SUMMARY OF THE GEOLOGY, GEOCHRONOLOGY, AND GEOCHEMISTRY OF THE ABIQUIU AREA, NORTH-CENTRAL NEW MEXICO

The Abiquiu area is located along the margin of the Colorado Plateau-Rio Grande rift in north-central New Mexico within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. We summarize the geology of the Abiquiu area, report newly determined ⁴⁰Ar/³⁹Ar dates and geochemical data for rocks within the area and describe a newly discovered low-angle fault. Rocks exposed include continental Mesozoic strata of the Colorado Plateau, Cenozoic basin-fill deposits, and Tertiary volcanic rocks. Mesozoic units are in ascending order, the Upper Triassic Chinle Formation, Middle Jurassic Entrada Sandstone and Todilto Limestone Member of the Wanakah Formation. Cenozoic rocks include the Eocene El Rito Formation, newly named Oligocene conglomerate of Arroyo del Cobre, Oligocene-Miocene Abiquiu Formation, and Miocene Chama-El Rito and Ojo Caliente Members of the Tesuque Formation of the Santa Fe Group. Intrusive and extrusive rocks include the basaltic dike of Red Wash Canyon (Miocene), Cerrito de la Ventana basaltic dike (Miocene), Lobato Basalt (Miocene), Sierra Negra Basalt (Miocene), Servilleta Basalt (Miocene), El Alto Basalt (Pliocene), and dacite of the Tschicoma Formation (Pliocene). Quaternary deposits consist of ancestral axial and tributary Rio Chama deposits, landslide colluvium, Holocene floodplain alluvium, fan and pediment alluvium. The predominant faults are Tertiary normal faults which displace rocks basinward and minor Mesozoic thrust faults. A low-angle fault, referred to here as the Abiquiu fault, separates an upper plate composed of the transitional zone of the Chama-El Rito and Ojo Caliente Members of the Tesuque Formation from a lower plate consisting of the Abiquiu Formation or the conglomerate of Arroyo del Cobre. The upper plate is distended into blocks that range from about 0.1 to 3.5 km long that may represent a larger sheet that has been fragmented and partly eroded.

New ⁴⁰Ar/³⁹Ar ages for intrusive and extrusive rocks include the following: (1) El Alto Basalt, 2.82±01 Ma; (2) Servilleta Basalt, 3.45±0.06 Ma; (3) Sierra Negra Basalt, 5.44±0.06 Ma; (4) Lobato Basalt, 9.57±0.11 and 7.83±0.07 Ma; (5) Cerrito de la Ventana basaltic dike, 19.22±0.30 Ma; and (6) basaltic dike of Red Wash Canyon, 19.63±0.40 Ma. In-progress geochemical analyses by XRF (major element) and INAA (trace element) will help resolve the geochemical evolution for these rocks.