Lower Cambrian volcanism in the Hawker Group and the Billy Creek Formation, Arrowie Basin, Flinders Ranges, South Australia

Abstract

Abstract Widespread tuffs are known from the lower Cambrian succession of the Arrowie Basin, Flinders Ranges, mainly from the lower parts of both the Mernmerna Formation and the Billy Creek Formation. Up to 2.5 m of potassium-rich (K2O 6.32–9.60 wt%) rhyolite tuffs are known from the Mernmerna Formation, with the ‘Big Green Tuff’ from Bunyeroo Gorge yielding a TIMS age of 515.4 ± 0.2 Ma (i.e. unnamed Cambrian stage 3). The petrography of the Big Green Tuff, with numerous glass shards, suggests an explosive subaqueous pyroclastic deposit or ash-flow tuff. The presence of several layers of identical green tuff indicates that these were produced by a multi-phase volcanic event. There are up to thirty tuff horizons (5–50 cm thick) within the Warragee Member of the Billy Creek Formation. The tuffs within the Billy Creek Formation occur over a wide area, with an east–west extent of 250 km and a north–south extent of up to 80 km. It is suggested that they are in part ash-flow tuffs. A previous TIMS age for the Billy Creek Formation was 511.87 ± 0.14 Ma (i.e. unnamed Cambrian stage 4). The source of the tuffs, both from the Mernmerna Formation and the Billy Creek Formation is unknown. Detailed study of the whole-rock and trace-element geochemistry is required to fully document them. KEY POINTS Widespread tuffs have been known for about sixty years from the lower Cambrian Mernmerna and Billy Creek formations in the Flinders Ranges, but only a limited amount of work has been done on them. The potassium-rich rhyolitic tuffs of the Mernmerna Formation occur in several horizons up to 2.5 m thick, thus indicating a substantial volcanic event. Sources of the tuffs from both the Mernmerna and Billy Creek formations are unknown. The widespread distribution of both the Billy Creek Formation tuffs and the rhyolitic tuffs in the Mernmerna Formation and the uppermost Wilkawillina Formation indicates that the volcanic activity within the Arrowie Basin was widespread and at times intense.