COMPOSITIONAL TRENDS AND PROVENANCE FROM EOCENE SYNOROGENIC STRATA OF THE SAN JOSE FORMATION, EAST-CENTRAL SAN JUAN BASIN, NORTHWESTERN NEW MEXICO

Although there has been extensive prior work on Jurassic-Paleocene strata in the San Juan Basin of northwest New Mexico, very little is known about the youngest, early-Eocene strata of the San Jose Formation. Presented here are new sedimentological and stratigraphic data (e.g., measured stratigraphic sections and facies analysis) as well as provenance data (e.g., U-Pb detrital zircon geochronology, sandstone modal composition, clast counts, and paleocurrent trends) from four previously identified members of the San Jose Formations. Also included for comparison are similar new data from the directly underlying early-middle-Paleocene Nacimiento Formation.

The Cuba Mesa member of the San Jose, making up the basal portion of the formation and unconformably overlying the Nacimiento, contained primarily lenticular, amalgamated, and stratified channel sand bodies, with isolated floodplain silts. The Regina member contained extensive floodplain silt and mud deposits with isolated cross-stratified channel sand bodies, not exceeding 15m in thickness. Compositional trends in samples of the Cuba Mesa and Regina members have dominant occurrences of quartz (Q), with minor Feldspars (F) and lithic fragments (L) (Q=67%, F=26%, L=7%). Detrital zircon ages from the lower 2 members of the San Jose had significant peaks at 1700 and 1450 Ma with minor peaks at 150 and 200 Ma. Mazatzal and A-type granite age signatures suggest source input from local basement cored uplifts, with minor input from Cordilleran Arc sources.

The Llaves member (overlying the Regina) contained extensive amalgamated cross-stratified channel sand bodies, ranging in coarseness from medium sand to granular, and had bodies of silty floodplain deposits. The Tapacitos member (overlying the Llaves) contained small amalgamated and cross-stratified channel sand bodies, with extensive silt-mud floodplains. Compositional trends of the Tapacitos and Llaves members have dominant occurrences of quartz, with very minor occurrences of feldspars and lithics (Q=85%, F=9%, L=6%). Detrital zircon ages from the upper two units saw peaks at 1700 and 1450 Ma, with minor age peaks at 1100 and 300-500 Ma. Llaves and Tapacitos members show continued basement cored uplift source areas with a reduction in inputs from Cordilleran Arc and an increased input of recycled Mesozoic Eolianites.