Graduate Thesis Or Dissertation
 

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  • A sequence of volcanic, volcaniclastic, and epiclastic deposits from Oligocene to Recent age are exposed in the region from Lake Simtustus to Madras in central Oregon. The epiclastic sediments of the Oligocene John Day Formation are unconformably overlain by two flows of the middle Miocene Columbia River Basalt Group. The upper Miocene and lower Pliocene Deschutes Formation generally overlies the Columbia River Basalt. The Deschutes Formation includes ash flows, pumice and ash falls, lava flows and epiclastic deposits. An ashflow tuff with a composition similar to other Deschutes ash flows occurs between flows of Columbia River basalt suggesting that the formations interfinger. The two flows of Columbia River basalt have compositions typical of the Prineville Chemical Type. The flows contain an average of 1.24 weight percent P2051 and K20 is high compared to other types of Columbia River basalt. The Columbia River basalt and underlying rocks have been moderately deformed, producing broad anticlines and synclines and normal faults with small displacement. Northwest of Madras, the Columbia River basalt dips three or four degrees to the southeast. The Deschutes Formation and overlying rocks are generally flat lying and undeformed. No faults were found extending into the Deschutes deposits. The Pelton Basalt member is an 80 to 150 foot thick unit of tholeiitic basalt that occurs near the base of the Deschutes Formation. It contains numerous discontinuous flow units which represent separate lobes of a compound lava flow. No interbeds or paleosols between flow units were found. The basalt is probably the result of a single prolonged eruption. Six intraformational lava flows occur interstratified with volcaniclastic and epiclastic deposits. Five of these flows are basalt; one flow is basaltic andesite. The Deschutes volcaniclastic rocks generally occur in a four hundred foot thick interval above the Pelton Member and below rimforming basalt lavas. The best exposures of these deposits are at the Vanora Cliff and at the cliff on the north side of the mouth of Willow Creek. Rocks in both of these areas are exposed as the result of large landslides. The ash-flow tuffs range in composition from dacite to rhyolite and range from three to sixty feet thick. The deposits generally have restricted areas of exposure. The strikes of paleochannels filled by ash flows and the orientation of the long axes of clasts in underlying fluvial conglomerates indicate a west or southwest provenance for these deposits. Seven lithic-rich laharic-breccia deposits occur interstratified with the ash flows and epiclastic deposits. The laharic breccias are the most resistant and extensive volcaniclastic rocks in the mapped area. In certain outcrops, the breccias consist of as much as seventy percent angular to subrounded lithic clasts suspended in a finegrained matrix of glass shards. The deposits are more numerous, thicker, and more poorly sorted to the north and northwest indicating a provenance to the northwest. Many features of the deposits are similar to volcanic lahars but other characteristics indicate that the materials were subjected to temperatures above the Curie point of the clasts. Some of these units might have been produced by a combination of fluvial materials and a hot pyroclastic flow. A widespread sheet of tholeiitic basalt caps the Deschutes Formation. Similarity between eight chemical analyses, constant phenocryst mineralogy, and uniform normal paleomagnetic polarity of samples from Binder's Canyon to Round Butte Dam suggest that the rimforming flow is a single sheet of lava that originally covered the mapped area. Round Butte cinder cone and lava flow are the youngest Deschutes Formation deposits in the mapped area. The Round Butte deposits have a composition similar to other intraformational lavas and therefore represent a continuation of the volcanism that has occurred throughout the late Miocene and early Pliocene in the Deschutes Basin.
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