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.

During Leg 122 the JOIDES Resolution occupied two sites (762 and 763) on the central Exmouth Plateau. The drilling target in the older part of the cored section was a Neocomian wedge of clastic sediments, sourced from the south so that Site 763 was located proximally and Site 762 distally to sediment source. Multiple objectives of the leg included: (1) dating of final breakup of the southern Exmouth Plateau and its evolution from a juvenile to mature oceanic margin; (2) delineation of depositional sequences associated with progradation of the Neocomian clastic wedge; (3) refinement of Mesozoic and Cenozoic magnetobiostratigraphies; and (4) recovery of a complete Cretaceous/Tertiary boundary section. Well-constrained dating of the unconformities associated with tectonic events has narrowed considerably the age of the final fragmentation of the plateau to late Valanginian when Greater India broke away from the southwestern margin, initiating the formation of the Gascoyne and Cuvier abyssal plains to the west and southwest of the plateau. Sequence-stratigraphic concepts applied to the Berriasian Barrow Group sediments have revealed the presence of several depositional sequences (at least four in the cored part of Site 763) and provide a valuable case study of an ancient lowstand prograding complex with high clastic influx. Refined multifossil biostratigraphy for much of the recovered Cretaceous and Cenozoic section was possible. Attempts at paleomagnetic analyses produced an incomplete but partially useful magnetostratigraphic record of the Late Cretaceous and Cenozoic. An essentially complete Paleogene and nearly complete Neogene record is present at Site 762 which lent itself to detailed biostratigraphic zonation. Given the good palynological (microplankton) biostratigraphic resolution available for the Neocomian part of the section, but less than definite correlation to global biostratigraphic schemes, the timing of sea-level events on the central Exmouth Plateau appear to agree well with the eustatic cycle chart, suggesting a global influence superimposed on the local and regional tectonics.

Site 764 of the Ocean Drilling Program (ODP), drilled during Leg 122 in the Exmouth Plateau region, cored 200 m of Upper Triassic (Rhaetian) reef complex. This site, on the northern Wombat Plateau (northernmost Exmouth Plateau) represents the first discovery of Triassic reefal material near the Australian North West Shelf. Seismic reflection data through Site 764 show that the reef itself corresponds predominantly to a seismically poorly reflective zone. A number of regional unconformities appear to correspond, however, to traceable seismic horizons which pass with reduced amplitude through the reef, indicating stages of reef growth separated by erosion or non- deposition. Seismic facies around the edges of the reef are consistent with the deposition of wedges of prograding reef- derived detritus.Application of the seismic criteria for reef recognition established at ODP Site 764, to other seismic reflection data on the Wombat Plateau, demonstrates that a major Upper Triassic reef complex fringes the margins of the Wombat Plateau. The Wombat Plateau lies at the western end of the North West Shelf, which was part of the southern margin of a warm Tethys Ocean in the Late Triassic, at a palaeolatitude of 25- 30°S. Upper Triassic reefs are known from southeast Indonesia and Papua New Guinea, and now the Wombat Plateau, and may be common elsewhere along the outer margin of the North West Shelf. Upper Triassic reef complexes, with their associated reservoir, source and seal facies, could represent an exciting new petroleum exploration play for the entire North West Shelf. Facies analysis suggests that they are likely only on the outer shelf and slope. Shallow Triassic reef complexes are clearly identifiable using high resolution seismic reflection data. Seismic reflection data of lower resolution may well reveal the associated detrital carbonate wedges, which are more laterally extensive than the reefal core, deeper in the section.

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