| Abstract |
Sedimentary deposits of Cascadia Basin lap onto the Juan de Fuca Ridge to within 20 km of the spreading axis (Davis and Currie, 1993, doi:10.1139/e93-023). Collectively, these interbeds of hemipelagic mud, mud turbidites, silt turbidites, sand turbidites, and debris-flow deposits act as a relatively low-permeability barrier that inhibits the hydrothermal connection between underlying igneous crust and the overlying reservoir of ocean water. The primary purpose of Leg 168 of the Ocean Drilling Program (ODP) was to explore the causes and consequences of ridge-flank hydrothermal circulation (Shipboard Scientific Party, 1997c, doi:10.2973/odp.proc.ir.168.101.1997). One important aspect of this overall goal was to determine how changes in thickness of the sediment cover affect heat flow, fluid flow, fluid composition, and chemical alteration of the igneous crust. Fluid circulation through the sediment is influenced by a variety of textural parameters and intrinsic physical properties. Physical properties of sediments usually change in a predictable way with increasing depth and mechanical compaction, but different lithologies display different compaction gradients.
Shipboard scientists subdivided the sedimentary succession throughout the study area into three principal lithofacies units and subunits. In general, these sequences coarsen and thicken upward from a basal interval of hemipelagic mud through a unit of mud and silt turbidites into a unit of mud, silt turbidites, sand turbidites, and debris-flow deposits. The sediment index properties (bulk density, water content, porosity, and void ratio) were measured aboard the JOIDES Resolution (Shipboard Scientific Party, 1997b, doi:10.2973/odp.proc.ir.168.104.1997; 1997d, doi:10.2973/odp.proc.ir.168.105.1997; 1997a, doi:10.2973/odp.proc.ir.168.106.1997). Shorebased work was devoted to accurate measurements of grain-size parameters. To allow for valid cross-correlation, the samples analyzed for grain-size distributions were taken from core intervals immediately adjacent to those of the physical properties specimens. The main purposes of this report are to show how the grain size and physical properties data are interrelated, and to determine how lithology might affect hydrologic properties of the sedimentary cover.
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