Mississippian System

Type section and use of name: The Edwardsville Formation was named by Stockdale (1931, p. 220) for Edwardsville, Floyd County, Ind. The type section is along former Indiana Highway 62 a short distance northeast of the town near the center of the NE¼ sec. 1, T. 3 S., R. 5 E. Stockdale included strata between his Floyds Knob Formation below and his Harrodsburg Limestone above. The latter included the Ramp Creek as its basal member. The Floyds Knob was later designated as the basal member of the Edwardsville, and the Ramp Creek was given formational status (Nicoll and Rexroad, 1975, p. 3). The Edwardsville of present usage overlies the Spickert Knob Formation and underlies the Ramp Creek Formation. (See "Borden Group" for a discussion of obsolete terms that have been applied to this stratigraphic interval.

Description: Above the Floyds Knob Limestone Member, siltstone, sandy shale, and sandstone are the dominant lithologies along most of the Edwardsville outcrop, but limestone is present in places. The Edwardsville represents delta-platform sedimentation, and according to Stockdale (1931) is from 40 to 200 feet (12 to 61 m) thick in Indiana. It thins and disappears in the subsurface to the west and the south, where Borden rocks are undifferentiated by formation. The Riverside Sandstone of Hopkins (1896, p. 287), exposed in Fountain and Warren Counties, was included as a facies of the Edwardsville by Stockdale (1931, p. 295-297), who correlated the Riverside with the Edwardsville.

The Edwardsville is conformable with the underlying delta-slope sediments of the Spickert Knob Formation, although cessation of deposition must have occurred locally. The overlying Ramp Creek Formation, or the Muldraugh Formation along the distal margin of the Edwardsville, appears to be disconformable, because the boundary is marked by a sharp lithologic break and a bed of glauconite occurs at the top of the Edwardsville along most of its outcrop.

Correlation: The Edwardsville Formation is thought to correlate with part of the Keokuk Limestone of the Mississippi Valley because conodonts from the Floyds Knob Limestone Member at its base and from the lower part of the overlying Ramp Creek Formation compare closely with Keokuk conodonts (Nicoll and Rexroad, 1975, p. 16). Further, the famous Crawfordsville crinoid beds, consisting of fossiliferous siltstones and lesser amounts of crinoidal limestone and lying partly in the stratigraphic position of the Edwardsville in Montgomery County, Ind., are Keokuk in age according to Van Sant and Lane (1964, p. 30-33)