CONCLUSIONS

The major conclusions of our compositional study of Cascadia Basin sediments are as follows.

  1. Sand-sized grains from turbidites are compositionally homogeneous, both within and among the Leg 168 sites. Average detrital modes are Q = 35, F = 35, and L = 30; Qm = 46, P = 49, and K = 5; Qp = 16, Lv = 43, and Lsm = 41; and Lv = 52, Ls = 39, and Lm = 9. The generic source for these sands was a polymictic dissected arc.
  2. Relative abundance of clay minerals are similar for turbidite matrix and interbeds of mud; there are no systematic shifts in mineralogy as a function of the lithologic host. Average mineral abundance for mud deposits are smectite = 8%, illite = 40%, and chlorite + kaolinite = 52%. Clay-mineral source areas probably included igneous, metamorphic, and sedimentary rocks, plus reworked glacial deposits enriched in detrital chlorite. We attribute the erratic stratigraphic changes in clay mineralogy to surface currents, near-bottom nepheloid clouds, and low-density turbidity currents converging from several directions, as well as subtle changes imparted by glacial vs. interglacial weathering.
  3. Site-specific detrital sources included Vancouver Island and the Olympic Peninsula. Sediment discharge from the west coast of Vancouver Island probably moved south through Vancouver Valley. Detritus from southern Vancouver Island and the Olympic Peninsula (and perhaps the Fraser River drainage) moved toward Cascadia Basin via the Strait of Juan de Fuca. Turbidity currents and nepheloid-layer suspensions flowed down Barkley, Nitinat, and Juan de Fuca Canyons, spread over the northern apex of Nitinat fan, and then merged and mixed with Vancouver Valley sediments moving from the north.
  4. Detrital smectite decreases significantly toward the north as one moves away from the mouth of the Columbia River. This zonation in clay mineralogy probably contributes to complicated three-dimensional changes in diagenesis, fluid budgets, and physical properties inboard of the Cascadia subduction front. Smectite content near the Leg 168 study area appears to be too low, however, to have a measurable effect on the material properties of strata as they undergo diagenesis and deformation within (or below) the adjacent accretionary prism.

NEXT