Site 765

Abstract

Principal results: The early tectonic and paleoceanographic history of the Indian Ocean is poorly understood. For this reason, Leg 123 Site 765 was drilled in the northeastern Indian Ocean, off northwestern Australia for the following objectives:63 SITE 7651. To elucidate the paleoceanography, sedimentology, and magmatic processes related to the rifting of the early Indian Ocean.2. To constrain the rift to drift tectonic history of one of the Earth's oldest oceanic basins.3. To improve the late Mesozoic time scale, particularly with reference to the Southern Hemisphere.4. To provide a geochemical reference section of old oceanic crust, incorporating the bulk composition of the sediments and basement, for use in geochemical and petrological, global mass-balance models.Site 765 is located on a magnetic anomaly, interpreted as local marine magnetic anomaly M26, that according to the geomagnetic time scale is of late Oxfordian age. Backtracking predicts an initial ridge-crest water depth of about 2800 m, which is comparable to that of normal oceanic crust. The site is about 15 km seaward of the geophysical ocean/continent boundary separating Australia from the Argo Abyssal Plain, and approximately 350 km south of the Java Trench.Holes 765A, 765B, and 765C, located at 15°58.541'S, 117°3 4.495'E, in a water depth of 5723 m, were continuously cored for Cenozoic and Cretaceous, fine-grained, abyssal sediments. Oceanic basement was reached at 931 mbsf in Hole 765C. Average sediment recovery was 68%. The JOIDES Resolution then was moved approximately 30 m, where Hole 765D was drilled and cased through 924 m of sediments into volcanic basement. This hole then was continuously cored another 259 m into remarkably fresh basalt. Average core recovery in Hole 765D was 31%. Both Holes 765C and 765D were logged extensively using sonic, litho-density, and geochemical tools. Site 765 is the deepest cased drill hole in the oceans. With its reentry cone on the seafloor, this site is in perfect shape for future scientific operations.Using combined nannofossil, foraminifer, radiolarian, and dinoflagellate biostratigraphy, visual core descriptions, assisted by multivariate analysis of smear-slide data and sedimentary paleomagnetics, we distinguished seven successive stratigraphic units. From basement to the seafloor, these are as follows:1. Unit I: 0-189.1 mbsf, clayey calcareous turbidites, massive slumps and debris flows, and siliceous ooze of late Miocene to Pleistocene age.2. Unit II: 189.1-474.1 mbsf, calcareous turbidites with minor clay and a massive debris flow containing basaltic pebbles of early to late Miocene age.3. Unit III: 474.1-591.7 mbsf, varicolored zeolitic clay, redeposited calcareous sediments, and dark claystones of Cenomanian-early Miocene age, with stratigraphic hiatuses.4. Unit IV: 591.7-724.1 mbsf, siliciclastic and mixed lithology turbidites, nannofossil chalk, calcareous claystone, and zeolitic clay of early Aptian-Cenomanian age.5. Unit V: 724.7-859.2 mbsf, varicolored and dark gray radiolarian and rhodochro...

Structural evolution of the northern Bonaparte basin, northwest shelf Australia

“…The regional geology of the basins, including the Bonaparte and Browse Basins, has been described by numerous authors including Laws & Kraus (1974), Gunn (1988), Mory (1988), Patillo &Nicholls (1990), andLongley (2002). The geological evolution of the area started with rifting in the Paleozoic which controlled the distribution and thicknesses of the Mesozoic and Cenozoic sequences.…”