New Mexico Geological Society Annual Spring Meeting — Abstracts

The Taos Plateau in northern New Mexico contains multiple basaltic lava flows, some of which preserve underlying paleosols. Here we present the composition of two paleosols from the Servilleta Basalts north of Tres Piedras, New Mexico. Basalt stratigraphy suggests that these paleosols are approximately 4 million years old. The two paleosols are separated by a two-meter-thick lava flow. They contain a large amount of sand and silt-sized quartz, with considerable amounts of finer-grained matrix, mafic minerals, feldspar, and volcanic glass. The observed distinct increase in felsic mineral content in paleosols suggests that there were significant additions from outside the andesitic to basaltic Taos Plateau to the soils during pedogenesis. The presence of unweathered pumice and volcanic glass in the paleosol profiles is indicative of volcanism during pedogenesis, some of which might have been explosive. Many unweathered clasts from the paleosol B horizons display smectite argillans that suggest relatively arid pedogenic conditions. Argillans are discontinuous and <5 μm thick. Primary mafic minerals make up only about 13% of paleosol B horizon materials; where they exist, however, they are mostly to completely unweathered. Weathering of primary minerals comprises minor clay alteration along grain margins and fractures. Clasts of basalt within paleosol B horizon suggest incorporation of underlying bedrock materials into the soil profile during pedogenesis. Bioturbation is a possible explanatory mechanism. The upwards increase in quartz and upwards decrease in average clast grain size through both paleosol profiles are interpreted here as evidence of accretionary conditions during pedogenesis in a dust-rich environment.