BACK RIDGE, PATTERSON HILLS, GUADALUPE MOUNTAINS NATIONAL PARK: A REFERENCE SECTION FOR STRATA ABOVE THE CAPITANIAN STAGE GSSP (GUADALUPIAN SERIES, MIDDLE PERMIAN)

The Guadalupian Series and its component Roadian, Wordian, and Capitanian stage GSSPs were ratified in 2001 by the International Union of Geological Sciences-International Commission on Stratigraphy to be the international standard Middle Permian chronostratigraphic units. The GSSPs are located within Guadalupe Mountains National Park (GMNP) in west Texas, and for the most part meet all ideal criteria for international reference standards. However, the basal Capitanian GSSP is the weakest of the three because it is located near the top of a hill, resulting in less than a meter of overlying section available for study.

Here we present data for an equivalent section within GMNP. Back Ridge is a fault block in the Patterson Hills, which have previously received less geologic attention than the main body of the much larger Guadalupe Mountains. A complete section is well exposed on Back Ridge from the upper Cherry Canyon Formation (beginning 15m below the South Wells Member) through the upper Bell Canyon Formation (26.5m into the Rader Member). These basinal units are overlain by the prograded Capitan Formation (the reef facies), which completes the section to the top of the mountain. The Capitanian Stage GSSP is located within the lower Pinery Member on Nipple Hill. At Back Ridge, the equivalent horizon lies between 155m (youngest Jinogondolella aserrata recovered to date) and 162.5m (oldest J. postserrata recovered to date) above the base of the section. This leaves approximately 58m of overlying section available for future stratigraphic analyses of the lower Capitanian in its type area. The section lies on a broad mountain face that provides an expansive area for future sampling and other kinds of analyses. Fusulinids, brachiopods, and other fossil taxa are well represented in the section, and ammonoids have been recovered from equivalent strata nearby. Additional conodont taxa recovered from the section include J. palmata, J. errata, J. gladirobusta, and Pseudohindeodus ramovsi. Once we have pinpointed the chronomorphocline from J. aserrata to J. postserrata, we will recommend that the Back Ridge section be designated as the primary reference section for study of the basal through middle Capitanian Stage.