MAGNETIC STRATIGRAPHY OF THE UPPER MIDDLE EOCENE BACA FORMATION, WEST-CENTRAL NEW MEXICO

The Baca Formation in west-central New Mexico (near the towns of Quemado and Pie Town) consists of about 180 m of redbeds, mudstones and volcanic ashes deposited in a braided alluvial-fluvial system. It yields a fragmentary but important Duchesnean (late middle Eocene) mammalian fauna, including the primitive entelodont Brachyhyops, the creodont Hyaenodon (both of which first occur in the Duchesnean), plus the primitive artiodactyl Protoreodon and the brontothere Diplacodon (both Uintan holdovers), and fragmentary agriochoerids, camelids and protoceratids. The most complete section at Mariano Springs was sampled and the samples were subjected to both thermal and alternating field demagnetization. The samples yielded a stable single component of remanence which passed a reversal test, and was held largely in magnetite with minor goethite overprints. Nearly the entire section is of reversed polarity. Based on a K-Ar date of 38.0 Ma from the overlying Spears ignimbrite (part of the Datil volcanic group), we correlate the Baca Formation with Chron C17r (38.2-38.5 Ma), or middle Duchesnean. This coincides with the age of the Duchesnean Galisteo Formation in north-central New Mexico, suggesting that the two basins opened and filled at the same time.