Abstract Australia's western margin is adjacent to a low–moderate-relief, semi-arid hinterland extending from northern tropical to southern temperate latitudes. Swell waves occur throughout, and cyclonic storms and tidal influences decline from north to south. The margin is influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current. There is limited upwelling and localized downwelling of saline water on to the shelf. The North West Shelf (NWS) is an ocean-facing ramp with palimpsest sediments – formed during Marine Isoptope Stage (MIS) 3 and 4; stranded ooids and peloids formed early during the post-Last Glacial Maximum (LGM) sea-level rise – and Holocene particles. Changing oceanography during sea-level rise profoundly affected sediment character. The SW Shelf (SWS) comprises the subtropical sediment-starved Carnarvon Ramp in the north and the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south. The inner Carnarvon Ramp includes the Ningaloo Reef and hypersaline Shark Bay. The mid ramp is relict or stranded foraminifer-dominated sand, and represents attenuated carbonate production due to downwelling incursions of Shark Bay water on to the ramp; the outer ramp is planktic foraminiferal sand or spiculitic mud. Rottnest Shelf has coralline algal-encrusted hardgrounds, larger symbiont-bearing foraminifers with abundant cool-water elements including bryzoans, molluscs and smaller foraminifers. The SWS is transitional between warm- and cool-water carbonate realms.

Abstract The western margin of Australia provides a regional latitudinal and climatic gradient from the macrotidal tropical north to the microtidal temperate south, modulated by the poleward-flowing warm Leeuwin Current. Coral-reef systems, discontinuously developed during the late Tertiary-Quaternary, vary from fringing reefs to isolated reefs which rise from deep-ramp settings. Quaternary evolution of the reef systems is being documented using regional mapping, seismic imaging, coring and U-series dating. The well-constrained sea-level data from the Houtman Abrolhos carbonate platforms (at 28–29° S) have also been applied to the less known North West Shelf reefs. The Ningaloo fringing reef at 20–22° S records Holocene and last-interglacial phases of reef growth in a tectonically stable environment. It overlies Tertiary carbonates of the Cape Range, which is flanked by uplifted Plio-Pleistocene terraces and reefs. Scott Reef (at 14° S) is a macrotidal, isolated reef which overlies a carbonate platform and a major gas discovery. Seismic profiles reveal a last-interglacial (ca. 125,000 year) reef system, but reefs which apparently grew to sea-level are 30 m below present sea-level, indicating significant subsidence in the late Quaternary. Contemporary reefs grew during the Holocene in the accommodation space provided by subsidence and are up to 35 m thick. The Rowley Shoals (15–17° S) comprise one of the most perfect morphological series of reefs known, and these emergent, annular reefs rise from depths of 200–400 m. Seismic profiles suggest that late Quaternary subsidence has been an important control on reef growth, while differential subsidence has influenced reef morpho1ogy. Differential geomorphic and physical process settings, seismic stratigraphy, sea-level history, and subsidence are keys to patterns of reef growth which can be seen as responses to these controls.

Abstract: The continental shelf of southwest Australia is part of a passive continental margin which is open, wave-dominated and characterised by cool-water carbonate sedimentation. The Houtman Abrollios coral reefs comprise three shelf-edge carbonate platforms which together form the discontinuously rimmed AbroLhos Shelf at latitude 28-29.5°S. This shelf lies in a biotic transition zone between northern tropical and southern temperate environments. There is also a regional transition between warm-water sediments to the north and the cool-water carbonate shelves of southwestern and southern Australia. There is a marked contrast between Abrolhos platform facies and adjacent shelf sediments. Platform sediments are aragonite-dominated coral framestones and aragonite/Mg calcite sand sheet facies composed of coral and coralline algal debris, with lesser amounts of bryozoans, foraminifers and molluscs. Shelf sediments are dominated by bryozoans and coralline red algae, with lesser molluscs and foraminifers, and are Mg calcite and calcite-rich. There are no ooids, and Halimeda is present only in trace amounts on the shelf. The abundance of corals imparts a warm-water character to the platform deposits, whilst the shelf sediments are indistinguishable from other cool-water shelf deposits which extend from Australia’s southern margin to the western margin, at least to the latitude of the Abrolhos shelf. A poleward-flowing warm-water current (the Leeuwin Current) influences the biotic transition and the relatively high southerly latitude of the Abrolhos reef platforms, which are near the temperature limits for reef-building coral growth. During Tertiary to Quaternary time, there was a vertical transition from cool-water ramp sedimentation to reefal platform development near the shelf edge at the Abrolhos, as a result of Australia’s northward drift and establishment of the Leeuwin Current. Such facies transitions owe their existence to regional patterns of oceanographic circulation, driven in the long term by changes in paleolatitude, and have been an integral part of carbonate platform evolution during the Phanerozoic.