the work on PDFsand their orientations have confirmed that Woodleigh is an

impact structure This finding is in agreement with theconclusions of Mory et al (2000b)

The lack of any impact melt-bearing breccias in the W-1core suggests extensive erosion of the Woodleigh structure(Hough et al 2001) The Woodleigh basement samplespreserve pre-existing metamorphic textures and areextensively shocked with abundant planar deformationfeatures in quartz While the overall optical examinationsuggested that the shock level decreases with depth in thesamples studied based on a presumed decrease of the averagenumber of sets of PDFs per grain from higher-level to lower-level parts of the drill core this is not confirmed by statisticsof the number of sets of PDFs per quartz grain Even thesample from 3267 m depth still contains a significant numberof quartz grains with 1013 and 1012 orientations whichare generally taken to indicate average shock pressure on theorder of 15ndash25 GPa That some samples have dense PDFpatterns in nearly every felsic mineral grain (comparedescription of sample W-9 in the Appendix) suggests that theobserved shock textures relate to shock pressures as high as30 GPa and the abundance of diaplectic quartz andorfeldspar glass displayed by some samples requires shockpressures around 30ndash35 GPa The fact that quartzisotropization is more abundant in the upper part of this coreinterval is suggestive of a slight (maximum) drop-off of shockpressure from 30ndash35 GPa at the top to 25ndash30 GPa at thebottom of this core section Shock pressure distribution acrossa sample clearly is not homogeneous the observations favorsignificant heterogeneity at the cm scale

Some of our samples exhibit fracturing and formation ofbreccia veinlets at the microscopic scale which are believedto be related to the impact event However they are devoid ofsignificant melt Should melt be present at all such veinletsdo not constitute a major or even a significant componentover the width of the studied core interval In fact only 2 ofour samples have a significant number of these micro-veinsSome of the brecciation is clearly cataclastic but this has notled to a marked reduction of the competency of the drill-coreVertical shears and fractures present on the macro-scale arehighly localized and rare

In comparison with the uppermost section of crystallinebasement from the Ries crater in Germany the fracturing atWoodleigh is minimal At the Ries the granitic basement iscompletely dissected by vertical-subvertical fractures thatupon closer examination are cataclastic breccia and devoid ofmelt very similar to the rare fractures in these Woodleighbasement samples These breccia-filled fractures and micro-zones do not constitute pseudotachylite as claimed by earlierworkers The shock level of these Ries samples wasconsidered low (Pohl et al 1977 von Engelhardt 1997)Similar cataclastic fracture zones in impact structures havebeen considered occasionally as pseudotachylite (extensivediscussion in Reimold [1995 1998]) We have not found anypetrographic or structural (eg notable displacements along

Fig 7 CI chondrite-normalized noble metal plot of Woodleighsamples Chondrite PGE values are taken from Jochum (1996)Platinum-group metals and Au are plotted in order of decreasingmelting points No distinct meteoritic component in the Woodleighsamples is evident


such veins) evidence that supports the presence ofpseudotachylite (friction melt) in these Woodleigh samplesAlso we cannot show or reject unequivocally that at leastsome of these veinlets could represent altered shock melt

Although the Ries crater is very much smaller thanWoodleigh and is comparatively well preserved it providesan interesting comparison with regard to the nature ofbasement in the very center of the Woodleigh structure Thefractures in Woodleigh samples that contain brecciatedmaterial could be analogous to those in the central upliftedbasement at the Ries The fact that brecciation is much less atWoodleigh than at the Ries may be the result of acomparatively somewhat deeper level of the impact structureaccessed in the Woodleigh case with the higher-level materialhaving been removed by erosion

This interpretation does not support the hypothesis ofMory et al (2000a b) that impact melt might be present in theform of narrow veinlets reported by these authors to contain ameteoritic component Our interpretation of statements in thisearlier work is that the ldquopseudotachyliterdquo reported to show ameteoritic component would have to be regarded as injectedimpact melt rather than in situ formed friction melt(pseudotachylite) (Reimold and Koeberl 2000) How couldtraces of the projectile become mixed into pseudotachyliteformed in situ below the crater floor Volatilization (additionof a meteoritic component transported via a vapor phase) wassuggested and appears in theory to be an interesting processbut no supporting evidence exists to show that this processhas in reality lead to meteoritic contamination of basementrock samples or within-basement formed breccias atWoodleigh One would have to assume an extensive opensystem of pores or fractures along which meteoriticallycontaminated vapor could be injected to not insignificantdepth below the crater floor This depth level is indicated asalready discussed above by the lack of more extensive andsevere shock deformation as well as our observation thatveinlets that could have but have not necessarily originallycontained melt are relatively rare

Our chemical data provide good characterization of thecomposition of the target rocks In contrast to the limited dataof Mory et al (2000a) who only gave upper limits ofchemical abundances but then proceeded to makeinterpretations based on element ratios () our data can beused to characterize the target rocks properly

