ABSTRACT: The correlation of the Rader Limestone Member of the Bell Canyon Formation to its shelf equivalent has varied over the years. Some models have equated it with the Seven Rivers Formation, and others to the Yates Formation, or a position straddling both units. More recent workers have used sequence stratigraphic models to refine this shelf-to-basin correlation, placing the “lower†part of the Rader Limestone Member in the Y3 high frequency sequence within the Yates Formation. Despite the presence of bentonites in both the Rader Limestone Member and the Yates Formation, and the potential to add a numerical age datum to the sequence stratigraphic hierarchy, geochemical tephrochronologic techniques have not yet been applied to the deposits. To address these issues, apatite phenocryst trace element chemistry was determined by electron microprobe analysis for samples from three bentonites collected from outcrop localities in Guadalupe Mountains National Park and two from the Gulf PDB-04 research well. Results are interpreted to indicate a correlation between a bentonite 16.5m above the base of the Rader at Back Ridge in the Patterson Hills of Guadalupe Mountains National Park, and one at 1956ft core depth in the PDB-04 core. Careful examination of the gamma-ray log for the well indicates that the 1956ft interval is in the Y3 high frequency sequence of Osleger and Tinker (1999). Further refinement can be made to the third cycle set of this high frequency sequence. Determining whether the bentonite at Back Ridge is in the “lower†or “upper†part of the Rader Member will require additional work. This correlation links the primary reference section for the basal through middle Capitanian Stage being established at Back Ridge with the shelf defined sequence stratigraphic framework. The sample from the Rader is the focus of U-Pb radioisotopic dating (Nicklen et al. in preparation) and would provide the first shelf-to-basin absolute age datum for the Capitan depositional system.
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