GEOMORPHIC AND STRATIGRAPHIC IMPLICATIONS OF ASYMMETRIC SUBSIDENCE IN THE ALBUQUERQUE BASIN, RIO GRANDE RIFT, NEW MEXICO

Deposits and relict surfaces associated with the Plio-Pleistocene Ceja and Sierra Ladrones Formations in the Albuquerque basin of the Rio Grande rift in New Mexico, record diachronous filling in an actively subsiding rift basin. Deposition of older synrift strata ceased shortly after 6.3 Ma, when erosion dominated the basin flanks. During Pliocene time, the Ceja Fm buried Miocene unconformities as alluvial deposits onlapped the distal hanging-wall ramp until 3.0 Ma, when sediment accumulation rates decreased. Broad sheets of amalgamated Ceja channel deposits prograded into the basin after 2.6 Ma, and Ceja deposition ceased shortly after 1.8 Ma, leaving a relict constructional surface (Llano de Albuquerque) that was partially onlapped by the Sierra Ladrones Fm (QTs). Deposition of the ancestral Rio Grande (QTs) occurred near the eastern-border faults until 1.8-1.6 Ma, when rift-flanking (tributary) piedmont deposits (QTs) prograded away from rift-flank uplifts, moving the axial river towards its present location until shortly after 0.78 Ma, when the Rio Grande Valley incised. Reduced rates of sediment accumulation may be the result of decreased subsidence and sediment bypass, and the asynchronous progradation of margin-sourced detritus may reflect a geometric response to decreasing updip accommodation on the hanging-wall ramp. The fore-tilted geometry of the hanging-wall ramp would promote the progradation of amalgamated channels as Ceja sediment bypassed the basin margin. Although tectonism is clearly important in the development of sedimentary successions, the nearly 0.5-Ma lag between slower accumulation and extensive deposition of Ceja gravels suggests that progradation of hanging-wall derived sediments occurred in response to increased discharge by early Pleistocene time. A nearly 1-Ma lag between the progradation of Ceja gravels and Sierra Ladrones piedmont gravels suggests differences in depositional response times that may be related to the asymmetrical structure of half-graben basins.