Early Miocene carbonate ramp development in a warm ocean, North West Shelf, Australia

Although carbonate ramps are widely described from the geological record, there is still a debate on the relative influence of water temperature, trophic conditions and type of carbonate factories on their development. The ca 2400 km long Australian North West Shelf is among the largest Cenozoic carbonate provinces worldwide, and records a transition from an early Miocene ramp to a middle Miocene rimmed platform. This change is observable on publicly available seismic data, giving the opportunity to investigate environmental influences on platform evolution. This study combines macroscopic and petrographic descriptions of early Miocene strata cropping out in the Cape Range Anticline (North West Cape, southern end of the North West Shelf) and of time-equivalent well cuttings from the adjacent, offshore Exmouth Sub-basin. Particular emphasis is placed on the identification of larger benthic foraminifera at a broad generic level, because differing taxa have a limited range of habitable conditions that serve as environmental proxies. The results show that early Miocene strata are dominantly composed of larger benthic foraminifera with minor coralline algae in the proximal platform, grading to micropackstones in the more distal platform. A ramp margin is inferred from the lithological data on the basis of the lack of framework builders and the presence of open oceanic indicators. Facies shallow upward through individual outcrops, with a proximal to distal trend towards the north-west. These trends along outcrops are consistent with the seismic interpretations. Identification of taxa with warm, oligotrophic water affinity suggests that the ramp was formed in an oligotrophic and warm ocean, despite the absence of coral reefs. Changes of carbonate facies with depth do not seem to be associated with changes in ramp morphology, and the latter may have been controlled by physical oceanic parameters, such as offshore currents and waves.