ABSTRACT: Microfacies Analysis of the Leonardian-Guadalupian Lower Member of the San Andres Formation in the Southern Sacramento Mountains, Otero County, New Mexico
The Rio Bonito Member of the San Andres Formation records a transgression of the northwestern shelf during the late Leonardian. Late Leonardian to Guadalupian marine carbonates exposed in the Sacramento Mountains relate a marked change from equatorial tidal flat rocks of the middle Leonardian Yeso Formation. These rocks were deposited during a worldwide sea level lowstand. The Yeso-San Andres contact, previously thought to be a gradational boundary, is here interpreted as a flooding surface resulting from the eustatic sea level rise after the lowstand. Inundation of the northwestern shelf led to deposition of the thick Andres Formation marine limestone sequence within a shallow-lagoon or shelf setting. Depositional environments are predominantly subtidal and intertidal. Microfacies in lude packstones of comminuted bioclasts of normal saline affinities redistributed by light currents. These shoal upward at times to Dasycladacean algal grainstones interpreted as tidal bars prograding across the lagoon or shelf. More restricted wackestones and laminated mudstones occur at the base of the section and indicate a transition from tidal flat to submerged shelf.
Aggradation of sediment into the intertidal zone may have occurred cyclically during San Andres deposition. One such cycle is present over the interval exposed in the Sacramento Mountains. Intertidal rocks resemble tidal flat deposits of dolostone, carbonate mudstone, and a thin tongue of quartzarenite interpreted to be Glorieta Sandstone. These rocks were previously attributed to interginguing of the Yeso with the San Andres.
AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)