New Mexico Geological Society Annual Spring Meeting — Abstracts

For more than 70 years, the base of the Permian was recognized by a fusulinid biostratigraphic datum, the first occurrence (FO) of “Schwagerina,” which to many meant the FO of the inflated schwagerinids. In 1998, the base of the Permian was defined by the FO of the conodont Streptognathodus isolatus at the GSSP located at Aidaralash Creek in western Kazakstan approximately coeval with the FO of the fusulinid Sphaeroschwagerinafusiformis. However, it is now clear that with respect to longstanding fusulinid biostratigraphy, the FO of S. isolatus is diachronous, with the GSSP being among the youngest known FOs of the species, and that the taxonomy and evolution of S. isolatus are not agreed on. This calls into question the conodont-based definition of the base of the Permian. In North America, the base of the Permian had been identified as the base of the Wolfcampian Series since about 1939, based on the FO of Schwagerina in the North American sense. However, longstanding and widely accepted fusulinid correlations indicate that the conodont-defined Permian base at the Aidaralash GSSP is younger than the Wolfcampian base, and corresponds closely to the early-middle Wolfcampian boundary. This has created much confusion with respect to the base of the Permian in North America, particularly with regard to a vast literature developed for an oil industry understanding of the greater Permian basin. It also fueled an unnecessary debate over whether a new stage, “Bursumian,” was needed in the North American provincial chronostratigraphy to equal early Wolfcampian time, so that the system boundary would not be within a provincial “stage,” but correspond to a “stage” boundary. Instead, we need to reconsider the position of the base of the Permian and its current GSSP, and reposition the base of the Permian so as to respect longstanding usage and produce the most correlateable boundary. No conodont-based definition obviously meets these criteria, but a fusulinid-based definition does. Two choices are available--the LO of North American Schwagerina s. str. (= Thompsonites Bensh?),which defines the base of the Wolfcampian, or the LO of an inflated schwagerinid taxon, which defines the beginning of the middle Wolfcampian and is closer to the original base of the Asselian. I favor the FO of North American Schwagerina sensu stricto because it also marks a major biological event—the beginning of the explosive diversification of schwagerinids--and it retains longstanding and extensive North American usage that equates the base of the Wolfcampian with the base of the Permian. In New Mexico, the section of the Horquilla Formation at New Well Peak in the Big Hatchet Mountains of Hidalgo County provides an excellent place to undertake such redefinition because of its unparalleled fusulinid record through the Pennsylvanian-Permian boundary. A fusulinid-defined base of the Permian respects longstanding usage and produces a correlateable Permian base that is synchronous within current levels of biostratigraphic resolution.