St. Croixan and Canadian Series

Cambrian and Ordovician Systems

Type area and history of name: The Knox Dolomite was named by Safford (1869, p. 151) for about 4,000 feet (1,220 m) of heavy-bedded ridge-making dolomite in Knox County, Tenn., but in the one original publication the name was also applied to a shale and to a sandstone and was used as a group. These early ambiguities were resolved, for example, by renaming the Knox Shale as the Conasauga Shale and the Knox Sandstone as the Rome Formation. (For details see Wilmarth, 1938.) The association of the name Knox, at formation or group rank, with thick bodies of rock dominated by dolomite has been made throughout the central United States. Additionally in Illinois, however, the name evolved to "Knox Dolomite Megagroup" (Swann and Willman, 1961, p. 447), which contains relatively pure dolomite generally underlying the St. Peter Sandstone and overlying rocks of the Potsdam Sandstone Megagroup.

The earliest use of the name Knox in Indiana is unknown, but "Knox Dolomite" was the long-preferred use. (See Gutstadt, 1958a, for additional details.) In 1985, however, the term Knox Supergroup was introduced to Indiana use (Droste and Patton). Nomenclaturally, it consists in ascending order of the Potosi Dolomite, the Prairie do Chien Group (consisting of the Oneota and Shakopee Dolomites), and the Everton Dolomite.

Description: The Knox Supergroup in Indiana consists of the relatively pure dolomite lying conformably above the Potsdam Supergroup (Cambrian: St. Croixan) and underlying unconformable the Ancell Group (Ordovician: Chazyan and Blackriverian). At its base the Knox grades laterally into the Potsdam. In Indiana the Potsdam-to-Knox gradation embraces approximately the stratigraphic interval of the Franconian Stage.

Except for fragmented rocks in the chaotic structure exposed in the Kentland Quarry in Newton County, the Knox Supergroup is present everywhere only in the subsurface of Indiana. The Newton County exposures are of faulted blocks of the Shakopee Dolomite (Gutschick, 1983). The Knox ranges from less than 20 feet (6 m) to more than 4,500 feet (1,372 m) in thickness. Twenty feet (part of the Potosi Dolomite) is all that remains in the area of one northwestern Indiana well beneath the major erosional unconformity that separates the Knox from the overlying St. Peter Sandstone. In contrast in southwestern Indiana, more than 4,500 feet (1,372 m) of the Knox is preserved beneath the unconformity with the overlying St. Peter. There the Everton Dolomite (Whiterockian Stage) makes up the youngest part of the Knox Supergroup. In northeastern Indiana, where the preserved Knox is more than 600 feet (183 m) thick, the Oneota Dolomite part of the Knox (Canadian Stage) lies unconformable below the Joachim Dolomite (Blackriverian Stage).

Correlation: Given the constitution of the Knox noted above, its apparent time-stratigraphic range in Indiana is from the Dresbachian (Cambrian) to upper Whiterockian (Ordovician). For details of rock-unit correlation throughout the Midwest, see Droste and Shaver (1983, fig. 2) and Shaver and others (1985); see also the general discussion under "Potsdam Supergroup." In some southwestern states, corresponding rocks are called the Arbuckle and Ellenberger (variably with formation and group ranks).