Ordovician System
Type locality and use of name: The name Curdsville Bed (and Curdsville Substage) was given originally by Miller (1905) to 30 feet (9 m) of fossiliferous and partly cherty rocks exposed at Curdsville in Mercer County, Ky. There it was considered to be the lowermost unit of what is now called the Lexington Limestone. After a period of use at different ranks in Kentucky. It was assigned member status by Black, Cressman, and MacQuown (1965). Their name, the Curdsville Limestone Member (Lexington Limestone), is here adopted with slight modification and assigned to corresponding rocks in Indiana. An Indiana reference section is designated as the pertinent rocks cored in Indiana Geological Survey drill hole 133 on the Robbins farm in see. 1, T. 2 N., R. 1 W., Switzerland County. In this core the Curdsville is 15 feet (4.6 m) thick.
Description, distribution, and correlation: The Curdsville Member of the Lexington Limestone is the lower of two named subdivisions of the Lexington Limestone in Indiana. (See the article in this volume on the Point Pleasant Member.) It is generally a tannish-gray fossiliferous limestone. The Curdsville is present everywhere the Lexington is present except in Floyd and Harrison Counties. It is zero foot thick in Floyd and Harrison Counties and thickens northeastward to a maximum of 20 feet (6 m) in northeastern Dearborn County. The Curdsville thins and becomes absent through facies change with overlying rocks of the Lexington.
The Curdsville Member is a distinctive lithologic unit as recognized in both cores and samples. It represents a sharp lithologic break from the micritic limestones of the underlying Plattin Formation of the Black River Group, but it apparently has a conformable contact with the Plattin. It is overlain conformably everywhere by an argillaceous limestone within the Lexington.
Although the lowermost few feet of the Trenton Limestone in southern Indiana resembles the Curdsville Member in color and composition, that part of the Trenton does not everywhere stand in sharp contrast to the overlying Trenton rocks. For this reason, the Curdsville is given member status only in the Lexington in Indiana. The Curdsville Member correlates with rocks known by the same name and mapped by Cressman (1973) in north-central Kentucky and, as is implicit from the discussion above, with other rocks of the Lexington that are not named as members.