High Resolution Biostratigraphy of the El Centro Member (Cutoff Formation), in South Shumard Canyon, Guadalupe Mountains National Park, West Texas: Potential Reference Section for the Cisuralian-Guadalupian Transition

The basal Guadalupian Global Stratotype Section and Point (GSSP) is located in Stratotype Canyon along the western escarpment in the Guadalupe Mountains National Park, and is based upon the conodont biostratigraphy of the Cutoff Formation, particularly the First Appearance Datum (FAD) of Jinogondolella nankingensis from Mesogondolella lamberti (Glenister et al, 1992; Lambert et al., 2000). Concerns have been raised about the GSSP in Stratotype Canyon, due to the poor abundance of other marine fauna in the section. The purpose of this research is to enhance the biostratigraphic resolution around the basal Guadalupian GSSP located in Stratotype Canyon by proposing a reference section to the GSSP location along South Shumard Canyon. Bulk rock samples were collected from three new measured sections taken from outcrops of the El Centro Member. Analysis of samples acquired from the El Centro Member show that the transitional morphocline of M. lamberti to J. nankingensis, which defines the basal Guadalupian GSSP, is exhibited in the El Centro member exposed in South Shumard Canyon. A significant radiolarian population comprising seven genera was also recovered. The important radiolarian index fossil Pseudoalbaillella globosa, as noted by Zhang et al. (2017), was identified throughout the El Centro Member in South Shumard Canyon. The second measured section, in which the majority of the microfossils were found, will be proposed as the primary reference section for the basal Guadalupian GSSP in Stratotype Canyon.