FLUVIAL DEPOSITIONAL ENVIRONMENT OF A PLEISTOCENE MEGAFAUNAL LOCALITY, PANACA, NEVADA

Reconstructions of Neogene depositional environments in the Basin and Range usually focus exclusively on geomorphology. Here we present sedimentary and paleontological data for a site at the Wilkin Quarry, Panaca, Nevada. A horn core from the Pleistocene longhorn bison (Bison latifrons) was recovered from sediments within the quarry, constraining the age of the deposit to the Rancholabrean Land Mammal Age (~200 ka to 11.5 ka). This is the first reported B. latifrons occurrence in Nevada. Fragmented mammoth tusk material was also recovered, and is presumed to be from the Columbian mammoth (Mammuthus columbi). Fine-grained sediments were screenwashed for invertebrate microfauna, but no material was identified. Plant macrofossils were recovered from finer grained sediments; macrofossil and palynological analyses are in progress. In an attempt to assess the paleoenvironment of this time interval, sedimentological data were collected by undergraduate students in a paleontology course.

A quarry face approximately 53 m long and 7 m high was photographed to evaluate alluvial facies architecture. Three stratigraphic profiles were measured to determine lithology, texture, sedimentary structures, and paleocurrent. The profiles indicate 3 to 5 fining-upward successions within a 7 m interval with cycle thicknesses of 1 to 2 m. The cycles consist of a basal, moderately to poorly sorted gravel; the upper portion of each cycle consists of a moderately to poorly sorted gravelly sand or silty sand. Paleocurrent data measured from cross-strata and clast imbrication indicate bimodal paleoflow directed to the northeast and southwest. We interpret these sedimentary packages to represent deposition in gravel bed rivers with episodic discharge. Overbank deposits are poorly preserved due to frequent channel migration and avulsion. The dominant flow direction indicated by paleocurrent data was to the southwest, which is consistent with modern local drainages. However, the northeast component of the bimodal paleocurrent distribution may represent localized sinuous reaches of the fluvial system. The topographic position of the quarry site suggests that these fluvial deposits form terraces representing an abandoned floodplain of the modern Meadow Valley Wash.