Abstract

The mineralogy of sandy sediments deposited during the late Pleistocene and Holocene in southern Cascadia Basin and the adjoining Blanco Fracture Zone displays systematic regional and temporal trends. The light-mineral constituents of these sands are lithic, arkosic, or volcanic in composition. They are mainly derived from extrusive and intrusive basic igneous rocks. Four deep-sea heavy-mineral provinces (A, B, C, and D) are distinguishable in the area investigated. Province A lies in the Blanco Fracture Zone--Gorda Ridge transition area. Textural and mineralogical compositions of these deposits suggest local submarine volcanism. The volcanism may be associated with sea-floor spreading along the young crestal zone of Gorda Ridge. Province B is restricted to the late Pleistocene sediments of western Cascadia Abyssal Plain and Vancouver Valley; the most probable sediment source is Vancouver Island and vicinity. Sandy sediments from the island no longer reach the deep-sea environments, because they are presently being trapped behind the sills of the glacially-scoured inlets of the island. Province C includes the late Pleistocene and Holocene deposits in southeastern Cascadia Basin and Cascadia Channel; the sediments were derived from the Columbia River drainage. During Holocene time, the supply of coarse-grained detritals from the Columbia River diminished as sea level rose, and locally derived sediments from the metamorphic terrane of the Klamath Mountains were deposited at the base of the continental slope off southern Oregon and northern California (Province D), overriding the influence of the Columbia River sedimentation.

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