Rockford Limestone
  

(CBR)

Kinderhookian and Valmeyeran Series,

Mississippian System


Type section and synonyms: The first written reference to the "Goniatite limestone of Rockford" was by Owen and Norwood (1847, p. 5). In time the name Rockford became associated with the goniatite-bearing limestone (Goniatite Limestone in several older reports), and the fossiliferous exposure in the bed of the East Fork White River at Rockford, Jackson County, Ind., became accepted as the type section. This exposure is in the SW¼SE¼ sec. 6, T. 6 N., R. 6 E. Meek and Worthen (1861, p. 167) are generally credited with formalizing the name Rockford Limestone by having referred to the "Rockford Goniatite bed."

Description: Limestone dominates the lithology of the formation, but shale, siltstone, and

dolomite are also present. The limestone is typically gray, fine grained, argillaceous, ferruginous, and sparingly fossiliferous. It has a characteristic green mottling, weathers to a rusty brown, and is argillaceous or dolomitic in places, particularly in the upper part to the north. Echinodermal debris and other fossils, such as the cephalopods, are concentrated in places. To the north thin gray-green shales are interlaminated with the limestones. In several exposures in southern lndiana an upper unit of bluish-gray to yellowish-brown shale containing calcareous nodules is present.

The Rockford is exposed in a belt extending northward from the southernmost exposure in New Albany, Floyd County, to a point a short distance north of Rockford. From this point the Rockford outcrop forms a northwestward-trending arc to the Indiana-Illinois state line in central Newton County, northwestern Indiana. Exposures are not found along most of the northern part of the belt because of glacial drift but are present in places in Benton and Jasper Counties. The Rockford is present in the subsurface of much of the state west and south of the outcrop. As shown by Lineback (1970, fig. 15) and Bassett and Hasenmueller (1980), however, the formation is absent from several places, apparently by nondeposition. It is commonly 2 or 3 feet (0.6 or 0.9 m) thick along the southern part of the outcrop belt, but it is thicker in the subsurface and to the north. The greatest recorded thickness is about 22 feet (6.7 m) (Melhorn, 1958, p. 196).

The Rockford Limestone overlies the New Albany Shale with apparent conformity, although the lithologic change is abrupt. It is in turn overlain by the New Providence Shale with apparent conformity in southern Indiana, and in places near the Ohio River the Rockford has been removed by pre-New Providence erosion. Except near the Ohio River at the southern margin of the Rockford, physical evidence of the unconformity is limited, and so in part recognition of the unconformity rests on faunal evidence.

Correlation: Conodont studies of the Rockford Limestone (Rexroad and Scott, 1964) show that it consists of strata of both the Kinderhookian and Valmeyeran Series. The Kinderhookian part is in the Siphonodella isosticha-S. cooperi Assemblage Zone and correlates with the upper part of the Chouteau Limestone of the Mississippi Valley. The Valmeyeran part, which is present from Scott County northward, is in the Gnathodus semiglaber-Pseudopolygnathus multistriatus Assemblage Zone and correlates with the Meppen Formation of Illinois and with its lateral equivalents in the lower part of the Burlington Limestone and the Fern Glen Formation.

The cephalopod Protocanites lyoni (Meek and Worthen) is found in the Rockford Limestone (Lineback, 1963). This fossil indicates correlation of the Rockford, therefore, with the Chouteau Limestone and the Northview Shale of the Mississippi Valley standard Mississippian section and generally with rocks of late Tournaisian age (Tn3c in the European standard) that are circumpolarly distributed in the Northern Hemisphere (Miller and Collinson, 1951, p. 481 Gordon, 1964, p. 283-284).






 
 
 
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