Wainman, Dr Carmine1, McCabe, Prof Peter1, Reynolds, Dr Peter1
1Australian School of Petroleum and Energy Resources, University of Adelaide, Adelaide, Australia
Volcanogenic rocks are important components of the Late Triassic to Early Cretaceous infill of the Great Australian Superbasin, including the widespread deposition of air-fall volcanic ash (tuff) preserved in the Jurassic Walloon Coal Measures (WCM) of the Surat Basin. With the paucity of known igneous bodies in eastern Australia, these tuff beds provide important clues on the tectono-magmatic environment of eastern Gondwana during the Mesozoic. To better understand the source and character of the volcanic province, age-constrained tuffs (168 to 148 Ma) were analysed in detail. Bed thickness, petrography (supported by XRD data), zircon crystal size and their geochemistry were documented. New datasets reveal these buff-coloured tuffs, mostly preserved within coal seams, are between 0.01 and 2 m thick with sharp lower and upper contacts. They are dominated by splintery, angular quartz clasts (approx. 10–100 μm in diameter) supported in an amorphous, white-buff coloured matrix consisting of clay minerals (predominantly smectite). The same beds are poorly sorted, lightly compacted and lack any structure. Tuff isopach maps from the WCM show elongate lobes that thin from current day northeast to southwest (5 m to <1 m). Dated zircon crystals average 170 μm in size, are euhedral to tabular in shape, and have moderate U values (100 to 1000 pm) and elevated Y values (>500 pm). Integrating these datasets demonstrate that these tuffs were (1) produced from volcanoes fed by intermediate to felsic magmas, (2) that the prevailing paleowind direction was from east-southeast to west-northwest and (3) sourced from volcanoes approximately 280 to 1000 km away which erupted with a volcanic explosivity index (VEI) of approximately 8. We infer that these tuffs originated from a long-lived (late Palaeozoic to Cretaceous) continental arc related to the westward subduction of the paleo-Pacific oceanic crust beneath eastern Australia. These tuffs were most likely derived from the Whitsunday Igneous Association as supported from similar studies from Early Cretaceous strata of the Eromanga Basin. These previous findings will help better constrain the timing of when eastern Gondwana transitioned from a convergent to a divergent margin, and define future targets for ocean drilling to locate the parent igneous bodies in the Tasman Sea.
I am a Visiting Research Fellow at the University of Adelaide. I completed my PhD at the same university and received an MSci in Geology from the University of Southampton. My research focuses on Permian and Jurassic coal-bearing strata in Australia, and the Mesozoic evolution of Australia’s southern margin.