LEGS 122 and 123

Exmouth Plateau and Argo Abyssal Plain

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.

Drilling at Site 765 during Leg 123 documented that the Argo Abyssal Plain may be 20 m.y. younger than previously supposed. Detailed microfossil stratigraphy for silty claystone and volcanic ash layers overlying fresh volcanic flows dates the basement, previously assigned magnetically to the Oxfordian, as latest Berriasian-Valanginian. Stratigraphic determinations and geomagnetic polarity in the basal sediments at Leg 123 Site 766 indicate that seafloor spreading between Australia and greater India started at 134 Ma (late Valanginian). The presence of a mid- ocean ridge basalt (MORB) type of tholeiitic intrusive sequence at the base of a sandy sedimentary section implies that the ocean/continent boundary must have been rifted, buried by continental sediments, and intruded by MORB-type magmas immediately before the formation of true oceanic crust a few kilometers seaward.

Sedimentation was relatively high (1.5-4 cm/k.y.) during the early oceanic-subsidence phase but decreased by a factor of 2 in the post-Aptian, reflected in a change from deposition of terrigenous material at river deltas to open-ocean blanket deposition. A major hiatus occurred at the end of the early Eocene, signifying the onset of vigorous deep-water circulation, owing to the formation of Antarctic deep water that eroded the abyssal seafloor. In mid-Tertiary time, several factors combined to rapidly increase abyssal sedimentation in the Argo region by a factor of 2-6. Canyons, like Swan Canyon that align with the main southwest-northeast fault trend on the margin, became active sediment conduits and led to catastrophic Neogene abyssal sedimentation with massive debris flows, representing in part the tail of a large prograding clastic fan on the Exmouth Plateau

The two sites drilled during Leg 123 yielded a detailed magnetic polarity sequence for M13-M0, Valanginian to Aptian, the first direct relationship recorded between the M-series of geomagnetic reversals and multiple biostratigraphy in the Southern Hemisphere. These sites also represent the first in a series of reference sites where geochemical studies of the terrigenous and volcanogenic clays will allow fingerprinting of the bulk composition of the earth's crust.

Basalts erupted at the Indian Ocean ridge axes have subtle geochemical differences from those in the Pacific and Atlantic oceans; each region must have a distinct mantle source. The lavas recovered during Leg 123 have preserved fresh glass, making isotopic evaluation of the earliest Indian Ocean mantle reservoirs possible, and indicate significant differences in incompatible element geochemistry from basalts erupted in the modern Mid-Indian Ridge axis.