Carboniferous–Permian facies and tectono-stratigraphic successions of the glacially influenced and rifted Carnarvon Basin, western Australia

The Carnarvon Basin of Western Australia is a rift basin that contains a thick (up to 5 km) succession of late Carboniferous–early Permian glacially influenced marine sedimentary strata. These rocks accumulated in near-polar paleolatitudes along the uplifted and glaciated margin of the west Australian Shield (Pilbara Craton). Three stratigraphic successions (I, II, III) can be recognised, each characterised by distinct facies associations that record different stages in the tectonic evolution of the basin and associated changes in the rate of basin subsidence and sediment accommodation. A lowermost succession (I) comprises rapidly deposited (30 m/Ma) glacially influenced marine strata (Lyons Group) containing palynomorphs of Westphalian–Tastubian (early Sakmarian) and possibly older age. Strata are dominated by subaqueously deposited sediment gravity flow facies. Succession II is composed of richly fossiliferous cool water shales (Callythara and Cordalia formations) that record much reduced sedimentation rates (2 m/Ma). In turn, shales are overlain by an uppermost succession (III) of shallow marine, wave- and storm-influenced sandstone (Moogooloo Sandstone). Comparison with other rift basin fills indicates that Succession I likely records initial basin infilling where abundant coarse debris was produced by faulting and glaciation of the adjacent Pilbara Craton. Shales of Succession II mark a phase of ‘sediment underfilling’ characterized by rapid tectonic subsidence, an increase in relative sea level and reduced sediment supply. Shallow water sandstone facies of Succession III record a late stage in the tectonic cycle when subsidence rates had decreased and sediment supply outpaced accommodation. Such successions, where found in other late Paleozoic basins, are widely interpreted in terms of glacioeustatically driven changes in sea level resulting from deglaciation events across Gondwana. Instead, the three successions within the Carnarvon Basin are argued to reflect a dominantly tectonic control on sedimentation and preservation.