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Thick, massive beds of nonmarine limestone are exposed in the Elephant Mountain quadrangle, Brewster County, Texas. This Eocene carbonate ranges from 1 to 70 m (3 to 230 ft) in thickness, and is intercalated with the three lowermost formations of the Buck Hill volcanic group: the Pruett tuff, the Crossen Trachyte, and the Sheep Canyon Basalt. The Pruett Formation (Eocene), which is composed mainly of volcanic tuff and tuffaceous nonmarine limestone, uncomformably overlies the Boquillas limestone (Cretaceous). These interbedded tuffs, and intercalating lavas of the lower Buck Hill group represent several interruptions during the formation of this lacustrine deposit.
The carbonate rocks consist primarily of pure to impure silty limestones with no appreciable dolomite or evaporite content. The unit contains charophytes, ostracodes, algal stromatolites, oncolites, possibly Equisetum, silicified wood, stromatolific tufa, and fresh-water gastropods. These features probably represent a shallow, low energy, sublittoral to littoral marginal facies which were deposited in a relatively humid environment. The thick lower beds of this nonmarine limestone were most likely deposited in a drainage basin formed by block faulting in the Late Cretaceous to early Tertiary. In contrast, the thinner beds in the upper portion of this deposit may have accumulated in local depressions formed in the surrounding volcanic flows.
The true size of the lake basin is unknown due to the lack of exposure, but the Pruett tuff interbedded with limestone has been found in the subsurface approximately 90 km (56 mi) northwest of the study area.
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