Subaqueous mass flow origin for Lower Permian diamictites and associated facies of the Grant Group, Barbwire Terrace, Canning Basin, Western Australia

Abstract

The intracratonic Canning Basin is Western Australia’s largest sedimentary basin (>400 000 km2) and has experienced repeated episodes of Phanerozoic extension and subsidence, resulting in deposition of a number of first‐order ‘megasequences’. A major phase of basin extension and sedimentation (Grant Group) occurred in the Late Carboniferous/Early Permian when Australia lay at high palaeolatitudes. Facies analysis of 5000 m of drill core from 25 continuously cored wells in Grant Group strata on the fault‐bounded Barbwire Terrace in the northern Canning Basin identified three facies associations (FAs). These record the predominance of fault‐generated, subaqueous mass flow and sediment reworking. The lowest association (FA I; up to 355 m thick) rests unconformably on tilted older strata and consists of coarse‐grained, subaqueously deposited, sediment gravity flow facies. These include fault‐generated breccias, massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization. FA I is overlain abruptly by relatively fine‐grained deposits of FA II (up to 140 m thick), which consist of laminated to thin‐bedded mudstone and sandstone turbidites, recording an abrupt increase in relative water depths. In turn, these facies coarsen upwards and are transitional into shallow‐water, swaley cross‐stratified and rippled sandstones of FA III (up to 125 m thick). The overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment (FA I), a subsequent phase of rapid subsidence, increasing water depths and ‘sediment underfilling’ (FA II) and, finally, a regressive phase of shoreface progradation. The occurrence of rare striated clasts in FA I suggests reworking of glacial sediment, but no direct glacial influence on sedimentation can be identified.