New Mexico Geological Society Annual Spring Meeting — Abstracts

The southern Mesa del Sol and adjacent Llano de Manzano are shattered into more than a dozen tilted fault blocks between the Rio Grande valley and Hubbell Bench on the eastern side of the Isleta Reservation. Maximum surface offsets of the faults are about 15 m, but underlying unit separations are much larger, with 10-30-m differences in thicknesses. Angular unconformities and disconformities within and between fault blocks are recognized by geometric relationships, clast compositions, and sequences of dated or geochemically fingerprinted volcanic fragments. These unconformities occur between (1) upper Arroyo Ojito Formation (Pliocene), (2) a transitional fine-sand unit (Plio-Pleistocene), (3) the overlying Sierra Ladrones Formation (lower Pleistocene exposures), (4) within the Sierra Ladrones Formation, and (5) between the Sierra Ladrones Formation and the overlying piedmont deposits of the Llano de Manzano (lower to middle Pleistocene).

Volcanic tephra, reworked ash, crossbedded pumice, clasts of rhyolitic tuff, and obsidian are correlated between blocks using ⁴⁰Ar/³⁹Ar ages and geochemical compositions. Ages range from >3 Ma to 1.21±0.03 Ma. Five geochemical groups of pumices and several unique specimens are distinguished in the Pleistocene ancestral Rio Grande deposits (upper Sierra Ladrones axial facies) and are correlated to samples from other areas along a 190-km reach of the Rio Grande, from Santo Domingo to Socorro.

The lowest occurrence of obsidian pebbles (Rabbit Mountain, 1.428±0.003 Ma) on either side of the west-dipping Palace-Pipeline fault zone on the north side of Hell Canyon indicates at least 38 m of offset since early Pleistocene time. East of the fault axial deposits thin to only 32 m, and the underlying transitional unit and upper Arroyo Ojito Formation are exposed. Hanging-wall blocks west of the fault exhibit two angular unconformities, with increasingly more section removed to the west. South of Hell Canyon, westernmost piedmont deposits of the Llano de Manzano truncate all but the lowest 14 m of the axial Sierra Ladrones Formation.