Type section: Gray (1978, p. 12) specified a section described by Malott (1925, p. 123-124) near Mount Pleasant as the type section (Gray, 1986). The type section is located in SE¼ sec. 6 and NW¼ sec. 5, T. 5 S., R. 1 W., Perry County, Indiana (Derby quadrangle); the altitude of the basal contact is 475 ft (144.8 m) and the upper contact is 710 ft (216.4 m) (Gray, 1978, p. 15).
History of usage:
Overview: The Mount Pleasant Sandstone Member was named for Mount Pleasant in northeastern Perry County, Indiana (Malott, 1925, p. 112; Gray, 1986). The name was initially applied at formational rank by Malott (1925, p. 112); Malott (1931, p. 222) later suppressed the name in favor of the Illinois term “Degonia Sandstone” (Gray, 1986). Definition of the latter unit (Swann, 1963, p. 41; Atherton, Collinson, and Lineback, 1975, p. 162) is not entirely clear and is at variance with Malott's original usage (Gray, 1986). For these reasons, Malott's original name and concept were reintroduced for outcrop use by Gray (1978, p. 12) (Gray, 1986).
In Malott's initial understanding, the Mount Pleasant in places was confused with basal sandstone of the Mansfield Formation (Morrowan), which in many places lies directly on the Mount Pleasant or appears to take its place stratigraphically as a result of pre-Mansfield erosion (Gray, 1978, p. 12; Gray, 1986). At the type section, Malott (1925, p. 123) identified 23 ft (7 m) of Mount Pleasant sandstone, but only the lower 14 ft (4 m) is now so assigned, and at the Shank Hill locality (Malott, 1925, p. 118-119) the entire ledge (35 ft, 11 m) is now considered to belong to the Mansfield (Gray, 1986). As presently understood, the Mount Pleasant Sandstone Member is a ledge- and waterfall-forming sandstone, thinly stratified to cross stratified, quartzitic and in places calcareous, and 3 to 18 ft (1 to 5 m) thick (Gray, 1986). It is widely present from the type locality southward to the Ohio River but is not known north of Perry County (Gray, 1986). It lies 55 to 85 ft (17 to 26 m) above the base of the Tobinsport Formation and about 230 ft (70 m) above the Glen Dean Limestone (Gray, 1986).
As judged by its position within the section, the Mount Pleasant Sandstone Member is probably stratigraphically equivalent to some part of the Degonia Sandstone of the standard Chesterian section (Gray, 1986). The internal stratigraphy of the Degonia is not well understood, however, and there is at present no way to determine what part of that formation is represented by the Mount Pleasant (Gray, 1986). Gray (1986) noted that the term "Mount Pleasant" has not been applied in the subsurface.
Regional Indiana usage:
Illinois Basin (COSUNA 11)
Geologic Map Unit Designation:
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.
Atherton, Elwood, Collinson, Charles, and Lineback, J. A., 1975, Mississippian System, in Willman, H. B., Atherton, Elwood, Buschbach, T. C., Collinson, Charles, Frye, J. C., Hopkins, M. E., Lineback, J. A., and Simon, J. A., Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, p. 123-163.
Gray, H. H., 1986, Mount Pleasant Sandstone 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. 96-97.
Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7th ed.): Washington, D.C., U.S. Geological Survey, 289 p.
Malott, C. A., 1925, The upper Chester of Indiana: Indiana Academy of Science Proceedings, v. 34, p. 103-132.
Malott, C. A., 1931, Geologic structure in the Indian and Trinity Springs locality, Martin County, Indiana: Indiana Academy of Science Proceedings, v. 40, p. 217-231.
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.
Swann, D. H., 1963, Classification of Genevievian and Chesterian (Late Mississippian) rocks of Illinois: Illinois State Geological Survey Report of Investigations 216, 91 p.
For additional information, contact:
Nancy Hasenmueller (firstname.lastname@example.org)Date last revised: April 5, 2017