Mid-Cretaceous calcareous and siliceous microfossils from the basal Gearle Siltstone, Giralia Anticline, Southern Carnarvon Basin.

A diverse assemblage of foraminifera, radiolaria, and calcareous nannoplankton is recorded from basal beds of the Gearle Silstone in its type area The calcareous nannofossils place the strata within the temperate-latitude CC8a Subzone and the Southern Ocean Sollasites falklandensis Subzone; the planktonic foraminifera indicate that the beds belong to the Hedbergella planispira Zone of the Australian region. The biozones suggest an early Albian age (later than the earliest Albian). Abundant calcareous and siliceous plankton and the Marssonella Association of benthonic foraminifera indicate normal-marine depositional conditions in a water depth of about 100 m. The transition from Windalia Radiolarite to Gearle Siltstone may reflect a marine transgressive pulse that reactivated bottom-water circulation and facilitated a significant increase in the calcareous biogenic component of the sediment. Deposition of the basal Gearle Siltstone was coincident with a major increase in bathymetry in the Papuan, Laura, Carpentaria, Eromanga and Surat Basins in eastern Australia.