Ashmore Reef's sedimentological response to the Holocene sea level rise

Positioned at the transition zone of the major hydrocarbon provinces of Browse and Bonaparte Basins, Ashmore Reef is built on what is thought to be Pleistocene antecedent topography. This mature, ovoid, shelf-edge reef experiences the seasonal oceanic influences of Indian Ocean and of the Indonesian-Through-Flow. The model for its development is derived from the post glacial (past 11,000 years) relative sea level curve, C14 dated facies changes and the reef growth phases extrapolated from the One Tree Reef model (Marshall & Davis 1982). A thorough visual examination of the reef was augmented with a series of 12 vibro-cores through algal-foraminiferal sand and coral, across the bioturbated platform. Changes in the lagoonal sediment facies were carbon dated giving dates ranging from 970 to 2020 (~70 years) BP. They indicated a major bio-facies change from robust vertical coral columns to an algal dominated reef crest and reef flats as sea level stabilised at ~2000 BP. Ashhmore Reef is presently characterised by high biodiversity and extensive coral growth, broad reef flats littered with coral boulders, and three vegetated cays. An extensive series of highly mobile and heavily bioturbated biogenic sand sheets adjoin two lagoons. Both are within a pronounced ovoid, algal-cemented, reef rim. The sediments comprised principally of Halimeda sp., coral fragments, foraminifera, molluscs and a range of coralline algae that infill the lagoon at up to 0.73 cm/yr. The three vegetated cays are capped with guano and all have wash-over deposits of pumice, wood, shell and coral.