Cenozoic Geology Of The Seven Devils Mountains Idaho

Author : Daniel R. Sarewitz
Publisher :
ISBN 13 :
Total Pages : 288 pages
Book Rating : 4.:/5 (93 download)

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Download or read book Geology of a Part of the Heavens Gate Quadrangle, Seven Devils Mountains, Western Idaho written by Daniel R. Sarewitz and published by . This book was released on 1982 with total page 288 pages. Available in PDF, EPUB and Kindle. Book excerpt: Metamorphosed volcanogenic rocks of the Permian and Triassic Seven Devils Group are exposed in the northern part of the Heavens Gate 7.5 minute quadrangle, western Idaho. These rocks originated in a volcanic arc at a destructive plate margin. In the north, rocks that probably belong to the Lower Permian Hunsaker Creek Formation comprise abundant volcaniclastic lithologies and sparse keratophyre and quartz keratophyre flows. To the south, the Middle to Upper Triassic Wild Sheep Creek Formation comprises volcaniclastic rocks interbedded with abundant spilite and keratophyre flows. Trace element analyses of these flow rocks indicate that most are transitional, probably calc-alkaline, island arc volcanic rocks. Some flow rocks may be island arc tholeiites. The lithologic and geochemical character of these rocks is different from that of the Wrangellia terrane, with which they have been correlated. The Seven Devils Group is overlain along a low angle thrust fault by the lower to middle Norian Martin Bridge Formation. This unit is composed primarily of sheared marbles, with local occurrence of unsheared micrite. The Martin Bridge Formation is overlain along the Rapid River thrust by the Upper Triassic Lucille Formation and by the Riggins Group (age unknown). Riggins Group in this area includes metavolcanics of the Lightning Creek Schist and metasediments of the Squaw Creek Schist. They both overlie the Martin Bridge Formation along a single plane of the Rapid River thrust in the southern part of the field area. To the north, the Seven Devils Group, Martin Bridge Formation, Lucille Formation, and Lightning Creek Schist are juxtaposed along imbrications of the Rapid River thrust. Structural elements of the study area indicate that the Rapid River thrust may have affected rocks significantly farther to the west than had been previously recognized. Bedding-parallel ductile shear fabrics occur in volcaniclastic rocks throughout the field area. The intensity of ductile shearing increases toward the low angle thrust fault that separates the Seven Devils Group from the Martin Bridge Formation. This thrust is interpreted to be an integral part of the Rapid River thrust system. An east-west-trending tear fault that separates Wild Sheep Creek Formation from Hunsaker Creek Formation postdates low-angle thrusting, and probably indicates the occurrence of structurally lower thrust faults that have not yet been documented. Tonalites of probable Late Jurassic to Early Cretaceous age postdate ductile shearing and thrust faulting. This may indicate that deformation of this area occurred during the regionally significant Late Jurassic pulse of the Nevadan orogeny. This event may record the collision of the Seven Devils volcanic arc with North America.