The Tookoonooka marine impact horizon, Australia: sedimentary and petrologic evidence

Ejecta from the large subsurface Tookoonooka impact structure have been found in the Lower Cretaceous strata of the extensive Eromanga Basin of central Australia. Observations from 31 wells spanning 400,000 km2 of the basin provide compelling evidence for the presence of a marine impact horizon of regional extent. Drill core was examined to determine the sedimentary context of the Tookoonooka impact event, the presence of ejecta, and the nature of the impact horizon. The base of the Wyandra Sandstone Member of the Cadna-owie Formation is an unconformity commonly overlain by very poorly sorted sediment with imbricated pebbles, exotic clasts, and occasional boulders. The basal Wyandra Sandstone Member is bimodal: a fine sand mode reflects an ambient sediment contribution and a coarse mode is interpreted to be impact-derived. Wells Thargomindah-1 and Eromanga-1, within four crater radii of Tookoonooka, contain distinctive clastsupported breccia-conglomerate beds at the base of the Wyandra Sandstone Member. Clasts in these beds include altered accretionary and melt impactoclasts, as well as lithic and mineral grains corresponding to the Tookoonooka target rock sequence, including basement. Petrographic evidence includes shock metamorphosed quartz and lithic grains with planar deformation features. These breccia-conglomerates are in stark contrast to the underlying, laterally persistent, unimodal Cadna-owie sediments and overlying shales deposited in an epeiric sea. The base of the Wyandra Sandstone Member is therefore interpreted to be the Tookoonooka impact horizon. The timing of the impact event is confirmed to be the Barremian-Aptian boundary, at 125 ± 1 Ma. The Wyandra Sandstone Member preserves both impact ejecta and postimpact marine sediments.Katherine A. Bron and Victor Gosti

Eroding abodes and vanished bridges: historical biogeography of the substrate specialist pebble-mound mice (Pseudomys).

Aim: To determine whether the pronounced ecological importance of pebble mounds to pebble-mound mice (Pseudomys) is manifest in their continental biogeography.\ud \ud Location: Northern Australia.\ud \ud Methods: A GIS-based comparison was made between the habitats contained within the potential climatic distributions of mice, representing a null hypothesis of no habitat selection, and their actual distributions based on all known location records.\ud \ud Results: All species had a clear preference for hilly, rocky landscapes with a surficial cover dominated by bedrock. Simple vegetation communities with relatively open eucalypt overstorey and grassy understorey were preferred. Highly degraded rocks and aggradational surfaces and plains were avoided. The extent of the summer monsoon may be important in determining the southern limits of the group's distribution. Major disjunctions between species were attributable to the presence of clay plains and sand sheets.\ud The behavioural requirement of pebble-mound mice for mounds determines their population distribution pattern and the distribution of the different species within the genus.\ud \ud Main conclusions: The behavioural need for pebble mounds drives the distributional pattern of populations and species of pebble-mound mice. The initial spread of pebble-mound mice probably occurred during the late Pliocene or earliest Pleistocene. There has predominantly been degradation of the potential distribution of the group since that time due to the stability of Australian landscapes and Pleistocene planation and sand sheet development over large areas of northern Australia. This process is ongoing, and past regions of rocky contact between current distributions have disappeared, while the distributional limits of several species are steadily being reduced by erosion of hills and the spread of dune fields

Molecular systematics and biodiversity of oniscidean isopods in the groundwater calcretes of central Western Australia

Abstract not availableMohammad Javidkar, Steven J.B. Cooper, Rachael A. King, William F. Humphreys, Terry Bertozzi, Mark I. Stevens, Andrew D. Austi

Gold and pathfinder elements in ferricrete gold deposits of the Yilgarn Craton of Western Australia: A review with new concepts

Secondary mineral deposits have played an important role in the global mineral resource economy for over 50 years, with lateritic Au, Al, Fe and Ni deposits having a significant input to global metal production and reserves. In the Yilgarn Craton of Western Australia, a deeply weathered mantle is commonly capped with 2–10 m of lateritic residuum (residual lateritic gravels and duricrust) and/or ferricrete (Fe oxide-cemented sediment), which formed under seasonally humid tropical and sub-tropical climates during the Cenozoic. The principal constituents of these units are goethite, hematite, maghemite, kaolinite and quartz. They are commonly overlain by younger, 2–10 m thick transported cover, deposited under later semi-arid conditions. Both ferricrete and lateritic residuum may host exploitable secondary gold deposits, typically small (<500,000 ounces) and of low grade (<1–5 g/t Au). The lateritic residuum deposits overlie weathered and fresh primary mineralization, whereas ferricrete deposits overlie uneconomic primary mieralization or barren saprolite and bedrock. Despite numerous studies, many questions remain about the behaviour and evolution of Au in the complex polygenetic systems that form lateritic residuum and ferricrete. In particular, why is it difficult to locate significant primary mineralization associated with highly Au-anomalous ferricrete? Understanding the mechanisms of enrichment of Au and pathfinder elements in ferricrete will assist future discovery. Accordingly, to obtain conclusive evidence for processes of anomaly formation, a combination of detailed field observations with state-of-the-art microscopy have been conducted at three of the larger deposits (Moolart Well, Mt Gibson and Bulchina). The aim of this review is to integrate these recent results with the results of earlier studies to trace the path of Au and pathfinder elements and associated dispersion processes in the ferricrete environment