The Moresby detachment (Figs. F1, F2) at the western tip of the Woodlark rift basin is one of the few known examples of seismically active low-angle normal faults (Abers, 1991; Wernicke, 1995; Abers et al., 1997). It therefore provides the opportunity to study the poorly understood mechanics and evolution of such faults. Ocean Drilling Program (ODP) Leg 180 was designed to address three different aspects of this problem by drilling three types of sites along a north-south transect across the Moresby detachment (Taylor, 1999; Shipboard Scientific Party, 1999b).
Site 1108 objectives were to drill the fault zone at depth to study its in situ physical properties and state of stress. Difficult drilling conditions and hydrocarbon safety concerns required the abandonment of this goal. The fault zone was drilled instead at Site 1117 on the northern slope of the Moresby Seamount, where it is exposed. This confirmed that it is a major shear zone (Shipboard Scientific Party, 1999b).
Southern Sites 1116 and 1114 were drilled to study the nature and deformation of the footwall and basement of the detachment (Shipboard Scientific Party, 1999b).
Sites 1118, 1109, and 1115 recovered 1.7 km of core in the synrift, and, in the case of Hole 1115C, prerift sediments of the northern margin of the basin. These three sites document the subsidence history of the hanging wall of the detachment and the interaction of sedimentation with extensional tectonics through numerous instances of soft sediment deformation associated with normal to oblique normal faulting (Shipboard Scientific Party, 1999b). These sites are also integrated with seismic reflection data, which allows extension of the borehole observations to a regional scale (Goodliffe et al., 1999; Goodliffe et al., this volume).
Four boreholes (Holes 1114A, 1118A, 1109D, and 1115C) were logged with the Formation MicroScanner (FMS), providing 2.3 km of continuous oriented high-resolution images of the sediment infill of the Woodlark rift basin. A preliminary analysis of these FMS data done on board is reported in Shipboard Scientific Party (1999b, 1999c, 1999d, 1999e)
FMS and core data are complementary for structural studies in many respects. Important detailed structures such as slickensides are only observed in core, but FMS images are oriented in space, whereas cores are not oriented in the horizontal plane (for vertical wells).
Core recovery is often incomplete and biased with respect to lithology or fracturation, whereas FMS data are continuous but do not cover the upper 100 m of boreholes, where the bottom-hole assembly is located during logging operations. The resulting coverage for the four sites is summarized in Table T1. In the footwall Site 1114, where recovery is low, the FMS images cover an interval that is 4.3 times larger than the recovered core length. In the northern margin Sites 1118, 1109, and 1115, where core recovery is high, FMS data cover an interval that is 1.1 to 1.6 times longer than the recovered core length.
A comprehensive structural analysis conducted postcruise is presented in this paper for the northern margin Sites 1118, 1109, and 1115 and in a companion paper for the footwall Site 1114 (Louvel et al., this volume).
The main goal of the FMS structural analysis is to provide the bedding and fracture dip direction that is missing from core observations, to study the organization of these structures, and to relate them both to regional tectonics and to the results of core analysis.
This, however, requires the identification of lithology because it affects the expression of strain. A few typical FMS image facies related to specific lithologies were indeed identified. The exceptional amount of FMS data this identification is based on and its integration within seismic profiles suggested a secondary goal, to present a catalog of detailed images of these typical facies and full borehole coverage images that relate them to the lithologic and logging units defined by the Shipboard Scientific Party. The full borehole coverage can be used to locate other studies of the Woodlark drill holes, and the catalog of larger-scale FMS facies provides a comparison basis for facies encountered in other sedimentary basins. This goal became possible because the new electronic format of the Ocean Drilling Program (ODP) Scientific Results does not place severe limitations on color figures.
The method and the resulting documents will be presented first. The data from each of the three sites and their interpretation will be discussed next. The discussion will proceed from south to north, that is, from the depocenter to the northern border of the basin. Finally, the results concerning fracturation, bed deformation, and the role of lithology will be discussed.