The Rowley Shoals atolls: Remnants of a Miocene great barrier reef on the north-west Australian margin

The tropical North West Shelf of Australia hosts a diverse range of modern reefs. Six shelf edge isolated atolls are present north of 18°S including: Ashmore Reef, Scott Reef and Seringapatam Reef, and three Rowley Shoals. The Ningaloo Reef is a fringing reef around the North West Cape at 22°S. All of these reefs are the remnants of a vast 2000 km long barrier reef that drowned during the Late Miocene (∼10 Ma). Despite extensive hydrocarbon exploration in the region, the history of these isolated reefs is not well known. Seismic analyses combined with stratigraphic analyses of International Ocean Discovery Site U1464 near the Rowley Shoals has revealed that these modern isolated atolls have a complex evolution related to climate and tectonism as they managed to survive on their Miocene barrier reef foundation. The first Miocene reefs (∼17 Ma) near the Rowley Shoals were isolated small, mound-shaped features. These evolved into a barrier reef by the Middle Miocene (∼16 Ma). However, by the Late Miocene (∼10 Ma) this barrier reef backstepped landward, evolved into isolated mounds/atolls and drowned prior to the Miocene-Pliocene boundary (∼6 Ma) largely due regional tectonic subsidence. Early Pliocene reef expansion (∼4.6 Ma) led to the growth of four isolated atolls (the Rowley Shoals) related to local faulting and Early Pliocene warmth. Subsequently a second Pliocene reef growth phase occurred from ∼3.5 to 3 Ma when eastern Indian Ocean sea surface temperatures cooled by ∼4 °C due to Indonesian Gateway constriction and a reduced Leeuwin Current. By the Pleistocene (∼2.4 Ma) one the four Rowley Shoals had drowned. Strong sea level variability, together with Indonesian Throughflow constriction and reduction in intensity of the Leeuwin Current after 2.4 Ma may have led to enhanced cooling and regional upwelling. These factors may have been sufficient to cause local drowning of the southerly fourth Rowley Shoal while the more northerly three Shoals survived until present. • The three modern West Australian Rowley Shoals are remnants of a 2000 km + barrier reef. • These isolated atolls persisted while subsidence and changed oceanographic conditions drowning the underlying barrier reef. • A fourth shoal drowned during the Pliocene due to a reduced Leeuwin Current and Indonesian Throughflow.