Imo Formation

The Imo Formation, or Imo Shale, is a geologic unit in northern Arkansas that dates to the Chesterian Series of the late Mississippian. The Imo is considered to be a member of the upper Pitkin Formation, and is the most recent Mississippian age rock in Arkansas. The Imo Shale unconformably underlies the Pennsylvanian age Hale Formation

Imo Formation Stratigraphic range: Carboniferous: Mississippian
Type	Formation
Unit of	Pitkin Formation
Underlies	Hale Formation
Location
Region	Arkansas
Country	United States
Type section
Named for	Imo, Searcy County, Arkansas
Named by	Mackenzie Gordon Jr.[1]

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 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",[2] and in 2010 was classified as a member of the Mississippian age Pitkin Formation.[3]

Paleontology

Cephalopods

Anthracoceras A. discus[4] Cravenoceras C. mapesi[4] Delepinoceras D. bressoni[4] Eumorphoceras E. imoense[4] E. richardsoni[4] Fayettevillea F. bransoni[4] F. friscoense[4]

Metadimorphoceras M. saundersi[4] Peytonoceras P. ornatum[4] Rhadinites R. miseri[4] Somoholites S. cadiconiformisj[4] Stenoglaphyrites S. involutum[4] Syngastrioceras S. imprimis[4]

Flora

• Stacheoides

S. tenuis[5]

Foraminifera

• Earlandia[5]
• Endothyra[5]
• Eosigmoilina

E. explicata[5]

E. rugosa[5]

• Monotaxinoides[5]
• Neoarchaediscus[5]
• Priscella[5]
• Trepeilopsis[5]
• Zellerina

Z. discoidea[5]

Ostracods

• Cavellina[6]
• Cornigella[6]
• Gortanella[6]
• Healdia[6]
• Healdioides[6]
• Hollinella[6]
• Monoceratina[6]
• Sargentina[6]
• Youngiella[6]

See also

• List of fossiliferous stratigraphic units in Arkansas
• Paleontology in Arkansas

References

1. Gordon, Jr., Mackenzie (1964). "Carboniferous cephalopods of Arkansas". U.S. Geological Survey Professional Paper. 460: 34–38.
2. Gordon Jr., Mackenzie; Stone, Charles G. (1973). "Correlation of Carboniferous rocks of the Ouachita geosyncline with those of the adjacent shelf (abstract)". Geological Society of America Abstracts with Programs. 5: 259.
3. Hutto, R. S.; Smart, E. (2010). "Reinstatement of Imo as a member of the upper Mississippian Pitkin Limestone after recent mapping in north-central Arkansas (abstract)". Geological Society of America Abstracts with Programs. 42 (2): 70.
4. Saunders, W. Bruce; Manger, Walter L.; Gordon Jr., Mackenzie (1977). "Upper Mississippian and Lower and Middle Pennsylvanian ammonoid biostratigraphy of northern Arkansas" (PDF). Oklahoma Geological Survey Guidebook. 18: 117–137. Retrieved 1 February 2018.
5. Brenckle, Paul (1977). "Foraminifers and other calcareous microfossils from Late Chesterian (Mississippian) strata of northern Arkansas" (PDF). Oklahoma Geological Survey Guidebook. 18: 73–87. Retrieved 3 February 2018.
6. Sohn, I. G. (1977). "Late Mississippian and Early Pennsylvanian ostracoda from northern Arkansas - a preliminary survey" (PDF). Oklahoma Geological Survey Guidebook. 18: 149–159. Retrieved 31 January 2018.

• Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Archived from the original on 31 July 2014. Retrieved 8 July 2014.