VOLCANIC DEBRIS FROM THE JEMEZ MOUNTAINS CONSTRAIN THE AGE, SEDIMENT-SOURCE HISTORY, AND DEFORMATION OF FLUVIAL FACIES OF THE ANCESTRAL RIO GRANDE (UPPER SANTA FE GROUP), CENTRAL ALBUQUERQUE BASIN, NEW MEXICO

The early Pleistocene Rio Grande followed the topographically lowest path in half-grabens adjacent to the active Hubbell Spring fault zone and faults farther west on Isleta Reservation. Adjacent SE-tilted blocks expose as many as 40 m of fluvial deposits with one or more angular unconformities and somewhat different stratigraphy, so units of volcanic ash, fluvially reworked ash, crossbedded pumice, clasts of rhyolitic tuff, and obsidian were sampled to determine whether similar sequences could be correlated between blocks, or whether the same volcanic materials were recycled at multiple levels on all blocks. Microprobe analyses of glass and 40Ar/39Ar ages helped differentiate between the similar-looking pumice and ash samples. Ages range from 1.71± 0.04 Ma to 1.21± 0.03 Ma. Geochemically, all the glass samples appear to be derived from a single magmatic source, but some are distinguishable by minor differences in major and minor elements, and the chemical variability within a sample (particularly Fe). Five geochemical groups of pumices were distinguished and could be correlated to samples from other areas along a 190-km reach of the Rio Grande, from Santo Domingo subbasin to Socorro constriction. Whereas some pumice compositions appear to have a limited stratigraphic range (<10 m), others recur at many levels, implying a long-term sediment source upstream. The lowest exposed pumice on two fault blocks west of the Palace-Pipeline fault zone (PPFZ) yielded an age of 1.64±0.04. Rabbit Mountain obsidian first occurs several meters above the first pumice at a level 25 m below a widespread 1-m thick reworked ash dated 1.55 ±0.08 Ma. The most impressive clasts near the top of the section are >3-m boulders of upper Bandelier Tuff (1.22 Ma).

The lowest occurrence of obsidian on either side of the PPFZ on the north side of Hell Canyon indicates at least 38 m of offset since early Pleistocene time. Surface offset is about 15 m. Hanging-wall blocks west of the fault exhibit two angular unconformities, with increasingly more section removed to the west and north. Surface offset on the east-dipping McCormick Ranch fault is about 15 m as well, but the base of the obsidian-bearing unit is beneath current exposures in the graben between this fault and the Hubbell Spring fault.