New Mexico Geological Society Annual Spring Meeting — Abstracts

The Black Butte 7.5-minute quadrangle in the SE part of the Albuquerque Basin of the Rio Grande rift reveals a complex geologic history of structurally-controlled fluvial and alluvial deposition and incision that is masked by low relief. The quadrangle extends eastward from the Rio Grande valley across uplands toward Abo Canyon and the northern Los Pinos Mountains. Drainages entering the southern basin all turn toward Veguita, NM, down a gravity gradient towards a regional gravity low. The dominant topographic feature on the quadrangle is Black Butte (Turututu), a horst of Oligocene-lower Miocene basaltic andesite lava flows underlain by three ash-flow tuffs. The majority of the quadrangle area is composed of subplanar Quaternary sediments, including the top and terraces of the ancestral Rio Grande, alluvial fans, local tributaries, their terraces, and Holocene valley fills. The distinctive suites of clast compositions are (1) ancestral Rio Grande (ARG) pebbly sands, sandstones, and gravels from the north that include 1.4-Ma obsidian; (2) broad constructional and inset fans of the Abo drainage; (3) broad constructional fans from the Palo Duro drainage; (4) inset fans of Pino Draw; and (5) smaller alluvial fans and inset fans from Los Pinos Mountains. Smaller drainages rework sediments from these sources. Alluvial fans and inset fans from the Abo drainage dominate the eastern half of the quadrangle and along the northern part parallel to the present Abo valley. Palo Duro fans dominate the southwestern part of the quadrangle. Smaller fans dominate the southeastern corner. ARG-sourced pebbly sand covers nearly horizontal steps and slopes on the west and west-central parts of the quadrangle. Subtle north-northeast-trending topographic steps from Maes Arroyo to Abo Arroyo are Quaternary normal fault scarps eroded to form laid-back gentle slopes. Three strands within the down-to-the-west Maes-Abo (normal) fault zone (MAFZ) drop planar levels of ARG deposits. The prominent north-northeast-trending down-to-the east Military Road (normal) fault zone (MRFZ) cuts SW from the modern Abo drainage through Abo fan deposits that predate maximum level of Rio Grande aggradation, is cut by Pino Draw, and eventually pinches out or is buried by Palo Duro fan deposits or more recent small Los Pinos piedmont deposits. The MRFZ in the central area of the quadrangle has a young graben at its base that deforms recent small drainages and an intermediate terrace in Abo Arroyo. Deposits from Abo Arroyo spill out around the MRFZ in a set of inset fans, with the youngest levels burying traces of the MAFZ. More recent (post-faulting) alluvial fans are being deposited on ARG surfaces by small drainages that cut through wind-gaps in the MRFZ horst. Terraces of Pino Draw are not preserved through the MRFZ, affecting possible correlations of terraces east and terraces west of the fault. ARG deposits (<1.4 Ma) abut the west side of the MRFZ before being dropped by the MAFZ to the west. This highest level of ARG deposits is likely to be at the maximum level of aggradation that occurred at approximately 800 ka.