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Results for “Journal of the Geological Society of Australia”

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USGS1981

U-Th-Pb systematics of some granitoids from the northeastern Yilgarn Block, Western Australia and implications for uranium source rock potential

The Mount Boreas‐type granite and spatially associated syenitic granitoid of Western Australia yield Pb‐Pb ages of 2370 ± 100 Ma and 2760 ± 210 Ma, respectively. Th‐Pb ages, although less precise, are concordant with these ages, and therefore the apparent ages are interpreted to be the crystallisation ages for these two units. U‐Pb ages are variable and for the most part anomalously old, which suggests a Cainozoic uranium loss. However, this loss is generally small (< 3 μ g/g); therefore, neither granitoid in its fresh state provides a good source for nearby calcrete‐hosted uranium deposits. The possibility remains that the Mount Boreas‐type granite that has been completely weathered during the Tertiary could have been a source for the calcrete‐type uranium deposits in W.A. Although the Mount Boreas‐type granite is highly fractionated, it does not bear a strong geochemical imprint of a sedimentary precursor. This feature contrasts it with apparently fresh granitoids from other parts of the world that have lost large amounts of uranium (∼ 20 μ g/g ) and are associated with large roll‐type and other low temperature‐type uranium deposits.

Journal of the Geological Society of Australia↗

USGS1958

Contact metamorphism adjacent to a teschenite intrusion

Shale adjacent to the upper contact of an annular teschenite intrusion was converted to andalusite hornfels in an aureole 2–4 feet wide. At some points along the contact there is no evidence of anhydrous recrystallization. Rarely, magmatic reaction with small shale xenoliths resulted in formation of cordierite‐sillimanite (?) hornfelses, and locally a very small‐scale movement of magmatic constituents took place across the contact before consolidation of the teschenite. Very severe deuteric alteration of teschenite adjacent to the contact resulted in large losses of Si, Fe, Mg, alkalis, and probably Ca, some of which were recombined in clays and carbonates in the more porous shales and in joints, but analyses of uniformly fine‐grained shales indicate little change in bulk composition. Mineralogical evidence indicates a maximum contact temperature within the range 500°‐600° C, and a maximum intrusion temperature within the range 1000°‐1200° C.

Journal of the Geological Society of Australia↗

USGS2005

Geology of five small Australian impact craters

Here we present detailed geological maps and cross-sections of Liverpool, Wolfe Creek, Boxhole, Veevers and Dalgaranga craters. Liverpool crater and Wolfe Creek Meteorite Crater are classic bowlshaped, Barringer-type craters, Liverpool was likely formed during the Neoproterozoic and was filled and covered with sediments soon thereafter. In the Cenozoic, this cover was exhumed exposing the crater's brecciated wall rocks. Wolfe Creek Meteorite Crater displays many striking features, including well-bedded ejecta units, crater-floor faults and sinkholes, a ringed aeromagnetic anomaly, rim-skirting dunes, and numerous iron-rich shale balls. Boxhole Meteorite Crater, Veevers Meteorite Crater and Dalgaranga crater are smaller, Odessa-type craters without fully developed, steep, overturned rims. Boxhole and Dalgaranga craters are developed in highly follated Precambrian basement rocks with a veneer of Holocene colluvium. The pre-existing structure at these two sites complicates structural analyses of the craters, and may have influenced target deformation during impact. Veevers Meteorite Crater is formed in Cenozoic laterites, and is one of the best-preserved impact craters on Earth. The craters discussed herein were formed in different target materials, ranging from crystalline rocks to loosely consolidated sediments, containing evidence that the impactors struck at an array of angles and velocities. This facilitates a comparative study of the influence of these factors on the structural and topographic form of small impact craters. ?? Geological Society of Australia.

Boxhole crater, Dalgaranga crater, Liverpool crate↗

USGS2014

Revised Wonoka isotopic anomaly in South Australia and Late Ediacaran mass extinction

The global Late Ediacaran Shuram–Wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean, assuming that all Ediacaran carbonate is marine. Disregarding carbonate in South Australia shown here to be palaeosol or palaeokarst, the synchronous marine organic–carbonate excursion is only −8‰ for δ 13 C organic and −6‰ for δ 13 C carbonate, and lasted less than a million years. This revised magnitude and duration is comparable with perturbations across the Permian–Triassic boundary, and correlative with a global Late Ediacaran acritarch mass extinction. Like Permian–Triassic isotopic excursions, the revised organic–carbonate Wonoka excursion may also have been a greenhouse palaeoclimatic warm spike, which terminated valley incision and glacioeustatic drawdown during the mid-Ediacaran Fauquier Glaciation, and preceded chill of the Late Ediacaran Billy Springs Glaciation.

Flinders Ranges↗