New Mexico Geological Society Annual Spring Meeting — Abstracts

The combination of ⁴⁰Ar/³⁹Ar geochronology and microbeam geochemical analyses is aiding development of a chronologic framework for numerous tephra layers within Rio Grande basin-fill sequences, from northern New Mexico to EI Paso. Tephra layers are intercalated with Miocene, Pliocene, and Quaternary sediments and can be divided into three typical lithologies: 1) coarse, proximal, pyroclastic-fall deposits, 2) fine, distal, pyroclastic-fall deposits, and 3) fluvially transported concentrations of pumice. Many of the coarse and fine pyroclastic-fall deposits have been fluvially reworked. Previous work using bulk geochemical fingerprinting methods had identified the sources for some of the Quaternary fine, distal, pyroclastic-fall deposits (Izett, xxx).

Advances in ⁴⁰Ar/³⁹Ar methods have made possible direct dating of many of the coarser and some of the finer tephra deposits. Laser-fusion of single sanidine crystals as small as 0.25 mm allows identification and rejection of older contaminant grains. In ashes containing tinier sanidines, laser-fusion of small (1 mg) groups of crystals also aids in assessment of contamination. Step heating of biotite separates allows assessment of alteration. Even in samples that are not presently dateable by ⁴⁰Ar/³⁹Ar methods, electron and ion microbeam techniques allow precise geochemical tingerprinting of major and trace elements in individual glass shards as small as 20 µm.

Coarse pumice deposits intercalated with axial river facies primarily represent reworked ignimbrite and plinian pumice derived from Jemez eruptions. The coarsest and most extensive pumice deposits include catastrophic flood deposits at sporadic locations extending from Isleta to Rincon, and fluvial pumice concentrations extending as far south as EI Paso.These deposits have been dated at 1.6 Ma and geochemically correlated by microbeam techniques with the lower Bandelier ignimbrite. They apparently formed after damming of the Rio Grande by the lower Bandelier ignimbrite and subsequent dam failure. Other pumice concentrations in southern Rio Grande basin-fill sequences have ⁴⁰Ar/³⁹Ar ages of 3.1 and 1.3 Ma, probably derived, respectively, from Puye and Cerro Toledo eruptions.

Sequences of coarse pyroclastic fall deposits intercalated with basin-fill deposits have been successfully ⁴⁰Ar/³⁹Ar dated in the San Lorenzo Canyon area (14.5 Ma), in the Silver Creek area (15.6-14.6 Ma), in the Albuquerque Basin (15.6-13.6 Ma), and in the Chamita Formation in the Espanola Basin (6.95-6.75 Ma). The San Lorenzo and Silver Creek ashes were apparently produced by eruptions associated with rhyolite domes in and near the Magdelena Mountains, and the Chamita Formation ashes were produced by eruptions associated with the Bearhead Rhyolite.

⁴⁰Ar/³⁹Ar dating of fine grained (<0.25 mm) ashes has proven problematic, because many contain older contaminant feldspars, but are too fine for single-crystal analysis. In at least one case, a non-reworked ash exposed west of Socorro in Blue Canyon, it is clear that the contaminant feldspars were incorporated at the time of eruption. Successfully dated fine ashes include White Ash #2 (15.6 Ma) and White ash #4 (15.4 Ma) in the Espanola Basin, and a Quatemary ash west of Luis Lopez (1.2 Ma). Successful dating of other fine ashes will require either improved separation techniques to remove very fine K-feldspar contaminant grains or improvements in mass spectrometry to allow single-crystal dating of silt-sized grains.

Microbeam geochemical fingerprinting has been much more successful in correlating, and thereby indirectly dating, tine ashes. Ashes in the Luis Lopez and Socorro areas have been geochemically linked with lower Bandelier plinian and other Jemez eruptions. Some of the tine ashes in the Espanola basin represent distal fall facies of eruptions in Nevada and Idaho.