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

Abstract

The Kalkarindji continental flood basalt province of northern Australia erupted in the mid Cambrian (c. 511-505Ma). It now consists of scattered basaltic lava fields, the most extensive being the Antrim Plateau Volcanics (APV) – a semi-continuous outcrop (c. 50,000km2) reaching a maximum thickness of 1.1km. 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 (15–20m thick) forming the upper crusts of rubbly pāhoehoe lava units 25–40m thick; covering 18,000–72,000km2 and an estimated volume of 1,500–19,200km3. 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 × 104–2.11 × 105 Tg total CO2 at a rate of 867 Tg a-1 and 9.07 × 103–1.16 × 105 Tg SO2 at 476.50 Tg a-1. 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.