The currently available PGE data for Woodleigh samplesdo not represent clear evidence for the presence of meteoriticcontamination in the basement granitoids and gneisses asreported by Mory et al (2000a 2001) The apparent Rh and Ptfractionation noted by Koeberl et al (2001) was most likelycaused by an anomalous reagent blank and is not believed torepresent meteoritic contamination Some mafic gneissescontain elevated PGE concentrations but these are thought tobe primary chemical features of the basement rather than acontribution from the projectile One granite sample (WL12)

shows a PGE anomaly albeit without accompanyingenrichment in Ni or Cr that may merit further investigationDespite the widespread evidence for shock metamorphism inthe basement rocks the suggestion that they containcontamination from the projectile in the form of melt orcondensed vapor as suggested by Mory et al (2000a)remains unsupported and as explained aboveunsubstantiated

The oxygen isotopic composition of Woodleighbasement samples fails to identify any meteoriticcontamination which is no surprise considering that only 1of meteoritic contamination typically occurs in target rocks inimpact events (eg Koeberl et al 1998) Even a system thatroutinely analyzes small (mg) quantities of meteorites(Franchi et al 1999) is not suitable for the detection of suchdilute contamination in impact target rocks The oxygenisotopic compositions that we obtained represent typicalcrustal values We did note the presence of volatiles in thesesamples upon fusion which agrees with Mory et al (2000a)who also noted volatile enrichment in Woodleigh materialWe consider this to be a likely result of secondary and post-impact alteration leading to volatile-rich clay mineralformation dominantly in the zones of brittle failure

SUMMARY AND CONCLUSIONS

Detailed petrographic and chemical including oxygenisotopic and platinum group element analysis of basementsamples from the W-1 borehole into the central uplift of theWoodleigh structure yielded these results

1 The presence of ample evidence of shock metamorphismin the form of PDFs and diaplectic glasses confirms theimpact origin of the Woodleigh structure At best a slightdecrease of shock deformation degree of about 5 GPa issuggested by our PDF and quartz isotropizationobservations and is also indicated by decreasingmesoscopic and microscopic fracturing and brecciationintensity with depth

2 No diagnostic evidence for the presence of any type ofmelt breccia (impact melt injection or in situ-producedfriction melt (ie pseudotachylite) was observed Somenarrow cataclastic veins do exist but anythingresembling aphanitic vein filling is also heavily altered tosuch an extent that the primary phase(s) can no longer beidentified

3 Major and trace element analyses provide a precisedatabase characterizing the basement lithologicalvariability None of these samples displays any evidencesupporting the presence of a meteoritic component asclaimed by earlier workers

4 Similarly oxygen isotopic analysis does not provide anysupport for the earlier claim that a meteoritic componentis present in samples from this drill core section

5 PGE concentration patterns also do not provide clear


evidence for the presence of a meteoritic componentFractionation patterns range from barely to stronglyfractionated relative to chondritic compositions In theabsence of a likely carrier phase for a meteoriticcomponent (impact melt injection) in this particulargeological setting (below the crater floor in the centraluplift) the presence of a meteoritic component would behighly unlikelyIn conclusion Woodleigh represents a large impact

structure However much work remains to be done withregard to just about every aspect of this structure its actualsize its age its internal structure the possible presence ofimpact breccias and the presence and composition of ameteoritic contribution This study provides a detailedaccount of the petrography of the W-1 drill core and a base ofquantitative chemical data onto which future work can build

AcknowledgmentsndashWe are grateful to the Director of theGeological Survey of Western Australia for permitting us tostudy the Woodleigh 1 core and to sample it for this work DrMartin Miller and Jenny Gibson of the PSSRI at The OpenUniversity are thanked for the stable isotope analyses Thiswork was supported by the Austrian Science Foundationproject Y58-GEO (to C Koeberl) the Royal Society andPPARC of the UK (to R M Hough) the LeverhulmeFoundation (to I McDonald) and the National ResearchFoundation of South Africa and the Research Council of theUniversity of the Witwatersrand (to W U Reimold) Criticalreviews by James Whitehead and an anonymous referee areappreciated This is University of the Witwatersrand ImpactCratering Research Group Contribution No 60

Editorial HandlingmdashDr Richard Grieve

REFERENCES

Brace T D and Wilton D H C 1990 Platinum-group elements in theArchean Florence Lake Group central Labrador The CanadianMineralogist 28419ndash429

Buchanan P C Reimold W U and Koeberl C 2002 Melt veins inthe Jackalsfontein L6 meteorite from South Africa (abstract1073) 33rd Lunar and Planetary Science Conference CD-ROM

Dressler B O 1990 Shock metamorphic features and their zoningand orientation in the Precambrian rocks of the Manicouaganstructure Quebec Canada Tectonophysics 171229ndash245

Engelhardt W v 1997 Suevite breccia from the Ries impact craterGermany Petrography chemistry and shock metamorphism ofcrystalline rock clasts Meteoritics amp Planetary Science 32545ndash554

