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Miocene and younger sedimentary rocks crop out on the continental shelf and slope off the central coast of Oregon. The predominant lithology is diatomaceous siltstone which contains different quantities of glass shards, Radiolaria, Foraminifera, sponge spicules, and glauconite. Glauconite sandstone was collected from the northern part of Heceta Bank. Angular, poorly sorted, volcanic sandstones were obtained in one sample from Heceta Bank and from several samples taken near the base of the continental slope.
Foraminifera contained in the rocks exposed on the continental shelf suggest that the rocks were deposited in water of bathyal depth. Thus, since late Tertiary time, rocks on this part of the continental shelf have been uplifted possibly as much as 5,000 ft.
The lithology and the faunal content of the rocks suggest that deposition during late Tertiary time occurred either on the continental slope or in one or more isolated basins somewhat removed from the continent. Subsidence contemporaneous with deposition resulted in thick accumulations of Tertiary sedimentary rocks. Late Tertiary and Quaternary tectonism resulted in the uplift and deformation of the Tertiary rocks in the area of the present continental shelf. Erosion and subsequent burial of portions of the Tertiary section occurred during late Quaternary fluctuations of sea-level.
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