Geologic summary of the Abiquiu quadrangle, north-central New Mexico
— Florian Maldonado and Daniel P. Miggins

We summarize the geology and present a geologic map of the Abiquiu quadrangle, report newly determined ⁴⁰Ar/³⁹Ar dates for volcanic rocks within and contiguous to the quadrangle, and describe a newly discovered low-angle fault. The Abiquiu quadrangle is located along the margin of the Colorado Plateau-Rio Grande rift in north-central New Mexico and occurs within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Rocks exposed include continental Mesozoic rocks of the Colorado Plateau, Cenozoic basin-fill deposits, and Tertiary volcanic rocks. Mesozoic units are the Upper Triassic Chinle Formation, and Middle Jurassic Entrada Sandstone and Todilto Limestone Member of the Wanakah Formation. Cenozoic rocks include the Eocene El Rito Formation, Oligocene conglomerate of Arroyo del Cobre, Oligocene- Miocene Abiquiu Formation, and Miocene Chama-El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ~15-8 Ma), El Alto Basalt (Pliocene; ~3 Ma), and dacite of the Tschicoma Formation (Pliocene; ~2 Ma). Quaternary deposits consist of inset ancestral axial and tributary Rio Chama deposits and landslide colluvium, and Holocene floodplain, fan and pediment alluvium. The predominant faults are Tertiary normal faults displacing 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 Ojo Caliente Sandstone and Chama-El Rito 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 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded.