Lithostratigraphy: Maquoketa Shale Group >>Brainard Shale
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Cincinnatian Series >>Richmondian Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Brainard Shale (Calvin, 1906, p. 60, 97), the "upper shale" formation in the Maquoketa Group, is named for an exposure near Brainard, Fayette County, Iowa.
Extent and Thickness
The Brainard Formation occurs throughout the area of the Maquoketa Group, except in local areas where it is truncated by the sub-Silurian unconformity. Although widely present near the surface in the mounds and ridges in northwestern Illinois, it is generally covered by slumped material from the overlying Silurian dolomite formations, and actual outcrops are small and scarce. It is exposed locally in northeastern Illinois, particularly near Channahon and Ritchey in Will County, near Elgin in Kane County, and in small areas in Calhoun and Jersey Counties. The Brainard Shale is 75-100 feet thick where it is not deeply truncated by the sub-Silurian unconformity.
In the outcrops the Brainard Formation consists of greenish gray to green shale, partly dolomitic, and locally silty. In subsurface it contains beds of siltstone, and locally limestone or dolomite. It is generally much lighter colored and softer than the Scales Shale. Two thin bentonite beds locally occur near the top in southeastern Illinois.
The Brainard is commonly fossiliferous, and the presence of Cornulites in the upper part suggests equivalence to the Elkhorn Formation at the top of the Richmondian strata in Indiana and Ohio.
CALVIN, SAMUEL, 1906, Geology of Winneshiek County: Iowa Geological Survey, v. 16, p. 37-146.
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