New Mexico Geological Society Annual Spring Meeting — Abstracts

In New Mexico (NM) no fossil mammals of Oligocene age are known, but there are three areas in which Duchesnean (late Eocene) fossil mammals are relevant to the local placement of the Eocene-Oligocene boundary. Fossil mammals from the upper part of the Galisteo Formation in north-central NM and from the Baca Formation in west-central NM indicate that these strata are of Duchesnean age. These age determinations are consistent with Oligocene K/Ar rates from the Espinaso (overlying the Galisteo) and Spears (overlying the Baca) Formations. Therefore, the Eocene-Oligocene boundary can be placed at or near the Galisteo-Espinaso contact in north-central NM and at or near the Baca-Spears contact in west-central NM.

A brontothere jaw from just west of the Winston graben in the northern Black Range (southwestern NM) is derived from a unit stratigraphically equivalent to the Rubio Peak Formation and previously termed "Spears (?) Formation" or "early andesite volcanic sequence". This jaw represents a relatively advanced though small brontothere (length M₁= 4.6 cm, no posteanine diastemata, P₁ absent, P_3-4 molariform) that pertains to the typical late Eocene genus Duchesneodus. It indicates a Duchesnean age for the horizon from which it was derived that is consistent with K/Ar dates from other areas that suggest the Rubio Peak Formation transgresses the Eocene-Oligocene boundary.