Engelhardt W v Arndt J Fecker B and Pankau H G 1995 Suevitebreccia from the Ries Crater Germany Origin cooling historyand devitrification of impact glasses Meteoritics 30279ndash293

Franchi I A Akagi T and Pillinger C T 1992 Laser fluorinationof meteoritesmdashSmall sample analysis of d17O and d18O(abstract) Meteoritics 27222

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structures

Contribution No 954 Houston Lunar and Planetary Institute120 p

Golding S D Uysal I T Glikson A Y Mory A J Baublys K Aand Glikson M 2001 Stable isotopic studies and isotopic datingof impact related alteration minerals Woodleigh impactstructure Carnarvon Basin Western Australia (abstract 3509)11th Annual V M Goldschmidt Conference

Golding S D Uysal I T Baublys K A Glikson A Y and Mory AJ 2002 Stable isotope geochemistry of impact related alterationphases from the Woodleigh impact structure Western Australia(abstract) 12th Annual V M Goldschmidt ConferenceGeochimica et Cosmochimica Acta 66A283

Greenough J D and Owen J V 1992 Platinum-group elementgeochemistry of continental tholeiites Analysis of the LongRange dyke swarm Newfoundland Canada Chemical Geology98203ndash219

Grieve R A F 1991 Terrestrial impact The record in the rocksMeteoritics 26175ndash194

Grieve R A F Rupert J Smith J and Therriault A M 1995 Therecord of terrestrial impact cratering GSA Today 5189ndash196

Henkel H Reimold W U and Koeberl C 2002 Magnetic andgravity model of the Morokweng impact structure South AfricaJournal of Applied Geophysics 49129ndash147

Hocking R M Moors H T and van de Graaff W J E 1987Geology of the Carnarvon basin Western Australia WesternAustralia Geological Survey Bulletin 133 289 p

Hough R M Lee M R and Bevan A W R 2001 Shocked quartzand more Woodleigh impact structure Western Australia(abstract) Meteoritics amp Planetary Science 36A84ndashA85

Huber H Koeberl C McDonald I and Reimold W U 2001Geochemistry and petrology of Witwatersrand and Dwykadiamictites from South Africa Search for an extraterrestrialcomponent Geochimica et Cosmochimica Acta 652007ndash2016

Iasky R P and Mory A J 1999 Geology and petroleum potential ofthe Gascoyne platform Southern Carnarvon basin WesternAustralia Western Australia Geological Survey Report 69 46 p

Iasky R P Mory A J and Blundell K A 2001 The geophysicalinterpretation of the Woodleigh impact structure SouthernCarnarvon basin Western Australia Western AustraliaGeological Survey Report 79 41 p

Jochum K 1996 Rhodium and other platinum-group elements incarbonaceous chondrites Geochimica et Cosmochimica Acta 603353ndash3357

Kenkmann T Stoumlffler D and Hornemann U 2000 Formation ofshock-induced pseudotachylites along lithological interfacesMeteoritics amp Planetary Science 351275ndash1290

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small sample analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1998 Identification of meteoritical components inimpactites In Meteorites Flux with time and impact effectsedited by Grady M M Hutchison R McCall G J H andRothery D A Special Publication 140 London GeologicalSociety of London pp 133ndash152

Koeberl C Reimold W U McDonald I and Rosing M 2000Search for petrographic and geochemical evidence for the lateheavy bombardment on Earth in early Archean rocks from IsuaGreenland In Impacts and the early Earth edited by Gilmour Iand Koeberl C Lecture Notes in Earth Sciences 91 BerlinSpringer-Verlag pp 73ndash97

Koeberl C Hough R M Boamah D French B M McDonald Iand Reimold W U 2001 Woodleigh impact structure AustraliaShock petrography and geochemical studies (abstract)Meteoritics amp Planetary Science 36A101


Langenhorst F and Poirier J P 2000 Anatomy of black veins inZagami Clues to the formation of high-pressure phases Earthand Planetary Science Letters 18437ndash55

Langenhorst F Poirier J P Deutsch A and Hornemann U 2002Experimental approach to generate shock veins in single crystalolivine by shear melting Meteoritics amp Planetary Science 371541ndash1553

McDonald I 1998 The need for a common framework for collectionand interpretation of data in platinum-group elementgeochemistry Geostandards Newsletters 2285ndash91

McDonald I Andreoli M A G Hart R J and Tredoux M 2001Platinum-group elements in the Morokweng impact structureSouth Africa Evidence for the impact of a large ordinarychondrite projectile at the Jurassic-Cretaceous boundaryGeochimica et Cosmochimica Acta 65299ndash309

Miller M F Franchi I A Sexton A S and Pillinger C T 1999High precision d17O measurements of oxygen from silicates andother oxides Methods and applications Rapid Communicationsin Mass Spectrometry 131211ndash1217

Mory A J Iasky R P Glikson A Y and Pirajno F 2000aWoodleigh Carnarvon basin Western Australia A new 120 kmdiameter impact structure Earth and Planetary Science Letters177119ndash128

