Blackoak Coal Member of Kalo Formation of Cherokee Group of Des Moines Supergroup. The Blackoak Coal Member, here named, is the lower of two members of the Kalo Formation (new name) of the Cherokee Group in south-central Iowa. Unit extends over a significant portion of the study area, and, to the south and west, splits into two or more closely spaced beds. The names Manbeck and Hastie were applied to coal beds in Polk County by Landis and Van Eck (1965) and may refer at least in part to the Blackoak. Contains abundant pyrite nodules and fusain. Is the biostratigraphic equivalent of the Pope Creek Coal Member of the Abbott Formation. Overlies the Kilbourn Formation (new name); underlies silty, carbonaceous shale below the Cliffland Coal Member (new name). Thickness is highly variable, ranging from 4.5 inches at the type section to 5.7 feet. Age is Middle Pennsylvanian (late Atokan).
Type section: along an intermittent tributary of Cedar Creek in an abandoned rock quarry, in NW/4 SE/4 sec. 31, T. 75 N., R. 17 W., Mahaska Co., IA. Named from Blackoak Twp., Mahaska Co., IA. [Extends into Appanoose, Marion, Polk, Van Buren, Wapello, and Warren Cos., southern and south-central IA.]

Source: US geologic names lexicon (USGS Bull. 1565, p. 33-34); supplemental information from GNU records (USGS DDS-6; Denver GNULEX).