ESTABLISHING A TEPHROCHRONOLOGIC FRAMEWORK FOR THE MANZANITA LIMESTONE IN THE MIDDLE PERMIAN TYPE AREA AND ADJACENT PORTIONS OF THE DELAWARE BASIN, WEST TEXAS, USA

A series of highly altered layers of volcanic tephra (bentonite) occur throughout the Delaware Basin in west Texas and southeastern New Mexico. Four are present in the Manzanita Limestone Member of the Cherry Canyon Formation, though not all are recognized at every locality. Given their rapid deposition over widespread areas, these bentonites are significant because they can be used to address numerous geologic problems, including issues of litho- and biostratigraphic correlation, deficiencies in the global time scale, and uncertainty regarding tectonomagmatic reconstructions. In this study we use bentonites in establishing tephrochronologic framework with which these problems can be addressed.

To establish this framework, the trace element chemistry of individual apatite grains (~30 per sample) from several bentonites was determined using electron microprobe analysis. These bentonites were collected from Manzanita Limestone localities in and near Guadalupe Mountains National Park (GMNP) and from a suspected Manzanita locality approximately 33 km into the Delaware Basin. Two bentonites occur at one of these localities, Nipple Hill in GMNP. This is particularly significant because Nipple Hill serves as both the type locality for the Manzanita Limestone, as well as the Late Guadalupian (Capitanian) Global Boundary Stratotype Section and Point (GSSP) in the Pinery Limestone Member of the overlying Bell Canyon Formation. A second locality is a roadcut in the nearby Patterson Hills. This roadcut contains all four of the bentonites recognized in the Manzanita, and was used to assess whether the trace element chemistry of the apatite phenocrysts could be used to differentiate between multiple beds occurring in stratigraphic succession.

Results indicate that apatite grains from individual bentonites within the Manzanita have distinct trace element chemistries, allowing for correlation of beds between localities. The two bentonites from Nipple Hill have apatite trace element chemistries that match the lowest two bentonites from the Patterson Hills roadcut and are interpreted as being correlative. This interpretation is extended to the single bentonite from the suspected Manzanita locality, as its trace element chemistry matches that of the upper Nipple Hill and second lowest roadcut bentonites.