Reference section of the Lower Permian San Andres Formation, Sierra County, New Mexico
— Karl Krainer, Spencer G. Lucas, and Richard J. Brose

In the northern San Andres Mountains, Sierra County, New Mexico, the reference section of the Permian San Andres Formation is a better exposed section than the nearby type section, also located in the San Andres Mountains. At the reference section, the San Andres Formation is ~ 130 m thick, has an eroded top and overlies a very thin Glorieta Sandstone. We divide the reference section of the San Andres Formation into a lower, hydrocarbon-bearing bedded-limestone interval, a middle thicker-bedded and massive limestone interval and an upper interval of bedded limestone with some chert. Petrographic study reveals that the most common microfacies of the San Andres Formation at the reference section is bioclastic wackestone, and that muddy textures dominate the section. Ammonoids indicate that the reference section of the San Andres Formation is of late Leonardian age. Our data, particularly detailed lithostratigraphy and petrography, do not support a previous interpretation of the reference section as a transgressive systems tract overlain by a highstand systems tract, both composed of numerous parasequences. Instead, our data suggest that limestone was deposited in a normal marine environment of dominantly low to moderate water turbulence within the photic zone with open circulation. Well washed grainstones formed under high-energy conditions are rare, and only a few shallowing-upward parasequences are developed. The reference section of the San Andres Formation is an incomplete section of the lower Rio Bonito Member of the formation. It records the initial transgression of the San Andres seaway over the Glorieta-Coconino erg, followed by the development of shallow marine platform environments across much of New Mexico. Hydrocarbons are restricted to the lowermost San Andres westerly to the New Mexico/Arizona border, but the mechanism by which they were sealed is not certain. It may have been the high-energy dispersal of clays into the Northwest Platform of the Delaware Basin during late Leonardian time.