Mory A J Iasky R P Glikson A Y and Pirajno F 2000b Responseto ldquoCritical Comment on A J Mory et al 2000 WoodleighCarnarvon basin Western Australia A new 120 km diameterimpact structurerdquo by W U Reimold and C Koeberl Earth andPlanetary Science Letters 184359ndash365

Mory A J Pirajno F Glikson A Y and Coker J compilers 2001GSWA Woodleigh 1 2 and 2A well completion reportWoodleigh impact structure Southern Carnarvon basin WesternAustralia Western Australia Geological Survey Record 20016147 p

Myers J S 1990 Pinjara orogen In Geology and mineral resourcesof Western Australia Western Australia Geological SurveyMemoir 3 pp 265ndash274

Myers J S 1993 Precambrian history of the West Australian Cratonand adjacent orogens Annual Review of Earth and PlanetaryScience 21453ndash485

Myers J S and Hocking R M 1998 Geological map of WesternAustralia 1250000 13th edition Perth Western AustraliaGeological Survey

Naumov M V 2002 Impact-generated hydrothermal systems Datafrom Popigai Kara and Puchez-Katunki impact structures InImpacts in Precambrian shields edited by Plado J and PesonenL J Impact studies Volume 2 Berlin Springer-Verlag pp 117ndash171

Pirajno F 2001 Appendix 3 Petrology In GSWA Woodleigh 1 2 and2A well completion report Woodleigh impact structure SouthernCarnarvon basin Western Australia compiled by Mory A JPirajno F Glikson A Y and Coker J Western AustraliaGeological Survey Record 20016 pp 41ndash64

Pohl J Stoumlffler D Gall H and Ernstson K 1977 The Ries impactcrater In Impact and explosion cratering edited by Roddy D JPepin R O and Merrill R B New York Pergamon Press pp343ndash404

Reimold W U 1995 Pseudotachylite in impact structuresmdashGeneration by friction melting and shock brecciation A reviewand discussion Earth-Science Reviews 39247ndash265

Reimold W U 1998 Exogenic and endogenic breccias A discussionof major problematics Earth-Science Reviews 4325ndash47

Reimold W U and Koeberl C 2000 Critical comment on ldquoA JMory et al Woodleigh Carnarvon basin Western Australia Anew 120 km diameter impact structurerdquo Earth and PlanetaryScience Letters 184353ndash357

Reimold W U Koeberl C and Bishop J 1994 Roter Kamm impactcrater Namibia Geochemistry of basement rocks and brecciasGeochimica et Cosmochimica Acta 582689ndash2710

Reimold W U Koeberl C Brandstaumltter F Kruger F J ArmstrongR A and Bootsman C 1999 Morokweng impact structureSouth Africa Geologic petrographic and isotopic results andimplications for the size of the structure In Large meteoriteimpacts and planetary evolution II edited by Dressler B O andSharpton V L Special Paper 339 Boulder Geological Society ofAmerica pp 61ndash90

Reimold W U Koeberl C Johnson S and McDonald I 2000 EarlyArchean spherule beds in the Barberton Mountain Land SouthAfrica impact or terrestrial origin In Impacts and the earlyEarth edited by Gilmour I and Koeberl C Lecture Notes inEarth Sciences 91 Berlin Springer-Verlag pp 117ndash180

Reimold W U Armstrong R A and Koeberl C 2002 A deepdrillcore from the Morokweng impact structure South AfricaPetrography geochemistry and constraints on the crater sizeEarth and Planetary Science Letters 201221ndash232

Renne P R Reimold W U Koeberl C Hough R and Claeys P2002 Comment on ldquorsquoK-Ar evidence from illitic clays of a LateDevonian age for the 120 km diameter Woodleigh impactstructure Southern Carnarvon Basin Western Australiardquo by ITUysal S D Golding AY Glikson A J Mory and M GliksonEarth and Planetary Science Letters 201247ndash252

Short N M and Gould D P 1996 Petrography of shocked rocksfrom the central peak at the Manson impact structure In TheManson impact structure Iowa Anatomy of an impact crateredited by Koeberl C and Anderson R R Special Paper 302Boulder Geological Society of America pp 245ndash265

Stoumlffler D 1972 Deformation and transformation of rock-formingminerals by natural and experimental shock processes IBehavior of minerals under shock compression Fortschritte derMineralogie 4950ndash113

Stoumlffler D 1974 Deformation and transformation of rock-formingminerals by natural and experimental shock processes IIPhysical properties of shocked minerals Fortschritte derMineralogie 51256ndash289

Stoumlffler D and Langenhorst F 1994 Shock metamorphism of quartzin nature and experiment I Basic observation and theoryMeteoritics 29155ndash181

