MID-UPPER MIOCENE SEDIMENTARY STRATA IN THE WESTERN ESPANOLA BASIN, NEW MEXICO

The Española basin is a west-tilted half graben filled by the Santa Fe Group. Basin-fill in the western part of the basin, west of the Rio Grande, are largely obscured by Plio-Pleistocene Bandelier Tuff and Cerros del Rio volcanic rocks. However, numerous groundwater monitoring and municipal water-supply wells provide important information about late Miocene sedimentation and tectonics in this area. Two depositional environments were present during the late Miocene: west-sloping alluvial fans and a south-sloping basin-floor. East-flowing streams from the Jemez volcanic field deposited intermediate to felsic volcanic gravel and sand on the alluvial fans (>600 m thick). The upper part of these deposits provides a correlatable marker interval in the subsurface and contains crystal-poor rhyolite ash, pumice, and lava detritus. Glass compositions and ⁴⁰Ar/³⁹Ar results indicate these deposits represent reworked Bearhead Rhyolite tephra (~6.8–7 Ma). The alluvial fan deposits interfinger laterally eastward with basin-floor riverine deposits of the Chamita Formation. The Chamita Formation (~6–13 Ma) is primarily comprised of sands and gravels deposited by two merging, south-flowing rivers: the ancestral Rio Chama (Hernandez Member) and ancestral Rio Grande (Vallito Member). Basalts and phreatomagmatic deposits intercalated within the Chamita Formation cluster between 11.5–13.2 Ma and 8.8–9.3 Ma, indicating sedimentation and mafic volcanism were coincident. Earlier investigations miscorrelated parts of the late-Miocene deposits with the Pliocene Puye Formation and Totavi Lentil, respectively. But a major unconformity separates the late Miocene alluvial slope and basin floor deposits from the overlying Pliocene Puye Formation. Chemical and ⁴⁰Ar/³⁹Ar results confirm petrographic observations that the lowermost Puye deposits contain abundant Rendija Canyon Rhyodacite detritus and have a maximum age of about 5 Ma.

Miocene Santa Fe Group outcrops in the vicinity of the Rio Grande dip gently westward, consistent with dip directions in most of the half graben. However, late Miocene beds to the west, beneath the Pajarito Plateau, dip to SSW. We interpret the south dips as indicative of increasing southward displacement along the east-down Pajarito fault, which serves as the master fault in this area.