New In-Situ Geophysical Measurements from Indian Ocean off Northwestern Australia

D. A. Castillo, R. Buffler, B. O'Neill, T. Bolmer

Site 765, drilled during Ocean Drilling Program (ODP) Leg 123 in the eastern Indian Ocean off northwestern Australia, provided an opportunity to conduct several in-situ geophysical experiments onboard the JOIDES Resolution. The site penetrated a 900-m thick Cenozoic and Cretaceous-Upper Jurassic sedimentary section before penetrating oceanic crust. An extensive suite of Schlumberger logs and four downhole experiments were conducted. The first experiment, a vertical seismic profile, recorded detailed velocity information for a thick sedimentary column and oceanic crust for the first time during an ODP leg. Data analyses provide the capability to correlate the borehole information directly with seismic reflection data at the site, extrapolate the drilling results regionally, and image the oceanic crust below the borehole.

The second major experiment, another ODP first, was aimed at measuring the in-situ stresses using hydraulic fracturing and the acoustic borehole televiewer imaging system to document the nature of interplate stresses. The proximity of this site to a trench-plate boundary provides an opportunity to examine several proposed models regarding the origin of stresses in the oceanic lithosphere such as slab-pull forces (Java Trench) and ridge-push forces (Southeast Indian Ridge). The third test was a permeability/porosity experiment to study the migration of seawater and other pore fluids circulating through the crust. Permeability experiments provided an opportunity to determine if pore-fluid circulation exists in this ancient oceanic crust and, if so, to what degree it is dependent upon th fracture system. The final in-situ measurement documented the thermal structure within this section of the Indian plate.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.