1.Evaluation of overall prism consolidation and velocity-porosity relationships: LWD data obtained at five sites during Leg 171A and two sites during Leg 156 provide an overview of prism consolidation. Two reference sections at Site 1044 and 1045 establish a baseline for evaluating consolidation of both the offscraped and underthrust sections up to 6 km east of the deformation front. Comparisons of the density curves from the reference sites enable calculation of underthrust sequence consolidation because of loading by the overlying accretionary prism at the LWD sites west of the deformation front. Velocity-porosity relationships could not be evaluated because of the absence of an LWD velocity tool that could measure sediments with such low velocities and the lack of time to conduct wireline velocity measurements.
2.Correlation of physical properties of faults with displacement and fluid flow: We found no correlation between consolidation and amount of underthrusting.
3.Evaluation of the consolidation state of sediments in and around faults: The localization of the décollement zone beneath the northern Barbados accretionary prism is controlled by the physical properties of the incoming sedimentary section, providing a classic example of the principle of sedimentary inheritance. A siliceous mudstone interval at reference Sites 1044 and 1048 (mean depth range = 178 to 198 mbsf) is characterized by a wet bulk density (mean = 1.47 g/cm3) that is lower than the mudstones immediately above and below and comparable to the surface sediment. This lower Miocene low-density layer biostratigraphically correlates to the décollement zone at five ODP sites that penetrate through the accretionary prism.
4.Determination of the origin of the negative polarity reflections in fault zones: LWD data were obtained through a negative polarity reflection, thereby documenting the in situ physical properties that produce these distinctive reflections.
In summary, Leg 171A partially accomplished Objective 1 by determining prism consolidation. However, we did not establish velocity-porosity relationships of these sediments. We tested the prediction of Objective 2 and found no correlation between consolidation and the amount of underthrusting; but we were successful in determining the consolidation state of sediments around the décollement (Objective 3). We recorded logs through a negative polarity reflection, established that it is probably a residual fluid accumulation, and now have documented the physical property variations that produce these distinctive reflections (Objective 4).
To 171A References
To 171A Table of Contents