Swisher C C Grajales-Nishimura J M Montanari A Margolis SV Claeys P Alvarez W Renne P Cedillo-Pardo E MaurrasseF J M Curtis G H Smit J and McWilliams M O 1992Coeval 40Ar39Ar ages of 650 Ma from Chicxulub crater meltrock and Cretaceous-Tertiary boundary tektites Science 257954ndash958

Taylor S R and McLennan S M 1985 The Continental crust Itscomposition and evolution Boston Blackwell Science Inc 312 p

Uysal I T Golding S D Glikson A Y Mory A J and Glikson M2001 K-Ar evidence from illitic clays of a late Devonian age forthe 120 km diameter Woodleigh impact structure SouthernCarnarvon basin Western Australia Earth and PlanetaryScience Letters 192281ndash189

Zhou M F 1994 PGE distribution in 27 Ga layered komatiite flowsfrom the Belingwe greenstone belt Zimbabwe ChemicalGeology 118155ndash172


APPENDIX

PETROGRAPHIC DESCRIPTIONS OF THE SAMPLE SUITE USED IN THIS INVESTIGATION

The samples are listed in order of increasing core depth

W-7 Well-Banded Biotite-Quartz-Feldspar Gneiss1945 m Depth

This sample is composed of 2 lithologies alternating atabout 1 cm wide spacings namely a biotite-poor felsiclithology and a biotite-gneiss with ~25 vol of quartz andfeldspar In the biotite-poor phase quartz generally displays1ndash2 sometimes 3 sets of PDFs Many grains appear toastedFeldspar appears largely isotropic but at high magnificationwe find that a significant proportion of each apparentlyisotropic grain is still crystalline but characterized by stronglyreduced birefringence This is interpreted to indicate incipientconversion of feldspar to diaplectic glass In plane polarizedlight this feldspar is brownish and mottled probably theresult of specific targeting of strongly shocked feldspar byalteration fluids Those feldspar (alkali and plagioclase)crystals that are still largely crystalline show sets of PDFsBiotite in this band is only partially kinkbanded Locallygranophyric intergrowths of quartz and feldspar that representan inherent feature of the gneiss are observed

In the biotite-rich zone biotite kinkbanding issignificantly stronger than in the felsic band However onaverage quartz (feldspar is virtually absent) displays a lowernumber of sets of PDFs per grain No isotropization of felsicminerals is noted Locally cataclasis is noted (very angularfragments of a quartz grain) Both bands are cut by narrowcarbonate veinlets and carbonate fills slivers between biotite-rich laminations A few narrow quartz veinlets also cross-cutthe sample Quartz from these veinlets is also shocked (PDFs)and consequently pre-dates the impact event Accessoryphases identified include apatite and possibly somemonazite

W-6 Biotite-Rich Gneiss 200 m Depth

This sample is a well-banded biotite-quartz-feldspar(plagioclase + alkali feldspar) gneiss with a several mm-widezone dominated by biotite Garnet is an accessory mineralThe sample is transected by a number of carbonate micro-veins with carbonatization also occurring in wider zonesadjacent to such veins Up to 1 cm wide quartz-rich bands arepresent that display ribbons and wisps of biotite Theseleucocratic bands indicate that plastic deformation occurredthat also generated randomly oriented fold structures Thesample is quite strongly shock deformed biotite displaysabundant kinkbanding with more than one kink system beingthe rule Quartz is characterized by the general development

of dense systems of PDFs typically with 2ndash3 sets developedper grain but a significant number of grains showing morethan 3 sets Feldspar is strongly altered probably as the resultof extensive microstructural damage due to shockmetamorphic overprint Only a few relatively fresh revealremnants of PDFs A good number of feldspar grains areisotropic (diaplectic glass) A cm-sized area of the thinsection appears as a microbreccia with small angular orrounded grains in a largely isotropic matrix However thenature of this matrix cannot be resolved optically due toextensive hydrothermal alteration of this area Other placesappear largely isotropic with small crystalline remnants Therare garnet displays cleavage as has been described frommany other occurrences of shocked rock (eg Dressler 1990)

W-8 Amphibolitic Gneiss 2199 m

In hand specimen this greenish sample displays whiteflecks and lenses up to 5 mm wide It also contains up to 2mm-sized black biotite crystals A subhorizontal fabric isindicated by slight color differences indicative of minerallayering An up to 2 mm-wide calcite-pyrite veinlet cross-cutsthe sample but was not sampled for petrographic analysis Itconsists of calcite and pyrite

Up to 5 mm large amphibole porphyroblasts are presentbetween fine- to medium-grained felsic minerals Intensepolysynthetic twinning is noted in amphibole and presumablyrepresents shock deformation In addition this mineral hasexperienced strong brittle deformation in the form of planarfracturing imparting an intersecting cleavage and fracturingthat yields a blocky microtexture Sometimes displacementsare observed along the cleavage which then would be betterdescribed as micro-faulting The in comparison to theamphibole relatively finer-grained plagioclase occurringmainly in the form of rounded grains is completelysericitised Quartz contains abundant PDFs often in a largenumber of sets (up to 6 sets per grain) Strong undulousextinction in quartz and locally development of shockmosaicism occur The sample is cut by a lt50 mm wide veinletof melt that could represent either shock or friction melt Thevein margins are very straight No obvious displacement ofthe host rock on either side of the vein is noted The veinfilling is aphanitic but zoned with the material along the veinmargins being slightly lighter colored than the interior Noincrease in shock deformation in the direct vicinity of the veinwas observed The vein closely resembles so-called ldquoshockveinsrdquo in meteorites for example in Jackalsfontein asdescribed recently by Buchanan et al (2002) The vein is cutand displaced by a late carbonate vein

