Reversal magnetostratigraphy and radioisotopic geochronology of the Plio-Pleistocene Camp Rice and Palomas Formations, southern Rio Grande rift
— Greg H. Mack, Stephen L. Salyards, William C. McIntosh, and Michael R. Leeder

Alluvial-fan and axial-fluvial sediment deposited during the most recent stage of crustal extension in the Rio Grande rift of southern New Mexico is referred to as the Palomas Formation in the Palomas Basin and the Camp Rice Formation in basins to the south. These formations can be accurately dated and correlated by (1) radioisotopically dated basalt flows, fallout ashes, and pumice-clast conglomerates interbedded within or inset against the formations, and (2) high-resolution reversal magnetostratigraphy, some sections of which incorporate dated volcanic rocks as chronologic tie points. The base of the formations correspond to either the Gauss (late Pliocene) or Gilbert (early Pliocene) geopolarity chrons, depending upon position within the basins, and the ancestral Rio Grande may have arrived in southern New Mexico as early as 5 Ma. The age of the top of the formations, corresponding to the La Mesa and Cuchillo geomorphic surfaces and to the initiation of downcutting by the ancestral Rio Grande and its tributaries, is at or very near the Matuyama-Brunhes geopolarity chron boundary, at 0.78 Ma.