William G. D. Gray

B. Sc. Honours Thesis

Sediment Consolidation and Permeability at the Central Oregon Margin, Ocean Drilling Program Sites 891 and 892

(PDF - 10.5 Mb)

Sediment hydraulic conductivity is a critical and highly variable control on the consolidation of accretionary prism materials. The consolidation state and pore pressure conditions within an accretionary prism ultimately determine its geometry and rate of deformation. This study uses back-pressured consolidometer and flow-pump permeameter testing with Shipboard physical property data to provide in situ hydraulic conductivity values for Ocean Drilling Program (ODP) Sites 891 and 892. These data augment a very limited data set for accretionary prism material and improve understanding of prism water budgets. Results from this study indicate an in situ hydraulic conductivity range of 10-5 to 10-7 cm/s for sediments at the central Oregon portion of the Cascadia convergent margin. These low hydraulic conductivities are insufficient to dissipate excess pore pressures developed within underthrust sediments at this location. This study identifies excess pore pressures developed within underthrust sediments at this location. This study identifies excess pore pressures in the range of 2.5 to 4.5 MPa for underthrust sediment. These findings support models that rank tectonic loading, rather than tectonic compression, as the largest influence on overpressuring. Excess port pressures of this magnitude indicate a high potential for sediment failure, such as fold and fault formation, and thus, deformation early in accretion history.

Keywords: Cascadia margin, accretionary prism, ODP, hydraulic
conductivity, consolidation, overconsolidation ratio, excess pore pressure
Pages: 166
Supervisor:  Kate Moran