The only observed unambiguous fractures (frac 1) are all located in the dolerite at the bottom of Hole 1109D (Table T3; Fig. F28). They dip 10°-35°, mainly to the north, but with a few, probably conjugates, to the south (Fig. F30). The lesser quality frac 2 distribution is more dispersed, as expected, but with a similar mean orientation. This orientation is subparallel to that of the Moresby detachment fault. Whether these faults are related to the regional north-south extensional stress field raises the question of their unfavorable low dip, as in the case of the detachment, but for much smaller structures with no large gouge. Steeper fractures, dipping ~60°, observed in core indicate that the FMS data undersamples the steep fractures.
Very few fractures are observed in the sediments, despite numerous observations in core, and all of them belong to the lower confidence frac 2 category. This disappointing result may be due to three causes: (1) the incomplete coverage of the borehole wall by the slim FMS used in ODP operations; (2) low resistivity contrast between the fractures and the formation, which would suggest that these fractures are closed; and (3) typical geometrical undersampling of steep structures by a vertical borehole. However, the few identified fractures are encouragingly often located within core-defined fracture zones (Holes 1118A and 1115C). They are concentrated within the prerift and earliest synrift sediments in Hole 1115C.
The number of bedding structural measurements (Table T3) on the FMS images is comparable to those in core. For each hole, the lower confidence bed 2 distribution is more dispersed than the bed 1 distribution, as expected, but displays similar mean orientations (Figs. F4, F29, F55).
Bedding is mostly subhorizontal (<15° dip), except in a few intervals (Figs. F3, F28, F54). Most of those intervals correspond to zones where soft-sediment deformation or fractures were observed in core. Many also include a few of the fractures that were identified on the FMS images. Slumping is observed on the FMS images and, in one instance, is correlated with the corresponding core feature (Fig. F14; Section 180-1118A-21R-4).
The orientation analysis determines a few fold axes that are always subhorizontal (within 5° of horizontal) and are summarized in Table T4. These fold axes can be interpreted either as being orthogonal to the depositional slope for gravity sliding or as striking along a fault that caused the bed rotation by its movement. However, in the case of synsedimentary extensional tectonics, such as that of the Woodlark Basin, the sedimentary slope may be controlled by large normal faults, and therefore gravity sliding along this slope and tectonic bed rotations due to fault movements may produce structures with similar axes. This is compatible with the core observation of soft-sediment deformation and slumping associated with extensional faulting.
The 250- to 280- and 275- to 340-mbsf intervals of Hole 1118A and the 190- to 202-mbsf interval of Hole 1109D yield similar northeast-east folding axis (Table T4). Because soft-sediment gravity deformation is the most likely cause of these folds, the axis direction may be orthogonal to the depositional slope.
The 570- to 596- and 840- to 890-mbsf intervals of Hole 1118A and the 622- to 634-mbsf interval of Hole 1115C, which correspond to core-defined fracture zones, yield consistent east-west fold axes and north- or south-dipping associated fractures (Table T4). Even if these fractures are not necessarily those responsible for the folding, it is reasonable to infer that both the folding and the faulting are the expression of the regional north-south extension.
At Site 1115 the angular unconformity between prerift and synrift sediments is very small. The FMS analysis suggests a possible slight angular unconformity within the prerift sediments at ~695 mbsf, that is, below fracture zone 2. Intervals of dipping beds are mostly located below 480 mbsf, that is within the prerift or early synrift sediments.
Sandy or silty intervals often correspond to borehole enlargement and poor recovery. FMS analysis shows that they are frequently the locus of dipping beds and/or fractures, and core observations show them frequently associated with fracture zone and/or soft sediment deformation. All this suggests that they are mechanically weak and concentrate deformation. The fact that these intervals are poorly recovered suggests that core analysis missed a large amount of hanging wall deformation. FMS data do not compensate for this because they reveal too small a proportion of the fracturation in the sediments.
Fracture zone 2 in Hole 1109D and fracture zone 3 in Hole 1115C occur in the similar lithologic Unit VIII organic-rich claystone that also seems to concentrate deformation.
Most outstanding sediment lithologies are related to an increase in sand or carbonate content against a background clay content. The analysis of the FMS dynamic image identified typical facies related to these increased sand or carbonate contents that are summarized in Table T5.
The static FMS image is very resistive in carbonates but is relatively conductive in sands and seems more sensitive to carbonate than to clay content. This can be verified with conventional logs. The shallow spherically focused electrical measurement (SFLU) is the closest to FMS measurement in terms of resolution and depth of investigation, the photoelectric effect (PEFL) is a direct indication of carbonate content, and clay content can be estimated by the difference (DPORO; Eq. 2) between neutron (APLC) and lithodensity (DPHI) porosity (Shipboard Scientific Party, 1999a). SFLU is represented as a function of PEFL and DPORO for each borehole in Figure F79; a strong correlation of resistivity (SFLU) with carbonate content (PEFL) is clear at Sites 1115 and 1109 and is still visible at Site 1118. No simple correlation appears between clay content (DPORO) and resistivity (SFLU).