New Mexico Geological Society Annual Spring Meeting — Abstracts

 At Otero Mesa in Otero County, southern New Mexico, the Otero Mesa Formation of the Lower Permian Yeso Group is ~ 52 m of red-bed siliciclastic mudstone and ripple-laminated sandstone that is above the lowermost strata of the Yeso Group, which comprise an ~ 51 m thick interval of gypsum, gysiferous siltstone and dolomite. In 2012, we collected fossil plants and trace fossils in the Otero Mesa Formation at 17 localities along the western flank of Otero Mesa in sections 2, 11 and 14, T22S, R10E. These fossils occur primarily at two stratigraphic levels in the lower part of the Otero Mesa Formation. The lowest fossiliferous bed is ripple-laminated sandstone ~ 14 m above the base of the formation; the most fossiliferous bed is a similar sandstone ~ 6 m higher. The trace fossils occur in very fine grained sandstone or siltstone beds that have abundant ripple laminations (usually climbing ripples). Fossils from the Otero Mesa Formation are walchian conifers, invertebrate ichnofossils (Augerinoichnus helicoidalis, Dendroidichnites irregulare, Scoyenia gracilis) and tetrapod footprints (Batrachichnus salamandroides, Dromopus lacertoides, Dimetropus ichnosp.). The ichnoassemblage of the Otero Mesa Formation closely resembles those of the Abo and Robledo Mountains formations to the west, and well represents the Dimetropus ichnocoenosis of the Batrachichnus ichnofacies. It supports correlation of the Otero Mesa Formation to the lithologically similar Lee Ranch Member of the Abo Formation; both units yield ichnoassemblages of the Dromopus biochron. In contrast, ichnoassemblages of the lower part of the Yeso Group (Arroyo de Alamillo Formation) in central New Mexico (primarily Socorro County) are dominated by captorhinomorph tracks (especially of Varanopus), have only rare Batrachichnus and Dromopus and lack Augerinoichnus. The change in ichnoassemblages is the boundary between the Dromopus (older) and Erpetopus (younger) biochrons. Correlation of the Lee Ranch Tongue and Otero Mesa Formation, supported by lithologic similarity and trace-fossil biostratigraphy, identifies a diachronous base of the Yeso Group in the Sacramento Mountains-Otero Mesa area. Correlation of the Lee Ranch Tongue and Otero Mesa Formation to the Deer Mountain Red Shale Member of the Alacran Mountain Formation in the Hueco Mountains of West Texas identifies a single, red-bed interval during the late Wolfcampian and thus is the most parsimonious event stratigraphic correlation.