Seismic Stratigraphy of the Site 766 Area, Western Margin of the Exmouth Plateau, Australia

Abstract

A grid of seismic reflection data was used to develop a seismic stratigraphic framework for the Site 766 area. Five seismic sequences were defined and correlated directly with the lithologic units drilled at Site 766. The results of the seismic stratigraphic analysis were used to summarize the geologic history of the site area plus discuss some of the more important regional relationships in the vicinity of the site area. A contour map on acoustic (volcanic) basement plus the wedge-shaped, onlap-fill geometry of the overlying Valanginian-Hauterivian (VH) sandstone/siltstone sequence suggest initial deposition in a deep-water (800-1000 m), asymmetrical rift basin along the western Exmouth Plateau. Although intruded by oceanic volcanics, basement below the site area may be a narrow zone of thin, rifted continental crust rather than true oceanic crust. The VH unit sediments were derived from shallow-water shelves and volcanic landmasses on the thermally uplifted adjacent plateau and possibly were fed into the basin through canyon systems to the south. During the Barremian (BA sequence) the site area and adjacent margins began to subside rapidly after the emplacement of oceanic crust in the Cuvier and Gascoyne basins, and only distal hemipelagic muds were deposited. From Aptian through Cenomanian time (AC sequence) the area continued to subside and received mainly pelagic sediments in a deep-water, open marine setting. Starved, open-marine conditions continued throughout the Late Cretaceous (UK sequence) and the Cenozoic (CZ sequence), with the final disappearance of clay taking place in the Campanian. Strong deep-sea current systems periodically swept the margin throughout the Cretaceous and Cenozoic, as indicated by the overall abbreviated sedimentary section drilled and the numerous erosional unconformities seen on the seismic sections.