Age:

Mississippian

Type designation:

Type locality: The Sanders Group was named by Smith (1965) to include the Harrodsburg and Salem Limestones (Shaver, 1970; Rexroad, 1986). The type locality is near Sanders in Monroe County, Indiana, but a specific type section was not designated by Smith (1965, p. 9) (Shaver, 1970; Rexroad, 1986).

Reference sections: Smith’s (1965, p. 9) reference sections are in tributaries of Ramp Creek within 0.5 miles (0.8 km) east and northeast of Sanders and in an abandoned quarry in the center of the E½NW¼SE¼ sec. 33, T. 8 N., R. 1 W. (Shaver, 1970; Rexroad, 1986).

History of usage:

The group was expanded by Nicoll and Rexroad (1975, pl. 2, p. 4), who raised the Ramp Creek Limestone Member (Muldraugh Formation) to formational rank and included it and the laterally equivalent Muldraugh Formation in the Sanders Group (Rexroad, 1986).

Description:

The Sanders Group consists of a variety of carbonate rocks in complex facies relationships (Rexroad, 1986). The Ramp Creek and Muldraugh Formations at the base of the group are dominantly a mixture of fine-grained dolostone and of limestone that in places contains abundant echinodermal and bryozoan fragments (Rexroad, 1986). Cherty and siliceous intervals are common, and minor amounts of siltstone and shale are present (Rexroad, 1986). Above that interval in the Harrodsburg Limestone well-cemented bioclastic calcarenites and calcirudites are dominant over argillaceous limestone, dolosiltites, and shale (Rexroad, 1986). The abundance of geodes and chert decreases upward in the group (Rexroad, 1986). The Salem Limestone except for the Somerset Shale Member at its base is dominated by porous calcarenite, although it contains a wide variety of other kinds of limestone (Rexroad, 1986).

Distribution: The group crops out along the margin of the Illinois Basin in an irregular arc from Fountain County to southern Harrison County (Rexroad, 1986). It is present throughout the subsurface west and south of the outcrop belt (Rexroad, 1986). Along the central and southern parts of the belt, the Sanders generally ranges between 120 and 150 ft (37 and 46 m) in thickness, but it is thinner toward the north (Rexroad, 1986). It thickens abruptly off the margin of the Borden delta deposits, principally because of thickening of the Muldraugh Formation within the resulting topographic basin, and it reaches a maximum thickness of about 510 ft (150 m) in the subsurface in Posey County (Rexroad, 1986).

Boundaries:

The Sanders Group overlies rocks of the Borden Group with a depositional hiatus marked by a sharp lithologic break and in most places by a zone of glauconite at the top of the Borden (Rexroad, 1986). Throughout most of its extent it is overlain conformably by the St. Louis Limestone, although local hiatuses are possible. Along its northern margin, however, the group is truncated by pre-Pennsylvanian erosion and is unconformably overlain by the Mansfield Formation (Morrowan) (Rexroad, 1986).

Correlations:

The Sanders Group is middle Valmeyeran in age (Rexroad, 1986). The names “Ramp Creek, Harrodsburg, and Salem” are used in Illinois, although the first two are considered to be members of the Ullin Formation and "Muldraugh" is considered to be a junior synonym of "Ramp Creek" (Rexroad, 1986). The units are not precisely isochronous throughout, however (Rexroad, 1986). In the Kentucky part of the Illinois Basin, the name “Salem” is also used, but the Warsaw Limestone of Kentucky use approximates the Harrodsburg and all but the lower part of the Ramp Creek-Muldraugh section in Indiana (Rexroad, 1986). The latter two formations are equivalent to the Fort Payne Formation in adjacent Illinois and Kentucky (Rexroad, 1986). The oldest part of the Sanders Group is in the Gnathodus texanus-Taphrognathus Assemblage Zone (conodonts), and the rest is in the Taphrognathus varians-Apatognathus Assemblage Zone (Nicoll and Rexroad, 1975, p. 2, 3; Rexroad, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Sanders Group
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Sanders Group
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: Sanders Group

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Msn

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:

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.

Nicoll, R. S., and Rexroad, C. B., 1975, Stratigraphy and conodont paleontology of the Sanders Group (Mississippian) in Indiana and adjacent Kentucky: Indiana Geological Survey Bulletin 51, 33 p.

Rexroad, C. B., 1986, Sanders Group, 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. 136–137.

Shaver, R. H., 1970, Sanders Group, 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. 160–161.

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.

Smith, N. M., 1965, The Sanders Group and subjacent Muldraugh Formation (Mississippian) in Indiana: Indiana Geological Survey Report of Progress 29, 20 p.