Data report: consolidation, permeability, and fabric of sediments from the Nankai continental slope, IODP Sites C0001, C0008, and C0004: Expeditions 315 and 316

Abstract

We conducted constant rate of strain (CRS) consolidation tests on 16 whole-round samples obtained during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 as part of Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 1 drilling. Samples were taken from slope sediments and the underlying accretionary wedge cored at IODP Sites C0001, C0004, and C0008 in the vicinity of a major out-of-sequence thrust fault (termed the megasplay). We conducted tests in parallel at the Rock and Sediment Mechanics Laboratory at The Pennsylvania State University (USA) and the geotechnical laboratory at the University of Missouri (USA), with the objectives of (1) defining the compression behavior and hydraulic properties of the sediments and (2) estimating in situ effective stress and pore pressure conditions from laboratory stress-strain behavior. In addition, we conducted quantitative fabric analyses at the University of Missouri on samples taken immediately adjacent to the CRS test specimens using an environmental scanning electron microscope. The samples exhibit similar consolidation and hydraulic properties. The samples exhibit a general trend of increasing fabric development with depth at Sites C0004 and C0008, with samples from the accretionary prism at Site C0001 and slope sediments in the footwall of the megasplay fault at Site C0004 exhibiting a higher degree of grain alignment. The compression index ranges from 0.419 to 1.058 for the entire suite of samples, with most values ranging from 0.6 to 0.8. Intrinsic permeability decreases systematically with increasing effective axial stress and decreasing porosity and varies log-linearly with porosity. The permeability-porosity trends for specimens from the three sites are similar and exhibit no systematic variation with depth. Estimated values of in situ hydraulic conductivity follow a similar trend at all three sites and decrease with depth from values of 3.3 × 10–10 to 1.0 × 10–9 m/s at ~50 meters below seafloor (mbsf) to 3.3 × 10–11m/s at 440 mbsf. We estimated maximum preconsolidation stress (Pc′) using two separate techniques and consistently obtained values comparable to calculated values of hydrostatic vertical effective stress at Sites C0001 and C0008. Similarity of these values is consistent with normal consolidation and hydrostatic pore fluid pressures. In contrast, Pc′ values for Site C0004 document apparent overconsolidation.