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40 | Frac Sand Insider May 2015 www.rockproducts.com Geology 7. Surface exposures of frac sand source units in the upper Midwest. Shown are units that contain the Middle and Upper Ordovician St. Pe- ter Sandstone in Wisconsin, Minnesota, Iowa, and Illinois; the Cambrian "undivided" that includes the Upper Cambrian and Lower Ordovician Jordan Formation and the Upper Cambrian Wonewoc and Mount Simon Formations (lav- ender) in Wisconsin, Minnesota, and Illinois; the Jordan Formation (red) mapped separately in Iowa and Upper Peninsular and the Formation (pink) (a partial equivalent of the Wonewoc Formation) mapped in Upper Penin- sula the banks of the Mississippi River in Iowa, thin exposures of the Wonewoc Formation (pink) and the Mount Simon Formation are mapped. Note that the Wisconsin "driftless area" is outlined in dark rivers - kansas, and Oklahoma. Frac sand source units shown are the Middle and Upper Ordovician St. Peter Sandstone in Missouri and Arkansas, and the Middle Ordovician Oil Creek Formation (pink) of the Simpson Group—a partial St. Peter equivalent in Oklahoma. Towns appear as red dots. Stratigraphically, the St. Peter Sandstone in the northern Mid- west is a formation within the Ancell Group (Mudrey and others, 1987). From evidence in outcrops in Sauk County, Wisconsin, the St. Peter Sandstone is subdivided, in descending order, into the Tonti and Readstown Members (Clayton and Attig, 1990) (Figure 3). In this area, the younger Tonti Member makes up most of the St. Peter Sandstone, and consists of very pale brown to yellowish red, fne- to medium-grained, quartzose sand, the coarser grains of which tend to be rounded, but in many places the grains exhibit faceted quartz overgrowths that produce a conspicuously sparkly surface in outcrop (Clayton and Attig, 1990). In outcrop, the sand- stone is hard due to silica cement (Clayton and Attig, 1990). The Readstown Member, by contrast, is a breccia of pebbles, cobbles, and boulders of sandstone that resembles that of the overlying Tonti Member (Clayton and Attig, 1990). In the area from north-central Illinois to southeastern Missouri, the subdivisions of the St. Peter include the fne-grained sands of the Tonti Member as the lower unit and the medium-grained sands of an overlying member known as the Starved Rock Member (Vi- socky and others, 1985). North of Jefferson County, Missouri, fne- to coarse-grained, pink to reddish-brown sandstone with varying amounts of shale, chert, and dolomite fragments constitute a basal unit that underlies the Tonti Member that is referred to as the Kress Member (Suter and others, 1959). In some parts of the subsurface of northern Missouri, the Kingdom Shale Member separates the Tonti and Starved Rock Members (Missouri Department of Natural Resources, 2014). The St. Peter Sandstone has been correlated with sandstone as far south and west as the Simpson Group of Oklahoma and west Texas (Suhm and Ethington, 1975), and with outcrops of the Burgen Sandstone of eastern Oklahoma (Buttram, 1913). From the excellent exposures near Pacifc, Missouri (Figure 9), the St. Peter Sandstone has been described as an ultra-pure (>99 wt. % silica in places), well-sorted, friable, fne to medium grained, rounded, highly spherical, and characteristically frosted, quartzose sandstone with an average thickness of 80 to 100 ft. (24 to 30 m) (Missouri Department of Natural Resources, 2014). F ure 9. Outcrop of dune facies of St. Peter Sandstone at old mine entrances in Pacifc, Missouri. from Kbh3rd at Wikipedia, Public Domain, from Wi- kimedia Commons at -