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www.rockproducts.com Frac Sand Insider May 2015 | 39 Geology (Suhm and Ethington, 1975). This sand unit is referred to as the Connell Sandstone Member in west Texas. The Oil Creek Formation and its equivalents have characteristics similar to the Ordovician St. Peter Sandstone of the Mississippi Valley (Suhm and Ethington, 1975) and are generally considered to be partially stratigraphically equivalent to the St. Peter Sandstone (Dapples, 1955). 5. column of the Ordovician Simpson Group subdi- visions as in common in west Texas and Oklahoma. (No scale is implied.) The six principal frac sand source units are subsequently de- scribed as to their petrology, stratigraphy, areal extent, and paleo- environmental characteristics. The units are organized by formation name listed in order of importance as a frac sand source for each region. The list begins with the most important sources of "Northern White" sand of the Midwest, that is, the St. Peter and the Jordan (Maslowski, 2012). These are followed by the Wonewoc and Mount Simon, also of the Midwest; the Hickory "Brown" or "Brady" sand, of importance primarily in Texas; and the Oil Creek, a partial St. Peter equivalent in Oklahoma. Stratigraphic nomenclature conforms to that provided by the references, as cited, except where names of units are modifed to conform to U.S. Geological Survey con- ventions. Midwest frac sand sOurces St. Peter Sandstone The Middle and Upper Ordovician St. Peter Sandstone and the partly stratigraphically equivalent sandstones of the Simpson Group are widespread in areal extent, both in the surface and sub- surface, occurring in southern Wisconsin, Minnesota, Illinois, Iowa, Michigan, Indiana, Kentucky, Missouri, Nebraska, Kansas, Arkan- sas, Oklahoma (Dake, 1921; Dapples 1955; Cole, 1965; Mai and Dott, 1985; Davis, 2011), west Texas (Suhm and Ethington, 1975; Jones, 2009), and Ohio and Tennessee (Dake, 1921). The pres- ent distribution of the uneroded sandstones of the St. Peter and Simpson attests to the widespread original depositional extent that covered much of the mid-continent region (Figure 6). Fi e 6. Areal extent (surface and subsurface) of the Ordovician St. Peter Sand- stone in the upper and central Midwest and partly equivalent units within the Simpson Group in Kansas, Oklahoma, and west Texas. (Modifed from Dake, 1921; Dapples, 1955; Cole, 1965; Jones, 2009; Davis, 2011.) In the upper Midwest, where the St. Peter Sandstone is un- ambiguously distinguished, it is in near-surface deposits in parts of western, southwestern, and south-central Wisconsin; southeastern Minnesota; northeastern Iowa; and northern Illinois (Mai and Dott, 1985) (Figure 7). In the central Midwest, surface exposures of the St. Peter Sandstone are limited to a narrow arc-shaped feature ex- tending from central to southeastern Missouri (Davis, 2011) and the highlands of the Ozark Plateau in northern Arkansas (Glick and Frezon, 1953) (Figure 8).