Lower Mississippian Lithostratigraphy, Northern Arkansas

Lower Mississippian lithostratigraphic units in northern Arkansas are (ascending order) the Bachelor, St. Joe, and Boone Formations. These formations disconformably overlie Middle Ordovician to Upper Devonian strata and are overlain disconformably by Meramecan or Chesterian strata. The Bachelor Formation is generally a thin (less than 0.3 m), persistent, orthoquartzitic sandstone with common to abundant phosphatic pebbles overlain by a green silty shale. In northwestern Arkansas, the Bachelor Formation commonly lacks sandstone. The Bachelor Formation has been confused previously with the Sylamore (Upper Devonian) and older sandstone units. Although commonly regarded as a member of the Boone Formation, the St. Joe Limestone should be raised to formation rank in accordance with the earlier proposal of Cline (1934). The St. Joe Limestone in northwestern Arkansas can be subdivided into (ascending order) the Compton, Northview, and Pierson Members which are recognized as formations in Missouri. In the type region, northcentral Arkansas, subdivision of the St. Joe is precluded by lack of the shaly Northview Member. A marked color change from gray to red from northwestern to northcentral Arkansas is accompanied by a general increase in the allochemical constituents. The St. Joe Boone boundary is taken to be at the first persistent chert. This contact generally coincides with a thin calcareous shale unit and a marked decrease in carbonate grain size