New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Pennsylvanian and Lower Permian rocks in the Dona Ana Mountains, consist of (in ascending order) the Panther Seep (which may cross the Pennsylvanian -Permian boundary), the Bursum and the Hueco Formations. Deposition of these rocks was on the inactive western margin of the Orogrande Basin and represent a change from a low -energy, intertidal environment to a more open marine environment.

Approximately 120 m of section and 49 thin sections were analyzed for this study. Thin sections corresponding to the Panther Seep and Bursum Formations are predominately mudstone and wackestones with a low diversity of fauna, mainly ostracods with few peloids, intraclasts, ooids, and fragments of echinoderms and mollusks (gastropods?) . Some of the mudstone/wackestones contain minor quartz sand which may
indicate a near shoreline setting. Also present are packstone/grainstones with greater amounts of terrigenous clastics (quartz) and more intraclasts with few if any fauna. Laminated wackestones may indicate algal mat growths in an intertidal to shallow restricted setting.

The base of the Hueco Formation is marked by a phylloid algae wackestone bank. It also contains ostracods, bryozoa, and Tuberitina with few Tetrataxis and Tubiphytes.Most of the thin sections are bioclastic wakestones containing ostracods, sponge spicules, phylloid algae, various types of forams, mollusks, gastropods, minor dasyclad algae, and few intraclasts. Few mudstones and some packstone/grainstones indicate a possible more open marine setting.

This section is interpreted as having been deposited in a low -energy, intertidal to lagoonal setting as evidenced by the abundant mudstone/wackestones with low fauna diversity, and the laminated algal mudstones. It was interrupted by periodic currents as seen in the terrigenous clastic influx in the packstone/grainstones of the Panther Seep and Bursum Formations. Faunal diversity in the lower Hueco represents a more open marine setting, which changed to a restricted lagoonal to shoreline setting as evidenced by a pebble/granular conglomerate bed and presence of petrified logs in the field. Overall small transgression- regression cycles are represented by interbedded shale and limestones and lithology variations in the section.