The geology of the southeastern Cuddy Mountain district, western Idaho Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v692tb07m

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  • The thesis area includes 54 square miles of the southeastern Cuddy Mountains in west-central Idaho. The oldest rocks within the area are plutonic phases of the Jurassic Cuddy Mountain complex. These and Cretaceous (?) intrusive and associated (?) extrusive rocks and nearby Triassic to Early Jurassic metasedimentary and metavolcanic rocks were covered by late Tertiary basalt flows. They have been exhumed by Quaternary glacial and stream erosion. The Cuddy Mountain intrusive complex consists of gabbro, quartz diorite, porphyritc granodiorite, and associated mafic and silicic dikes, in order of decreasing age. Hydrothermal alteration is apparently related to late residual fluids derived from the porphyritic granodiorite intrusive and is 'primarily of the propylitic (epidote, chlorite, and calcite) and silicious (quartz) types. Country rocks and the intrusive complex were regionally metamorphosed to the greenschist facies in Middle Jurassic time. A porphyritic rhyolite dike post-dates the regional metamorphism and is suggested to be related to the Cretaceous Idaho Batholith. A welded ash-flow tuff unconformably overlies a part of the intrusive complex and is postulated to be the extrusive equivalent of the porphyritic rhyolite dike. Columbia River Basalts were erupted during Middle Miocene to Early Pliocene time and unconformably rest on all pre-Tertiary rocks. Both Picture Gorge and Yakima members of the Columbia River Basalt are present and their combined thickness exceeds 1,000 feet. The lowest flow of the Picture Gorge member is chemically similar to an older series of basalt flows (High-Titania Alkali-Olivine Basalts) in north-central Oregon. The Cuddy Mountains probably existed as a topographic high prior to extrusion of the Columbia River Basalt. However their present expression, although modified by erosion, has resulted from Late Tertiary and Quaternary uplift by vertical to steeply dipping normal faults that were accompanied by minor folding.
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  • File scanned at 300 ppi using ScandAll PRO 1.8.1 on a Fi-6770A in PDF format. CVista PdfCompressor 5.0 was used for pdf compression and textual OCR. Plate: Master map file scanned at 600 dpi, 24-bit color on a Paradigm ImagePRO GxT 42 HD (OEM version of ColortracSmartLF Bx 42). Image manipulated by SmartLF1.3.05.
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