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COUGAR POINT TUFF

Type Section Information

The Cougar Point Tuff was named by Coats (1964) for exposures below Cougar Point on the East Fork of the Jarbidge River in T. 47 N., R. 59 E.

Geologic Age

The Cougar Point Tuff is Miocene in age. Three K-Ar dates of the Cougar Point Tuff in the Owyhee Quadrangles are 9.6 +/- 2 Ma, 12.2 +/- 0.8 Ma, and 14.2 +/- 0.4 Ma (Armstrong and others, 1980; Coats, 1985). The Cougar Point Tuff both overlies and interfingers into air-fall tuff named the Tuff of Jenny Creek, and wedges out to the south into air-fall tuffs and sedimentary rocks which have been mapped with the Humboldt Formation (Coats, 1985). The Cougar Point is generally unconformably overlain by the basalts of the Big Island Formation.

General Lithology

The Cougar Point is described as a complex cooling unit with several individual rhyolitic to rhyodacitic ash-flow units. Individual cooling units are as much as 150 feet thick and often have vitrophyric bases which grade upward into dense felsophyric textured tuff (Coats, 1985).

Where crystalline, the tuff is a reddish or yellowish-brown to brownish-gray or orange-red, and is a purplish gray to dark brown or black where glassy and fractured. Much of the Cougar Point Tuff is strongly welded and vesicular with abundant elongated and flattened vesicles (Coats, 1964). Phenocrysts form 2 to 10 percent of the unit and include quartz, sanadine, plagioclase, and pigeonite or hypersthene, with accessory zircon, magnetite, and apatite. The thicker welded tuffs often show total recrystallization of the glass to sanidine and tridymite, and often show sharp anticlinal folds that die out downward in the section (Coats, 1985).

In the Mountain City and Owyhee Quadrangles an angular unconformity exists within the tuff. White and brown air-fall tuff and volcanic loess overlie welded tuffs and in turn are overlain by welded tuff. The source vents for the Cougar Point Tuff have not been defined but may at least in part be related to dike swarms exposed in the northeastern Mountain City Quadrangle, southwest of Contact, and at Soldier Cap in the Soldier Cap Quadrangle (Coats, 1985).

Average Thickness

The Cougar Point Tuff is about 1,250 feeet thick along the Jarbidge River type locality, and reaches 1,500 feet in thickness across the state border in southern Idaho (Coats, 1964). Areal Distribution

The areal distribution of the Cougar Point Tuff is poorly documented (Coats, 1985). The Cougar Point forms most of the northern third of the Jarbidge Quadrangle, is present in the Owyhee and Mountain City Quadrangles, near Contact, and in quadrangles to the south including the Soldier Cap, Burner Hills and Wild Horse. It is also extensively exposed within Idaho as the Idavada Volcanics (Malde and Powers, 1962).


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Last modified: 09/12/06