The Giant Lavas of Kalkarindji: rubbly phoehoe lava in an ancient continental flood basalt province

The Kalkarindji continental flood basalt province of northern Australia erupted in the mid Cambrian (c. 511 505 Ma). It now consists of scattered basaltic lava fields, the most extensive being the Antrim Plateau Volcanics (APV) a semi-continuous outcrop (c. 50,000 km) reaching a maximum thickness of 1.1 km. Cropping out predominately in the SW of the APV, close to the top of the basalt succession, lies the Blackfella Rockhole Member (BRM). Originally described as ‘basaltic agglomerate’ the BRM has, in recent years, been assumed to be explosive tephra of phreatomagmatic origin, thus providing a potent vehicle for volatile release to the upper atmosphere. Our detailed field investigations reveal that this basaltic agglomerate is, in reality, giant rubble collections (15 20 m thick) forming the upper crusts of rubbly pāhoehoe lava units 25 40 m thick; covering 18,000 72,000 km and an estimated volume of 1,500 19,200 km. These flows, rheologically but not chemically, distinct from the majority of Kalkarindji lavas, indicate a fundamental change in eruption dynamics. A low volatile content, induced high amounts of pre-eruptive degassing causing super-cooling and an increase in crystal nucleation and viscosity. A more viscous lava and a consistently faster rate of effusion (analogous to that of Laki, Iceland) created the flow dynamics necessary to disturb the lava crust to the extent seen in the BRM. Volatile release is estimated at 1.65 × 10 2.11 × 10 Tg total CO2 at a rate of 867 Tg a and 9.07 × 10 1.16 × 10 Tg SO2 at 476.50 Tg a. These masses accounted for 0.5% of Cambrian atmospheric conditions whilst limiting factors reduced the effect of volatile delivery to the atmosphere, thus any potential global impact caused by these flows alone was minimal.