Middle Miocene tropical oligotrophic lagoon deposit sheds light on the origin of the Western Australian coral reef province

Rosine Riera,Julien Bourget,Eckart Håkansson,Victorien Paumard,Moyra E.J Wilson

doi:10.1016/j.palaeo.2021.110501

Rosine Riera, Julien Bourget + Show 3 more

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https://doi.org/10.1016/j.palaeo.2021.110501

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Abstract

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.

Highlights

Western Australia is a modern coral reef province located along the western margin of a continent with coral reefs and coral colonies devel oping respectively at latitudes as far south as 29◦S and 32◦S (Collins, 2002; Fairbridge, 1950; Gallagher et al, 2017a; Hatcher, 1991)
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
2016) and an increase in polar ice sheet volume during the Miocene Climatic Transition (Flower and Kennett, 1994; Zachos et al, 2001) are a likely cause of these repeated lowstands. Tropical taxa such as hermatypic corals, Halimeda sp. and larger benthic foraminifera was present on the North West Shelf during the middle Miocene

Summary

Introduction

Western Australia is a modern coral reef province located along the western margin of a continent with coral reefs and coral colonies devel oping respectively at latitudes as far south as 29◦S and 32◦S (Collins, 2002; Fairbridge, 1950; Gallagher et al, 2017a; Hatcher, 1991) This latitudinal extent is unusual, as elsewhere coral reefs do not develop at latitudes higher than 2◦S along western margins of continents (Kiessling, 2001), and as, for comparison, the Pacific atolls are restricted to the tro pics (Darwin, 1842; Hatcher, 1991). During the middle Miocene (i.e., 16–15 Ma; McCaffrey et al, 2020), the seismic reefs were forming a ~2000 km long reef tract, possibly concomitant with the development of the ‘Little Barrier Reef’ – another seismic reef - in the Bight Basin (Feary and James, 1995; O'Connell et al, 2012; Fig. 1)

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