The Cougar Point Tuff, Southwestern Idaho and Vicinity

The Cougar Point Tuff is a sequence of densely welded, rhyolitic ash-flow tuff cooling units that erupted from the Bruneau-Jarbidge eruptive center during late Miocene time. By the end of Cougar Point Tuff volcanism, the eruptive center had developed into a large structural and physiographic basin. The exposed part of the Cougar Point Tuff consists of outflow-facies rocks that extend beyond the margins of the eruptive center. Thicker intracaldera facies of the welded tuff units may be hidden beneath younger rhyolite and basalt lava flows and sediments which fill the basin. The Cougar Point Tuff is best exposed’ in the canyons of the Bruneau River and the East and West Forks of the Jarbidge River near the Idaho-Nevada border. At Black Rock escarpment in Bruneau Canyon, eight units are exposed and have an aggregate thickness of 400-475 meters. At Cougar Point in the East Fork of Jarbidge Canyon, six of the units are exposed with a thickness of about 250 meters. The various Cougar Point Tuff units are quite similar in composition and in the temperature and mode of eruption. Where relatively thick, all are simple cooling units, but some, traced into areas where they are thin, change into compound cooling units or even bifurcate into composite sheets. In a vertical section through a unit, the typical zones from base to top consist of (1) a basal layer of thinly bedded air-fall ash, (2) a vitrophyre layer at the base of the ash-flow cooling unit, (3) a massive, relatively thick, lithoidal central zone of the ash-flow sheet in which most structures have been obliterated by matrix crystallization, and (4) an upper zone of the ash-flow sheet with abundant primary flow marks and secondary folds. Quartz, sanidine, plagioclase, augite, pigeonite, fayalite, magnetite, and ilmenite are the principal phenocryst minerals, and are accompanied by accessory zircon, monazite(? ), and apatite. Hornblende and ‘Idaho Bureau of Mines and Geology, Moscow, Idaho 83843. 2Amoco Production Company, P.O. Box 50879, New Orleans, Louisiana 70 150. biotite are essentially absent, suggesting that the magmas were relatively hot and dry. The absence of hypersthene and the presence of pigeonite, the hightemperature Ca-poor pyroxene, in nearly every unit mean that the magmas were hotter than the pigeonitehypersthene inversion boundary. Plagioclase-clinopyroxene-opaque oxide cumulophyric aggregates occur in most units; these are interpreted to be fragments of the rocks melted to form the magmas. Chemically analyzed samples from locations many kilometers apart in each unit reveal that each has a relatively narrow compositional range. Overall, the Cougar Point Tuff becomes increasingly femic upwards, but local reversals in the trend exist. The tuff has been divided into three compositional cycles based on these local reversals. In each cycle the composition became increasingly femic upwards, as successive magma batches were erupted. The emplacement of each unit was a distinct volcanic event, separated by the deposition of sedimentary layers. In some cases sufficient time elapsed between eruptions for Earth’s magnetic field to reverse its polarity. A potassium-argon age of 11.3 million years is reported for one of the lower Cougar Point Tuff units. Initial strontium isotope ratios indicate a predominantly, if not exclusively, crustal origin for the Cougar Point Tuff, being similar in that respect to other rhyolitic rocks in the Snake River PlainYellowstone Plateau volcanic province. In all probability, the Cougar Point Tuff magmas originated in deep crustal rocks that were partially melted as successive large batches of basaltic magma were injected into the crust. The silicic magmas that gave rise to the ash-flow units are interpreted to have been relatively hot and dry as they rose through the crust, so that the disruptive vesiculation which propelled hot ash high into the atmosphere during their eruption occurred at comparatively shallow depths.