Pg. 284, 286, 288-289 (fig. 1), 292-297. Oklan series (Oklan epoch). Analysis of physical characters and paleontological features of Pennsylvanian rocks in North America leads to conclusion that stratigraphic cleavage at two positions within system has paramount significance. Three series recognized: Ardian -Lower Pennsylvanian, Oklan -Middle Pennsylvanian, and Kawvian -Upper Pennsylvanian. Oklan series includes all Pennsylvanian deposits between Morrowan and Missourian rocks. Boundaries marked by disconformities which are clearly distinguished at many places but obscure in others. In some areas, Oklan formations rest disconformably or nonconformably on pre-Pennsylvanian rocks ranging in age from Mississippian to Precambrian. Divisible into two stages: Atokan (or Derryan) and Desmoinesian. Rocks of Oklan series are more widespread than those of either older or younger divisions of Pennsylvanian. Series contains several faunal zones which have world-wide distribution and which are readily identified. Time equivalent is Oklan epoch.
Name Oklan is based on abbreviation of State of Oklahoma and refers to the thick fossiliferous succession of Middle Pennsylvanian deposits in east-central and northwestern Oklahoma.

Source: US geologic names lexicon (USGS Bull. 1200, p. 2813-2814).