Wed 16th November @ noon, Rm 312.222 

Abstract

Following the breakup of Australia from Greater India in the Early Cretaceous, the North West Shelf of Australia (NWS) evolved as a passive margin. The combination of a wide continental shelf and relatively limited clastic influx resulted in the progradation of carbonate strata, and these deposits  constitute excellent archives of Australia’s past climate and sea levels. Neogene carbonates of the NWS are generally dominated by “heterozoan” carbonate assemblages (e.g., carbonate production controlled by organisms that usually strive in sub-tropical to colder oceans and/or areas associated with high nutrient input). However there is widespread evidence for the Miocene development of a coralgal “photozoan” (tropical, light-dependant) reef system.

This presentation will review the sedimentological and geophysical evidence of a Miocene coralgal barrier reef system that extended over at least 2000 km, from the Exmouth sub-basin to the NE Bonaparte Basin. The high quality 3D seismic datasets that cover most of the North West Shelf allow reconstructing the stratigraphic architecture and geomorphology of this reef system at very high resolution. Biostratigraphic and Strontium isotope dating on more than 30 wells along the margin, together with field evidence (Cape Range) allow constraining for the first time the timing of reef development across all the NWS basins. The analysis shows that in general reef development initiated at ca. 18-17 Ma, at the onset of the Mid Miocene Climatic Optimum global warming. An extensive barrier reef rimmed the North West Shelf by 15-14 Ma, at a time where paleoclimate reconstructions indicate global mean surface temperatures up to 3-4° higher than today and atmospheric C02 concentrations ranging from pre-industrial levels (250 ppm) to 500 ppm (e.g., same range than those projected in the coming centuries). This barrier system may have persisted until 11-10 Ma, during a time interval marked by relatively high Sea Surface Temperatures in the Indian Ocean despite the onset of global cooling and the expansion of Antarctic ice. The slow demise of this barrier reef system after 11-10 Ma and most importantly after 7-6 Ma probably results from a complex interaction between global changes in oceanography (due to plate tectonic reorganization in greater SE Asia, including the Banda Arc) and local changes in subsidence rates and/or clastic influx as the result of collision.

This Miocene “Great Barrier Reef” appears as a very useful analogue to understand long-term carbonate reef response to climate changes and coral reef development in a predicted warmer future.

Biography

Julien Bourget is a senior lecturer in Marine and Petroleum Geology at the School of Earth Sciences and the Associate Director of the Centre for Energy Geoscience of the University of Western Australia. Julien obtained his MSc and PhD from the University of Bordeaux (France) in 2006 and 2009, respectively. Following his PhD he held a postdoctoral fellowship at the Australian School of Petroleum (University of Adelaide) from 2010 to 2011. His research expertise is in Sequence and Seismic Stratigraphy with a primary focus on modern 3D seismic interpretation techniques and their applications for understanding the geodynamic and sedimentary evolution of the margins of the Indian Ocean (including the North West Shelf of Australia) in the Mesozoic and Cenozoic.