New Mexico Geological Society Annual Spring Meeting — Abstracts

The north-striking, 19 km-long Cliff fault passes 1.5 km west of the Sevilleta National Wildlife Refuge headquarters. My study uses stratigraphic relationships to interpret tectonic activity of the Cliff fault during the early Pleistocene. Previous work had suggested the latest movements occurred during the late and middle Pleistocene (Machette, 1978), but early Pleistocene activity remained uncertain. It is necessary to use Santa Fe Group stratigraphy because potential early Pleistocene activity on the Cliff fault antedates preserved geomorphic surfaces or fault scarps. The Cliff fault is the easternmost of a trio of Quaternary normal faults in the southwestern Albuquerque Basin, the other two being the east-down Loma Pelada and Loma Blanca faults to the west. Because these faults have been mapped as extending southward into the northern Socorro Basin, interpreting their long-term displacement behavior is important in understanding how strain is transferred between the Albuquerque and Socorro Basins. Long-term (10⁶-10⁷yr) paleoseismic records of the fault trio can also be used to evaluate if fault activity has migrated towards the center of the basin. The Cliff fault offsets strata of the Sierra Ladrones Formation, a Pliocene-early Pleistocene deposit composed mainly of sand with lesser gravel and silt-clay. The studied sediment lies stratigraphically above a sample locality of a 3 Ma tooth fossil. My field research illustrates several lithofacies in the Sierra Ladrones formation, including two different types of channel fills, floodplain deposits, hyper-concentrated or debris flow deposits, and possible colluvium and slope wash deposits. The two different types of channel fills are: (1) sandstone-dominated and trough cross-stratified, with southerly paleoflow and abundant chert clasts, and (2) gravel-dominated with mostly tabular-lenticular bedding, southeasterly paleoflow, and 10% - 20% chert clasts. Floodplain deposits consist of interbedded fine sand and clay. Paleosols are locally observed and characterized by accumulations of calcium carbonate. Hyper- concentrated flow and debris flow sediments are matrix-supported pebbly sand deposits that lack sedimentary fabric within a given bed. We group these lithofacies into the following lithofacies assemblages: axial-fluvial, distal alluvial fan, and fault-scarp sediments. Using clast counts and paleocurrent data, the alluvial fan deposits are correlated with the Rio Salado paleo-drainage. The axial-fluvial assemblage includes floodplain deposits and channel fills associated with: (1) a S-flowing Rio Puerco and (2) relatively large, gravelly channels exiting the Rio Salado fan and flowing E-SE-S on the basin floor. In contrast to interpretations by Machette (1978), we did not find evidence of deposits derived from the Joyita Hills. Suggestions for early Pleistocene movement of the Cliff fault are mainly inferred from the immediate hanging wall. Certain lithologic units increase in thickness and/or coarsen laterally towards the fault. Also, local angular unconformities are also observed on the immediate hanging wall. Lastly, the southward displacement gradient along the fault, inferred from our mapping near the Refuge headquarters, suggests that the Cliff fault does not extend as far south as previously mapped.