Graduate Thesis Or Dissertation
 

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  • The southern half of the upper Nehalem River basin contains the most complete section of lower to middle Tertiary marine sedimentary and volcanic rocks in northern Oregon. Determination of stratigraphic relationships of six formations, and their depositional environments and provenance is the chief objective of this paper. More than 75 percent of the sedimentary rocks comprising the Cowlitz, Keasey, Pittsburg Bluff and Scappoose Formations is tuffaceous arkosic sandy mudstone and siltstone. Much of this has been incorrectly called "shale" in the past. Goble Volcanics, consisting of basaltic lava, breccia, dikes and irregular bodies are interbedded with and intruded into the late Eocene lowermost Cowlitz Formation, made up of immature volcanic siltstone and lithic arkose. Conglomerate is locally associated with the volcanics. The lithologic identity between the mudstone and muddy volcanic arenite of the uppermost 1,500 feet of the Cowlitz and the lowermost Keasey Formations as previously mapped indicates that they should both be called lower Keasey. The sandy mudstone of the Pittsburg Bluff Formation is easily distinguished from the lithic arkose with minor siltstone of the overlying Scappoose Formation. Smectitic clays present in nearly all rocks were formed diagenetically from volcanic detritus. Kaolin is restricted to the mudstone of the uppermost Cowlitz Formation. The lithologic similarity of the four sedimentary units is so pronounced that it is recommended that they be combined to form the Nehalem Group. The Columbia River Group overlies the sedimentary rocks unconformably in the eastern part of the area. It once had wide distribution, as shown by lateritic soil, remnant patches of basalt, and feeder dikes. The sediments accumulated in an environment which changed from shallow marine, flanking volcanic islands, in the late Eocene, to shallow shelf and delta during the Oligocene. Besideslocal volcanic sources of these sediments, there were volcanic sources to the east and plutonic igneous and metamorphic sources in the northern Rocky Mountains.
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