Pennsylvanian stratigraphy in the northern Oscura Mountains, Socorro County, New Mexico
Spencer G. Lucas and Karl Krainer
The Pennsylvanian strata exposed in the northern Oscura Mountains are a classic section because M. L. Thompson used it as a key reference section in his 1942 monographic synthesis of the Pennsylvanian in New Mexico. This section is ~ 334 m thick, and we assign it to the Sandia, Gray Mesa, Atrasado and Bursum formations. The Sandia Formation is thin (~ 13
m), overlies Precambrian granite and is mostly shale and sandy limestone. The lowest cherty limestone ledge marks the base of the overlying Gray Mesa Formation, which is ~ 162 m thick. Most of the Gray Mesa is limestone (63% of the section), the remainder is covered slopes (shale?). Most of the limestones are cherty, and numerous fusulinid packstones are present. In the northern Oscura Mountains, we are not able to subdivide the Gray Mesa Formation into members, and we abandon Thompson’s Coane Formation as an unrecognizable unit that is part of the upper Gray Mesa Formation.
The Atrasado Formation is ~ 121 m thick and includes (in ascending order) the type sections of the Veredas, Hansonburg and Keller groups of Thompson, names that should be abandoned as chronostratigraphic, not lithostratigraphic units. The facies ranges from siliciclastic sandstone and fine-grained conglomerate to bedded fossiliferous limestone of an open marine shelf to massive algal limestone of a mound facies formed just below wave base in quiet water in the photic zone. We recognize six formations named by Thompson as members of the Atrasado Formation (in ascending order), the Adobe, Council Spring, Burrego, Story, Del Cuerto and Moya members. The Bursum Formation overlies the Atrasado Formation and is 35 m of marine limestone and red-bed clastics. Abo Formation red-bed siliciclastics overlie the Bursum Formation and are of presumed earliest Permian age.
Fusulinids indicate that the Desmoinesian-Missourian boundary is in the upper Gray Mesa Formation, the Missourian-Virgilian boundary is ~ the base of Del Cuerto Member and the Wolfcampian (Newwellian) base is in the Bursum Formation. The Pennsylvanian succession in the northern Oscura Mountains is a relatively thin section largely because the Atrasado Formation is unusually thin. The Atrasado Formation represents most of Missourian-Virgilian time but encompasses no more than eight transgressive-regressive cycles of deposition. Clearly, Atrasado deposition in the northern Oscura Mountains was heavily influenced by tectonic activity in the ancestral Rocky Mountain foreland, particularly of the nearby Pedernal uplift.
- Lucas, Spencer G.; Krainer, Karl, 2009, Pennsylvanian stratigraphy in the northern Oscura Mountains, Socorro County, New Mexico, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 153-166.