Although volumetrically insignificant relative to the dominant siliciclastics of continental rift basins, strata of chemical origin can provide important information. Mack et al. explore the origin of a syndepositional geothermal opal deposit in Pliocene axial-fluvial facies of the southern Rio Grande rift, New Mexico, using field and geochemical observations. The data suggest that the opal and calcite precipitated from high-temperature fluid of either a deep crustal or an advecting bedrock source. These results provide a conceptual model for recognition of similar deposits in ancient continental rifts, and insights into their hydrologic and thermal evolution.
Mixed geothermal and shallow meteoric origin of opal and calcite beds in Pliocene–lower Pleistocene axial–fluvial strata, southern Rio Grande Rift, Rincon Hills, New Mexico, U.S.A. by Greg H. Mack, Matthew C. Jones, Neil J. Tabor, Frank C. Ramos, Sean R. Scott, and James C. Witcher