The uppermost Esmeralda Formation in Clayton Valley, Nevada, consists of about 330 ft (100 m) of tuffaceous sediment deposited in a closed basin formed by Basin and Range normal faulting about 7 m.y. ago. Five closed-basin lithofacies can be defined on the basis of lithology and sedimentary structures: fluvial, debris flow, shallow lacustrine, spring, and playa. The fluvial lithofacies consists of light-gray to brown, trough-cross-bedded sandstone, lenticular clast-supported conglomerate, and irregularly bedded siltstone and mudstone. The debris-flow lithofacies is made up of pale olive-gray to white, poorly sorted, mud-supported conglomerate and gray sandstone, which commonly displays load structures and convolute bedding. The shallow-lacustrine lithofacies consists of w ite, light-gray, and greenish-gray, thin to medium-bedded, laminated mudstone, vitric tuff, and diatomite. The spring lithofacies is laminated travertine with convolute bedding. The playa lithofacies is made up of massive gray mudstone, reworked sandstone and siltstone, and mud-clast conglomerate.

Further information regarding the depositional environment can be deduced from studying the petrology of the rocks. The tuff beds and tuffaceous sandstones contain clinoptilolite, opal-CT, and phillipsite as alteration products of vitric material. The laminated mudstone of the shallow-lacustrine lithofacies contains hectorite, calcite, gypsum, halite, clinoptilolite, and opal-CT, but the massive mudstones of the playa lithofacies contain no hectorite. It is concluded that the hectorite and calcite precipitated from alkaline waters in a shallow, spring-fed lake. During playa deposition, the alkaline environment disappeared either as a result of increasing salinity or the deterioration of the spring source.

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