Foraminiferal characterization of the Morrowan-Atokan (lower Middle Pennsylvanian) boundary
Designation of a lower Atokan (lower Middle Pennsylvanian) boundary stratotype is necessary because the traditional type Atokan Series is inadequate as a chronostratigraphic standard of reference. Foraminiferally, the boundary between the Atokan and the underlying Morrowan Series (Lower Pennsylvanian) is best defined by the appearances of the primitive fusulinaceans Eoschubertella and Pseudostaffella . Recrystallized staffellids ( Staffella and Nankinella ) appear at or near this level at some localities. The appearance of the fusiform fusulinid Profusulinella is considerably higher, and its range defines an informal middle Atokan interval. Upper Atokan rocks are characterized by the range of Fusulinella below the appearance of Beedeina and/or Wedekindellina . Foraminiferal successions across the Morrowan-Atokan boundary have been documented at many localities in the south-central and western United States and in the Arctic. The merits of these sites as potential boundary stratotypes are evaluated herein. The Arctic is unsuitable as a reference area for the Atokan Series because taxon ranges there are seemingly atypical of ranges elsewhere in North America. Of the remaining sites where successions have been reported, those in southern New Mexico (type Derryan area), southeastern Arizona (Pedregosa Basin), southern Nevada (Las Vegas area), and east-central Idaho (Lost River and Lemhi Ranges) exhibit the best foraminiferal faunas in apparently continuously deposited strata.