New Mexico Geological Society Annual Spring Meeting — Abstracts

Geologic mapping in the northern Santo Domingo Basin, and adjacent Jemez Mountains, elucidates late Miocene to Quaternary patterns of subsidence, sedimentation and volcanism in the structural transftion between the Albuquerque and Espanola basins.

Three structural domains occupy a symmetrical graben. The westem domain consists of west-tilted upper Miocene volcanic and sedimentary strata unconformably overlain by Pliocene(?) gravel and lower Pleistocene Bandelier Tuff. This domain is west of the down-to-the-east Pajarito fault and includes NNW-trending faults defining a late Miocene graben along which most eruption of Bearhead Rhyolite was focused and in which the coeval pyroclastic and alluvial deposits of the Peralta Tuff Member are principally preserved. Late Cenozoic displacement along the Camada fault zone (5 km west of the Pajarito fault) is perhaps comparable to the Pajarito fault. The central domain is between the Pajarito fault and a prominent down-to-the-west fault that Kelley (1977) mapped as a northward extension of the San Francisco fault. Within this domain, attitudes in the conformable upper Miocene to lower Pleistocene alluvial section, which encloses the lower Bandelier Tuff, are near horizontal. The eastern domain, bounded on the east by the La Bajada fault, consists of a gently east-tilted section of exposed Pliocene and lower Pleistocene alluvium interbedded with intrabasinally erupted basaltic lava flows and hydromagmatic tuff.

Strong westward tilting in the late Miocene is suggested to account for the presence of > 7 Ma ancestral Rio Grande gravel at least 10 km west of the present river. The lateral transition between this gravel and volcaniclastic piedmont deposits of the Cochiti Formation steps eastward at higher stratigraphic levels. Pliocene subsidence was greater in the central and eastern domains, leading to erosional beveling of tilted strata in the westem domain while deposition was continuing farther east. Evidence is lacking for post-early Pleistocene movement on the San Francisco and La Bajada faults but both the Pajarito and Camada faults have remained active until at least the late Pleistocene suggesting a return to principal westward tilt. Terrace preservation along only the east side of the Rio Grande suggests net westward migration of the channel since the middle Pleistocene, perhaps in response to westward tectonic tilting. The see-saw subsidence pattern has caused widespread distribution of hydrostratigraphically important axial quartzite-rich gravel across the basin. The principal faults continue northward (Pajarito and [?] Camada) or southward (San Francisco and La Bajada) as major structures at or near the margins of the Espanola and Albuquerque basins, respectively, suggesting that this narrow zone of latitudinal overlap of east-and west-facing faults is a low-relief accommodation zone between the two large rift basins.