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. lasky, bears a number of parallels with that of the Chlcxulub 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 Ajona - 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 Woodlelgh 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 degrees 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 Conning Basin

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-feldspor-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 omega{10 (1) over bar3}, a lesser abundance of sets parallel to pi{10 (1) over bar2}, and some sets parallel to the basal plane (0001) and r,z {10 (1) over bar1}, consistent with pressures about or over 20 GPa. Feldspar-hosted FDFs 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, Cc, 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, Cc, 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 DGE (platinum group elements) may support the former model

Directions in cardiovascular medical: the renin system in the diagnosis of hypertension

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3D structural and stratigraphic model of the Perth Basin, Western Australia: Implications for sub-basin evolution

The history of rifting and breakup of eastern Gondwana is recorded by the development and fill of the Perth Basin in Western Australia. Knowledge of the structural architecture and stratigraphic geometry of the Perth Basin is essential to understand the evolution of the Western Australian margin and its applications to hydrocarbon and geothermal prospects, and effective aquifer management. This study integrates existing, publicly available geological, gravity, magnetic and digital elevation data to develop the first refined, regional structural and stratigraphic interpretation of the entire onshore and offshore Perth Basin, Western Australia. This new 3D model offers formation depth and thickness predictions in areas of sparse or no data. The model shows significant heterogeneity in the preserved formation thicknesses and depths at both local and regional scales. These differences may have resulted from differential subsidence and/or differential exhumation, but the formation geometries alone cannot distinguish between these two models if any erosion has occurred. Only the Lower to Middle Jurassic stratigraphy has been minimally eroded and thereby records the net subsidence. This stratigraphic interval shows that subsidence was broadly hinged from south to north, with a greater subsidence rate in the southern and central Perth Basin. Localised differences in thicknesses across adjacent sub-basins were likely controlled by differential displacement along sub-basin bounding faults during subsidence and, subsequently, during exhumation episodes. This new 3D model of the entire Perth Basin provides a framework for numerical simulations of fluid and heat flow and large-scale tectonic analysis, such as stratigraphic forward modelling of the southwestern Australian margin

Normal fault linkage and reactivation, Dampier Sub-basin, Western Australia

The Northern Carnarvon Basin of Western Australia has experienced a polyphase deformation history during the breakup of Gondwana. Extension during the Carboniferous–Permian and a subsequent Early Jurassic rift event imposed two distinct fault systems, separated by a several kilometre-thick Triassic sedimentary sequence. Inboard areas, where Triassic sequences are thinner, Jurassic faults both detach above and also penetrate into Permian sequences. Other large-scale faults demonstrate a vertical hard/soft linkage between the two fault systems. In outboard areas where the Triassic is thicker, the relationship is less clear owing to the lower resolution of Permian sequences in seismic data. Here we undertake fault displacement analysis on three faults on the southern margin of the Exmouth Plateau to investigate the growth mechanism of Jurassic-aged faults and possible structural influence of deeper Permian faults. We find evidence of low-throw faults restricted to Mesozoic strata as more complex-segmented faults that have nucleated at a depth below that resolvable on seismic data. When considered in a regional context, the nature of faults in this study suggest oblique reactivation of the NE-trending Permian fabric, under east–west-oriented extension