|St. Peter Sandstone, Illinois Basin
The St. Peter Sandstone is a mature quartz arenite that contains little or no silt and clay, except near its perimeter. Lamar (1928; his fig. 10) provided results of grain-size analysis for a number of St. Peter Sandstone samples in which the percent clay ranged from 0 to 5.8 with a mean of about 2.5 percent. Because clay is a minor constituent of the St. Peter Sandstone, we simply assign the percent shale to be 2.5 throughout the central Illinois Basin.
Lamar, J. E., 1928, Geology and economic resources of the St. Peter Sandstone of Illinois: Illinois Geological Survey Bulletin No. 53, 175 p.