Type locality: The Elwren Sandstone was named by Malott (1919, p. 11) for a series of exposures "in the cuts of the Illinois Central Railway" near Elwren, Monroe County, Indiana (Gray, 1970, 1986). The name was later changed to Elwren Formation because the unit is not dominantly sandstone but includes much shale and siltstone (Gray and Perry, 1956, p. 1,005; Gray, 1986).
The Elwren Formation includes thin-bedded fine-grained sandstone, cross-bedded sandstone, and green-gray and red-brown shale and mudstone (Gray, 1970, 1986). It ranges from 20 to 60 ft (6 to 18 m) in thickness (Gray and others, 1957, pl. 2; Gray, 1970, 1986).
The Elwren is overlain conformably by the Beech Creek Limestone (Stephensport Group) or disconformably by the Mansfield Formation (Morrowan) (Gray, 1970, 1986). It overlies the Reelsville Limestone conformably except in a few places where a sandstone body in the Elwren rests disconformably on the Reelsville or on the underlying Sample Formation (Malott, 1952, p. 14; Gray, 1970, 1986). The hypothesis of a regional disconformity at this horizon was rejected, however, by Gray and Perry (1956), who considered this relationship to be a local aspect of what later became known as the West Baden clastic belt (Sullivan, 1972; Gray, 1986).
Malott (1931, p. 222; 1952, p. 14) considered the Elwren Formation to be equivalent to the middle part of the Paint Creek Formation of Illinois, even though he was aware that in subsurface usage it was commonly called the Cypress Sand, but Swann and Atherton (1948), among others, demonstrated equivalence of the Elwren to the type Cypress Sandstone of southern Illinois as well as to the Cypress of subsurface usage (Gray, 1986). The Elwren equivalent in the subsurface is the Cypress Formation, which is recognized from southern Parke County southwestward (Gray, 1986).
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.
Gray, H. H., 1970, Elwren Formation, 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. 53–54.
Gray, H. H., 1986, Elwren Formation, 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. 43–44.
Gray, H. H., and Perry, T. G., 1956, Local lithologic discontinuities in Reelsville (lower Chester) Limestone of Indiana: American Association of Petroleum Geologists Bulletin, v. 40, p. 1,005–1,009.
Gray, H. H., Dawson, T. A., McGregor, D. J., Perry, T. G., and Wayne, W. J., 1957, Rocks associated with the Mississippian-Pennsylvanian unconformity in southwestern Indiana: Indiana Geological Survey Field Conference Guidebook 9, 42 p.
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., 1919, The "American Bottoms" region of eastern Greene County, Indiana–a type unit in southern Indiana physiography: Indiana University Studies, v. 6, no. 40, 61 p.
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.
Malott, C. A., 1952, Stratigraphy of the Ste. Genevieve and Chester formations of southern Indiana: Ann Arbor, Michigan, Edwards Letter Shop, 105 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.
Swann, D. H., and Atherton, Elwood, 1948, Subsurface correlations of lower Chester strata of the Eastern Interior Basin: Journal of Geology, v. 56, p. 269–287.
For additional information, contact:
Nancy Hasenmueller (firstname.lastname@example.org)Date last revised: November 22, 2016