New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro del Medio drainage in the Jemez Mountains contains more than 10 laterally continuous alternating light-dark sediment bands. This study hypothesizes that color variations reflect changes in organic matter which in turn reflects repeated cycles of forest fire sedimentation. To test this hypothesis, we are characterizing sediment samples from dark and light bands to test for changes in the mineralogy and in the size, shape, and sorting of the grains that could correlate to changing sediment source and/or depositional environment. Foundational results indicate that grain type and size were consistent within and between the bands throughout the profile indicating an unchanging sediment source and depositional environment through time. The fine to medium sediment size indicates that the depositional environment was one of a low to moderate energy, i.e. low gradient stream. The sub-rounded to sub-angular grain shape indicates that the sediment was not far traveled from the source region. The high amount of quartz and feldspar in the samples is consistent with the sediment eroding from the Cerro del Medio rhyolite dome. The high amount of organic matter in alternating dark bands suggests that the source region was repeatedly replenished with organic material and is consistent with forest fire cycles.