Graduate Thesis Or Dissertation
 

Geology of part of the northeast quarter of the Mitchell quadrangle, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m326m534t

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  • The thesis area covers 165 square miles including the greater part of the northeastern quarter of the Mitchell quadrangle in north-central Oregon. The three stratigraphic units exposed in the area consist of a variety of Tertiary volcanic rocks including the Clarno Formation, the John Day Formation, and the Columbia River Basalt. The Clarno Formation of Eocene to Early Oligocene age is the oldest rock unit in the thesis area. From oldest to youngest, the mapped units include volcanic boulder conglomerates, andesite and basalt flows and flow breccias, tuffaceous sandstones intercalated with lava flows, and twelve plugs and dikes of andesite and basalt. The composition, lack of sorting, stratigraphic and structural features, and the included flora indicate that the volcanic boulder conglomerates were deposited as mudflows on a terrain of high relief in a humid, subtropical climate. Andesite lavas were extruded intermittently during deposition of the mudflows resulting in complex intertonguing of the two rock types. Later, Clarno lavas poured from numerous isolated fissures and covered the volcanic boulder conglomerates. Near Middle Oligocene time the formation was folded into a broad NE-trending anticline which swings sharply eastward in the north-central half of the area. Following uplift, the Clarno rocks were eroded into a surface of moderate relief and thick fossil soils developed on scoriaceous portions of the lava flows. The John Day Formation of Late Oligocene to Early Miocene age consists of ash flows and ash falls of acid to intermediate pyroclastic material which were deposited unconformably on the deformed and eroded Clarno rocks. These pyroclastics were erupted from unknown local sources, and probably from distant volcanoes to the west. Much of the material deposited on the pre-Miocene anticlinal crest was reworked by streams and formed thick tuff beds on the flanks of the fold. In Middle Miocene time the consolidated John Day Formation was slightly folded parallel to pre-Miocene anticlinal trends and underwent erosion. The Columbia River Basalt was extruded from fissures onto the tuffs during Middle Miocene time. Following deposition the thick layers of basalt were gently folded as the pre-Miocene anticlinal trend again served as the pattern for mild uplift and deformation in latest Miocene time. Rocks younger than Middle Miocene were either never deposited on the basalt plateaus of the area, or have been completely removed by erosion. In Pleistocene and Recent time the plateaus were dissected and the flows were eroded from the anticlinal crest. Landslide, fluvial, and terrace deposits were laid down in the valleys. White volcanic ash was transported by wind from an unknown source and intercalated with Recent fluvial and slope wash deposits.
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file details PattersonRobertL1966.pdf 2017-08-22 Público Descargar