Graduate Thesis Or Dissertation
 

A stratigraphic study of the marine Cretaceous rocks near Mitchell, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pn89db05k

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  • Nearly 9,000 feet of Cretaceous mudstones, shales, sandstone, and conglomerates are exposed along the northeast-trending Mitchell anticline of central Oregon, These rocks are divided into two formations on the basis of criteria developed in this investigation. The lower formation, hereby designated the Meyers Formation, is 4,716 feet thick at its type section, and consists typically of mudstone and siltstone with thin interbeds of sandstone. This formation rests unconformably on Permian metasedimentary rocks and is conformably overlain by strata hereby designated the Frizzell Formation. The Frizzell Formation is 4,167 feet thick at the type section and consists of three sandstone and conglomerate units separated by thin mudstone units. It is unconformably overlain by terrestrial volcanic rocks of the Clarno Formation of Eocene age. Rapid facies changes are common in both the Meyers and Frizzell Formations. A late early Cretaceous age has been established for these formations by ammonites and other molluscs that they contain. The Meyers Formation is Early to Late Albian and the lower part of the Frizzell is Late Albian in age; no fossils have been found in the upper part of the Frizzell Formation. The basal sandstone unit of the Meyers Formation was deposited in a littoral or upper neritic marine environment. Abundant ammonites and the paucity of macroscopic benthonic fossil types indicate that the thick mudstone units of the Meyers Formation were deposited in a bathyal environment. Thin sandstone beds, possibly deposited by turbidity currents, are interbedded with the sandstones and mudstones. The lower part of the overlying Frizzell Formation is shallow marine in origin; however, the upper part of this formation displays channeling, oxidized zones, large -scale cross -bedding, lateral pinchouts, and associated features indicative of fluvial rather than marine sedimentation. The composition of pebbles and sand grains in these strata indicate that supracrustal rocks were the primary source for the sediments. A progressive increase in granitic detritus higher in the stratigraphic section implies the gradual uncovering of plutonic rocks in the source area. A topographic high to the south and southeast of the Mitchell area may have been the source of most materials in the Cretaceous rocks. The implied current directions support this conclusion, for sole markings suggest that the depositing currents moved N. 10 -15°W.
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