Hydrostatic consolidation tests of undeformed, clay-rich samples from the Barbados accretionary prism, Leg 156

Abstract

Geomechanical tests of three well-characterized, clay-rich, macroscopically undeformed samples document the loading history and permeability of two late Miocene samples above the décollement zone and a late Oligocene sample below the décolle-ment zone of the Barbados accretionary prism. These samples are similar in all aspects studied. About two-thirds of each sample is clay, of which most is mixed-layer illite/smectite. Permeabilities are low, ranging from 5 × 10 -18 to 1 × 10 -19 /m 2 . All samples are underconsolidated and may have been overpressured since shortly after deposition, especially the late Miocene sample near the deformation front (Site 949). Alternatively, the apparent low consolidation state may arise from disruption of grain contacts by early deformation in the accretionary prism, thereby erasing any earlier stress history.

Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai

“…At Barbados, inferred pore pressures are in good agreement with the results of long‐term monitoring at Sites 948 and 949 (same location as Site 1046, 2.2 km from the trench) [ Foucher et al , 1997; Becker et al , 1997]. A single consolidation test result from the base of the decollement at Site 948 yields a value of P ′ c that is also in good agreement with inversion results [ Vrolijk et al , 1998]. At Nankai, P ′ c estimates for two samples from underthrust sediments at Site 808 indicate significant underconsolidation and are in excellent agreement with inversion results, with inferred pore pressures ≥4.9–6.3 MPa above hydrostatic near the base of the section [ Karig , 1993] (Figure 8b).…”

“…Direct measurements of pore pressure from instrumented boreholes at Sites 948 and 949 (same as Site 1046) are shown for comparison (dark gray vertical bars) [ Foucher et al , 1997; Becker et al , 1997]. An indirect pore pressure estimate derived from P ′ c for samples from Site 948 is also shown (open square) [ Vrolijk , 1998]. In all plots, the gray shaded area shows the position of the decollement, and the solid gray and black lines denote hydrostatic and lithostatic pore pressures, respectively.…”

Acoustic and mechanical properties of Nankai accretionary prism core samples

“…Although consolidation studies usually focused on the yield point, i.e., the effective stress for which plastic deformation resumes [ Feeser et al , 1993; Moran et al , 1993; Morgan and Ask , 2004; Vrolijk et al , 1998], there has been little attention paid to the volume shrinkage when very low stresses, much below yield, are applied, either during resaturation of the sample or during the first steps of the test itself. This deformation was significant only for shallow material in Sites C0006 (samples 20X, 25X, 30X, 35X) and 7 (8X and 10X), i.e., for trench facies sediments, which have a lower clay fraction than other tested samples.…”

The ability of rock physics models to infer marine in situ pore pressure

“…This result is consistent with other regional CORK studies that suggest elevated fluid pressures in the Barbados decollement zone [e.g., Becker et al ., ]. Consolidation tests at Site 948 at a depth of 438 mbsf suggest sediments are underconsolidated and have a low permeability (10 −18 −10 −19 m 2 ), consistent with the onset of overpressure development following burial [ Vrolijk et al ., ]. Finally, high‐quality, high‐resolution measurements of pore pressure at Site 948 using field‐based consolidation curves derived from logging‐while‐drilling (LWD) bulk densities at the site suggest pore pressure ratio values of 0.70–0.92 exist from 530 to 570 mbsf at 948 [ Saffer , ].…”