The Bight Basin: a tale of three deltaic megasequences

Abstract

The Bight Basin developed during the Jurassic and Cretaceous in response to repeated periods of extension and thermal cooling leading up to, and following, the onset of seafloor spreading between present-day Australia and Antarctica. The bulk of the resulting sedimentary basin fill includes up to 15 km of Middle Jurassic–recent sediments comprising three deltaic megasequences: White Pointer, Tiger and Hammerhead. High quality seismic 3D data have enabled detailed mapping of the megasequences, evaluation of the nature of infill and assessment of implications for hydrocarbon prospectivity. The Cenomanian White Pointer Megasequence succeeds a period of widespread mudstone deposition in the basin. Growth fault and complex styles of deposition are a function of the high accumulation rates. The growth faulting gradually ceased towards the top of the megasequence allowing for the delta to advance into the basin. Renewed tectonic activity during accumulation of the Turonian–Santonian Tiger Megasequence led to formation of the Outer High trend in the basin. Basal transgressive mudstones during early Tiger deposition are succeeded by a set of progradational–retrogradational units. The base of the Santonian–Maastrichtian Hammerhead Megasequence records widespread subaerial erosion and formation of incised valleys depicting a distinct drop in relative sea level most likely in response to the start of the Australian and Antarctic breakup. This event is followed by a widespread regional flooding event, before the Hammerhead delta built out, first in a highly progradational and subsequently in a more aggradational style. At the end of the sequence, the delta steps back due to decreasing sediment input. Detailed seismic stratigraphic mapping has improved the understanding of the gross depositional environment developed throughout the three megasequences, and importantly also helped identification of the main reservoir fairways and their distribution through time.