Abstract

Although originally proposed as a series, many workers treat Ochoan as a stage equivalent to the Late Permian. As the stratotype of a stage, Ochoan strata are well described and studied and are very accessible, so they meet two of the key requirements of a stratotype of a chronostratigraphic unit. Nevertheless, the Ochoan strata fall far short of an ideal stratotype for a Late Permian stage because they generally lack fossils, radiometric dates and geomagnetic-polarity data with which to correlate. Given that much of the Ochoan section is anhydrite/gypsum or halite, lack of these data is real and not going to be overcome by further work. Furthermore, estimates of the duration of early Ochoan evaporite deposition (Castile Formation = 200,000-300,000 years) and intra-Ochoan unconformities suggest that Ochoan strata represent only a very incomplete record of Late Permian time. The strong possibility also exists that some of the youngest Ochoan strata are Triassic in age.

We conclude that the Ochoan strata do not provide a suitable stratotype for a chronostratigraphic unit. Furthermore, use of Ochoan as a regional stage is problematic, because outside of the southern High Plains of the western United States, there are no known correlative rocks in North America. Using Ochoan as a chronostratigraphic unit therefore has little utility.

We recommend that Ochoan be dropped from the chronostratigraphic hierarchy, and we have redefined it as a lithostratigraphic unit, the Ochoa Group. The Ochoa Group consists of the Castile, Salado, Rustler and Quartermaster (=Dewey Lake) Formations. It is an evaporitic succession, as much as 1700 m thick, dominated by anhydrite and halite. The age of the Ochoa Group is Late Permian-Early Triassic?