ABSTRACT

The Hawkesbury Sandstone is a Triassic sheet sandstone, extensively exposed in the Sydney Basin, New South Wales, particularly along the coast near Sydney. Unidirectional paleoflow in the sandstone, its freshwater biota, and abundant mudrock intraclasts indicate fluvial deposition. Sheet morphology, low paleocurrent variance, abundant erosion surfaces, and the paucity of in situ mudrocks point to a braided fluvial system.

Three facies assemblages have been recognized: stratified sandstone, massive sandstone, and a minor mudrock assemblage. The stratified sandstone assemblage is dominated by stacked sets of planar cross-strata and minor trough cosets in sequences 6-23 m thick, bounded by erosion surfaces. Significant paleocurrent changes between channel sequences indicate that they were initiated by the avulsion of major channel systems. In some cases the channel sequences fine upward, with planar cross-stratal sets overlain by trough sets that decrease in magnitude upwards, fining up into mudrocks. The massive sandstone assemblage occurs principally as massive sandstone in elongate erosional features oriented transverse to paleoflow. The massive sandstone commonly contains large mudrock intraclasts and is attributed to failure of high channel banks and/or large bedforms during falling-water stages. The mudrock assemblage comprises rippled and horizontally laminated fine sandstone, siltstone, and shale, with minor mudstone. It is attributed primarily to floodplain deposition, but abandoned channel fills are also present.

The Hawkesbury Sandstone does not conform to existing models for braided-fluvial deposition in that planar cross-strata accumulated in deeper parts of channels, whereas trough cross-strata formed in shallower water. The large scale of planar cross-strata (sets up to 7.5 m thick), mudrock intraclasts (up to 38 m long), bank-collapse scars (up to 11 m deep) and abandoned channel fills (up to 18 m deep) show that the Hawkesbury River was very large, with deep main channels and high but variable discharge.