GEOLOGY OF THE ABIQUIU QUADRANGLE, NORTH CENTRAL NEW MEXICO

The Abiquiu 1:24,000 scale quadrangle is located along the Colorado Plateau-Rio Grande rift margin in north central New Mexico, within the Abiquiu embayment, an early extensional basin. Rocks exposed include continental Mesozoic rocks characteristic of the Colorado Plateau and Cenozoic basin-fill alluvial deposits, and volcanic rocks. Mesozoic units are Triassic Chinle Formation, and Jurassic Entrada Sandstone and Todilto Formation. Cenozoic rocks are Eocene El Rito Formation, Oligocene-Miocene Abiquiu Formation (upper and lower members), Miocene Chama-El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation of the Santa Fe Group. Volcanic rocks include the Lobato Basalt (10-8 Ma), El Alto Basalt (~3 Ma), and intermediate lavas of the Tschicoma Formation (7-2 Ma). Quaternary deposits consist of inset ancestral axial and tributary Rio Chama alluvium, Holocene floodplain alluvium, fan and pediment alluvium, and landslide colluvium. Axial terrace benches are found approximately 30 m to 115 m above the present Rio Chama floodplain. One set of terraces at about 115 m height is as old as 640 ka, based on presence of the Lava Creek B ash east of the map area. Tributary gravels beneath Lobato and El Alto Basalts contain clasts of a fluvial system that probably represents several ancestral Rio Chama courses. Tops of these gravels are about 580 m and 395 m, respectively, above the modern Rio Chama. Clasts consist mostly of subrounded to well-rounded Paleozoic quartzite and granite, Tertiary volcanics, and traces of Pedernal chert. Gravels containing quartzite and Pedernal chert are also present under the Sierra Negra (~5 Ma) and Black Mesa (3-4 Ma) lavas located northeast and east of the map area, respectively. The Cañones fault zone separates the Abiquiu embayment from the Colorado Plateau. Time of development of the embayment is constrained by the presences of two basalts on the Colorado Plateau with centers in the embayment. The Lobato Basalt (10-8 Ma) is displaced across the Cañones fault and the El Alto Basalt (~3 Ma) is not.