Evolution of the Central Exmouth Plateau: A Post-Drilling Perspective

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. INTRODUCTION Ocean Drilling Program (ODP) Leg 122 on the Exmouth Plateau was one of the rare occasions when scientific drilling sampled an old passive continental margin. Drilling on a margin with known hydrocarbon occurrence, without the benefit of a protective bottom-hole assembly and riser system, elicited obvious safety concerns. But the scientific objectives were compelling enough that a drilling strategy was designed to core near previously drilled, but relatively poorly sampled, exploratory industry holes that did not have significant hydrocarbon shows. This unprecedented experiment yielded an important record of the late-rift and postbreakup sedimentary history and paleoenvironmental evolution of this margin. Site 762 was drilled approximately 2 km northwest of the industry well Eendracht-I to a depth of 940 meters below seafloor (mbsf), and Site 763 was drilled near well Vinck-I to a depth of 1036.6 mbsf, about 2 km southeast of the well site (Fig. 1). The availability of well-log data from these and other commercial wells and an extensive network of high-quality seismic data on the Exmouth Plateau were important factors in choosing this area as a case study of the development of sediment-starved passive margins, where various models of 1 von Rad, U. , Haq, B. U. , et al., 1992. Proc. ODP, Sci. Results, 122: College Station, TX (Ocean Drilling Program). 2 Division of Ocean Sciences, National Science Foundation, Washington, DC 20550, U.S.A. 3 Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada. 4 Bureau of Mineral Resources, Canberra ACT 2601, Australia. 5 Bundesanstalt für Geowissenschaften und Rohstoffe, D-3000 Hannover, Federal Republic of Germany. margin evolution could be tested. Several such models have been proposed (e.g., Falvey, 1974; Mauffret and Montadert, 1987; Lemoine and Trümpy, 1987) that envision the mechanism of margin development in terms of uplift and subsidence cycles, or varying thermal response of oceanic and continental lithosphere. For example, recent seismic data from Exmouth Plateau show large rotated fault blocks and two prominent landward-dipping, subhorizontal, mid-crustal detachment faults under the central plateau (Williamson et al., 1990). Mutter et al. (1989) suggest that the outer part of Exmouth Plateau (near Site 762) was produced by lithospheric thinning and pure shear (high-angle normal faults and McKenzie-type stretching), and postdates the thin-skinned deformation with simple-shear detachment systems (Wernicke-type deformation) under the central part of the plateau (near Site 763). GEOLOGICAL BACKGROUND Exmouth Plateau is a part of the northern Carnarvon Basin, which is structurally subdivided into Dampier, Barrow, and Exmouth sub-basins, and whose stratigraphy is detailed by Hocking et al. (1987). The basin is itself a part of the Westralian Superbasin, which received up to 10 km of sediments from the Permian through the Mesozoic. The Westralian Superbasin formed a part of the northern margin of Gondwana bordering the Neo-Tethys Ocean (Bradshaw et al., 1988), characterized largely by Triassic clastic sedimentation. Segmentation of the superbasin into platforms and troughs occurred in the Jurassic. Thick marine sequences filled the troughs, while high areas such as the central and western Exmouth Plateau received little or no sediments (Barber, 1982, 1988; Exon and Willcox, 1978; Boyd et al., this volume; and Exon et al., this volume, chapter 12). Structurally the Exmouth Plateau is dominated by highangle faults, trending northeast-southwest or north-south, that