New Mexico Geological Society Annual Spring Meeting — Abstracts

We report an important late Pliocene (Blancan) vertebrate fauna in the upper Arroyo Ojito Formation, below Mesa de Sol and south of Tijeras Arroyo in the Albuquerque basin, Bernalillo County, New Mexico. The Mesa del Sol Local Fauna (MdS LF) has the most diverse Blancan small-mammal fauna from New Mexico. The productive microvertebrate site is in fine-grained sediments and has a limited stratigraphic extent (10 cm thick, <2 m wide). The MdS LF contains 18 species of vertebrates, primarily smallbodied taxa, including frog/toad, small tortoise, two snakes, two birds, and 12 mammals. The small-mammal fauna includes 11 taxa: a shrew; the mole Hesperoscalops; a rabbit; and eight species of rodents, including the kangaroo rat Prodipodomys, the pocket gopher Geomys, the woodrat Neotoma, the cotton rat Sigmodon medius, the pygmy mouse Baiomys, the extinct mouse Calomys (Bensonomys) arizonae, the extinct vole Mimomys meadensis, and the extinct muskrat Pliopotamys meadensis. The only large-mammal fossil in the fauna is an articulated partial hind limb of the horse Equus scotti.

The mammalian genera Hesperoscalops, Prodipodomys, Calomys (Bensonomys), Mimomys, and Pliopotamys are indicative of the Blancan land mammal age (~4.5-1.8 Ma). The association of Calomys (Bensonomys) arizonae, Mimomys meadensis, Pliopotamys meadensis, and Sigmodon medius in the MdS LF further restricts its age to "medial" or late early Blancan (~3 Ma). The presence of the microtine rodents Mimomys and Pliopotamys is significant because there is a well-established biochronology for North American Plio-Pleistocene faunas based on this group. The co-occurrence of Mimomys meadensis and Pliopotamys meadensis is indicative of the Blancan IV microtine age (~3.2-2.6 Ma). Broadly correlative Blancan IV faunas are Benson and Clarkdale in Arizona and Sanders in Kansas. The MdS LF is approximately 45 m below an unconformable contact with the early Pleistocene (early Irvingtonian) Sierra Ladrones Formation. To the south the Arroyo Ojito Formation contains fluvially reworked 2.6-2.7 Ma pumice pebbles, further constraining the MdS LF to the younger part of the Blancan IV age.