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

Richard M. Chamberlin, James S. Harris and Margaret I. Onimole,

New Mexico Geological Society Guidebook 43, p.313-320, 1992

Abstract - We present a geochemical map showing the estimated silica (eSiO₂) content of 496 NURE stream-sediment 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 low-silica (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 the 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 the 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.