New Mexico Geological Society Annual Spring Meeting — Abstracts

Neogene basin fill within the Rio Grande rift and Basin and Range contain distinctive light-colored beds less than 2 m thick, which are often referred to as “white beds”. These beds originate from a variety of processes, most commonly precipitation of opal from laterally flowing geothermal fluids, and volcanic ash fall, with or without digenetic alteration. Three distinct “white beds” are exposed within the Camp Rice Formation (Pliocene-lower Pleistocene) of Rincon Arroyo of south-central New Mexico. Preliminary field observations have determined these beds are laterally persistent and change in composition northward from fossiliferous silica deposits to microcrystalline carbonate and finally to massive clay.

Previously published research has suggested the “white beds” originate from siliceous sinter deposits associated with a geothermal system, or the precipitation of silica from a spring-fed cienega. However, neither of these hypotheses has been researched in detail, nor have they suggested a mechanism for the changes in composition that occur laterally within the beds.

The purpose of this study is to investigate origin of these beds using field work, petrology, and geochemistry. Presently, field observations have documented the locations of where compositional changes occur and their geographic extent. Measured sections have determined the “white beds” were deposited within a predominately fluvial setting, and have given insight into upper and lower contacts of each bed. In the near future, petrology will be used to observe the mineralogy and texture of each composition within the beds. Samples that are too fine-grained to be studied using petrography will be analyzed using x-ray diffraction, and scanning electron microscopy.