New Mexico Geological Society Annual Spring Meeting — Abstracts

Recently completed geologic mapping of the Canada 7.5’ quadrangle corroborates previous preliminary interpretations of the rift faults continuing northward from the Santo Domingo basin into the southern Jemez Mountains. Most notable are the continuation of the Camada and Peralta faults to form the western margin of the Bearhead basin, the full extent of which is now mapped. The Bearhead basin is a 7- kilometer wide, NNW- striking, west-tilted asymmetric basic bounded on the west by the Peralta fault and on the east by the Media Dia fault. Northeast-striking faults related to the Jemez lineament also played an important role in the formation of the basin and its subsequent uplift and dissection.

Although basin-margin and intrabasinal faults locally displace the Pleistocene Bandelier Tuff by 80 meters, or perhaps more, basin subsidence primarily occurred in the late Miocene. This period of basin subsidence is documented by the accommodation of at least 700 m of the Peralta Tuff Member of the Bearhead Rhyolite and Cochiti Formation strata with depositional ages between approximately 7.0 and 6.2 Ma, based on 49Ar-39Ar ages of the rhyolitic tephra. Correlative strata are thin to absent adjacent to the western and eastern margins of the basin. Contemporaneous rhyolite intrusion and extrusion to form the Bearhead Rhyolite was locally focused along the basin-bounding faults and along northeast-striking intrabasinal faults. The basin terminates to the north in a region of poorly exposed and largely older intrusive rocks that contain screens of pre-Jemez sedimentary rocks. These relationships imply the presence of one or more northeast-striking, down-to-the south faults that form a structural margin to the basin.