Stanley Shale

The Stanley Shale, or Stanley Group, is a Mississippian stratigraphic unit in the Ouachita Mountains of Arkansas and Oklahoma. First described in Arkansas in 1892,^[4] this unit was not named until 1902 by J.A. Taff in his study of the Ouachita Mountains of Oklahoma.^[3] Taff assigned the town of Stanley in Pushmataha County, Oklahoma as the type locality, but did not designate a stratotype. After introduction into Arkansas in 1909 by Albert Homer Purdue,^[5] the unit was redefined in 1918, when the formation known as the Fork Mountain Slate was abandoned and partially combined into the Stanley Shale.^[6] As of 2017, a reference section for the Stanley Shale has yet to be designated.

The Stanley Shale is recognized as a geologic formation with two sub-units in Arkansas, the Hatton Tuff Lentil and the Hot Springs Sandstone Member,^[1] however, several others have been proposed. These include the Chickasaw Creek Member,^[7] Chickasaw Creek Tuff,^[8] Gap Ridge Sandstone Member,^[9] Moyers Member,^[7] Parker Hill Sandstone Member,^[9] Polk County Ash Bed,^[10] and Tenmile Creek Member.^[7] In 1963, it was first proposed to raise the rank of the Stanley Shale Formation to Group status in Arkansas,^[11] with proposed sub-units consisting of the Chickasaw Creek Formation, Moyers Formation (with the Gap Ridge Sandstone and Parker Hill Sandstone Members), and Ten Mile Creek Formation.^[12] However, the proposal was not accepted and the unit remained as a formation.

In Oklahoma, this unit is recognized as a group called the Stanley Group composed of three formations: the Chickasaw Creek Shale Formation, the Moyers Formation, and the Tenmile Creek Formation. The Hatton Tuff Lentil, recognized in Arkansas, is also in Oklahoma as a unit of the Tenmile Creek Formation. Several informal members have been noted including the Smithville chert lentil, the Faith chert member, and the Chickasaw Creek tuff among others ^[2]

Mining in the Stanley Shale is limited to cinnabar, barite, and quartz. Cinnabar is primarily used for mercury, however, production in Arkansas essentially ended by the mid-20th century. Barite mining, mostly useful for oil and gas drilling fluid, is ongoing.^[13]