PERMIAN CHRONOSTRATIGRAPHY IN NORTH AMERICA

Derek Ager, in the Nature of the Stratigraphical Record, indicated that at certain times in Earth history there are persistent facies (e.g. Mississippian grainstone). Except for earliest Permian cyclothems, there are no persistent facies within Permian successions. This variability, combined with profound provincialism, has led to significant chronostratigraphic challenges, but a Permian System with three series (Cisuralian, Guadalupian, Lopingian) and nine stages (Asselian, Sakmarian, Artinskian, Kungurian, Roadian, Wordian, Capitanian, Wuchiapingian, Changhsingian) has been developed. The formal definitions for these stages are nearly complete. These definitions do not replace local terminology and time scales, but rather provide a standard for comparison and calibration. Regional units do not need to be redefined, unless they have been adopted in a revised form in the ICS International Geologic Time Scale. In North America, the Permian was traditionally subdivided into the Wolfcampian, Leonardian, Guadalupian and Ochoan on the basis of successions in West Texas. The occurrence of the conodont Streptognathodus isolatus in the upper Gaptank Formation indicates that the base of the Wolfcampian does not exactly coincide with the base of the Permian. Conodont biostratigraphy indicates that the Leonardian correlates with the Upper Artinskian to Kungurian. The base of the Guadalupian is redefined by the FAD of Jinogondolella nankingensis within the El Centro Member of the Cutoff Formation in Guadalupe Mountains National Park where the Guadalupian forms impressive cliffs of reef limestone, but to the north, the series is represented by thin, phosphatic, sandy chert facies in Western Canada and cool-water, sandy limestone and glauconitic sandstone in the Arctic. In China, the Maokou Formation includes a thick platformal limestone succession. Correlation among these regions requires a variety of tools, including carbon isotopes, because key conodont species are either missing, or first occurrences may be diachronous. The Ochoan, which includes varved evaporites of the Castille Formation, as well as restricted marine facies in the Rustler Formation, indicates duration of only a few hundred thousand years and as such represents the very end of the Guadalupian and a fraction of the Lopingian Series.