Age:

Mississippian

Type designation:

Type section: The first written reference to the "Goniatite limestone of Rockford" was by Owen and Norwood (1847, p. 5) (Rexroad, 1986). 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, Indiana, became accepted as the type section (Rexroad, 1986). This exposure is in the SW¼SE¼ sec. 6, T. 6 N., R. 6 E. (Rexroad, 1970, 1986).

History of usage:

Meek and Worthen (1861, p. 167) are generally credited with formalizing the name “Rockford Limestone” by having referred to the "Rockford Goniatite bed" (Rexroad, 1986).

Description:

Limestone dominates the lithology of the formation, but shale, siltstone, and dolostone are also present (Rexroad, 1970, 1986). The limestone is typically gray, fine grained, argillaceous, ferruginous, and sparingly fossiliferous (Rexroad, 1970, 1986). 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 (Rexroad, 1970, 1986). Echinodermal debris and other fossils, such as the cephalopods, are concentrated in places (Rexroad, 1970, 1986). To the north thin gray-green shales are interlaminated with the limestones (Rexroad, 1970, 1986). In several exposures in southern Indiana an upper unit of bluish-gray to yellowish-brown shale containing calcareous nodules is present (Rexroad, 1970, 1986).

The Rockford is commonly 2 or 3 ft (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 (Rexroad, 1986). The greatest recorded thickness is about 22 ft (6.7 m) (Melhorn, 1958, p. 196; Rexroad, 1970, 1986).

Distribution: 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 (Rexroad, 1970, 1986). From this point the Rockford outcrop forms a northwestward-trending arc to the Indiana-Illinois state line in central Newton County, northwestern Indiana (Rexroad, 1986). 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 (Rexroad, 1986). The Rockford is present in the subsurface of much of the state west and south of the outcrop (Rexroad, 1986). As shown by Lineback (1970, fig. 15) and Bassett and Hasenmueller (1980), however, the formation is absent from several places, apparently by nondeposition (Rexroad, 1986).

Boundaries:

The Rockford Limestone overlies the New Albany Shale with apparent conformity, although the lithologic change is abrupt (Rexroad, 1986). 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 (Rexroad, 1986). 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 (Rexroad, 1986).

Correlations:

Conodont studies of the Rockford Limestone (Rexroad and Scott, 1964) show that it consists of strata of both the Kinderhookian and Valmeyeran Series (Rexroad, 1970, 1986). 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 (Rexroad, 1970, 1986). The Valmeyeran part, which is present from Scott County northward, is in the Gnathodus semiglaber-Pseudopolygnathus multistriata 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 (Rexroad, 1986).

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; Rexroad, 1970, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: none
Formation: Rockford Limestone
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: none
Formation: Rockford Limestone
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: none
Formation: Rockford Limestone
Kankakee Arch (COSUNA 14)
Supergroup: none
Group: none
Formation: Rockford Limestone

Misc/Abandoned Names:

Goniatite Limestone, Goniatite limestone of Rockford, Rockford Goniatite bed

Geologic Map Unit Designation:

Mr

Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural features in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Bassett, J. L., and Hasenmueller, N. R., 1980, Map of Indiana showing thickness of the Ellsworth Member of the New Albany Shale (Devonian and Mississippian): Morgantown, W. Va., U.S. Department of Energy, Morgantown Energy Technology Center, METC\EGSP Series No. 810.

Gordon, Mackenzie, Jr., 1964, Carboniferous cephalopods of Arkansas: U.S. Geological Survey Professional Paper 460, 322 p.

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7^th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Lineback, J. A., 1963, Age of the Rockford cephalopod fauna (Mississippian) of southern Indiana: Journal of Paleontology, v. 37, p. 939–942.

Lineback, J. A., 1970, Stratigraphy of the New Albany Shale in Indiana: Indiana Geological Survey Bulletin 44, 73 p.

Meek, F. B., and Worthen, A. H., 1861, Remarks on the age of the Goniatite Limestone of Rockford, Indiana, and its relation to the "Black Slate" of the western states, and to some of the succeeding rocks above the latter: American Journal of Science, ser. 2, v. 32, p. 167–177.

Melhorn, W. N., 1958, Revision of Mississippian-Devonian boundary in White and Benton Counties, Indiana: Indiana Academy of Science Proceedings, v. 67, p. 194–198.

Miller, A. K., and Collinson, Charles, 1951, Lower Mississippian ammonoids of Missouri: Journal of Paleontology, v. 25, p. 454–487.

Owen, D. D., and Norwood, J. G., 1847, Researches among the Protozoic and Carboniferous rocks of central Kentucky made during the summer of 1846: St. Louis, Keemle and Fields, Printers, 12 p.

Rexroad, C. B., 1970, Rockford Limestone, in Shaver, R. H., Burger, A. M., Gates, G. R., Gray, H. H., Hutchison, H. C., Keller, S. J., Patton, J. B., Rexroad, C. B., Smith, N. M., Wayne, W. J., and Wier, C. E., Compendium of rock-unit stratigraphy in Indiana: Indiana Geological Survey Bulletin 43, p. 140–142.

Rexroad, C. B., 1986, Rockford Limestone, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 124–125.

Rexroad, C. B., and Scott, A. J., 1964, Conodont zones in the Rockford Limestone and the lower part of the New Providence Shale (Mississippian) in Indiana: Indiana Geological Survey Bulletin 30, 54 p.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.