New Mexico Geological Society Annual Spring Meeting — Abstracts

The Abiquiu quadrangle is located within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift near the eastern margin of the Colorado Plateau-Rio Grande rift in north-central New Mexico. The geology, newly determined ⁴⁰Ar/³⁹Ar dates and geochemical data, and a newly discovered low-angle fault are described. Rocks exposed in the quadrangle and contiguous areas include continental Paleozoic and Mesozoic strata of the Colorado Plateau, Cenozoic basin-fill deposits, and Tertiary volcanic rocks. Paleozoic units include the Late Pennsylvanian to Early Permian Cutler Group, undivided. Mesozoic units are, in ascending order, the Upper Triassic Chinle Group, undivided, and 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-Pliocene), Servilleta Basalt (Pliocene), El Alto Basalt (Pliocene), and dacite of the Tschicoma Formation (Pliocene). Quaternary deposits consist of ancestral axial and tributary Rio Chama deposits, landslide colluvium, and Holocene floodplain alluvium, fan and pediment alluvium. Faults in the quadrangle are Tertiary normal faults that displace rocks towards the rift 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±0.05 Ma (isochron age), 2.86±0.05 Ma (plateau age); (2) Servilleta Basalt, 3.45±0.06 Ma (isochron age), 3.69±0.45 Ma (plateau age); (3) Sierra Negra Basalt, 5.44±0.06 Ma (isochron age), 5.56±0.12 Ma (plateau age); (4) Lobato Basalt/dike, 10.05±0.07 Ma (isochron age), 10.11±0.13 Ma (plateau age); 9.57±0.11 Ma (isochron age), 9.51±0.21 Ma (plateau age); and 7.83±0.07 Ma (isochron age), 7.87±0.10 Ma (plateau age); (5) Cerrito de la Ventana basaltic dike, 19.22±0.30 Ma (isochron age), 19.58±0.09 Ma (weighted mean age); and (6) basaltic dike of Red Wash Canyon, 19.63±0.40 Ma (isochron age). In-progress geochemical analyses by XRF (major element) and INAA (trace element) will help resolve the geochemical evolution for these rocks.