Sediment Consolidation and Permeability at the Barbados Forearc

Abstract

Sediments cored from the Barbados Ridge accretionary complex at ODP Sites 671, 672, and 676 are progressively underconsolidated with depth. Very low measured intergranular permeabilities contribute substantially to the probable excess pore-water pressures that account for the underconsolidated behavior. The estimated pore pressures immediately overlying the incipient decollement at the reference site are nearly equal to the overburden stress. Data show that the pore fluids within the fine-grained sediments of the complex are absorbing the tectonic shock of underthrusting as they are accreted, effectively leading to "lubricated subduction" of the underthrust Oligocene and older sediments. Lateral compression and dewatering by fracture permeability is taking place in the accreted section despite apparent underconsolidation, resulting in limited pore-volume reduction and sediment strengthening from east to west on the ridge.

Permeability and consolidation characteristics from Hole 949B, northern Barbados Ridge

“…When presented in terms of OCR, the values are small, less than one (the normally consolidated state), and decrease with depth with OCRs of 0.4, 0.2, and 0.1. These values are comparable to those measured previously at Barbados from Leg 110 (Taylor and Leonard, 1990) and at other modern accretionary complexes (Moran et al, 1993;.…”

“…We develop a generalized function by compiling permeability data for the northern Barbados Ridge that are estimated from onedimensional consolidation tests from a previous study (Taylor and Leonard, 1990). The hydraulic conductivity data are presented as a function of void ratio and show two distinct generalized relationships (Fig.…”

“…It is also a crucial parameter in all numerical models of accretionary prism hydrology. Although studies have used core samples to determine matrix permeability (Taylor and Leonard, 1990), the number of measurements is still far too small to provide representative coverage of the different lithologies and structural domains within the Barbados accretionary prism. Another important parameter relevant to our understanding of accretionary prism dynamics is sediment deformation behavior, which is a function of compressibility and strength, as well as the in situ stress conditions or stress history.…”

Physical Properties of Sediment and Rocks from the D'Entrecasteaux Collision Zone and North Aoba Basin, Vanuatu Island Arc

“…Lower matrix permeabilities and possible excess pore pressure in certain sedimentary units (Leonard, 1991) appear to channel fluid through more permeable, tectonically fractured lithologies. The low water content of the chalk below 200 mbsf at Site 829 is approximately half that of claystones and mudstones of similar depth at Site 671 in the Barbados complex (Mascle, Moore, et al, 1988;Leonard, 1989;Taylor and Leonard, 1990;Ask, 1991). This difference is due, in part, to sediments being incorporated into the complex from the DEZ that are more coarse-grained, less porous, and more permeable than the hemipelagic clay-dominated Barbados forearc.…”

Anomalously high porosities in the proto‐decollement zone of the Barbados Accretionary Complex: Do they indicate overpressures?

“…Furthermore, previous researchers had inferred near-lithostatic pore pressures directly above the proto-decollement zone based on consolidation tests on cores (Taylor and Leonard, 1990). In contrast, previous studies, based on results from Deep Sea Drilling Project (DSDP) Leg 78a, concluded that sedimentation rates alone were not sufficient to produce overpressures (Marlow et al, 1984;Shi and Wang, 1986).…”