INTRODUCTION

During Ocean Drilling Program (ODP) Leg 180, a series of holes was drilled in the Woodlark Basin to characterize the composition and in situ properties (stress, permeability, temperature, pressure, physical properties, and fluid pressure) of an active low-angle normal fault zone (Taylor, Huchon, Klaus, et al., 1999). The western Woodlark Basin is characterized by a lateral variation from active continental rifting to seafloor spreading within a small region, and in the northern margin of the basin, the Woodlark Rise represents a downflexed prerift sedimentary basin and basement sequence that is unconformably onlapped by synrift sediments. A north-south transect of three deep holes (Sites 1109, 1115, and 1118), representing a spatial proxy for the temporal variability of tectonic activity in the area, was drilled on the Woodlark Rise.

In this paper we present results of isotopic (⁸⁷Sr/⁸⁶Sr and ¹⁸O) and trace element (Rb and Ba) analyses in interstitial water (IW) from the three sites and the trace element composition and mineralogy of corresponding sediments at Site 1109. A subset of squeeze cakes remaining after IW was recovered from whole-round cores collected at Site 1109 was selected on the basis of volcanic matter content. The clays were analyzed for their trace element content to evaluate how the presence and alteration of volcanic minerals impact the chemical composition of the IW.

Volcanic matter in sediments of the Woodlark Rise is present as discrete ash layers and dispersed volcaniclastic sand, as well as large-body intrusions or basement igneous rocks deep in the holes (Taylor, Huchon, Klaus, et al., 1999). Sediments cored at Site 1109 record progressive subsidence from subaerial to lagoonal then shallow- to deep-water marine settings between the latest Miocene (~8 Ma) and the late Pleistocene. The hemipelagic Pliocene sediments (i.e., the upper ~550 meters below seafloor [mbsf]) include an abundance of volcanic ash layers and dispersed volcanic material, much of which is quite fresh and unaltered (Taylor, Huchon, Klaus, et al., 1999). Interpretations of the data reported here will be presented elsewhere (De Carlo et al., unpubl. data).