Mississippian System

Type and reference sections and history of name: The name Ramp Creek Member was given by Stockdale (1929b, p. 240) to a part of his Lower Harrodsburg Limestone. The name was later modified by Smith (1965) to "Ramp Creek Limestone Member" and assigned to the Muldraugh Formation. (See fig. 1.) Still later, this unit was elevated to formation status by Nicoll and Rexroad (1975) and assigned as the lowest unit in the Sanders Group. The type section is along a stream in the NW¼NW¼ sec. 35, T. 8 N., R. 1 W., Monroe County, Ind., but the formation is now very poorly exposed there. Three reference sections of the Ramp Creek Formation and the overlying Harrodsburg Limestone within 5 miles (8 km) of the type Ramp Creek are: (1) the road cut on Indiana 37 on the north bluff of Clear Creek about 1 mile (1.6 km) northeast of Harrodsburg, mostly in the SW¼SW¼ sec. 20, T. 7 N., R. 1 W.; (2) the cut on former Indiana 37 less than one-quarter mile (0.4 km) west of the location noted immediately above and along a north-south line through the center of the SE¼ sec. 20, T. 7 N., R. 1 W.; and (3) the road cut along the dam-access road at Monroe Lake in the NE¼SE¼ sec. 28, T. 7 N., R. 1 W. (for example, Nicoll and Rexroad, 1975).

Nicoll and Rexroad (1975, p. 5-8) also recognized that the Ramp Creek and Muldraugh Formations are parts of a single depositional unit. They retained both names, however, and limited the name Ramp Creek to sediments deposited on the topset surface of the Borden delta. (See "Borden Group.") The Ramp Creek and Muldraugh units had also been equated by Lineback (1966), who used the older name Ramp Creek Member for the unit, which he considered to be the lower part of his Ullin Limestone.

Description: The Ramp Creek Formation is dominantly a carbonate unit consisting of interbedded very fine grained dolomite and limestone but containing small amounts of siltstone and shale. Chert is common to abundant, and geodes are numerous, especially in the dolomite. The limestone is generally coarse bioclastic calcarenite and calcirudite. Although the formation is known to range from 16 to 34 feet (5 to 10 m) in thickness, it is characterized by a relatively uniform thickness of about 20 to 25 feet (6 to 8 m) and tends to be even bedded.

The Ramp Creek has been recognized on outcrop in northern Putnam County in west-central Indiana and can be traced from there to southern Harrison County on the Ohio River, where abrupt thickening and facies changes mark its passage into the Muldraugh Formation. In the subsurface it extends to the southwest from the outcrop to the margin of the Borden deltaic platform (see "Borden Group"), where it merges with the Muldraugh, which is the off-delta basin facies. The boundary between the Ramp Creek and the underlying Borden Group is marked by a sharp lithologic break and in most places by a zone of glauconite at the top of the Borden that probably represent a brief hiatus in sedimentation. The Harrodsburg Limestone conformably overlies the Ramp Creek.

Correlation: Nicoll and Rexroad (1975, p. 16, 17) correlated the uppermost part of the Ramp Creek Formation with the lowermost part of the Warsaw Shale of the Mississippi Valley on the basis of the presence of conodonts of the Taphrognathus varians-Apatognathus Assemblage Zone. They found conodonts indicative of the Gnathodus texanus-Taphrognathus Assemblage Zone in the rest of the formation and correlated it with the upper part of the Keokuk Limestone of the Mississippi Valley. As indicated above, the Ramp Creek in Illinois is the lower member of the Ullin Limestone, and geologists of the Illinois State Geological Survey have generally applied this Illinois scheme to Indiana.