Triassic to Cenozoic evolution of the NW Australian continental margin and the birth of the Indian Ocean (preliminary results of ODP Legs 122 and 123)

The northwest Australian continental margin is a very old, sediment-starved NE Gondwanan margin with the opportunity to study the early structural and depositional evolution during the Triassic to Jurassic rifting stages (Neo-Tethys), the earliest Cretaceous transition from rifting to drifting, and the early tectonic, volcanic, and paleoceanographic history of the Indian Ocean. Ocean Drilling Program (ODP) Legs 122 and 123 drilled eight sites in the area of the Exmouth Plateau and the adjacent abyssal plains: Sites 759-761 and 764 studied the Triassic to Cenozoic evolution of Wombat Plateau, a small sub-plateau of the northern Exmouth Plateau; Sites 765 penetrated almost 1 km of Cretaceous to Cenozoic sediments of the nearby Argo Abyssal Plain and 270 m of the oldest Indian ocean crust; Sites 762 and 763 studied the southern central Exmouth Plateau, whereas Site 766 was drilled into intrusive volcanics with MORB affinity under the adjacent Gascoyne Abyssal Plain at the foot of the western escarpment of the plateau. The major discoveries during the drilling of the two continental margin-deep ocean basin transects led to a better understandingof the early rift history of this margin during late Permian to late Triassic times (stretching, block-tectonic movements, fluviodeltaic/-shallow-water carbonate environments, major Rhaetian carbonate buildup, early rift volcanism); of the rift-drift transition processes (major blockfaulting during Jurassic times with local uplift, rift flank tilting and subaerial erosion forming a post-rift unconformity); of the age and paleoenvironment of the oldest part of the Indian Ocean (possibly not earlier than late Berriasian to early Valanginian breakup, 20 Ma younger than expected); of the »juvenile ocean stage« (transgressive, condensed early Neocomian belemnite sands, calcisphere chalks and bentonites on Wombat Plateau); of the »mature ocean stage« (post-mid-Cretaceous eupelagic chalks); of the geochemical nature and origin of old oceanic crust and the overlying sediments in a »geochemical reference hole«; and of the Mesosoic chrono-(bio-magneto-stratigraphy) and the age and causes of major relative sea level fluctuations, especially during late Triassic and early Cretaceous times.