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38 | Frac Sand Insider May 2015 www.rockproducts.com Geology PRINCIPAL SOURCES OF FRAC SAND Brief Overview The upper Midwest (north-central midcontinent) of the United States has the principal supply of the premium frac sand that the in- dustry calls "Northern White" or "Ottawa White" sand in near-surface exposures that make it economic to mine. This 99.8 percent pure silica sand brings premium prices and is mined from the Middle and Upper Ordovician St. Peter Sandstone, the Upper Cambrian and Lower Ordovician Jordan Formation, and the Upper Cambrian Wonewoc and Mount Simon Formations in Wisconsin, Minnesota, Illinois, and nearby states (Zdunczyk, 2007; Runkel and Steenberg, 2012). These units are widespread sheet sands deposited in the early Paleozoic continental interior seaway, whose lithologic char- acteristics and stratigraphic relationships are well illustrated in ex- posures in Sauk County, Wisconsin (Clayton and Attig, 1990). The following discussions of these principal frac sand source units are organized according to order of importance as frac sand sources within geographic regions. Stratigraphic columns are provided to maintain a sense for their correct age and stratigraphic context. The Middle and Upper Ordovician St. Peter Sandstone (Figure 3) is an areally extensive marine coastal sandstone representing the onset of Sloss's (1963) Tippecanoe sequence that was deposit- ed on the deeply eroded unconformable surface of the Ordovician Oneota Dolomite of the Prairie du Chien Group that forms the top of the Sauk sequence in Wisconsin (Sloss, 1963; Clayton and Attig, 1990). The underlying Upper Cambrian and Lower Ordovician Jor- dan Formation and Upper Cambrian Wonewoc and Mount Simon Formations are among the widespread marine sands of the earlier phases of the Sauk sequence deposited on the unconformable Precambrian surface of the North American Craton (Clayton and Attig, 1990) (Figure 3). In central Texas, deposits mined for frac sand are referred to as "Brown" or "Brady" sand. These are local quartz arenites that are within the subarkosic to arkosic Upper Cambrian Hickory Sand- stone Member of the Riley Formation in the Llano uplift region (Kyle and McBride, 2014) (Figure 4). Texas the Upper Cambrian Hickory Sandstone Member of the Riley For- mation that unconformably overlies the Proterozoic basement. (Modifed from Kyle and McBride, 2014; modifed after Barnes and Bell, 1977). Oklahoma's principal mined source of frac sand is the basal sand member of the Middle Ordovician Oil Creek Formation of the Simpson Group (Figure 5) that crops out in south-central Oklahoma GEOLOGICAL DESCRIPTIONS OF CURRENT AND POTENTIAL SOURCES OF FRAC SAND IN THE UNITED STATES 3. column the units in Sauk County, Wisconsin. (No scale is im