Brainard Shale
  

Maquoketa Group,

Ordovician System


Type locality area use of name in Indiana: The Brainard Shale was named by Calvin (1906, p. 60, 97) for exposures of bluish-gray shale near Brainard, Fayette County, Iowa. This formation has been widely recognized in Illinois (Templeton and Willman, 1963; Kolata and Graese, 1983), where it has been assigned to the Maquoketa Group. Gutstadt (1958a, p. 76) recognized the Maquoketa in northwestern Indiana and noted that it consisted of "upper and lower shale units separated by interbedded shale and limestone or dolomite units," but he did not apply names to the divisions. Gray (1972b, fig. 5 and p. 20-21) adopted the name Brainard Shale for the uppermost formation in the Maquoketa Group in western Indiana.

Description and correlation: The Brainard Shale is restricted to subsurface usage in Indiana and depends for its recognition on identification of the underlying Fort Atkinson Limestone. Gray (1972b) recognized the Brainard primarily in northwestern Indiana, but later work by John B. Droste (oral communication, 1983) shows that the Fort Atkinson and, therefore, also the Brainard are identifiable over a wider area that includes much of northern, central, and southwestern Indiana. Flugeman and Pope (1983) have recognized thin outliers or tongues of Brainard Shale on the outcrop in southeastern Indiana.

Over most of the area of its recognition, the Brainard Shale is 75 to 100 feet (23 to 30 m) thick, but in a narrow area from central Indiana northward to the southern shore of Lake Michigan it is thin to absent because of erosion before deposition of overlying Silurian rocks. The Brainard consists principally of gray to greenish-gray shale that contains a few thin interbeds of limestone. It is overlain in central and western Indiana by the Sexton Creek Limestone and in northeastern Indiana by the Manitoulin Dolomite Member of the Cataract Formation.

The Brainard overlies the Fort Atkinson Limestone through nearly all of its known extent, but in the outliers mentioned above the Brainard rests on a hard ground developed on the underlying Whitewater Formation (Flugeman and Pope, 1983). Regional significance of this feature is unclear according to Willman and others (1975, p. 86), fossils in the upper part of the Brainard in Illinois suggest equivalence to the Elkhorn Formation of former Indiana usage (uppermost Richmondian), whereas Flugeman and Pope concluded that "the Maquoketa Group is in part younger than the entire Cincinnatian Series in southeastern Indiana."






 
 
 
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