New Mexico Geological Society Annual Spring Meeting — Abstracts

North-trending normal fault traces and scarps cutting deposits between the Loma Blanca and Cliff faults in the southwestern Albuquerque Basin of the Rio Grande rift complicate interpretations of basin structure, but may lead to better understanding of ages of fault offsets. Deposits in a gravity-defined graben between the larger faults include a 1553-m-high drainage divide between the Rio Puerco (north) and Rio Salado (south). Clasts along the divide and farther north show that the Rio Salado used to enter the Rio Puerco valley down to an elevation of 1494 m, where sediments of the two streams may interfinger. The high divide appears to be on a horst between several poorly exposed faults. The top of the down-to-the-east hanging wall of the Loma Blanca fault is locally about 1564 m whereas the top of the eroded footwall of the Cliff fault (down-to-the-west) is 1553 m, which is the same elevation as the top of the ancestral Rio Grande deposits to the east and the top of the Rio Puerco-Llano de Albuquerque deposits to the north. The northern part of the Loma Blanca fault offsets alluvial-fan deposits of late-middle Pleistocene age graded toward intermediate terrace levels of the Rio Puerco. Farther east, the same alluvial deposits are offset by east- and west-facing scarps 1-4 m high. The exposed northerly trace of the Cliff fault is about 6 km long between the Rio Salado and Rio Puerco valleys and appears to have at least 75 m of offset. The Cliff fault does not offset the highest terraces along the Rio Puerco and Rio Salado valleys. Two high levels (1540 and 1534 m elevations) of Rio Salado terraces bevel both tilted beds of the hanging wall and horizontal beds of the footwall, with at least 15 m removed below 1551 m elevation. West of the Cliff fault, the slightly lower terrace (estimated to be at least 600 ka) is offset by three subparallel north-south faults, forming a small horst and narrow graben. Farther west are suspected fault scarps that mark the western edge of the larger horst on the drainage divide but these probably are truncated and buried by a later, lower terrace of the Rio Salado. Although others mapped a Rio Puerco fault in the Rio Puerco valley to the north and projected it into the area, the numerous faults with small offsets suggest that a large, down-to-the east fault does not cut the surface here. As in other parts of the Albuquerque basin, younger fault scarps appear to be active toward the center of the broader graben.