New Mexico Geological Society Annual Spring Meeting — Abstracts

Pliocene (Blancan) vertebrate fossils are common in the Palomas Formation near Elephant Butte Lake (EBL), Sierra County, south-central New Mexico. There are four Blancan local faunas (LF) from this region; Elephant Butte Lake LF, from sites around EBL; Cuchillo Negro Creek LF, 6 km west of EBL; Truth and Consequences (TC) LF, 1 km west of TV; and Palomas Creek LF, 5 km southwest of TC. These sites are mostly composed of large mammals derived from axial river gravels; however, the TC LF has produced a diverse microvertebrate fauna from mudstones. Age-diagnostic large mammals from the vicinity of EBL include: horse (Equus simplicidens), giant camel (Gigantocamelus spatula), and gomphothere (Stegomastodon primitivus=S. rexroadensis) from the EBL LF; borophagine canid (Borophagus hilli); E. simplicidens, camel (Camelops), and S. primitivus from the Cuchillo Negro Creek LF; and two horses (Nannippus peninsulatus, E. cumminsii) and mastodont (Mammut raki) from the Palomas Creek LF. Small mammals from the TC LF (lagomorphs- Hypolagus vetus, Notolagus lepusculus; rodents- Geomys minor, Neotoma quadriplicata) indicate an early Blancan age. K/Ar dates of 4.2 Ma on a basalt from the lower Palomas Formation and 2.9 Ma on the Mitchell Point basalt from above most of the fossil sites constrain the age of the EBL faunas to early Blancan (2.7- 4.9 Ma). The biostratigraphic ranges of certain mammals (B. hilli, H. vetus, N. lepusculus, G. minor, N. quadriplicata, N. pennisulatus, E. simplicidens, S. primitivus) further restrict the age of these sites to late early (=medial) Blancan ( ?3.0-3.6 Ma).