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www.rockproducts.com Frac Sand Insider May 2015 | 41 Geology In northwestern Wisconsin, the St. Peter Sandstone is 25 me- ters (m) (82 ft) thick (Mudrey and others, 1987). Elsewhere, the formation is highly variable in thickness. In Missouri, the St. Peter Sandstone averages 80 to 100 ft. (24 to 30 m) (Missouri Depart- ment of Natural Resources, 2014). In most of northern Illinois, its thickness ranges from 100 to 200 ft. (30 to 60 m); however, north of the Sandwich fault zone, a maximum thickness range of 400 to 600 ft. (120 to 180 m) is reached (Visocky and others, 1985). The highly variable thickness is the result of deposition on both erosional channels and karsted surfaces in the underlying carbonate beds (Visocky and others, 1985). Color White, with occasional shades of pink and green. Weathered surfaces are a dirty gray or brown and hardened at many localities Grain Size <0.075 mm (No. 200 U.S. Standard Sieve Series size) to 2 mm (No. 10 U.S. Standard Sieve Series size) Hardness Individual sand grains are hard, 7 on Mohs scale Cementation Soft, friable, easily disaggregated Specifc Gravity Estimated 2.24 assuming 15% porosity (spe- cifc gravity of quartz is 2.65) Porosity Estimated 10–15% Solubility Quartz has a very low solubility except when exposed to pH >9, or when in contact with hy- drofuoric acid; thus, the St. Peter Sandstone is stable under ordinary environmental conditions. Table 1. Properties of St. Peter Sandstone in Missouri. (Missouri Department of Natural Resources, 2014). Despite bearing the industry name "Northern White," the St. Peter can be white to pale yellow, buff to tan, depending upon its iron content (Mudrey and others, 1987). In exposures in Missouri, bedding within the St. Peter Sandstone is indistinct, and the unit often appears massive throughout. Locally, however, the rock may be cross bedded, ripple marked, and is generally porous, perme- able, and mostly nonfossiliferous in Missouri (Missouri Department of Natural Resources, 2014). Marine fossils are observed in the unit, however, as far north as Minneapolis, Minnesota (Dake, 1921). The mineralogic and textural maturity of the St. Peter Sand- stone is attributed to multicyclical marine shoreface and coastal aeolian processes acting upon sediment derived from older Cam- brian and Precambrian sands (Mazzullo and Ehrlich, 1983; Winfree, 1983; Dott and others, 1986; Dott, 2003). Dake (1921) points to the Upper Cambrian Potsdam Sandstone as a possible prove- nance for the St. Peter Sandstone. The unit is not uniformly homogeneous throughout its geo- graphic extent, as depositional facies changes and diagenetic al- teration have impacted its mineralogic purity, grain size and shape, and friability. For example, quartz overgrowths are observed in the unit in areas of southwestern and south-central Wisconsin, and in southeastern Minnesota (Kelly, 2006; Kelly and others, 2007; Winfree, 1983; Clayton and Attig, 1990), making the grains less rounded/spherical in places. For these reasons, in the case of the St. Peter Sandstone, as with all the other units discussed in this re- port, not all surface exposures of the formation shown on the Figure maps and in Plate 1 are suitable for frac sand mining. Jordan Formation The Upper Cambrian and Lower Ordovician Jordan Formation is within the Trempealeau Group (Ostrom, 1967) and is a marine regressive sandstone (Runkel, 1994) that crops out in southeastern Minnesota, west-central Wisconsin, and northeastern Iowa (Thom- as, 1992). It has also been recognized in Illinois (Clayton and Attig, 1990) and extends into the Upper Peninsula of Michigan where the Jordan is partly equivalent to the Trempealeau, which, at this locality, is of formation rank (Figure 7) (Plate 1). Note that the surface map extent of the Jordan shown on Plate 1 is actually Cambrian "undivided" in Wisconsin and Minnesota. This means that the Jor- dan is combined with other Upper Cambrian strata that include the Wonewoc and Mount Simon Formations as well as nonsand-bear- ing units; therefore, each of these Upper Cambrian units is overrep- resented on the Figure map and in Plate 1. Throughout much of Wisconsin, the Jordan Formation con- tains primarily two quartzose sandstone members (Mudrey and oth- ers, 1987; Clayton and Attig, 1990; Runkel, 1994) (Figure 3). The uppermost unit is the Van Oser Member, which is quartzose, white to brown to yellow or orange, fne to medium grained, poorly sorted, medium to thin bedded, cross bedded, with calcite-cemented nod- ules, is iron cemented in places, may be locally interbedded with the underlying unit, and 9 to 15 m (30 to 50 ft) thick (Mudrey and others, 1987). In extreme western Wisconsin, the Van Oser is known as a medium- to coarse-grained, well-rounded quartz arenite that is thick bedded, contains cross bedding and calcareous concretions, and is easily disaggregated, with a thickness of around 45 ft. (14 m) (Ostrom, 1987). The Van Oser is interpreted as a higher-ener- gy, marine intertidal sand deposited as the sea shallowed (Runkel, 1994). Underlying the Van Oser Member, the Norwalk Member is quartzose, white, fne grained, rounded, moderately sorted, medi- um bedded, with a trace of garnet, and 15 to 18 m (50 to 60 ft) thick (Mudrey and others, 1987). In extreme western Wisconsin, the Nor- walk is a fne- to very fne-grained feldspathic sandstone (Ostrom, 1987; Runkel, 2000). It is interpreted as a low-energy, below wave base, marine deposit (Runkel, 1994). The Van Oser Member is the more suitable of the Jordan sub- units in Wisconsin for use as frac sand, as it is coarser grained and higher in silica than the Norwalk Member (Runkel and Steen- berg, 2012; Brown, 2014). The Van Oser is highly prized for its high yield of 20/40 mesh quartz sand (Runkel and Steenberg, 2012). An important consideration when mining the Van Oser sand is the approximately 100-ft (30-m) thick resistant dolomite of the Oneota Dolomite of the Lower Ordovician Prairie du Chien Group that exists as overburden in many places in Wisconsin (Runkel and Steenberg, 2012). For this reason, the Van Oser is preferentially extracted from eroded hilltops in Barron and Chippewa Counties; from old quarries and ridgetops in Dunn, St. Croix, and Buffalo Counties; and from underground mines in Pierce County, Wisconsin (Brown, 2014). In southeastern Minnesota, the Van Oser Member is interfn-