The western Woodlark Basin is a propagating Neogene rift extending eastward from Papua New Guinea and the D'Entrecasteaux Islands (Fig. F1). In the segment of the rift north of the Pocklington Rise and Moresby Seamount, the basin consists of an asymmetric graben atop a north-dipping detachment fault (Taylor et al., 1999; Taylor, Huchon, Klaus, et al., 1999). Sites 1118, 1109, and 1115 were drilled along a transect that extends northward from the basin axis (Fig. F2). Present water depth (~2300 m) and thickness of the basin-fill deposits (~850 m) are greatest at Site 1118.
The floor of the rift basin experienced a phase of rapid subsidence beginning at ~3.9 Ma (late early Pliocene) (Takahashi et al., this volume). Until ~3 Ma in the middle Pliocene, accumulation rates ranged from ~480 m/m.y. at Site 1118 to ~300 m/m.y. at Sites 1109 and 1115 (the Pliocene is divided into three epochs following Shipboard Scientific Party, 1999). Water depth reflected this strong subsidence by increasing rapidly from <150 m before ~3.9 Ma (outer neritic) to 500-2000 m by ~2.5 Ma (middle bathyal). During the phase of rapid subsidence and deepening, rift-basin facies consisted of marine shales punctuated by thin- to thick-bedded sandy and silty turbidites derived from the Papuan Peninsula, D'Entrecasteaux Islands, and Trobriand arc (Robertson et al., in press).
Sandy and silty turbidites interbedded with shales are particularly amenable to study using high-resolution microresistivity logs like the Schlumberger Formation MicroScanner (FMS) tool and its successor, the Formation MicroImager. This is because of the sharp contrast in resistivity between the coarser basal divisions of turbidites and the associated shales. For example, Hiscott et al. (1992, 1993) and Pirmez et al. (1997) used FMS images to document facies architecture and bed-thickness distributions in turbidites from the Izu-Bonin forearc and the Amazon submarine fan, respectively. In the case of these earlier studies, the microresistivity images permitted intervals of low core recovery in Ocean Drilling Program (ODP) boreholes to be described bed by bed just like the intervals that were successfully cored and described at sea.
The lower to upper Pliocene succession in the western Woodlark Basin consists of rhythmically interbedded muds, silt turbidites, and sand turbidites. This interbedding can be appreciated from standard downhole logs as well as from FMS images. Gamma ray profiles are not suitable as a guide to textural variations because of radioactive sands in the Woodlark Basin succession. Instead, the difference between neutron porosity (APLC) and lithodensity tool porosity (DPHI), designated as DPORO, serves as a more reliable indicator of clay content and texture (Shipboard Scientific Party, 1999; Célérier et al., this volume). DPORO varies from -1.8 in uniformly sized sand to 0.4 in clay; poorly sorted sands and silts have intermediate values. Downhole plots of DPORO show a serrated signature at all drill sites because of the fine-scale interbedding of sand and silt turbidites with basinal muds (Fig. F3).
Continuous FMS images in turbidite successions allow the assembly of large data sets that can be used to investigate the nature and origin of particular types of bed-thickness populations like those conforming to exponential or power-law distributions (e.g., Drummond and Wilkinson, 1996; Beattie and Dade, 1996; Malinverno, 1997). Even in the western Woodlark Basin, where core recovery was generally high (commonly >80%), borehole images provide a powerful complement to cores because (1) there are no gaps in the data, and (2) sand bed thicknesses can be determined even in intervals where rotary coring (including extended core barrel coring) has produced drilling "biscuits" (Shipboard Scientific Party, 1995) and core disturbance that prevent accurate determination of bed thicknesses and sedimentary structures.
FMS images from the uppermost lower Pliocene to lowermost upper Pliocene turbidite successions at Sites 1118, 1109, and 1115 were studied with a number of goals in mind: (1) to prepare uninterrupted bed-by-bed sections for studies of sediment architecture, (2) to assess the nature of bed-thickness populations, (3) to investigate turbidity-current frequency as a function of basin tectonics and accumulation rates, and (4) to assess temporal and spatial variability in turbidite facies in a rift setting. The oldest deposits considered at each site date from the time when water depth increased to >150 m (from Takahashi et al., this volume), corresponding to 850 meters below seafloor (mbsf) in Hole 1118A (~3.6 Ma), 570 mbsf in Hole 1109D (~3.9 Ma), and 410 mbsf in Hole 1115C (~3.8 Ma).