Record Details
Geology of the Sturgill Peak Area, Washington County, Idaho
ScholarsArchive at Oregon State University
| Field | Value |
|---|---|
| Title | Geology of the Sturgill Peak Area, Washington County, Idaho |
| Names |
Skurla, Steven John
(creator) Field, Cyrus W. (advisor) |
| Date Issued | 1974-04-08 (iso8601) |
| Note | Graduation date: 1974 |
| Abstract | The oldest rocks of the Sturgill Peak area are herein named the grassy slope phyllites, and are of Permian (?) age. They are divided into a phyllitic member, consisting of complexly folded and highly sheared phyllites and phyllitic cherts, and a limestone member, containing sheared limestone, silicified limestone, and chert. The grassy slope phyllites are in fault contact with the underlying sedimentary rocks of Jurassic age. The grassy slope phyllites may correlate with the Burnt River Schist of northeast Oregon. Andesitic to dacitic welded tuffs and volcanic flows of the Upper Triassic Seven Devils Volcanics have a minimum thickness of 3,200 feet in the Sturgill Peak area. Rare interbeds of metaconglomerate, volcaniclastic wackes, and metachert are present. The Jurassic Sturgill Peak rhyolite welded tuff was deposited unconformably on the Seven Devils Volcanics, The welded ash flow origin of the unit is documented by relict eutaxitic textures. The unit is up to 700 feet thick, but is characterized by highly variable thick, nesses that are attributed to an interval of erosion that followed deposition of the welded tuff. The Mann Creek slates, an estimated 3, 000 feet in thickness, consist predominantly of slates, with lithic and feldspathic wackes, and limestone. They tectonically overlie the rhyolite welded tuff and Seven Devils Volcanics. The contact is marked, in part, by the conglomeratic schist lithology that was formed by shearing. The conglomeratic schist is correlated with the Red and Green Conglomerate (Brooks, 1967) and shearing is attributed to faulting along the Bayhorse Thrust (Livingston, 1932). The Mann Creek slates are correlated with the Lucile Series of the Cuddy Mountains (Slater, 1969) on the basis of similar lithology. Between Early to Middle Jurassic and Early Cretaceous time, a small serpentinite body was emplaced along the thrust fault that separates the Mann Creek slates and the grassy slope phyllites. The occurrence of ultramafic rocks suggests that this fault may be of regional importance. The Sturgill Peak intrusive complex is believed to have been emplaced in Early Cretaceous time. Members of the complex, in order of emplacement, are hybrid gabbro (containing secondary quartz), quartz diorite, granodiorite, and mafic and felsic dikes and quartz veins. The Early Cretaceous age is based on two 120 million year K-Ar age dates from the granodioritic phase, as reported in Henricksen, Skurla, and Field (1972), Because of problems involving the dated rock, the complex could be older. Intrusion of the complex resulted in contact metamorphism of the country rocks to the hornblende-hornfels facies, and the formation of minor hydrothermal mineral deposits containing small amounts of copper, silver, and gold. Uplift and erosion during Cretaceous and Tertiary time exposed the intrusive complex. In Early to Middle Miocene time (Waters, 1961) flows of Picture Gorge Basalt completely buried the basement rocks of the Sturgill Peak area. In Middle to Late Miocene to Early Pliocene time (Waters, 1961) flows of Yakima Basalt were deposited, with slight angular unconformity, on the underlying Picture Gorge Formation. Pre-Tertiary rocks were uplifted above the Columbia River Basalts by major northwest and northeast-trending normal faults in late Tertiary and Quaternary time. In terms of plate tectonic concepts, the Seven Devils Volcanics, Mann Creek slates, and grassy slope phyllites are considered to be an island arc, arc-trench gap sediments, and deep sea sediments, respectively. Late Permian to Middle (?) Jurassic deformation is related to the separation of North American and African plates. Assuming the 120 million year age is correct, the Sturgill Peak intrusive complex may be related to subduction along the western margin of the North American plate in Early Cretaceous time, perhaps predating the separation of the European and North American plates. |
| Genre | Thesis/Dissertation |
| Topic | Geology -- Idaho -- Washington County |
| Identifier | http://hdl.handle.net/1957/45179 |
