The Imo Formation, or Imo Shale, is a geologic unit in northern Arkansas that dates to the Chesterian Series of the late

Mississippian Mississippian may refer to: * Mississippian (geology), a subperiod of the Carboniferous period in the geologic timescale, roughly 360 to 325 million years ago *Mississippian culture, a culture of Native American mound-builders from 900 to 1500 AD ...

. The Imo is considered to be a

member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

of the upper

Pitkin Formation The Pitkin Formation, or Pitkin Limestone, is a fossiliferous geologic formation in northern Arkansas that dates to the Chesterian Series of the late Mississippian. This formation was first named the "Archimedes Limestone" by David Dale Owen in ...

, and is the most recent Mississippian age rock in Arkansas. The Imo Shale unconformably underlies the Pennsylvanian age

Hale Formation The Hale Formation is a Formation (geology), geologic formation in northern Arkansas that dates to the Pennsylvanian (geology)#Subdivisions, Morrowan Series of the early Pennsylvanian (geology), Pennsylvanian.Various Contributors to the Paleobiolo ...

The stratigraphic placement of this interval has long been debated. It was introduced in 1964 as the "Imo Formation," representing an interval of shale that was presumably of Mississippian and possibly of

Pennsylvanian Pennsylvanian may refer to: * A person or thing from Pennsylvania * Pennsylvanian (geology) The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, in the International Commission on Stratigraphy, ICS geologic timesca ...

age. However, in a footnote in the first publication to use this designation, the author noted that the shale interval had been mapped into the Pennsylvanian Cane Hill Member of the Hale Formation and revoked use of the name "Imo Formation". The name was reintroduced in 1973 as the "Imo Shale", and in 2010 was classified as a

of the Mississippian age

Paleontology

Cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s

* ''Anthracoceras'' : ''A. discus'' * ''Cravenoceras'' : ''C. mapesi'' * ''Delepinoceras'' : ''D. bressoni'' * ''Eumorphoceras'' : ''E. imoense'' : ''E. richardsoni'' * ''Fayettevillea'' : ''F. bransoni'' : ''F. friscoense'' * ''Metadimorphoceras'' : ''M. saundersi'' * ''Peytonoceras'' : ''P. ornatum'' * ''Rhadinites'' : ''R. miseri'' * ''Somoholites'' : ''S. cadiconiformisj'' * ''Stenoglaphyrites'' : ''S. involutum'' * ''Syngastrioceras'' : ''S. imprimis''

Flora

*''Stacheoides'' :''S. tenuis''

Foraminifera

*''Earlandia'' *''Endothyra'' *''Eosigmoilina'' :''E. explicata'' :''E. rugosa'' *''Monotaxinoides'' *''Neoarchaediscus'' *''Priscella'' *''Trepeilopsis'' *''Zellerina (foram), Zellerina'' :''Z. discoidea''

Ostracods

* ''Cavellina'' * ''Cornigella'' * ''Gortanella'' * ''Healdia'' * ''Healdioides'' * ''Hollinella'' * ''Monoceratina'' * ''Sargentina'' * ''Youngiella''

References

* Carboniferous Arkansas Carboniferous southern paleotropical deposits {{Carboniferous-stub