ABSTRACT: Despite being recognized for many years, bentonites in the Guadalupian (Middle Permian Series) type area of West Texas and southeastern New Mexico have received little research attention. These important deposits offer opportunities for long distance stratigraphic correlation and high-precision radioisotopic age dating. In this study, apatite phenocryst chemistry is used to establish a tephrochronologic framework for the Manzanita Limestone Member of the Cherry Canyon Formation. Samples were collected from bentonites in one core and five field localities including Nipple Hill, which is the site of the Late Guadalupian (Capitanian Stage) GSSP and located in Guadalupe Mountains National Park. Apatite phenocrysts from these samples were analyzed for minor, trace, and rare earth element chemistry using electron microprobe techniques. Results indicate the presence of three patterns or trends of data that are repeated at multiple localities. These groups of data are interpreted to represent coeval deposits and are correlated among several localities to form a tephrochronologic framework. This framework links the strata on Nipple Hill with that of several other Guadalupian type area localities. Absolute age time control for the Guadalupian is poor, and the precise stratigraphic position of the existing radioisotopic date from Nipple Hill is uncertain. Based on a review of published reports and a new measured section of Nipple Hill presented herein, the most accurate position is in the Manzanita Member. That placement adjusts the only radioisotopic age constraint for the Capitanian GSSP lower in the Wordian Stage than previously reported. The bentonite correlations proposed herein constrain this position to the Mz-3 cycle of the Manzanita High Frequency Sequence and allow it to be traced to several outcrop localities, and across the Delaware Basin in the subsurface. Based on the correlation of a bentonite from its locus typicus near Casey’s Last Chance Well in Culberson County, Texas, the type specimen of the biostratigraphically significant ammonoid Newellites richardsoni is confirmed to have been recovered from the basal Manzanita Member (Wordian Stage), stratigraphically underlying the radioisotopic age date. The type locality of this ammonoid has been inconsistently reported in museum records and in the literature.


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