Type section: The name “Speeds Member” (of the Sellersburg Limestone) was given by Sutton and Sutton (1937, p. 326) to 1.5 ft (0.5 m) of shaly fossiliferous limestone that was exposed beneath the Silver Creek Limestone Member of Butts (1915) and above the Jeffersonville Limestone at Speed's Quarry near Sellersburg (Clark Military Grants 131 and 132, Clark County, Indiana) (Droste and Shaver, 1986).
History of usage:
In 1955, Patton and Dawson (p. 42) stated that the names “Silver Creek” and “Speed” are applied to lithofacies of the “Hamilton rocks” (North Vernon) and that these lithofacies may be used as members in the southern part of the outcrop belt. Droste and Shaver, 1986 noted that this unit makes up a lower part of the North Vernon Limestone. (See the North Vernon article for the evolution of North Vernon nomenclature affecting the Speed Member.)
Characteristically, the Speed Member consists of gray granular (poorly sorted) shaly thin-bedded fossiliferous limestone (Droste and Shaver, 1986). The thin-bedded, platy appearance on exposure is partly due to many brachiopod shells (Droste and Shaver, 1986). This unit, however, is a facies of the Silver Creek Member, so that characteristic Silver Creek lithology (gray dense and argillaceous to sublithographic limestone) is interbedded with characteristic Speed lithology in some places (Droste and Shaver, 1986).
The Speed unconformably overlies the Jeffersonville Limestone, including the Vernon Fork Member in some places (Droste and Shaver, 1986). It is overlain variably by its intimate facies, the Silver Creek Member, conformably, and by the Beechwood Member (upper North Vernon), unconformably (Droste and Shaver, 1986).
Speed rocks had been correlated with part of the Hamilton Group of New York even before their naming (Whitlatch and Huddle, 1932) on the basis of the brachiopod Stropheodonta demissa (Droste and Shaver, 1986). Conodonts from the upper Silver Creek equivalent of the Speed identify the Icriodus latericrescens latericrescens Zone (Orr, 1971, p. 20; Droste and Shaver, 1986). This zone has been thought to be early Givetian in age and is also found in the Skaneateles Formation (upper lower Hamilton) of New York (Droste and Shaver, 1986). Droste and Shaver (1986) noted that according to Dale Sparling (1983 and written communication, February 23, 1984) and on the basis of studies cited to Seddon (1970) and to Curtis Klug (1983), a Givetian age for the Speed and Orr's (1971) conodont zone is by no means assured; an Eifelian age is also possible. [See under "Silver Creek Member" for a listing of the midwestern correlatives of the Speed-Silver Creek interval (Droste and Shaver, 1986).]
Regional Indiana usage:
Illinois Basin Margin (COSUNA 12)
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.
Butts, Charles, 1915, Geology and mineral resources of Jefferson County, Kentucky: Kentucky Geological Survey, ser. 4, v. 3, pt. 2, 270 p.
Dawson, T. A., 1941, Outcrop in southern Indiana, pt. 1 of The Devonian formations of Indiana: Indiana Division of Geology, 48 p.
Droste, J. B., and Shaver, R. H., 1975, The Jeffersonville Limestone (Middle Devonian) of Indiana—stratigraphy, sedimentation, and relation to Silurian reef-bearing rocks: American Association of Petroleum Geologists Bulletin, v. 59, p. 393-412.
Droste, J. B., and Shaver, R. H., 1986, Speed 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. 147-148.
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.
Klug, C. R., 1983, Conodonts and biostratigraphy of the Muscatatuck Group (Middle Devonian), south-central Indiana and north-central Kentucky: Transactions of the Wisconsin Academy of Sciences, Arts and Letters, v. 71, Part 1, p. 79-112.
Patton, J. B., and Dawson, T. A., 1955, Stratigraphy, in Murray, H. H., compiler, Sedimentation and stratigraphy of the Devonian rocks of southeastern Indiana: Indiana Geological Survey Field Conference Guidebook 8, p. 16, 21-22, 25-26, and 37-43.
Seddon, George, 1970, Pre-Chappel conodonts of the Llano region, Texas: Texas Bureau of Economic Geology Report of Investigations 68, 130 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.
Sparling, D. R., 1983, Conodont biostratigraphy and biofacies of lower Middle Devonian limestones, north-central Ohio: Journal of Paleontology, v. 57, p. 825-864.
Sutton, D. G., and Sutton, A. H., 1937, Middle Devonian of southern Indiana: Journal of Geology, v. 45, p. 320-331.
Whitlatch, G. I., and Huddle, J. W., 1932, The stratigraphy and structure of a Devonian limestone area in Clark County, Indiana: Indiana Academy of Science Proceedings, v. 41, p. 363-390.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: January 12, 2021