Arthur J. Mory

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

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

K–Ar evidence from illitic clays of a Late Devonian age for the 120 km diameter Woodleigh impact structure, Southern Carnarvon Basin, Western Australia

Abstract Woodleigh is a recently discovered impact structure with a diameter of 120 km, and thereby represents the third largest proven Phanerozoic impact structure known after Morocweng and Chicxulub. K–Ar isotopic studies of fine-grained authigenic illitic clay minerals (

Carboniferous-Permian palynostratigraphy of west Australian marine rift basins: resolving tectonic and eustatic controls during Gondwanan glaciations

Abstract The longest interval of cold climate in Earth’s Phanerozoic history spanned some 55 million years from the Namurian (late Carboniferous) to the Kungurian (early Permian) when glaciation affected much of high-latitude Gondwana. Uncertainty surrounds the timing of glaciation(s) and the frequency of interglacial episodes, ice volumes and the location of ice centres. This is because the ‘glacial’ record is preserved predominantly within marine strata deposited in intracontinental rift basins marginal to onshore ice centres on uplifted basement highs. The direct stratigraphic record of glaciation is meagre on adjoining basement highs, and extremely difficult to date. Many basins contain broadly similar threefold lithostratigraphic successions of lowermost glaciomarine diamictite facies, middle shales and uppermost carbonate and sandstone-rich, commonly coal-bearing, deltaic strata. In the absence of absolute age control and infrequent presence of macrofossils, such ‘deglaciation sequences’ are widely assumed to be correlative from basin to basin recording Gondwanan deglaciation(s) and a world-wide glacioeustatic sea level rise. A newly compiled biostratigraphic database for west Australia, using first appearance datums of spore-pollen types, allows correlation of Late Palaeozoic siliciclastic successions between seven basins (Bonaparte, Canning, Carnarvon, Collie, Gunbarrel, northern and southern Perth basins). These extend some 3800 km along Australia’s western continental margin adjacent to the west Australian shield. All but the Gunbarrel Basin contain tripartite glacially influenced successions (diamictite/shale/sandstone). Biostratigraphic data reveal sharp differences in ages and stratigraphic distributions, and marked variations in the thickness of coeval units. Diamictite/shale/sandstone successions accumulated diachronously from basin to basin recording a dominant tectonic control on subsidence and relative sea level. Middle shale units are not exact correlatives between basins and thus are unlikely to simply represent widespread climatically controlled glacioeustatic events. Instead, they are argued to record high relative sea levels during times of maximum synrift subsidence. This tectonostratigraphic model can be applied to other intracontinental rift-basin successions across Gondwana.

Woodleigh, Southern Carnarvon Basin, Western Australia: history of discovery, Late Devonian age, and geophysical and morphometric evidence for a 120 km-diameter impact structure

The discovery of the Woodleigh impact structure, first identified by R. P. Iasky, bears a number of parallels with that of the Chicxulub impact structure of K – T boundary age, underpinning complications inherent in the study of buried impact structures by geophysical techniques and drilling. Questions raised in connection with the diameter of the Woodleigh impact structure reflect uncertainties in criteria used to define original crater sizes in eroded and buried impact structures as well as limits on the geological controls at Woodleigh. The truncation of the regional Ajana – Wandagee gravity ridges by the outer aureole of the Woodleigh structure, a superposed arcuate magnetic anomaly along the eastern part of the structure, seismic-reflection data indicating a central > 37 km-diameter dome, correlation of fault patterns between Woodleigh and less-deeply eroded impact structures (Ries crater, Chesapeake Bay), and morphometric estimates all indicate a final diameter of 120 km. At Woodleigh, pre-hydrothermal shock-induced melting and diaplectic transformations are heavily masked by pervasive alteration of the shocked gneisses to montmorillonite-dominated clays, accounting for the high MgO and low K2O of cryptocrystalline components. The possible contamination of sub-crater levels of the Woodleigh impact structure by meteoritic components, suggested by high Ni, Co, Cr, Ni/Co and Ni/Cr ratios, requires further siderophile element analyses of vein materials. Although stratigraphic age constraints on the impact event are broad (post-Middle Devonian to pre-Early Jurassic) high-temperature (200 – 250°C) pervasive hydrothermal activity dated by K – Ar isotopes of illite – smectite indicates an age of 359 ± 4 Ma. To date neither Late Devonian crater fill, nor impact ejecta fallout units have been identified, although metallic meteoritic ablation spherules of a similar age have been found in the Canning Basin.

