Proceedings of the Ocean Drilling Program Volume 122 Scientific Results: Exmouth Plateau, Sites 759–764

This Scientific Results volume covers Leg 122 of the cruises of the Drilling Vessel JOIDES Resolution, Singapore, Republic of Sing., to Singapore, Republic of Sing., Sites 759–764, 28 June 1988–28 August 1988.

The Exmouth Plateau is a rifted and deeply subsided fragment of continental crust, covered by > 8 km of sediments, including ~ 1-2 km of post-breakup sediments. The present configuration of this region was initiated in the Late Triassic to Early Cretaceous by East Gondwanan rifting between northwestern Australia, greater India, and other Gondwanan fragments to the north. During Leg 122, core recovered from the Wombat Plateau (Site 759 to Site 761 and Site 764) yielded a composite record of sediments deposited during rifting in the Triassic and the Cretaceous and Cenozoic following the breakup. Two sites drilled on the Exmouth Plateau (Leg 122 Sites 762 and 763) recorded the Cretaceous to Cenozoic paleoenvironmental and passive-margin evolution of the western part of the central plateau, documenting a thick clastic shelf-margin wedge that prograded from a southern source area during the Early Cretaceous late-rift phase, and was there overlain by an Upper Cretaceous to Cenozoic pelagic (marine) sequence.

Deep crustal extension and thinning of the northern Exmouth-Wombat Plateau area occurred during the Permian followed by rifting in the Triassic. Carbonate rocks were deposited on the Wombat Plateau during the mid-Carnian in a southern embayment of a shallow Tethys Sea. The plateau periodically re-emerged until the Rhaetian when it developed a fully marine carbonate sequence resembling coeval strata in the western Tethys region of the Alps. During the Jurassic, northward tilting raised the Wombat Plateau above sea level again, and non-deposition/erosion prevented the preservation of Jurassic sequences before the onset of seafloor spreading in the Argo Abyssal Plain. The plateau subsequently subsided rapidly during the Early Cretaceous.

A major erosional unconformity constrains the time of breakup of the western and southern margins of the Exmouth Plateau to around the end of the Neocomian. Intervals of rising water depth are represented by condensed sections of thin glauconitic limestones and belemnite-rich mudstones. During the mid-Cretaceous, when the southern hinterland was drifting northwest with greater India, the southern supply of eroded rocks was cut off. Cyclic deposition of deep-water claystone during the middle to late Aptian marks the onset of hemipelagic deposition which was followed by a stagnant period of oxygen depletion producing black shale at the Cenomanian/Turonian boundary and post-Cenomanian deposition dominated by pelagic carbonate deposition which was continuous in the northern part of the basin and reached the south by the middle Eocene.

Important sequence boundaries on the Wombat Plateau between sediments of the middle and late Carnian, the Norian/Rhaetian boundary, and latest Rhaetian conform well with global changes as do sequence boundaries recognized in the Lower Cretaceous of the central Exmouth Plateau.