Mississippian System

Type locality: The Grove Church Shale was named by Swann (1963) for Cedar Grove Church, Johnson County, Ill. Sixteen feet (4.9 m) of shale and interbedded limestone are exposed in a road cut and in gullies about 1 mile (1.6 km) east of Lick Creek, Ill. No other surface exposures have been reported. Before designation as a separate formation, the Grove Church rocks were included in the subjacent Kinkaid Limestone. The subsurface reference section in Indiana for the Grove Church is the interval from 1,725 to 1,764 feet in the R. K. Petroleum Corp. No. 1 E. Bandy et at. well, NE¼NW¼NW¼ sec. 10, T. 6 S., R. 14 W., Posey County.

Description: The Grove Church Shale in Indiana is the uppermost formation of the Buffalo Wallow Group, is the youngest known Mississippian unit, and is recognized only in the subsurface of Posey County. The formation is predominantly drab fossiliferous shale in various shades of medium and dark gray and green: brownish and grayish maroon are subordinate colors. The shale is thin bedded to platy and quite soft and appears to be free of siliceous material. Several thin beds of limestone as thick as 2 feet (0.6 m) are inter bedded in the shale. The limestones are moderately fossiliferous light- to medium- brown and gray mudstones and wackestones packstones are present in subordinate amounts. Maximum known thickness of the Grove Church in Indiana is 50 feet (15 m). The formation conformably overlies the Kinkaid Limestone and may or may not conformably underlie rocks of Pennsylvanian age,

Correlation: The Grove Church Shale is known by the same name in Illinois. No biostratigraphic data have been reported from the Grove Church in the subsurface of Indiana, but the probable age relations of the Grove Church may be inferred from the data obtained in studies of surface exposures in Illinois. Fusulinids of the genus Millerella that are found in the Grove Church have affinities with Pennsylvanian forms (Cooper, 1947). The Grove Church in Illinois contains conodont of the Adetognathus unicornis Assemblage Zone, which are distinct from the fauna of the underlying Kinkaid Limestone (Rexroad and Burton, 1961). According to Rexroad and Merrill (1985) uninterrupted deposition in marine environments locally prevailed across the Mississippian-Pennsylvanian boundary in the Illinois Basin, that is, across the Grove Church-Caseyville formational boundary.