PALEONTOLOGIC EXPLORATION AND GEOLOGIC MAPPING OF THE RANCHOLABREAN AGE LAS VEGAS FORMATION, LAS VEGAS, NEVADA

The Las Vegas Formation, described from the Tule Springs area of the upper Las Vegas Wash, has yielded invertebrate and vertebrate fossils that comprise one of the most significant late Pleistocene assemblages from the southern Great Basin. Workers established that these and other fine-grained deposits throughout the southern Great Basin are indicators of elevated water tables and increased ground water discharge during the late Pleistocene. Analyses on these spring deposits have clarified paleoenvironmental conditions and constrained hydrologic changes through time. However, vertebrate paleontologic evidence recognized from these same high discharge lithologies is rarely reported upon and necessitates integration into this paleoclimatic framework.

The San Bernardino County Museum has expanded the data from the Las Vegas Formation across the upper Las Vegas Wash, augmenting early work from the 1960s. 526 vertebrate localities have been discovered since 1990. In an effort to incorporate these records into the larger paleoclimatic context of the last two glacial maxima, ongoing research focuses on integrating the abundant vertebrate paleontology record with GIS data, digital photography of three-dimensional stratigraphy, and traditional mapping. The Las Vegas Formation is composed of seven stratigraphically ascending units, designated A through G. Units B₂, D and E₁ have proven fossiliferous and have yielded radiocarbon dates to >40,000 ka, approximately 25,500 ka, and about 14,780 to 9,300 ka, respectively. The formation was first defined at Tule Springs and its units were subsequently extended beyond the original locality to other deposits in the region. Paleospring discharge features in these units demonstrate correlation of spring recharge and climate changes in the late Quaternary in this region.

Newly recognized fauna includes Rana sp., Masticophis sp., cf. Arizona sp., Marmota flaviventris, Neotoma cf. N. lepida and cf. Onychomys sp. The megafauna now contains a large bovid similar in size to Euceratherium. Additionally, we have recognized Bison from Unit E1; the youngest reliably dated record of this genus in the Mojave Desert/southern Great Basin. Radiocarbon dating (14,780 +/- 40 ka) confirms this locality is within the reported range of unit E₁ in the southern Great Basin.