Exmouth Plateau Revisited: Scientific Drilling and Geological Framework

Abstract

The Exmouth Plateau is a very large marginal plateau containing 10 km of Phanerozoic sediments and is underlain by continental crust that was stretched and thinned, probably in the Late Permian. For much of the Mesozoic it was part of the northern shore of eastern Gondwana and the southern shore of Tethys, and a large part of the Phanerozoic sequence consists of Triassic fluviodeltaic sediments. Late Triassic to Late Jurassic rifting caused block-faulting, and several large grabens have a thick fill of Jurassic shallow-marine carbonates and coal-measure sequences. A major post-rift unconformity, of a minimum Callovian-Oxfordian age, is commonly directly overlain at the Valanginian breakup unconformity by Neocomian transgressive marine sediments, whose character is controlled by the breakup history of the plateau margins. A microcontinent broke away from the northern margin in the Late Jurassic, leaving behind the oceanic crust of the Argo Abyssal Plain. Greater India broke away from the northwestern and southwestern margins of the plateau in the late Valanginian, leaving behind the oceanic crust of the Gascoyne and Cuvier abyssal plains. After breakup, the Exmouth Plateau was surrounded by abyssal plains on three sides, so terrigenous input greatly decreased. As the plateau sank, shallow-marine Early Cretaceous sedimentation gave way slowly to bathyal carbonate sedimentation. Slow sedimentation rates resulted in a Cretaceous-Cenozoic sequence above the post-rift unconformity that is very thin by global standards, so that the plateau is an ideal area to study the pre-rift and rift sequences by geophysical methods, dredging, and drilling. The plateau has been extensively studied by academia, government agencies, and the petroleum industry, and has a vast open file and proprietary data set. In 1988, the Ocean Drilling Program drilled at six sites on the plateau during Leg 122 and two sites on the adjacent abyssal plains during Leg 123, designed to address the geological history of this continental margin, as an exemplar for other, more heavily sedimented margins. The six Exmouth Plateau holes cored a total of 3370 m of Upper Triassic, Cretaceous, and Cenozoic strata, with excellent recovery in all but Upper Triassic carbonates. This paper introduces the results contained in 55 papers included in this volume, covering most aspects of passive margin evolution and paleoenvironment, biostratigraphy, magnetostratigraphy, chemostratigraphy, sequence stratigraphy, rift tectonics, eustatic sea-level fluctuations, cyclic sedimentation, and orbital forcing. This paper also outlines the paleogeographic development of the region from Late Triassic to Albian times.

Carbonate pelagites in the post-Gondwana succession (Cretaceous – Neogene) of East Timor

“…Here, Aptian to lower Albian radiolarian-rich carbonates are overlain by claystone, marl and carbonate units of the Albian to Neogene. On Exmouth Plateau, a broad continental terrace adjacent to the Barrow Basin, the Albian to Neogene succession consists of up to *900 m of carbonate pelagite in the form of friable chalk and marl in the lower part of the section and ooze in the upper few hundred metres (Exon et al 1992). On the Argo Abyssal Plain, situated north of Exmouth Plateau and southwest of Timor, the equivalent succession is made up of carbonate turbidites and claystone that remain uncemented (Dumoulin & Bown 1992).…”

“…As on Exmouth Plateau, subsidence of the Timor region may have taken place during the Early Cretaceous with bathyal pelagic deposition well established by the Aptian -Albian. On Wombat Plateau, which represents the northern part of the broader Exmouth Plateau, the pelagite succession rests on lowest Cretaceous belemnite-rich sandstone and chalk that lies unconformably above a Triassic Gondwana succession (Exon et al 1992). This appears to mirror the From analogy with Exmouth Plateau, it seems probable that at 10.4 -9.8 Ma GTS2004, a succession, perhaps ranging from 200 to 900 m thick, of middle to lower bathyal carbonate pelagite lay above a thin Upper Jurassic to lowest Cretaceous shallow-marine siliciclastic unit that rested unconformably on the upper part of the Gondwana Megasequence.…”

Fossil seaward-dipping reflector sequences preserved in southeastern Australia: a 600 Ma volcanic passive margin in eastern Gondwanaland

“…Most information about these margins is remotely sensed from a combination of multi-channel seismic (MCS), ocean bottom seismometer, gravity and magnetic data, with limited 'groundtruth' coming from sparse dredging and drilling (Ocean Drilling Program (ODP) Sites 642, 765, 915, 917, 918, 989 and 990: Eldholm et al 1989;Exon et al 1992;Larsen et al 1994;Fitton et al 2000). Knowledge of SDRS on other margins is from remotely sensed data alone, principally MCS and magnetic data (e.g.…”

Deformation of the Cablac Mountain Range, East Timor: An overthrust stack derived from an Australian continental terrace

“…The Upper Triassic-Lower Jurassic carbonate platform facies that forms so much of the Cablac Mountain Range probably developed on discontinuous small banks in the interior seas surrounded by basinal facies in similar conditions to their counterparts on the West Shelf of Australia (Exon et al, 1992;McCartain et al, 2006;. The Late Cretaceous and Early Eocene pelagites of the Australian-Margin Megasequence, found on the northern margin of the Cablac Range, were probably deposited on a continental terrace (similar to presentday Exmouth Plateau; Fig.…”

“…On the Wombat Plateau (Fig. 1), which represents the northern part of the broader Exmouth Plateau, the pelagite succession rests on lowest Cretaceous belemnite-rich sandstone and chalk that lies unconformably above a Triassic Gondwana succession (Exon et al, 1992). This appears to mirror the stratigraphic relationships reconstructed for Timor (Charlton and Wall, 1994;Harris et al, 2000;Charlton, 2002;Villeneuve et al, 2005).…”