Lacustrine sediments of Baca Formation, western Socorro County, New Mexico
— Steven M. Cather

The Eocene Baca Formation of New Mexico and correlative Eagar Formation and Mogollon Rim gravels of Arizona comprise a sequence of conglomerate, sandstone, mudstone, and claystone which crops out in discontinuous exposures along a west-trending belt from near Socorro, New Mexico, to the Mogollon Rim of Arizona. The western portion of the outcrop belt represents the synorogenic basin-fill deposits of a large intermontane basin (Baca basin; see Cather, 1983, fig. 1, this guidebook) present in western New Mexico and eastern Arizona during late Laramide time. The Baca basin is bounded to the north by the Lucero, Zuni, and Defiance uplifts, to the southwest by the Mo- gollon Highland and the Apache uplift, to the southeast by the Morenci uplift and to the east by the Sierra uplift. These Laramide uplifts, particularly the Mogollon Highland, were the dominant contributors of detritus to the basin.

This article summarizes the lithologic and sedimentologic characteristics of the lacustrine rocks of the Baca Formation in western Socorro County, New Mexico. It deals primarily with the Baca exposures in the Gallinas Mountains area (fig. 1), although some features of the lacustrine-system deposits exposed in the vicinity of the Bear Mountains are also discussed.