New Mexico Geological Society Annual Spring Meeting — Abstracts

Ninety-seven meters of the basal, massive- to medium-bedded, Rio Bonito Member of the San Andres Formation were sampled at Orendorf Peak, sec. 29, T.19 S., R.12 E., at an interval of 3 meters. The upper contact is the erosional surface and the lower gradational contact with sandstones and evaporites of the underlying Yeso Formation is poorly defined. Forty-two thin sections were studied to determine microfacies using J.L. Wilson's (1975) scheme. Deposition occurred on a shallow equatorial shelf. A tidal flat facies dominates the lower third of the section as indicated by rip-up clasts, algal mats, horizontal lamanat, and a sparse fauna. A thin sheet (2 meters) of quartz arenite, similar to the Glorieta Sandstone, stands out as a resistant bed within the easily weathered intertidal facies. Midway through the section, mudstones and wackestones containing ostracods, pelecypods, gastropods, and foraminifera represent slighly restricted marine conditions on the shelf as Permian seas transgressed. Massive cliff forming limestone in the upper third of the section is normal marine packstone and wackestone. Characteristic of the normal marine facies, abraded fragments of echinoderms, pseudopunctate, brachiopods, bryozoans, and trilobites were apparently deposited in swales below wavebase, but proximal to a surf zone. Thin dasycladacean algae/gastropod grainstones cap the normal marine facies, representing shoals which developed on the edge of the prograding carbonate platform. The progression upwards from tidal flat to normal marine facies may be just one cycle which was repeated many times through San Andres time.