W-9 Garnet-Biotite-Gneiss 220 m Depth

This is a coarse- (several mm) to medium-grained gneisswith distinctive banding into biotite-rich and biotite-poor


zones Our sample comprises a several mm-wide biotite-poorzone flanked by biotite-rich portions in which the biotite iswell-aligned Some feldspar crystals attain sizes in excess of 4mm Biotite is locally kinkbanded but not particularlystrongly Garnet (in hand specimen up to 1 cm in size)displays extensive fracturing in the form of well-developedcleavage (several sets of parallel fractures) that isinterconnected through numerous irregular fracturesparticularly emanating from inclusions in often radialarrangements Quartz displays distinct undulatory extinctionAll quartz crystals have PDFs frequently in a large number ofsets per grain (up to 6 sets were counted which issignificantly more than noted in the other samples) Feldsparis completely sericitised and locally saussuritized Inaddition to these alteration effects calcite microveining isubiquitous and fine- to medium-grained calcite aggregateshave locally replaced the rock The sample is clearly quitestrongly shocked but one can only speculate whether or notfeldspar has been converted to diaplectic glass Pyrite occursas an accessory phase

W-3 Gneiss Composed of a Biotite-Rich and a Biotite-PoorBand 2243 m Depth

This sample represents a strongly shocked banded gneissThe biotite-rich lithology comprises circa 80 vol biotite and20 vol quartz plusmn feldspar Biotite is strongly kinkbandedQuartz and feldspar display PDFs in quartz generally 1ndash-2sets per grain but not at the same abundance as observed inthe more felsic band Here quartz may have up to 4 sets ofPDFs per grain and on average displays gt1 set more thanquartz in the biotite-rich band Feldspars often show reducedbirefringence They are strongly altered possibly because ofthe strong microdeformation imparted on them by the impactevent Many altered domains have crystallographicallycontrolled angular shapes that should be related tocrystallographically controlled shock deformation In thebiotite-poor band feldspar often has remnants characterizedby reduced birefringence and remnants of sets of short butdensely spaced PDFs Similar to sample W-7 this specimenalso contains some pockets of myrmekitic intergrowth ofquartz and feldspar that are clearly a feature of the primaryrock and not related to impact deformation

W-5 Granite 240 m Depth

This medium- to coarse-grained granite is locally quitebiotite-rich All quartz grains carry PDFs on average 2ndash3 setsper grain occasionally more Feldspar displays a range ofshock effects from unshocked to reduced birefringence toremnants of PDFs that apparently occurred in vastly differentnumbers of sets per grain (from single to multiple) Manyfeldspar grains mostly plagioclase are cut by narrow shearsthat offset twinning (micro-faulting) or peter out in irregular

forms Many of these veinlets are filled by isotropic orcryptocrystalline material which is possible evidence ofshock or friction melting These veins are generally not widerthan 10 mm Locally these veinlets may also containsecondary phases such as carbonate andor phyllosilicates Afew isolated feldspar grains are completely isotropized

W-2 Microgranite 2455 m Depth

In hand specimen this microgranite shows a subduednear-parallel lineation Main phases are quartz alkalifeldspar and plagioclase biotite is abundant and somemuscovite and traces of opaques occur Feldspar is lesssericitised than in W-1 The rock has a weak fabric definedby subparallel arrangement of mineral long-axes A goodnumber of biotite flakes are intergrown with chlorite flakespresumably formed at the expense of biotite Shockdeformation is prominent but weaker overall than in W-1Much of the quartz but not all displays 1 to 2 sets of PDFsper grain but many unshocked crystals occur as well NoPDFs in feldspar were noted but several plagioclase crystalshave areas with well-developed mechanical twinningApatite is an accessory phase and may display somefracturing Biotite and muscovite are often kinkbanded butnot as intensely as in W-1

W-1 Biotite-Gneiss 2589 m Depth

This is a well-banded (either quartz or quartz-alkalifeldspar-rich or biotite plusmn quartz-rich bands) gneiss in whichquartz-rich areas frequently form lenses or ribbons (seenclearly at the macroscopic and microscopic scale) The biotiteis extensively kinkbanded all quartz displays single ormultiple sets of PDFs most grains have 1ndash3 sets oftenstrongly undulous extinction in quartz exists alkali feldspar isgenerally characterized by reduced birefringence due to theabundant occurrence of sets of short but densely occurringPDFs and no maskelynite was observed The thin sectionshows that the sample is strongly deformed due to tectonicstresses but no macrodeformation such as brecciation zonesare observed that could be related to the impact deformationIn contrast the hand specimen displays a sub-vertical fracturezone that displaces the host rock This zone also displaces anolder quartz vein Plagioclase is largely sericitised Anotherminor phase is sphene that generally occurs with opaquephases and only shows some irregular fracturing

