REINSTATEMENT OF IMO AS A MEMBER OF THE UPPER MISSISSIPPIAN PITKIN LIMESTONE AFTER RECENT MAPPING IN NORTH-CENTRAL ARKANSAS

The Imo Formation was proposed by Mackenzie Gordon in 1964 for sequence of late Mississippian shale with interbedded sandstone and conglomerate above the Pitkin Limestone and below the Cane Hill Member of the Hale Formation. Gordon found that this unit contains unequivocal Mississippian-age fossils, but abandoned the name in the same publication it was proposed due to concurrent mapping by E. E. Glick. Glick included this unit in his Cane Hill Formation which spanned the Mississippian-Pennsylvanian boundary. This was due to the difficulty delineating its upper contact in the field which is typically shale on shale. The name Cane Hill Formation was officially adopted by the Geologic Names Committee of the USGS in 1964, and the name Imo Formation officially abandoned. Nonetheless, the name Imo has been and still is unofficially applied to this interval by the geologic community as seen in various publications, and the name Cane Hill Formation has subsequently dropped from use. In the years since this unit was proposed and abandoned, it has been the subject of sometimes heated debate over its viability as a lithostratigraphic unit, but very little was done to try to map it in the field on a regional basis until recently.

Geologic mapping at the 1:24,000 scale for the STATEMAP Program (2006-2009) has delineated the Imo interval on seven (7) 7.5 Minute Quadrangles in north-central Arkansas. Lithologically, the unit consists of interbedded sequences of locally fossiliferous shales, sandstones, sandy fossiliferous limestones and conglomerates. To date, the Imo has been mapped in five (5) counties totaling roughly 73 square miles of outcrop area. The authors will propose that the Imo can be distinguished from the Cane Hill on the basis of lithology on a regional scale, and that the name Imo should be formally reinstated as an upper member of the Pitkin Limestone.