Timber Mountain Tuff, Southern Nevada, and Its Relation to Cauldron Subsidence
The Timber Mountain Tuff, southern Nevada, consists of three ash-flow sheets, the Rainier Mesa Member, Ammonia Tanks Member, and Cat Canyon sheet. The K-Ar age of all three is about 11 m.y. These sheets are lithologically, petrographically, and chemically similar and are distributed symmetrically in and around Timber Mountain caldera, an 18-by-20-mile elliptical collapse structure. The caldera formed during or after eruption of most of the Rainier Mesa, for it is the youngest unit of the caldera wall and is present as debris in the oldest caldera-filling deposits. One late Rainier Mesa ash flow postdates collapse. The Rainer Mesa extends eastward 35 miles and westward 50 miles over an area of 3200 sq miles. Its initial volume was more than 285 cu miles.
The Ammonia Tanks Member, with an initial volume of about 180 cu miles, overlies and is about co-extensive with the Rainier Mesa outside the caldera. The Ammonia Tanks partially fills the caldera, and onlaps the walls.
The Cat Canyon sheet is present only within the caldera on an uplifted central dome. At least 3000 ft of Cat Canyon is known (the base is not exposed). Its age relation to the Rainier Mesa is known only from indirect evidence; the Cat Canyon was contained in the already formed caldera and presumably buries the Rainier Mesa on the caldera floor.
The Ammonia Tanks Member and Cat Canyon sheet are distinguished from the Rainier Mesa by small but consistent petrographic differences.