Cenozoic Sedimentary Framework of the Gorda-Juan De Fuca Plate and Adjacent Continental Margin—A Review
L. D. Kulm, G. A. Fowler, 1974. "Cenozoic Sedimentary Framework of the Gorda-Juan De Fuca Plate and Adjacent Continental Margin—A Review", Modern and Ancient Geosynclinal Sedimentation, R. H. Dott, Jr., Robert H. Shaver
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Earlier studies show that: the area of western Oregon and Washington and the adjaccnt continental margin was the site of a large geosyncline during Ceuozoic time. The marine sandstone and silt stone and locally interbeded volcanic rocks reach a maximum thickness of about 8 km. Late Cenozoic continental shelf and upper slope deposits consist primarily of siItstone and mudstone and subordinate arkosic and lithic Wackes. Lower slope deposits are characterized by lithic, arkosic, or volcanic sand turbidites and mudstones.
During late Eocene time, the geosyncline was subjected to intense tectonism. The seaward part of the basin probably was destined by subduction in middle to late Cenozoic time. As the Coast Range was uplifted late in the Oligocene Epoch, deposition shifted westward into the structural basins in the vicinity of the present continental shell:. Several periods of deposition followed by uplift and erosion occurred on the continental shelf. At least two regional unconformities, late middle Miocene and Plioccne-Pleistocene, are interpreted from seismic reflection records of the Oregon shelf. The earlier unconformity probably is related to the worldwide change in lithospheric plate motion 10 million years ago; the later one to small-plate tectonics within the Gorda-Juan de Fuca plate.
Seismic reflection profiling in the Cascadia and Gorda Basins shows that a thick turbidite wedge occurs at the base of the continental slope in an elongate basement low between Vancouver Island and the Mendocino Fracture Zone. This wedge thins westward toward the Gorda and Juan de Fuca Ridges and reaches a maximum thickness of 2.5 km at the base of the continental slope in the vicinity of the Columbia River The configuration of the basement depression and its associated sedimentary fill are similar to that found in the Aleutian and Peru-Chile Trenches.
Two large submarine fans, Astoria and Nitinat, occupy a large portion of the Gorda-juan de Fuca plate and consist of middle to late Pleistocene turbidites. Sand turbidites of the Astoria Fan form a discordant contact: with the underlying, landward dipping silt turbidites of the abyssal plain. The fan sediments were deposited in a trenchlike depression at the base of the continental slope. With continued deposition, the fan sediments prograded westward as the depression filled.
Silt and sand turbidites, which comprise the bulk of the deposits of the Gorcla-Juau de Fuca plate, are intercalated with thin hemipelagic and pelagic deposits. These turbidites are characterized by lithic, arkosic, and volcanic sands. Paleoenvironmental analyses suggest that the Pliocene and Pleistocene abyssal-plain turbidites of the Gorda-Juan de Fuca plate are being uplifted and accreted to the lower continental slope. A maximum uplift of approximately 1,200 m has occurred on the Oregon continental margin since the late Miocene.
Although both the Cenozoic geosyncline and Cascadia Basin are dominated by terrigenous deposits, there are significant differences in the periodicity of volcanic activity and the configuration of the sedimentary bodies.