Age:

Mississippian

Type designation:

The name “Somerset Shale Member” was given by Butts (1922, p. 89, 104-107) to an upper part of the Warsaw Formation in Kentucky (Rexroad, 1986).

History of usage:

The name was derived from Somerset in Pulaski County, although Butts indicated that the unit was better exposed in glades about 2 miles (3.2 km) southwest of Colesburg (Rexroad, 1986). In 1939, Stockdale (p. 226, pls. 6, 25) traced the Somerset from its type area to southern Monroe County, Indiana, and interpreted it "as a basal argillaceous phase of the Salem limestone" (Rexroad, 1986). This interpretation has been followed by most geologists, including Nicoll and Rexroad (1975), but the section at Somerset may be stratigraphically higher than the one at Colesburg from which the stated stratigraphic position has been accepted (Rexroad, 1986).

Description:

On outcrop in southern Indiana and adjacent Kentucky, the Somerset Shale Member consists of as much as 16 ft (5 m) of dolomitic silty shale commonly containing gypsum- or quartz-filled geodes (Rexroad, 1986). It thins abruptly to the north and ranges from a few inches to little more than a foot (0.3 m) at its northern limits in Monroe County (Rexroad, 1986). Its subsurface extent has not been determined (Rexroad, 1986). In Kentucky similar shales with which the Somerset might be confused are present higher in the Salem (Rexroad, 1986).

Boundaries:

Feldman (1989) noted that the Somerset Shale Member appears to be a useful biostratigraphic boundary defined by the last occurrences of the genera Barycrinus, Cyathocrinites, and Actinocrinites and the base of the range of Batocrinus.

Correlations:

Studies by Howard Feldman (written communication, April 17, 1984) show that the bulk of fossils found in the Somerset Shale Member, including Globoendothyra baileyi, are also common to the limestone facies of the Salem (Rexroad, 1986). The presence of several species of the crinoid Batocrinus that he recorded from the Somerset and the rest of the Salem sets these units apart from the underlying Harrodsburg Limestone, from which that genus is absent. Cyathocrinites and Barycrinus are found in the Somerset but not in the overlying Salem beds (Howard Feldman, written communication, April 17, 1984; Rexroad, 1986). The latest record of these two genera is in rocks of Keokuk age (middle Valmeyeran), and so their presence in the Somerset probably represents the last remnant of the well-known crinoid fauna of Keokuk age (Rexroad, 1986). Although part of the restriction of the Somerset crinoids may be ecologic, the overlap of the genera mentioned above characterizes a very limited stratigraphic interval (Rexroad, 1986).

Regional Indiana usage:

Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Sanders Group
Formation: Salem Limestone
Member: Somerset Shale Member

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Mss

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:

Butts, Charles, 1922, The Mississippian Series of eastern Kentucky: Kentucky Geological Survey, ser. 6, v. 7, 188 p.

Feldman, H. R., 1989, Echinoderms of the Somerset Shale Member, Salem Limestone (Mississippian), in Indiana and Kentucky: Journal of Paleontology, v. 63, p. 900–912.

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, Somerset Shale Member, 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. 146.

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.

Stockdale, P. B., 1939, Lower Mississippian rocks of east-central interior (U.S.): Geological Society of America Special Paper 22, 248 p.