Abstract:
The primary scientific objectives of Leg 168 of the Ocean Drilling Program focused on how the physical and hydro-geochemical parameters of hydrothermal circulation change with increasing distance from the crest of Juan de Fuca Ridge. Turbidite sequences of Cascadia Basin lap onto juvenile igneous crust in this region, and continuous coring of the turbidites at seven sites revealed many interesting aspects of their depositional history, composition, physical properties, and provenance. All of the cored sedimentary successions display the same overall facies pattern of upward coarsening. In detail, however, unit thicknesses, rates of sediment accumulation, and recurrence intervals of turbidite depositional events vary considerably from site to site, as well as through time. These temporal and spatial variations occurred largely in response to the sheltering and deflecting effects of basement highs. Physical properties of the turbidites and interbeds of hemipelagic mud change somewhat in response to increasing burial and compaction, but the most pronounced differences in porosity and bulk density are related, in a fairly predictable way, to clay content and mean grain size. Relative abundances of clay minerals in turbidite matrix specimens are similar to those in hemipelagic muds. Values range from 0 to 35% smectite (mean = 8%), 18 to 59% illite (mean = 40%), and 29 to 78% chlorite (mean = 52%); compositional variations among sites are subtle. Sand-sized framework grains also show considerable homogeneity, both within and among sites. Average modal values are: Q = 35, F = 35, L = 30; Qm = 46, P = 49, K = 5; and Qp = 16, Lv = 43, Lsm = 41. Most of the sand data plot within the fields of dissected-arc or mixed tectonic provenance (magmatic arc + rifted continent), and the most likely detrital source areas are the Olympic Peninsula and Vancouver Island.