INVESTIGATIONS OF PRE-AND-INTRA-BANDELIER TUFF SEDIMENT DEPOSITS IN THE PONDEROSA AND JEMEZ SPRINGS 7.5 MINUTE QUADRANGLES, SANDOVAL COUNTY, NEW MEXICO

Fluvial gravels and alluvial debris deposits are present below and within the Bandelier and San Diego Tuff members in the southwestern Jemez Mountains. The investigations of these sediments have included compositional analyses of the various depositional layers, and their relationship to the relevant tuff members, at many outcrops throughout San Diego and Virgin canyons. Also, the imbrications of the fluvial sediments have been measured.

The imbrications generally indicate flow to the south and suggest a meandering stream system was present very near the modern system. The sediment loads contain mainly local materials of varying age from Quaternary to Precambrian and are deposited to the west of the present Jemez River system. Major compositional features include: Precambrian clasts from the Nacimiento Mountains; Paliza Canyon andesite, found mainly in the middle to northern portions of the San Diego Canyon area; basalt, rhyolite, and dacite, all of which are common lithics found within the tuff members, though this is not the only source of these clasts; and sandstones, mainly from the Permian Abo, Yeso and Glorieta formations, which often dominate the composition. Furthermore, the Abo and Yeso formations are often the bedrock bases of these gravel deposits. Tuff clasts and reworked tuff materials including pumice, ash, and obsidian are also present to varying degrees.

These fluvial, alluvial, and tuff deposits record the evolution of the Pleistocene paleogeography and paleoclimate of the southwestern Jemez Mountains. The gravels are illuminating previously shadowed aspects of the volcanic and environmental history of the area, proving to be a valuable asset in understanding a geologically complex area. Furthermore, these sediments have significant implications for the modern erosion and hydrologic systems of the region.