New Mexico Geological Society Annual Spring Meeting — Abstracts

Reduction features are commonly observed in redbeds, including the Abo Formation, yet are still not fully understood. In particular, the exactnature of the mineralogical changes accompanying reduction are poorly understood. By using the electron microprobe to gain back-scattered electron images and X-ray maps of elements, ours will be one of only a few studies to employ modem analytical techniques to study reduction. Fifteen thin sections were made of samples collected from the Abo type section near Mountainair, NM as well as from the Quebradas area east of Socorro, NM. The samples include a variety of reduction features and grain sizes. Reduction features included in the samples are reduction spheres, alternating layers of reduced and non-reduced rock, and shrinkage cracks filled with reduced sediment. The lithologies of the samples collected varies from mudstones to conglomerates. Preliminary studies of thin sections show a significant
difference in the hematite content between the white portions and red portions of the samples. Hematite is present as grain coatings on detrital grains in red portions, but is completely absent in the white portions. In some instances the amount of detrital opaques is equal between red and white portions. This contradicts observations from most . previous studies, and suggests that the source of iron for the hematite grain coatings in red portions. of the Abo Formation is not from the dissolution of Fe bearing opaques. Porosity and grain size are both greater in the white layers compared to adjacent red layers, yet in reduction spheres porosity and grain size are unchanged across the boundaries of the reduction sphere.