Graduate Thesis Or Dissertation
 

Geology and mineral deposits of the western Cuddy Mountain District, western Idaho

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  • The map area includes 16.5 square miles in the western part of the Cuddy Mountain District, which is located between the Wallowa Mountains of northeastern Oregon and the Salmon River Mountains of western Idaho. Folding, faulting, and more recent glacial and stream erosion have exhumed Triassic to Early Jurassic(?) sedimentary and volcanic rocks and Triassic(?) plutonic rocks. A composite stratigraphic section includes at least 10,000 feet of strata. The oldest exposed rocks in the thesis area are the Triassic Seven Devils Volcanics. This unit exceeds 2,000 feet in thickness and includes a complex assemblage of volcanic and volcaniclastic rocks. The Cuddy Mountain limestone, presumably correlative to the late Triassic Martin Bridge Formation, has been tectonically em­placed into younger units. The late Triassic(?) red conglomerate unconformably overlies the Seven Devils Volcanics. The red con­glomerate is at least 400 feet thick and is composed primarily of clasts of basaltic rocks with subordinate clasts of sedimentary and intrusive rocks. The late Triassic(?) porphyritic rhyolite tuff is at least 400 feet in thickness and is probably both ash flow and air fall in origin. The Lucile Series overlies the older rocks with angular unconformity and is thought to be of Early Jurassic(?) age. The Lucile Series exceeds 5,000 feet in thickness and is composed of shales, slates, limestones, sandstones, and conglomeratic schists. Flows of Miocene and Pliocene Columbia River Basalt exceed 1,000 feet in thickness and rest unconformably on all pre-Tertiary rocks. The major intrusive phases include gabbro (239 m.y.) and porphyritic granodiorite (201 m.y.). The pre-Tertiary units were regionally metamorphosed during Middle Jurassic time to the greenschist facies. Alteration of the country rock near the intrusives is primarily to the propylitic type. Contact metasomatic deposits of pyrite, specular hematite, and magnetite are localized in silicified limestone lenses within the Seven Devils Volcanics. Hydrothermal deposits of chalcopyrite and argentiferous galena are localized primarily in the manganese stained outcrops of the porphyritic rhyolite tuff near the red conglomerate contact.
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