Modeling petroleum generation in the Paleozoic of the Carnarvon Basin, Western Australia: Implications for prospectivity

Maturity and petroleum generation modeling of the Paleozoic succession in the Carnarvon Basin shows that most potential source rock intervals reached their maximum generation migration during the Carboniferous–Permian and could have charged traps developed during rifting in the middle Carboniferous to Early Permian except for the Peedamullah Shelf, where generation peaked during the Cretaceous. However, identification of such traps is challenging because it is difficult to differentiate between the deformation associated with middle Carboniferous–Early Permian rifting and that of the Early Cretaceous breakup rifting. Given the presence of suitable reservoir and sealing units in most of the Paleozoic, the prime risks for this section are the volume of available source rock, trap integrity because of the long period of preservation required, and relative timing of generation vs. trap formation, such that charging of younger traps requires secondary migration.The best Paleozoic oil-prone source beds identified in the Carnarvon Basin are thin beds in carbonate-dominated Silurian and Devonian units on the Gascoyne Platform, but Devonian source beds are restricted to the northern parts of the platform. The maturity of these units progressively increases from immature in the south-southeast to mature in the north-northwest, following increasing depth of burial in that direction. The best gas-prone source beds lie within the Lower Permian of the Merlinleigh Subbasin, and their maturity ranges from immature along the margins of the subbasin to overmature toward the center. Within the Upper Permian, the best source beds for oil and gas are in the Peedamullah Shelf, where they range from immature in the southeast to mature in the northwest.Commercial Paleozoic petroleum has yet to be discovered in the Paleozoic of the Carnarvon Basin, but the assessment of this part of the basin is limited because few, if any, of the exploration wells drilled to date were valid tests of these objectives.

Carboniferous–Permian facies and tectono-stratigraphic successions of the glacially influenced and rifted Carnarvon Basin, western Australia

Abstract The Carnarvon Basin of Western Australia is a rift basin that contains a thick (up to 5 km) succession of late Carboniferous–early Permian glacially influenced marine sedimentary strata. These rocks accumulated in near-polar paleolatitudes along the uplifted and glaciated margin of the west Australian Shield (Pilbara Craton). Three stratigraphic successions (I, II, III) can be recognised, each characterised by distinct facies associations that record different stages in the tectonic evolution of the basin and associated changes in the rate of basin subsidence and sediment accommodation. A lowermost succession (I) comprises rapidly deposited (30 m/Ma) glacially influenced marine strata (Lyons Group) containing palynomorphs of Westphalian–Tastubian (early Sakmarian) and possibly older age. Strata are dominated by subaqueously deposited sediment gravity flow facies. Succession II is composed of richly fossiliferous cool water shales (Callythara and Cordalia formations) that record much reduced sedimentation rates (2 m/Ma). In turn, shales are overlain by an uppermost succession (III) of shallow marine, wave- and storm-influenced sandstone (Moogooloo Sandstone). Comparison with other rift basin fills indicates that Succession I likely records initial basin infilling where abundant coarse debris was produced by faulting and glaciation of the adjacent Pilbara Craton. Shales of Succession II mark a phase of ‘sediment underfilling’ characterized by rapid tectonic subsidence, an increase in relative sea level and reduced sediment supply. Shallow water sandstone facies of Succession III record a late stage in the tectonic cycle when subsidence rates had decreased and sediment supply outpaced accommodation. Such successions, where found in other late Paleozoic basins, are widely interpreted in terms of glacioeustatically driven changes in sea level resulting from deglaciation events across Gondwana. Instead, the three successions within the Carnarvon Basin are argued to reflect a dominantly tectonic control on sedimentation and preservation.

Early Carboniferous Siphonodella (Conodonta) faunas from eastern Australia

Early to Middle Tournaisian conodont faunas with Siphonodella from ten sections in eastern Australia, between Gloucester in New South Wales and Rockhampton in Queensland, may be referred to the following ‘standard’ zones; 1 sulcata, 2 upper duplicata, 3 sandbergi, 4 lower crenulata and 5 isosticha-upper crenulata, in ascending order. In eastern Australia the first occurrences of Gnathodus cuneiformis, G. delicatus, G. typicus and Protognathodus cordiformis, near the base of the lower crenulata zone, are significantly earlier than in Europe and North America. Consequently the base of the isosticha-upper crenulata zone in eastern Australia is defined by the first appearance of G. punctatus rather than that of G. delicatus. On the present evidence it is difficult to reconcile some brachiopod and conodont occurrences in the Early-Middle Tournaisian of eastern Australia. Seventeen discrete conodont species are discussed, four of which are described informally: Dinodus sp. nov. A, Dinodus sp. nov. B, Pinacognat...

Conodont biostratigraphy of the Visean series in eastern Australia

Conodonts from Visean limestones of the Yarrol and Tamworth Belts of Queensland and New South Wales are, in general, sparsely preserved but widespread and about equally divided between endemic and cosmopolitan species. Patrognathus conjunctus sp. nov. is the commonest conodont in the early Visean and gave rise to Montognathus semicarinatus gen. et sp. nov. and to M. carinatus gen. et sp. nov., the trio being name-bearers for three zones based on endemic elements. The fourth and highest Visean zone has the mondial Gnathodus texanus and Gnathodus bilineatus as joint nominate species, the latter being included in the zonal name to emphasise the restricted definition we adopt for G. texanus. Adetognathus also probably evolved from Patrognathus to give a lineage of three new endemic species: — A. taphrognathoides, A. cannindahensis and A. subunicornis, all predating the earliest adetognathids of the northern continents. Cavusgnathus altifrons sp. nov. is intermediate in platform morphology and time-range betwe...

