MIDDLE MIOCENE LACUSTRINE STRATA AND FOSSIL KILLIFISH IN A VOLCANIC SETTING: THE ROCKS OF PAVITS SPRING, NEVADA TEST SITE, NYE COUNTY, NEVADA

Middle Miocene lacustrine strata on the Nevada Test Site, Nye County, Nevada are part of a succession of fluvial, lacustrine and volcanic lithologies that make up the informally named rocks of Pavits Spring. These lacustrine deposits are important in two ways: they record a lacustrine environment in a volcanic setting; and they contain a monospecific assemblage of a fossil killifish Fundulus sp, a plesiomorphic fundulidae. This fish is a possible ancestor to the modern pupfish that inhabit the region. In this report we document the depositional history sequence through a 120-meter stratigraphic interval within this unit.

The upper volcanic units intercalated within the rocks of Pavits Spring have been identified as part of the Crater Flat Group (13.2 Ma), which erupted from the Silent Canyon Caldera. The Silent Canyon Caldera is located within the Southwest Nevada volcanic field, which is located about 40 kilometers to the northwest of the study area. The alternating sequence of volcanic ash-flow and ash-fall tuffs, fluvial gravels and sands, and fine-grained laminated lacustrine rocks suggests that a small fluvial system was repeatedly dammed or partially filled by the volcanic tuffs. This sequence repeats several times within the 120-meter study section. The uppermost lacustrine deposits are silicified, and large chunks of the finely laminated mudstone are included within the base of the overlying ash-flow tuff in the western part of the study area.

The killifish fossils are found in this uppermost lacustrine deposit at 110-meters. Despite the killifish abundance, no other macroscopic fauna were seen in this or the underlying deposits. Modern cyprinidontids are well adapted to stressed environments, thus the monospecific assemblage, combined with the lithologic evidence, supports the conclusion that the lake that these fish occupied was a high stress, restricted basin-filling lake along the Rock Valley strike-slip fault system.