Proceedings of the Ocean Drilling Program Volume 123 Scientific Results: Argo Abyssal Plain/Exmouth Plateau, Sites 765–766

This Scientific Results volume covers Leg 123 of the cruises of the Drilling Vessel JOIDES Resolution, Singapore, Republic of Sing., to Singapore, Republic of Sing., Sites 765–766, 28 August 1988–1 November 1988.

Leg 123 of the Ocean Drilling Program (ODP) was undertaken in the northeast Indian Ocean, off the northwestern margin of Australia. This margin forms one of the oldest continent/ocean boundaries on Earth (approximately 160 Ma, Middle to Late Jurassic) and is comparable in age to the conjugate margins of eastern North America and northwestern Africa. ODP Legs 122 and 123 represent a combined study of the northwestern margin of Australia. Leg 123 investigated the outboard area of the Exmouth Plateau in the Argo and Gascoyne Abyssal plains with two drill sites, whereas the continental margin was drilled at five sites during Leg 122. 

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.