W-4 Garnetiferous Biotite-Gneiss 2643 m Depth

A well-developed gneissic fabric defined by stringers andgeneral alignment of biotite and stretching of other mineralsThe rock contains quartz and feldspar porphyroblasts up to 5ndash10 mm wide and often of rounded to ovoid shapes Biotite-richareas form stringers and narrow bands that flow around the


porphyroblasts No steep fracturing or veining as observed inthe previous sample are noted Garnet occurs in the form ofrare up to 15 mm large crystals The sample is stronglyshocked Quartz grains in their majority display PDFs veryfrequently as 2ndash4 sets per grain PDFs can also be discerned infeldspar (both plagioclase and K-feldspar) Both quartz andfeldspar crystals often show 1 set of grain-pervasivesubparallel and subplanar fractures Many felsic grains havea toasted (Short and Gould 1996) appearance The garnetcrystals are strongly fractured The biotite is generallykinkbanded though not as intensely as in W-1 Several shearfractures transgress this sample Apparently along and asidethese fractures some quartz and feldspar grains may havebeen transformed into glassy phases that are now partiallyannealed Whether this represents shock or friction melt cannot be resolved at this stage These possible melt patches aregenerally smaller than 025 mm

W-13 Amphibole-Biotite Gneiss 272 m Depth

This sample is similar to W-8 but is distinguished by asignificantly smaller grain size (fine- to medium-grained) anda somewhat higher content of biotite Amphibole crystals areinvariably altered They are strongly fractured generallyhaving (sub)planar fractures along distinct crystallographicplanes and often display twinning Feldspar is stronglysericitized and shock deformation cannot be discernedAbout 75 of all quartz grains display shock deformation inthe form of 1ndash2 sets of PDFs Three sets occur only rarelyThis sample is unique in our sample suite as it exhibits 2different types of melt veinlets Both types are up to 15 cmlong but not more than 50ndash100 mm wide The first type ischaracterized by very straight margins and has a compositenature its interior comprises an aphanitic phase that isaccompanied on both sides by a near-isotropic in plane-polarized light pink and high-refractive index phase whichwas apparently formed from host rock amphibole This phasehas not yet been identified Veins of the second type are

distinct by a meandering geometry and constantly changingwidth that gives them a distinct pinch-and-swell attributeLocally such veins contain recrystallized quartz Alsolocally along this type of vein microbrecciation is observedand annealing has affected the host rock Further work isnecessary to establish whether either of these vein typesrepresents a product of cataclasis plus friction melting andorshock melting We note that such veins have only beenobserved in mafic sample material of this collection


This sample represents a coarse-grained 2-feldspargranite that also comprises minor amounts of muscovite andchlorite Locally in the hand specimen brownishdiscoloration is noted which is interpreted as the result ofalteration of one of the 2 feldspar phases Both muscovite andchlorite are strongly kinkbanded A part of the felsic mineralsdisplay strong undulatory extinction PDFs are present insome quartz grains at 1ndash3 sets per grain Two or 3 sets pergrain are quite abundant However obviously the density ofPDFs of given sets is much reduced compared to the verytight spacings in samples from higher up in the drill core Incomparison with samples from higher levels this sample ischaracterized by strong brittle microdeformation Mostlyfeldspar but also some quartz has been victim to micro-faulting Displacements along these faults occur frequentlyand produce domino-style offsets Between pairs of micro-shear fractures feldspar may display kinkband-likecompression features (ldquocrumplingrdquo)

A general change in degree of shock deformation is notedfrom the upper sampled part of this drillcore toward thelowermost specimen In samples from higher levels localmelting but more distinctly isotropization are recorded andseemingly PDFs occur more abundantly at higher numbers ofsets per grain than in samples from lower depths The lowermostsample is characterized by brittle deformation of a kind that ismore strongly developed here than in the uppermost samples


Fig 1 a) Location map of the Woodleigh structure in western Australia b) major regional tectonic features and location of the Woodleighstructure and positions of the drilling sites in the central part of the impact structure (drill cores Woodleigh 1 and 2A) adapted from Mory etal (2001)

a

b






GEOLOGICAL SETTING















































W-7 61 130 1 51 9 0W-6 51 107 1 44 6 0W-8 31 60 4 25 2 0W-9 33 75 1 22 10 0W-3 51 104 2 45 4 0W-5 40 85 1 33 6 0W-2 63 129 2 56 5 0W-1 22 47 2 15 5 0W-4 51 104 4 42 4 1W-13 23 45 0 22 1 0W-12 42 92 0 31 11 0



