The Sequence Stratigraphic Evolution of the Exmouth-Barrow Margin, Western Australia
Young, Hamish C.
Doctor of Philosophy
University of Adelaide
The Exmouth-Barrow Margin evolved as a passive margin following the middle Valanginian (132.5 Ma) breakup of Greater India from western Australia. Five supersequences have been revealed by a regional sequence stratigraphic study. The interpretation utilises a new sequence stratigraphic model developed for mixed siliciclastic-carbonate lithofacies. A high degree of resolution is brought to the study by identification of thirty-seven regional sequence boundaries constrained by biostratigraphic, wireline and seismic data. Ditch cutting analysis, integrated into the new chronostratigraphic framework, provided detailed lithofacies maps.
The lithology of the margin grades from a siliciclastic-dominated claystone developed in the Albian late Cenomanian to a coarse-grained carbonate shelf deposited in the Oligocene-Recent. The initiation of carbonate sedimentation on the margin corresponds to the northward migration of the Australian continent into warmer latitudes combined with periods of increased oceanic circulation.
The five supersequences, named the Gallic, Senonian, Palaeogene, Middle Neogene and Pliocene, are based upon regional lowstand, transgressive and highstand phases. The siliciclastic-dominated Gallic Supersequence (Late Tithonian–latest Cenomanian) is the largest supersequence and extends across the entire the Northern Carnarvon Basin. The lowstand phase of the supersequence coincides with rifting between Greater India and west Australia during the Neogene and is characterised by a thick prograding delta complex in the Exmouth Margin. Breakup was followed by rapid thermal subsidence and the deposition of thick retrogradational claystones of the transgressive phase. The late Gallic Supersequence is heavily truncated by incised canyon systems of the Senonian Supersequence (latest Cenomanian– middle Maastrichtian). The evolution of the Senonian section corresponds to the initial separation of Australian from Antarctica and the first appearance of carbonates.
The Palaeogene Supersequence (middle Maastrichtian–late Early Miocene) dominates much of the Tertiary and is identified by a basinward shift of facies following a Maastrichtian–Paleocene sea level fall. Enhanced subsidence on the outer shelf during the Eocene resulted in a forced transgression and the deposition of carbonate mudstone-siltstones. A highstand during the Oligocene–Early Miocene formed the distinctive prograding carbonate shelf recognised throughout the North West Shelf.
The Middle Neogene Supersequence (late Early Miocene–Early Pliocene) is identified by an erosive base and the development of a thin lowstand fan on the outer shelf. The supersequence is characterised by backstepping barrier reefs of the Middle Miocene transgressive phase. A Late Miocene eustatic stillstand restricted reef development to the inner shelf, generating coarse-grained carbonate progrades by highstand-shedding. The final Pliocene Supersequence (Pliocene–Recent) was initiated by a eustatic fall during the Early Pliocene and was followed by the development of a transgressive, aggrading shelf. The aggrading wedge is observed globally on passive margins with varying lithology, sedimentation supply and rift-drift histories, and is interpreted to have been caused by load-induced subsidence.
Comparing the stratigraphic development of the Exmouth-Barrow Margin with several Australasian, Atlantic and Indian Ocean passive margins enabled the identification of two different types of subsidence: synchronous (including load-induced and tectonic) and diachronous (thermal) subsidence.
The subsidence history of the Exmouth-Barrow Margin was further complicated by the influence of buried antecedent topography. The slower burial rates of the Jurassic Alpha-Rankin Complex had the effect of compartmentalising subsidence during the middle Cretaceous to Recent, resulting in oversteepening and slumping of the Pliocene shelf edge and slope sediments.
The generation, migration, accumulation and preservation of hydrocarbons in the Exmouth-Barrow province are closely related to the subsidence and associated structural activity of the Late Cretaceous- Recent. Three major hydrocarbon phases were recognised: the Senonian, Palaeogene and Pliocene, which account for the majority of hydrocarbon fields in the province.