Abstract

Exposed in upper Pliocene fluvial strata in the eastern Rincon Hills and adjacent Jornada del Muerto Basin, southern Rio Grande rift, New Mexico, USA, are five laterally continuous beds 0.2 to 1.0 m thick of opal-CT, which straddle the East Rincon Hills normal fault. Four of these opal beds change basinward over ~ 1 km or less to white, friable, lithofeldspathic sand with opal glaebules, and then to microcrystalline calcite. The opal and calcite generally replaced fine sand at shallow depths, although two beds locally precipitated from standing water. ⁸⁷Sr/⁸⁶Sr values (0.71301 to 0.71724) of the opal beds are consistent with precipitation from an advecting, high-temperature geothermal fluid that flowed through bedrock and/or from a high-temperature geothermal fluid derived from a deep-crustal source. Carbon and oxygen isotopic values (δ¹³C_PDB = −7.1 to −0.6; δ¹⁸O_PDB = −9.1 to −5.6) of the calcite beds lie between and overlap those of Paleozoic carbonate and Pliocene paleosol calcite, suggesting precipitation from a mixture of a high-temperature geothermal fluid that interacted with Paleozoic carbonates and shallow, meteoric groundwater, for which paleosol calcite is a proxy. The geothermal fluids are interpreted to have upwelled along the East Rincon Hills fault, flowed laterally at or just below the surface, and precipitated opal in response to rapid cooling and evaporation. Downflow precipitation of opal glaebules occurred as a result of decreasing silica concentrations and perhaps by mixing with shallow, meteoric groundwater, from which calcite precipitated, probably by CO₂ degassing.