W-1 27 (154) 99 (565) 24 (137) 25 (143) 175W-2 54 (248) 116 (532) 7 (32) 41 (188) 218W-3 17 (73) 148 (635) 47 (202) 21 (90) 233W-4 45 (144) 189 (607) 61 (196) 17 (54) 312W5 18 (83) 131 (601) 32 (147) 37 (170) 218W-6 11 (37) 173 (588) 79 (269) 31 (105) 294W-7 44 (190) 121 (522) 12 (52) 55 (237) 232W-8 51 (323) 92 (582) 4 (25) 11 (70) 158W-9 32 (174) 106 (576) 19 (103) 27 (147) 184W-12 23 (148) 117 (755) 11 (71) 5 (32) 155W-13 53 (235) 147 (644) 9 (40) 16 (71) 225



















Bio Gneiss Granite


Am B Gneiss Granite


Total 10005 9920 9996 9918 9995 9990 9984 9992 10052 9908 9978





















1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037


Ir Ru Rh Pt Pd Au





DISCUSSION



























REFERENCES


























































APPENDIX








































GEOLOGICAL SETTING















































W-7 61 130 1 51 9 0W-6 51 107 1 44 6 0W-8 31 60 4 25 2 0W-9 33 75 1 22 10 0W-3 51 104 2 45 4 0W-5 40 85 1 33 6 0W-2 63 129 2 56 5 0W-1 22 47 2 15 5 0W-4 51 104 4 42 4 1W-13 23 45 0 22 1 0W-12 42 92 0 31 11 0



W-1 27 (154) 99 (565) 24 (137) 25 (143) 175W-2 54 (248) 116 (532) 7 (32) 41 (188) 218W-3 17 (73) 148 (635) 47 (202) 21 (90) 233W-4 45 (144) 189 (607) 61 (196) 17 (54) 312W5 18 (83) 131 (601) 32 (147) 37 (170) 218W-6 11 (37) 173 (588) 79 (269) 31 (105) 294W-7 44 (190) 121 (522) 12 (52) 55 (237) 232W-8 51 (323) 92 (582) 4 (25) 11 (70) 158W-9 32 (174) 106 (576) 19 (103) 27 (147) 184W-12 23 (148) 117 (755) 11 (71) 5 (32) 155W-13 53 (235) 147 (644) 9 (40) 16 (71) 225



















Bio Gneiss Granite


Am B Gneiss Granite


Total 10005 9920 9996 9918 9995 9990 9984 9992 10052 9908 9978





















1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037


Ir Ru Rh Pt Pd Au





DISCUSSION



























REFERENCES


























































APPENDIX













































































W-7 61 130 1 51 9 0W-6 51 107 1 44 6 0W-8 31 60 4 25 2 0W-9 33 75 1 22 10 0W-3 51 104 2 45 4 0W-5 40 85 1 33 6 0W-2 63 129 2 56 5 0W-1 22 47 2 15 5 0W-4 51 104 4 42 4 1W-13 23 45 0 22 1 0W-12 42 92 0 31 11 0



W-1 27 (154) 99 (565) 24 (137) 25 (143) 175W-2 54 (248) 116 (532) 7 (32) 41 (188) 218W-3 17 (73) 148 (635) 47 (202) 21 (90) 233W-4 45 (144) 189 (607) 61 (196) 17 (54) 312W5 18 (83) 131 (601) 32 (147) 37 (170) 218W-6 11 (37) 173 (588) 79 (269) 31 (105) 294W-7 44 (190) 121 (522) 12 (52) 55 (237) 232W-8 51 (323) 92 (582) 4 (25) 11 (70) 158W-9 32 (174) 106 (576) 19 (103) 27 (147) 184W-12 23 (148) 117 (755) 11 (71) 5 (32) 155W-13 53 (235) 147 (644) 9 (40) 16 (71) 225



















Bio Gneiss Granite


Am B Gneiss Granite


Total 10005 9920 9996 9918 9995 9990 9984 9992 10052 9908 9978





















1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037


Ir Ru Rh Pt Pd Au





DISCUSSION



























REFERENCES


























































APPENDIX





















































Bio Gneiss Granite


Am B Gneiss Granite


Total 10005 9920 9996 9918 9995 9990 9984 9992 10052 9908 9978





















1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037


Ir Ru Rh Pt Pd Au





DISCUSSION



























REFERENCES


























































APPENDIX




















































1 1332 685 -00801 1315 682 -00162 1213 632 00083 1531 792 -00464 1412 728 -00604 1483 768 -0037


Ir Ru Rh Pt Pd Au





DISCUSSION



























REFERENCES


























































APPENDIX





































DISCUSSION



























REFERENCES


























































APPENDIX























































REFERENCES


























































APPENDIX



































































APPENDIX



































Woodleigh impact structure, Australia: Shock petrography and geochemical studies

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Transcript of Woodleigh impact structure, Australia: Shock petrography and geochemical studies