Exmouth Plateau and Argo Basin

Abstract

INTRODUCTION Northwestern Australia from the Exmouth Plateau to the Scott Plateau forms one of the oldest oceanic margins in the world (155 Ma), with a relatively low sediment influx and a large biogenic component (Fig. 1). It is an ideal margin for comprehensive and integrated sedimentologic, biostratigraphic, paleobathymetric, and subsidence studies. Two Ocean prilling Program (ODP) legs are planned in this area: Leg 122, to drill a transect of three or four sites across the Exmouth Plateau, and Leg 123, to drill one site on the northern Exmouth Plateau and one site on the Argo Abyssal Plain (Fig. 2). The Exmouth Plateau off northwestern Australia is about 600 km long and 300-400 km wide with water depths .ranging from 600 to 4000 m (Fig. 1). ,. The plateau consists of rifted and deeply subsided continental crust i covered by a Phanerozoic sedimentary sequence about 10 km thick. It is • separated from the Northwest Shelf by the Kangaroo Syncllne, and is bound I to the north, west and south by Jurassic oceanic crust of the Argo, ; Gascoyne and Cuvier abyssal plains. The Canning and Carnarvon Basin { sediments extend over the plateau from the east and abut the Pilbara Precambrian block (Exon and Willcox, 1978, 1980; Exon et al., 1982; Exon and Williamson, 1986). The Argo Abyssal Plain is an extremely flat, about 5.7 km deep, abyssal plain located north of the Exmouth and Wombat plateaus and west of the Rowley Shoals and Scott Plateau (Fig. 1). It is underlain by the oldest (Late Jurassic M-26) oceanic crust known in the Indian Ocean, crust that is slowly being consumed by the convergence of Australia and the Sunda arc. Legs 122 and 123 Scientific Prospectus page 2 terrigenous material. This and the paleo-waterdepths (10-4000 m) make it a prime target for detailed studies of biostratigraphy, sediment facies, paleoenvironment and stratigraphic evolution owing to subsidence and sea. level fluctuations during the entire period from the Late Triassic through the Quaternary. This prospectus was compiled using the drilling proposals by von Rad, Exon, Symonds and Willcox (1984, ODP proposal 121/B); Arthur (SOHP deep stratigraphic test proposal, 1985, ODP proposal 211/B); von Rad, Exon, Williamson and Boyd (1986, ODP proposal 121/B revised); Gradstein (1986, ODP proposal 240/B); and Mutter and Larson (1987 ODP proposal 288/B); and Langmulr and Natland (ODP proposal 267/F, 1986). We also relied heavily on the pre-site survey information in the cruise reports of R/V Sonne cruise SO-8 (von Stackelberg et al., 1980) and Rig Seismic cruises 55 and 56 (Falvey and Williamson, 1?86; Exon and Williamson, 1986). BACKGROUND Tectonic and stratigraphic background The geological development of the Exmouth Plateau has been discussed