Sequence stratigraphy of a sandstone‐rich Permian glacial succession, Fitzroy Trough, Canning Basin, Western Australia

Abstract

The Canning Basin in northwest Australia contains hydrocarbon‐bearing Permo‐Carboniferous glacial successions, one of a number of such units that occur widely distributed across Gondwana. Up to 2.5 km of clastic sediment eroded by Permian ice sheets from the adjacent Precambrian craton was trapped in the Fitzroy Trough in the northeast part of the Canning Basin. These sedimentary rocks are between 60 and 80% fine to medium sandstone. During relative sea‐level lowstands, regionally extensive erosion surfaces with a relief of several hundred metres developed. Lowstand and early transgressive deposits consisting of sandstone and finer grained elastics that infill topographic lows were deposited on these erosion surfaces. Later transgressive deposits consist of glaciomarine mudstones and rain‐out diamictites, subaqueous outwash fans and deltaic deposits, composed mostly of sandstone. Alternatively, in some situations, the sequences consist entirely of aggrading sandy fluvial braidplain deposits. Some relative highstands may have resulted from isostatic loading of the basin by the ice sheet, so that highstand systems tracts in the Grant Group may correlate with relative lowstands in other basins. Englacial and proglacial lake outbursts deposited extensive sheets of massive, relatively well‐sorted sandstone in all systems tracts. The hydrology of these catastrophic floods meant that much fine sediment was washed out of the basin. This process, combined with the nature of the Precambrian basement adjacent to the Canning Basin produced the high proportion of sandstone in the Grant Group depositional system. Sequence architecture within the Grant Group was determined by interaction of eustasy with tectonic subsidence and sediment supply. Depositional space was created by thermal subsidence in response to Permian extension (which began at approximately 295 Ma) which probably also resulted in uplift of the source area on the Pilbara Craton. In general, sediment supply overwhelmed available depositional space. Glacial sediments produced before the initiation of thermal subsidence would have bypassed the Fitzroy Trough entirely. Late Palaeozoic glacial successions preserved in basins in Oman and Brazil, which lay at the opposite extremes of the Gondwana Plate, have remarkably similar architecture to those of the Canning Basin suggesting similar depositional controls and large‐scale tectonic linkages.