The Baca Formation and the Eocene-Oligocene boundary in New Mexico
— Spencer G. Lucas

The Eocene-Oligocene transition in New Mexico was a time of significant change in magmatism and sedimentation throughout the state. During the Paleocene and Eocene, continential sediments were shed from complex uplifts and deposited in adjoining basins as the result of amagmatic basement deformation (Laramide orogeny). However, during the Oligocene much of New Mexico was characterized by massive outpourings of intermediate to silicic volcanic rocks and associated intrusions. The precise timing of this magmatic-sedimentologic change- over varies throughout the state, but in general the beginning of the Oligocene corresponds to this fundamental transition (Lucas and Ingersoll, 1981).

The sequence of rocks that span the Eocene-Oligocene boundary in west-central New Mexico is exemplary of this transition. In this area, conglomerate, sandstone, mudstone, and claystone of the Eocene Baca Formation are fluvial and lacustrine deposits that were shed from the Defiance, Zuni, Lucero, Sierra, Morenci, and Mogollon uplifts and deposited inthe Baca basin (Chapin and Cather, 1981; Cather, 1983a, this guidebook, fig. 1). Volcaniclastic rocks (containing mainly clasts of quartz-latite to andesite composition) of the Spears Formation (Oligocene) overlie the Baca Formation in west central New Mexico and probably represent the eruptive products of stratovolcanoes in the north- ern part of the Mogollon Datil magmatic province (Elston and others, 1976). The Eocene age assigned here to the Baca Formation is based both on its fossil mammals and on radiometric dates from the Spears Formation. The purpose of this paper is to review the fossil evidence used to assign an Eocene age to the Baca Formation and to compare the fossil mammals from the Baca with temporally equivalent fossil mammals from elsewhere in New Mexico and western North America. In this paper, LSUMG refers to the Louisiana State University Museum of Geoscience (Baton Rouge) and UNM-MSB refers to the Museum of Southwestern Biology, University of New Mexico (Albuquerque).