TECTONICS, CLIMATE, AND THE TRANSITION FROM AGGRADATION TO INCISION OF THE UPPER SANTA FE GROUP, ALBUQUERQUE BASIN, CENTRAL NEW MEXICO

The age of uppermost Santa Fe Group sediment and onset of incision of the modern Rio Grande valley can only be determined by careful consideration of the origin and distribution of unconformities in rift basins. Local Pliocene unconformities are present along faulted basin margins, in updip positions of tilted half-graben basin floors, and along intrabasinal faults. In the Albuquerque basin, some of these unconformities are clearly demonstrated to correlate to conformable stratigraphic successions within the more rapidly subsiding parts of subbasins. Stratigraphic and geomorphic data from the Albuquerque basin preclude the establishment of an incised Pliocene Rio Grande, as suggested by Reneau and Dethier (1996) for the Española basin.

The western and eastern margins of the Santo Domingo subbasin commonly contain unconformity bounded deposits, such as the Plio-Pleistocene gravel of Lookout Park and the Tuerto formation. Comparably aged, conformable stratigraphic successions are exposed basinward on the hanging walls of the La Bajada and San Francisco faults. The Llano de Albuquerque (Calabacillas and Belen subbasins) represents an abandoned constructional basin-floor surface of probable late Pliocene age. Unconformities between the Arroyo Ojito and Sierra Ladrones Fms, exposed between Tijeras Arroyo and Hell Canyon Wash, are interpreted to be the result of local intrabasinal faulting and a consequence of earlier incision of the ancestral Rio Puerco fluvial system and abandonment of the Llano de Albuquerque near Albuquerque. The youngest terminal deposits are constrained by dated tephra between 1.05-0.66 Ma and constrain the initial downcutting of the Rio Grande, probably in response to climate change. South of White Rock Canyon, early Pleistocene entrenchment of the Rio Grande and its major tributaries marked the termination of widespread Santa Fe Group deposition. Smaller nonintegrated drainages, such as those on the Llano de Manzano, continued to aggrade locally into middle Pleistocene time. Therefore, erosional and constructional surfaces are diachronous within and between basins and results from one locality cannot be extrapolated throughout the rift.