Graduate Thesis Or Dissertation


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  • The Greenhorn Mountains contain a tectonic ally disrupted ophiolite and both arc-derived and pelagic sediments. Age of major sedimentary units within and bordering the thesis area is Early Permian, based upon dates of conodonts and fusulinids from contemporaneous but allocthanous limestones. Sediments near the south boundary of the thesis area previously considered Triassic to Jurassic are re-assigned to the upper-Early Permian Metamorphic grade varies in metasediments from prehnite-pumpellyite in the southern, proximal arc sediments to incipient (disequilibrium) upper greenschist facies in the pelagic series. Pillow lavas associated with Elkhorn Ridge Argillite of the thesis area are only partly spilitized and have original alkalic affinities, probably representing a seamount. High pressure, incipient blueschist metamorphism which retrograded with time to lower greenschist facies is recognized at Bennett Creek, and is correlated with the Mine Ridge Schist. Microprobe analyses of amphiboles and other phases from Bennett Creek, Mine Ridge Schist near Hereford, Oregon, and the lawsonite blueschist of Mitchell, Oregon indicate that in all units pressure decreased and temperature increased with time. Bennett Creek and Mine Ridge Schist metamorphism may be associated with tectonic overpressures followed by burial and increased temperatures with approach of the North American plate, or may be related to subduction. The Mitchell blueschist appears to be a product of subduction. Metagabbro and alpine peridotites of the ophiolite are structurally and lithologically equivalent to the Canyon Mountain Complex, and were tectonically emplaced during Late Triassic time. Late Jurassic intrusives similar to the Bald Mountain batholith range from norite to two-mica granodiorite. Seven discrete intrusions were recognized. Eocene olivine basalt and basaltic andesite exposed in the thesis area are alkalic in affinity and are equivalents of the Clarno Formation. Overlying andesites and dacites are probably Oligocene and may represent the equivalent of John Day volcanism. The Columbia River group consists of two Picture Gorge flows in the western-most portions of the thesis area. Extensive alpine glaciation during the Pleistocene incised deep, U-shaped valleys in the western thesis area. Morainal deposits are common at lower elevations in the area. Deposits of Mazama ash up to six feet in thickness occur along streams of the area.
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