Black River Group,
Type area and use of name in Illinois: The Pecatonica Formation (Hershey, 1894, p. 175 1897) was named for exposures along the Pecatonica River in southern Wisconsin and was classified in Illinois as the lowest formation of the Platteville Group. There it is overlain by the Plattin Subgroup of the Platteville Group (Templeton and Willman, 1963).
Description: The Pecatonica Formation as traced in Indiana from Illinois (Droste, Abdulkareem, and Patton, 1982) is the lower formation of the Black River Group and except for faulted blocks in the quarried Kentland structure (Gutschick, 1983) is known only in the subsurface in Indiana. The Pecatonica overlies the Ancell Group with minor erosional discontinuity generally and the Knox Supergroup with major unconformity where the Ancell rocks are missing because of nondeposition.
In northwestern Indiana medium- to dark-grayish-brown dolomite dominates the formation. Elsewhere in Indiana grayish-brown to dark-brownish-gray lithographic limestone and fine-grained burrow-mottled limestones and dolomites compose the Pecatonica Formation. Throughout most of Indiana a zone a few feet thick of dark argillaceous limestone or silty calcareous shale is near the base of the Pecatonica, and in northwestern Indiana this zone also contains thin sandy carbonates beds of dolomite showing fine- to medium-grained sand grains surrounded by dolomite.
The Pecatonica thickens from 30 feet (9 m) m northwestern Indiana to 130 feet (40 m) in southwestern Indiana.
Correlation: The Pecatonica Formation in Indiana is correlated with the Pecatonica Formation of Illinois and Kentucky and becomes parts of the Black River Group in Michigan and of the Black River Limestone in Ohio (Droste and Shaver, 1983; Shaver and others, 1985).