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The Western Australian margin is a unique coral reef province, with modern coral reef development occurring at latitudes as far south as 29°S. The genesis of this coral reef province may go back to the Oligo-Miocene, since geological features ~30 million-year-old and younger interpreted as coral reefs are known from offshore seismic surveys. The nature of these seismic reefs is, however, uncertain, as they are only sparsely sampled, and as time-equivalent outcrops are only present in a few remote and understudied locations. This study investigates middle Miocene shallow-water limestones with tropical fauna formed along the North West Shelf (NWS) between ~13 Ma and ~15 Ma, when the southward extension of the seismic reefs was at its maximum. The outcrops and cores investigated are dominantly composed of peloidal packstones and micritic floatstones containing larger benthic foraminifera, Halimeda sp. and scleractinian corals, including reef-building genera, that accumulated in a protected and oligotrophic, warm-water lagoonal environment. Climate was therefore warm during the middle Miocene acme of seismic reefs development, despite the NWS being located ~7° further south than its present position at that time. Results of this investigation also support the existence of a strong southward flowing Leeuwin-current-style oceanic circulation during the middle Miocene, which could have transported fauna from south-east Asia along the Western Australian margin. Development of the lagoon and seismic reefs may have also been promoted by middle Miocene aridification of the coast bordering the NWS, and by repeated eustatic-driven exposures of the NWS. [Display omitted] • Lagoon 15–13 Ma with taxa characteristic of tropical oligotrophic reef environments • Facies with micrite, scleractinian corals, Halimeda and larger benthic foraminifera • Outcrops time-equivalent to offshore Middle Miocene Western "Great Barrier Reef" • Time-equivalent to Miocene Climatic Transition, post-date Miocene Climatic Optimum • Climatic, paleoceanographic (proto-Leeuwin current), eustatic or tectonic controls [ABSTRACT FROM AUTHOR]