A stream-sediment geochemical map (from NURE data) showing bulk silica distribution within the Eocene San Jose Formation, San Juan Basin, New Mexico
— Richard M. Chamberlin, James S. Harris, and Margaret I. Onimole

We present a geochemical map showing the estimated silica (eSiO₂ ) content of 496 NURE streamsediment samples collected within the Eocene San Jose Formation. This computer-generated map essentially shows the bulk distribution of quartz within the formation. The formation is clearly divisible into a central lowsilica (low quartz) domain, a western intermediate-silica domain and a southeastern high-silica domain. Our follow-up stream-sediment sampling and analysis have verified the chemical patterns seen in the NURE data. Preliminary petrographic study of follow-up stream-sediment samples indicates that quartz/feldspar ratios are relatively constant within each silica domain, as defined by the NURE data. Some second-order noise within silica domains apparently reflects high concentrations of mudstone grains or heavy minerals in the NURE samples. Quartz/feldspar estimates from 44 thin sections of San Jose sandstones demonstrate a good correlation of granite-derived arkose with the low-silica domain, cherty subarkose with the intermediate-silica domain, and subfeldspathic arenite with the high-silica domain. We interpret the intermediate-silica, subarkosic strata and the high-silica, subfeldspathic strata as the products of southerly flowing early Eocene river systems that entered the basin near Aztec and Llaves, respectively; these rivers appear to have joined near Cuba. Preliminary field observations and second-order geochemical patterns suggest that the low-silica, arkosic strata represent relatively thin deposits of a younger river system which overlies the main body of San Jose strata. Additional data are needed to accurately define the geometry of low-silica arkosic strata within the San Jose Formation.