A stream-sediment geochemical map (from NURE data) showing bulk silica distribution within the Eocene San Jose Formation, San Juan Basin, New Mexico

We present a geochemical map showing the estimated silica (eSiO,) content of 496 NURE streamsediment samples collected within the Eocene San Jose Fonnation. This computer-generated map essentially shows the bulk distribution of quartz within the fonnation. The formation is clearly divisible into a central lowsilica (low quartz) domain, a western intennediate-s ilica domain and a ~outheastern high-sil ica domain. Our follow-up stream-sediment sampl ing and analys is have verified the chemical patterns seen in the NURE data. Preliminary petrographic study of follow-up stream-sedi ment samples inuicates that quartz/fe ldspar ratios are relative ly constant within each silica domain, as defined by the NURE data. Some second-order noise within silica domains apparently retlects high concentrations of mudstone grains or heavy minerals in the NURE samples . Quart z/feldspar est imates from 44 thin sections of San Jose sandstones demonstrate a good correlat ion 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-,ilica . subarkosic strata and the high-silica, subfeldspathic strata as the products of southerly flowi ng early E.ocene 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, arkos ic strata represent relatively thi n deposits of a younger river system which overlies the main body of San Jose strata. Additional data are needed to accurate ly define the geometry of low-si lica arkosic strata within the San Jose Formation.