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Tectonic, Sea-Level and Oceanographic Controls on the Distribution and Growth of Isolated Carbonate Build-Ups at the Margin of a Very Wide Platform (NW Australia)


Isolated carbonate build-ups (ICBs) represent attractive hydrocarbon exploration targets. They are often seen as long-term transgressive features; but their distribution within basins and growth history can be difficult to predict as they respond to the interplay between various tectonic, eustatic, and oceanographic parameters. Here we use a 3D seismic megasurvey (18,000 km2) combined with well data to understand the timing and mechanisms of formation of tropical Quaternary ICBs in NW Australia. At present-day, these ICBs are typically 1-30 km wide and form clusters of ∼ 150 build-ups, developing 2 to 85 km from the edge of a 650 km-wide continental shelf. The physiography of the area is analogous to some ancient carbonate systems that formed along very wide shelves and epeiric seas (i.e. Devonian-Carboniferous build-ups of the Caspian region). Results demonstrate that the structural evolution of the margin had a major impact on the distribution of the ICBs. Main fault activity commenced during the latest Miocene-Early Pliocene, corresponding to the initial collision of Australian Plate with Banda Arc. Fault activity peaked in the late Pliocene (∼3 Ma), and was associated with flexural reactivation of structural highs (uplift) and lows (subsidence) along the shelf-margin. Later, during the Quaternary, ICBs developed on the highs. This late Pliocene event is synchronous with the uplift of Timor. Penecontemporaneous signatures of ocean bottom currents (i.e. erosional surfaces, moats and contourite drifts) of early Quaternary age (∼2 Ma) suggest a change in oceanographic regime along the NW Australian margin. It is probable that the reactivation of the Australian margin and the uplift of Timor narrowed the path of the Indonesian Throughflow (ITF) warm-water current. Despite potentially good conditions for carbonate production (basement highs and warm water ocean currents), ICBs did not form until the Mid-Quaternary (ca. 0.9-0.6 Ma). This age corresponds to the onset of major sea-level fluctuations associated with repeated, high-amplitude (+120 m) deglacial rises, long-term highstands and slow falls. Thus, we infer that the NW Australia ICBs formed due to: (1) structural shaping of the margin; (2) oceanographic changes, and; most importantly, (3) onset of repeated high-amplitude transgressions reactivating the carbonate production along isolated highs following 4th-5th order lowstand exposures and allowing catching-up carbonate morphologies to develop.