Establishing a Tephrochronologic Framework for the Middle Permian (Guadalupian) Type Area and Adjacent Portions of the Delaware Basin and Northwestern Shelf, West Texas and Southeastern New Mexico, USA

Despite being recognized for many years, bentonites in the Middle Permian (Guadalupian Series) type area of west Texas and southeastern New Mexico, have received little research attention. As these important deposits act as geologic timelines, they can be used as tools for long distance stratigraphic correlation and high-precision radioisotopic age dating. An important problem that these bentonites can address is the lack of temporal control for the Guadalupian. This is an important time interval for significant changes in Earth's climate and biodiversity that include events leading up to, and potentially including, the first pulse of a double-phase mass extinction at the end of the Paleozoic. Also needed is better temporal constraint for key Guadalupian global chemostratigraphic and geomagnetic markers. In light of this, the duration of the Guadalupian stages and boundary age estimations need to be updated in order to assess cause and effect of these significant events. Refinement of the regional stratigraphic framework of units that comprise the Guadalupian type area is also necessary. Ambiguities are present regarding the shelf-to-basin relationships of units within the Capitan depositional system. Additionally, correlation of key sections of biostratigraphic and chronostratigraphic significance is needed. To address these problems, a tephrochronologic framework was established using apatite-based geochemical bentonite correlations in conjunction with high-precision zircon U-Pb radioisotopic dating. Electron microprobe analyses of apatite phenocrysts indicate that a geochemical fingerprint may be established for Guadalupian type area bentonties, allowing correlation of samples between localities. These correlations have linked several sections to the location of the GSSP for the base of the Capitanian at Nipple Hill in Guadalupe Mountains National Park. This fingerprinting has also led to a new shelf-to-basin timeline based on the interpreted correlation of a sample from the shelfal Yates Formation to the basinal Rader Limestone Member of the Bell Canyon Formation, providing biostratigraphic control for the bentonite within the Rader by extension. The calculation of new U-Pb ID-TIMS dates in the Guadalupian type area indicate the need for changes to the geologic time scale and the temporal relationships of global events. These new data suggest that current estimates of the Wordian-Capitanian boundary and probably the Roadian-Wordian boundary are too old. This extends the duration of the Wordian to c. 3 myr and decreases the duration of the Capitanian to c. 4 myr. Results here also provide age estimates for the Illawarra geomagnetic reversal (c. 266.5 Ma) and the onset of the Kamura cooling event (c. 262.5 Ma). The latter provides constraint on the proposed within Guadalupian (Capitanian) mass extinction, placing it temporally closer to the main extinction pulse at the Permo-Triassic boundary than previously understood