Plio-Pleistocene mammalian biostratigraphy and biochronology at Tijeras Arroyo, Bernalillo County, New Mexico (original) (raw)

Most of the vertebrate fossils from Tijeras Arroyo, located just south of the Albuquerque International Airport in Bernalillo County, are derived from the Sierra Ladrones Formation and are early Irvingtonian in age (Lucas et al., 1993). However, one locality (New Mexico Museum of Natural History and Science [NMMNH] site L-1458) at the base of the exposed stratigraphic section in Tijeras Arroyo (Fig. 1) has produced two species that are indicative of a Blancan age. The fossils from this site were derived from a sandstone comprising unit 1 in the stratigraphic section of Lucas et al. (1993, fig. 2). The lowermost part of the section in Tijeras Arroyo, including unit 1, was recently referred to the Ceja Member of the Arroyo Ojito Formation (Connell and Hawley, 1998; Connell et al., 1999). Both mammals identified from site L-1458 in the Tijeras Arroyo section, Hypolagus cf. H. gidleyi and Equus cf. E. cumminsii, are typical of Blancan faunas, and do not occur in the Irvingtonian. The extinction in the late Pliocene (about 2.2 Ma) of several characteristic Blancan genera, including Hypolagus, Borophagus, Rhynchotherium, and Nannippus, is considered one of the most important biochronological events in the late Blancan (Lindsay et al., 1984). The presence of Hypolagus thus indicates that site L-1458 is older than 2.2 Ma. Equus cf. E. cumminsii appears to be absent from early Blancan faunas, so L-1458 is probably middle or early late Blancan in age. Ten stratigraphically higher localities in Tijeras Arroyo have produced a significant vertebrate fauna of early Irvingtonian age (Lucas et al., 1993; Morgan and Lucas, 2000). More than 75 m of the Sierra Ladrones Formation are exposed in Tijeras Arroyo, consisting of sandstones, pumiceous sandstones, and gravels, with minor amounts of mudstone and diatomite. These sediments represent axial river deposits of an ancestral Rio Grande. The most distinctive lithologic chracteristic of these beds is the presence of reworked Guaje Pumice derived from the Bandelier Tuff, Ar/Ar dated at 1.61 Ma (Izett and Obradovich, 1994), in the units associated with an Irvingtonian fauna (units 3-8 of Lucas et al., 1993). An extensive flora of leaves and pollen from a localized volcanic ash bed was collected in the Tijeras Arroyo section (NMMNH Site L-1445). The Tijeras Arroyo flora indicates that the cottonwood forest or bosque currently found along the banks of the Rio Grande in New Mexico dates back to at least the early Pleistocene (Knight et al., 1996).