SEQUENCE STRATIGRAPHY OF A SANDSTONE-RICH PERMIAN GLACIAL SUCCESSION,FITZROY TROUGH, CANNING BASIN, WESTERN-AUSTRALIA

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 elasti c 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 exte nsive erosion surfaces with a relief of several hundred metres develop ed. Lowstand and early transgressive deposits consisting of sandstone and finer grained clastics that infill topographic lows were deposited on these erosion surfaces. Later transgressive deposits consist of gl aciomarine 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 fl uvial braidplain deposits. Some relative highstands may have resulted from isostatic loading of the basin by the ice sheet, so that highstan d systems tracts in the Grant Group may correlate with relative lowsta nds in other basins. Englacial and proglacial lake outbursts deposited extensive sheets of massive, relatively well-sorted sandstone in all systems tracts. The hydrology or: these catastrophic floods meant that much fine sediment was washed out of the basin. This process, combine d with the nature of the Precambrian basement adjacent to the Canning Basin produced the high proportion of sandstone in the Grant Group dep ositional system. Sequence architecture within the Grant Group was det ermined by interaction of eustasy with tectonic subsidence and sedimen t supply. Depositional space was created by thermal subsidence in resp onse to Permian extension (which began at approximately 295 Ma) which probably also resulted in uplift of the source area on the Pilbara Cra ton. In general, sediment supply overwhelmed available depositional sp ace. Glacial sediments produced before the initiation of thermal subsi dence 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 simil ar architecture to those of the Canning Basin suggesting similar depos itional controls and large-scale tectonic linkages.