New Mexico Geological Society Annual Spring Meeting — Abstracts

The Pennsylvanian strata exposed on the northern flank of the 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. The Atrasado Formation is ~ 124 m thick and includes (in ascending order) the Veredas, Hansonburg and Keller groups of Thompson, names that should be abandoned. 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. The formations named by Thompson work as local lithostratigraphic units at the member or bed rank. Thus, we recognize (in ascending order) the Adobe Member (33.5 m), Council Springs Bed (4.8 m), Burrego Member (16.4 m), Story Member (18.3 m), Del Cuerto Member (34.8 m) and Moya Member (16.5 m) of the Atrasado Formation. 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 DesmoinesianMissourian boundary is in the upper Gray Mesa Formation, the Missourian-Virgilian boundary is ~ base of Del Cuerto Member and the Wolfcampian (Newwellian) base is ~ base of Bursum.