New Mexico Geological Society Annual Spring Meeting — Abstracts

On-going investigations continue to elucidate the upper Miocene basin fill stratigraphy of the western Espanola basin. Three lithostratigraphic units are recognized under the Los Alamos using well-data. The youngest deposit is 120 m-think and composed of tuffaceous, fine sand (40Ar-39Ar ages of 7.0-7.5 Ma). Below this fine unit lies 420-500 m of alluvial fan deposits consisting of coarse, volcaniclastic sandy gravel shed from the Jemez Mountains. Intercalated basalts have 40Ar-39Ar ages between 8.45 and 8.97 Ma. An older-fine unit, consisting of silty sand with minor volcanic pebble beds, contains basalt flows with 40Ar-39Ar ages of 10.9 to 13.1 Ma.

West of Espanola, three new members of the Chamita Formation are recognized, The Hernandez Member (400-450 m think) consists of broad channel complexes of sandy gravel (volcanic clasts with 2-27% quartzite) intercalated with floodplain deposits. Up-section there are increases in the concentration of quartzite, the relative proportion of coarse channel deposits, and clast size. The Hernandez Member (6-12 Ma) was deposited by a south-southeat flowing river draining the Tusas Mountains and Abiquiu embayment. The Hernandez Member interfingers to the east with the Cejita Member, a unit deposited by a river draining the Penasco embayment and the San Luis basin. The Vallito Member generally underlies these other two members and consists of fluvial, fine to medium said and minor eolian deposits. Sparse pebbles consist of volcanic clasts. The Vallito Member may correlate with aprt of the older-fine unit under Los Alamos because of its overall texture, clast composition, and interpreted age (~9.5-13 Ma).