Untangling the stratigraphy of the upper Paleozoic Supai and Naco Groups of the Mogollon Escarpment

Steve Reynolds (President's Professor at Arizona State University) presented this talk to the members and guests of the Arizona Geological Society on 3 Dec. 2024. Presentation 47-minutes, Q&A 20-minutes.
The famous red rocks of the Mogollon Escarpment, from Sedona to White River, have traditionally been assigned the name Supai, correlating these rocks with the Supai Group of the Grand Canyon. The underlying limestone-bearing units have traditionally been called the Naco Formation in reference to the Naco Group of southeastern Arizona. Several workers, however, have proposed that the Supai nomenclature is largely not appropriate for rocks of the Mogollon Escarpment, based on proposed correlations and ages of the rocks.
Due to these proposals, the stratigraphic framework of the Pennsylvanian-Permian Naco and Supai Groups of the Mogollon Escarpment have been confusing, with conflicting names, diverse proposed unit boundaries, various correlations within and between regions, and generally sparse age constraints. As a result, our team of regional geologists, biostratigraphers, and paleontologists are finishing a major program aimed at providing a data-based stratigraphic framework. We have measured dozens of stratigraphic sections, focusing on the stratigraphic unit type sections of previous workers and on strata that have the highest likelihood of providing paleontological age constraints. We have collected and identified a diverse suite of fossils, including conodonts, fossil plants, fossil pollen, and fossil trackways. In conjunction with these studies, we have obtained detrital zircon analyses from key units. Our paleontologic results, specifically identification of conodonts, are not yet complete, but we have already made major discoveries regarding the ages and correlations of units. For example, our detailed evaluation of fossil plants and pollen has demonstrated that the conglomerate-bearing interval of the lower Supai, whose importance was emphasized by Wes Peirce, is Late Pennsylvanian in age at Promontory Butte and Fossil Creek. These conglomerates are within the transition between the Supai and Naco, indicating that the Supai is in part Late Pennsylvanian. This eliminates the possibility that these rocks, including the conglomerate-bearing unit near Sedona, are correlative with the Hermit Formation, as has been proposed by some workers. Another issue is that the Naco Formation of the Mogollon Escarpment is Middle and Late Pennsylvanian in age, whereas the Naco Group of southeast Arizona ranges from Early Pennsylvanian to late Early Permian. We are considering the option that the term Naco is therefore not appropriate for rocks along the Mogollon Escarpment.
We plan to publish a comprehensive study of the stratigraphy of the Supai and the Naco using our paleontological age constraints to discriminate between different possible correlations. We propose extending the stratigraphic name Supai Group to the Mogollon Escarpment, and elevating members proposed by prior workers, such as Jackson and Winters, to formation rank, as well as proposing some new names. In this talk we will outline stratigraphic problems, summarize key results, and discuss possible strategies for resolving the stratigraphic confusion.