High-quality log data provide an opportunity to evaluate relationships between logging and other rock properties. Multivariate methods of factor and cluster analysis produced fast, reliable, and objective definitions of logging units that correlate well with the lithologic units defined at the cored holes that are adjacent to the Leg 171A sites (Bücker et al., Chap. 2, this volume). Moreover, such objective unit-definition criteria were especially valuable to define log units at Sites 1045 and 1048, where no cores were available.
The Leg 171A LWD data immediately showed that the seismically defined décollement zone and proto-décollement zone correlate with a low-density interval in the logs (Fig. F4). The changes in impedance (velocity × density) at the top and bottom of this low-density interval either singularly or in combination result in the seismic reflection that characterized the décollement zone and proto-décollement zone (Bangs et al., 1999). Moreover, this low-density interval correlates with a radiolarian claystone in both the proto-décollement and décollement zones and with the deformed interval of the décollement zone as defined in the cores (Figs. F5, F6). Recognition of these correlations provided a basis for using the seismic data to broadly extend the LWD results and for numerous geologic interpretations of the logging results that are reported below.
The décollement zone is a structural feature of maximum disharmony that can be viewed in cores or in seismic data. In cores, the décollement zone thickness is defined by a zone of concentrated scaly foliation ranging up to 33 m thick (Labaume et al., 1997; Maltman et al., 1997). In seismic reflection images, the thickness of the décollement zone cannot be resolved, although models of the waveforms reflected from the décollement indicate the thickness of the associated low-density interval (see "Predicting Décollement/Proto-décollement Zone Density and Thickness throughout the Seismic Survey"). This low-density zone is not equivalent to the structural décollement and may either encompass the décollement zone where it is developing or be included in the décollement zone where it is more mature and more consolidated (Fig. F6).
The Leg 171A LWD data, although of excellent quality, have certain limitations. The LWD tools used during Leg 171A did not include a means of measuring sonic velocity because the available devices are unreliable in sediments with velocities <2000 m/s. The neutron porosity tool is also not effective in high-porosity sediments, and the data we collected was not extensively utilized. Moreover, conversions of LWD density to porosity are hampered by the presence of interlayer water in smectite, which is recorded by the density tool.
To address some of the limitations in the LWD data, Erickson and Jarrard (1999) used smectite-corrected, density-based porosities to calibrate a conversion between porosity and formation factor then calculated porosity from resistivity logs. By comparing resistivity-based porosities to velocities from vertical seismic profiles, Erickson and Jarrard (1999) estimated velocity profiles for Leg 171A sites. The velocities determined from resistivities can be compared to a wireline velocity log collected during Leg 156 (Shipboard Scientific Party, 1995). Both logs show similar velocities over scales of 10 to 50 m, but the resistivity-velocity log shows much more detailed character, mimicking changes in lithology that were recorded by the resistivity tool and not the wireline sonic velocity tool. Determination of accurate, detailed velocities and porosities from these sediments remains a problem that will hopefully be rectified by improved technology.