Geology of southern Black Mesa, Espanola Basin, New Mexico: New stratigraphic age control and interpretation of the southern Embudo fault system of the Rio Grande rift
Daniel J Koning, William McIntosh, and Nelia Dunbar
Studies involving geologic mapping, tephrochronology, and 40Ar/39Ar dating on southern Black Mesa have increased our understanding of late Miocene through Pliocene deposition and tectonism at the center of the Española Basin. A prominent landmark extending 21 km in a northeast direction, Black Mesa is capped by 3-20 m of 3.3-3.8 Ma Servilleta Basalt flows and forms a prominent landmark north of Española. These basalts overlie 1-20 m of Pliocene strata, including ancestral Rio Grande sediment. The Pliocene strata unconformably overlie upper Miocene strata of the Chamita Formation (Santa Fe Group). Strata pre-dating the Chamita Formation include the Ojo Caliente Sandstone Member of the Tesuque Formation (Santa Fe Group), which was deposited in an extensive dune field. The margin of this dune field retreated north northwestward between 11.0-11.5 and 9-10 Ma, concomitant with fluvial deposition by an ancestral Rio Grande (Vallito Member of Chamita Formation) and an unnamed river flowing southwest from the Peñasco embayment (Cejita Member of Chamita Formation). After 8 Ma, the distal toe of eastern piedmont-slope sediment (Cuarteles Member of the Chamita Formation) advanced westward to under the present-day mesa. Several faults and folds in the vicinity of Black Mesa include the northeast-striking Santa Clara and La Mesita faults of the Embudo fault system, the southern Ojo Caliente fault, and the west-plunging Chamita syncline. Pliocene strata thicken eastward across the Santa Clara fault, consistent with vertical displacement during Pliocene time. High slip rates on the southern Embudo fault system and the Chamita syncline occurred between 7-8 Ma (age of lower and upper Chamita tuffaceous zones) and 3.3-3.8 Ma (age of the Servilleta Basalt), based on a pronounced contrast in dips across an angular unconformity between these two stratums. An angular unconformity at the base of Pliocene strata correlates to an unconformity at the base of the 2-5 Ma Puyé Formation to the south, and supports tectonic tilting and deformation, but relatively low sediment preservation, between 3.8 and 6 Ma. Activity on the southern Embudo fault system and Chamita syncline decreased after 3.3-3.8 Ma. Slower tectonic subsidence of the basin and increased discharges along the adjacent Rio Grande and Rio Chama during the Pleistocene induced deep erosion of weakly resistant Santa Fe Group strata and resulted in topographic inversion of the basalt-capped Black Mesa.
- Koning, Daniel J; McIntosh, William; Dunbar, Nelia, 2011, Geology of southern Black Mesa, Espanola Basin, New Mexico: New stratigraphic age control and interpretation of the southern Embudo fault system of the Rio Grande rift, in: Geology of the Tusas Mountains and Ojo Caliente Area, Koning, Daniel J.; Karlstrom, Karl E.; Kelley, Shari A.; Lueth, Virgil W.; Aby, Scott B., New Mexico Geological Society, Guidebook, 62nd Field Conference, pp. 191-214.