New Mexico Geological Society Annual Spring Meeting — Abstracts

The southwestern part of the Albuquerque Basin of the Rio Grande rift between the Ladron Mountains and Rio Grande Valley is cut by three major and several lesser-known north-south normal faults with Quaternary offsets: Loma Pelada, Loma Blanca, and Cliff (from west to east). Each fault block exposes different sedimentary deposits ranging in age from mid-Miocene to early Quaternary. Deposits in the footwall of the west-down Cliff fault adjacent to the modern Rio Grande Valley consist of two kinds of southeast-directed channels and floodplain deposits of the ancestral Rio Puerco and Rio Salado and mixtures. The two channel types presumably joined the ancestral Rio Grande west of the Joyita Hills. The floodplain/basin-floor deposits are time- and in-part lithologically correlative with the broad fluvial fan of the Ceja Formation and Llano de Albuquerque to the north; they predate development of high-level terraces of the Rio Grande and modern west-east Rio Salado Valley to the south. Clasts are distinctive for the two types of stream channels, although locally some become mixed. Rio Puerco gravels commonly are well-rounded siliceous pebbles (at least 20 % chert) less than 8 cm long with a few larger pebbles; rare pebbles of 3.26-Ma Grants obsidian are present in upper exposures. Rio Salado clasts include larger, subangular-subrounded pebbles to boulders of limestone, sandstone, granite, quartzite, other Proterozoic metamorphic rocks, ash-flow tuffs, intermediate and basaltic volcanic rocks, and rare travertine. To determine the path(s) of the two channel types upstream from the exposures at the north end of the Cliff fault, we looked for similar suites of clasts between the Cliff and Loma Blanca faults and between the Loma Blanca and Loma Pelada faults. Northeast-directed Rio Salado deposits meet and overlie Rio Puerco deposits along the southern margins of the Rio Puerco Valley west of the Cliff fault and may be traced southwest to the east side of the Loma Blanca fault north of the Rio Salado Valley. Between the Loma Blanca and Loma Pelada faults, two possible levels of northeast-trending bluff-lines with Rio-Salado-type gravel deposits south of the bluffs suggest northeastward-directed paths toward the Rio Puerco. However, in the underlying deposits that clearly predate piedmont gravels shed from the Ladron Mountains, similar suites of “Rio Salado” clasts indicate northward transport. The transport direction shifts northeastward near AT&T road. North of AT&T road, clasts similar to the “Rio Salado” suite are directed southeast and probably were reworked from separate exposures northeast of the Ladron Mountains. Rio Puerco channels on both sides of the Loma Blanca fault are also directed southeast. We conclude that there may be several origins for clasts of the “Rio Salado suite” exposed in the footwall of the Cliff fault and that paleogeographic maps of fluvial contributors to deposits of the southern Albuquerque Basin may need revision.