Sedimentology and lithostratigraphy of Upper Eocene sponge-rich sediments, southern Western Australia

Late Eocene time in the Bremer and western Eucla Basins of southern Western Australia was a period of terrigenous clastic and abundant, unusual, biosiliceous sponge sedimentation. The Pallinap Formation (revised) consists of five units; 1 and 2 are basal sandstones, 3 and 4 are variably spiculitic mugstones, wailst the uppermost unit is spiculite and spongolite, and formaised as the Fitzgerald Member (new). The Pallinup Formation, plus coeval spiculites in palaeovalleys and carbonates in the western Eucla Basin, accumulated during one large-scale, transgressive-regressive relative sea-level cycle Drowned, low-gradient rivers supplied mud but little sand. Instead, sand was locally sourced via transgressive shoreface erosion of deeply weathered regolith. Regression terminated shoreface erosion, eliminated the sand source, and resulted in a river-supplied, clay-dominated shallow-marine deposit onal system. The unit 2-3 sandstone-mudstone transition, which would normally be interpreted as transgressive crowning, is in this case the result of regressive cessation of sand supply. The peak relative sea-level (highstand) horizon thus lies within unit 2 sandstones, a facies that would usually be considered wholly transgressive, and no highstand systems tract was deposited. The maximum flooding and downlap surfaces are the same horizon and cap the transgressive systems tract. They formed coincidentally or subsequent to peak relative sea-level, but prior to initiation of unit 3 mudstone deposition Upper uni- 2 plus unit 3 represent a condensed section systems tract, and unit 4 plus the Fitzgerald Member comprise a regressive systems tract.