Microchemistry and microstructures of hydrothermally altered shock-metamorphosed basement gneiss, Woodleigh impact structure, Southern Carnarvon Basin, Western Australia

Hydrothermally altered shock-metamorphosed gneisses consisting of relic igneous biotite – K-feldspar – Na-rich alkali feldspar – plagioclase – quartz assemblages (± accessory garnet, corundum, titanite, monazite, zircon), and showing extensive replacement by montmorillonite, illite, sericite, and to a lesser extent chlorite, calcite, epidote, zoisite and pyrite, occur in the basement core uplift of the Woodleigh impact structure, Western Australia. The rocks display extensive hydrothermal clay alteration, complicating identification of pre-hydrothermal and pre-impact textures and compositions. Analysis of quartz-hosted planar deformation features (PDFs) indicates a majority of indexed sets parallel to , a lesser abundance of sets parallel to , and some sets parallel to the basal plane (0001) and , consistent with pressures about or over 20 GPa. Feldspar-hosted PDFs form reticulate vein networks displaying checkerboard-like to irregular and serrated patterns attributable to preferential replacement of shock-damaged PDFs and/or perthitic twin lamella by clay minerals. The gneisses are pervaded by clay-dominated intergranular and intragranular veins of cryptocrystalline material that display marked departures from bulk-rock chemistry and from mineral compositions. XRD analysis identifies the cryptocrystalline components as illite – montmorillonite, illite and chlorite, while laser Raman analysis identifies high-fluorescence sub-micrometre clay assemblage, feldspar, quartz and minor mica. SEM/EDS-probe and laser-ICPMS analysis indicate low-K high-Mg clay mineral compositions consistent with montmorillonite. Quartz PDF-hosted cryptocrystalline laminae display distinct enrichments in Al, Mg, Ca and K. Altered intergranular veins and feldspar-hosted cryptocrystalline components show consistent enrichment in the relatively refractory elements (Al, Ca, Mg, Fe) and depletion in relatively volatile elements (Si, K, Na). The clay alteration retards determination whether clay-dominated vein networks represent altered shock-induced pseudotachylite veins, diaplectic zones and/or shock-damaged twin lamella, and/or result from purely mineralogical and chemical differentiation affected by hydrothermal fluids. Overall enrichment of the shocked gneiss and of the cryptocrystalline components in Mg and trace ferromagnesian elements (Ni, Co, Cr) may be attributed alternatively to introduction of siderophile element-rich fluid from the projectile, or/and contamination of hydrothermal fluids by MgO from dolomites surrounding the basement uplift. High Ni/Co and Ni/Cr and anomalous PGE (platinum group elements) may support the former model.

Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating

ABSTRACT The advent of chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-IDTIMS) has revolutionised U–Pb dating of zircon, and the enhanced precision of eruption ages determined on volcanic layers within basin successions permits an improved calibration of biostratigraphic schemes to the numerical time-scale. The Guadalupian and Lopingian (Permian) successions in the Sydney, Gunnedah, Bowen and Canning basins are mostly non-marine and include numerous airfall tuff units, many of which contain zircon. The eastern Australian palynostratigraphic scheme provides the basis for much of the local correlation, but the present calibration of this scheme against the numerical time-scale depends on a correlation to Western Australia, using rare ammonoids and conodonts in that succession to link to the standard global marine biostratigraphic scheme. High-precision U–Pb zircon dating of tuff layers via CA-IDTIMS allows this tenuous correlation to be circumvented—the resulting direct calibration of the palynostratigraphy to the numerical time-scale highlights significant inaccuracies in the previous indirect correlation. The new data show: the top of the Praecolpatites sinuosus Zone (APP3.2) lies in the early Roadian, not the middle Kungurian; the top of the Microbaculispora villosa Zone (APP3.3) lies in the middle Roadian, not the early Roadian; the top of the Dulhuntyispora granulata Zone (APP4.1) lies in the Wordian, not in the latest Roadian; the top of the Didecitriletes ericianus Zone (APP4.2) lies in the first half of the Wuchiapingian, not the latest Wordian; the Dulhuntyispora dulhuntyi Zone (APP4.3) is exceptionally short and lies within the Wuchiapingian, not the early Capitanian; and the top of the Dulhuntyispora parvithola Zone (APP5) lies at or near the Permo-Triassic boundary, not in the latest Wuchiapingian.