Origin of elevated sediment permeability in a hydrothermal seepage zone, eastern flank of the Juan de Fuca Ridge, and implications for transport of fluid and heat

Elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the Juan de Fuca Ridge. Consolidation and permeability tests of fine-grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site (primarily hemipelagic) have 10-15% higher porosity and 10x higher permeability at a given depth and 10-100x lower hydraulic impedance for the 40-m-thick sediment column. The correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result fi-om fluid flow but are intrinsic properties of the hemipelagic sediment. On the basis of measured sediment properties, fluid pressure at the top of basement is less than or equal to 5 kPa greater than hydrostatic. A simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops (4-20 km) suggests that local basement permeability is between 3x10(-13) and 3x10(-11) m(2), within the range measured in nearby boreholes. The measured sediment properties combined with other published data indicate that a few tens of meters of fine-grained terrigenous, hemipelagic, or calcareous marine sediment effectively seal the basement aquifer, whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow. The large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes.