Graduate Thesis Or Dissertation
 

Geology and structure of the Bear Creek area, Crook County, Oregon

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  • The Bear Creek area is situated at the boundry between the Basin and Range Province, Deschutes Basin and Blue Mountains Province. It contains rock units representing most of the Cenozoic geologic column. The major rock units exposed are the Clarno Formation, John Day Formation, Columbia River Basalt Group, Deschutes Formation and the Rim Basalts. The Bear Creek area exposes the southwestern most outcrops of the John Day Formation, which are typically fresher than the John Day rocks exposed elsewhere. In the Bear Creek area fluviatile sediments of John Day affinity have been reported for the first time. The most important unconformity in the thesis area is an angular break, between the John Day Formation and the Columbia River Basalt Group rocks. Rocks under the unconformity are folded with dips up to 40°, and the thesis area is on the limb of one of these folds. The Columbia River Basalt is very gently folded in the same pattern. The most prominent structures of the area are a number of north-south trending faults. Field evidence is presented which indicates that all of these faults are younger than the Columbia River Basalt / John Day angular unconformity. Five different sets of faults have been identified in the thesis area on the basis of orientation and sequence. The Bear Creek Faults are the oldest set of faults. These faults are high angle normal and/or reverse (possibly scissor) faults. Minor strike slip faults cut the Taylor Butte. These faults are at least younger than the John Day Formation. The Dam Fault is a high angle normal fault, and has the maximum amount of displacement associated with it. The Reservoir Fault trends in the northwest-southeast direction and is a high angle normal fault. The Alkali Flat Fault is the youngest fault exposed in the area, and is a north-south trending normal fault. It is my contention that these faults were developed in response to the extention and development of the Deschutes Basin.
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