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42 | Frac Sand Insider May 2015 www.rockproducts.com Geology gered with the Waukon Member (also of the Jordan Formation) and is overlain by the Sunset Point Member of the Jordan (Ostrom, 1987) that is a fne-grained feldspathic and dolomitic shallow-ma- rine deposit (Thomas, 1992) now referred to the Coon Valley Mem- ber of the overlying Oneota Dolomite (Clayton and Attig, 1990; Run- kel, 1994). Similarly to the subdivisions in Wisconsin, the Van Oser Member in Minnesota is described as a medium-grained quartzose sand; and the Norwalk and Waukon Members are very fne-grained feldspathic sands (Thomas, 1992). In parts of Minnesota and western Wisconsin, diagenetic pro- cesses such as quartz syntaxial and potassium feldspar epitaxial overgrowths, hematite precipitation, dolomitization, calcite pre- cipitation, calcite dissolution, and a second hematite precipitation have variously altered the original texture of the sands of the Jordan Formation (Thomas, 1992; Runkel and Steenberg, 2012). Jordan samples from Arcadia, in Trempealeau County, Wisconsin, show originally rounded quartz grains with second-stage rounding of quartz overgrowths indicative of multi-cycling, and authigenic feld- spar overgrowths that result in localized compromises to optimal frac sand properties (Runkel and Steenberg, 2012). Wonewoc Formation The Upper Cambrian Wonewoc Formation is the uppermost for- mation within the Elk Mound Group (Clayton and Attig, 1990) (Figure 3). The formation overlies the Eau Claire Formation, also of the Elk Mound Group. The Wonewoc Formation is observed in Wisconsin, Minnesota, Iowa, and Illinois (Figure 7) (Plate 1); although it is mapped as Cambrian "undivided" in these states but for Iowa, which means it is combined with other Cambrian units, as described above for the Jordan. Where the Wonewoc Formation crops out in west-central Wiscon- sin, it consists of two quartzose sandstone members. In descending order, they are the Ironton Member and the Galesville Member (Mudrey and others, 1987; Clayton and Attig, 1990). In northeastern Wisconsin, the narrow outcrop belt of Wonewoc Formation extends northward into the Upper Peninsula of Michigan, where it is known as the Munising Formation (Dott and others, 1986). In much of Wisconsin, the Ironton Member is quartzose, white to brown with iron staining, medium to coarse grained, subrounded, poorly sorted, wavy bedded, with vertical burrows present, and cal- cite cemented, and 5 to 18 m (16 to 60 ft) thick (Mudrey and others, 1987). Underlying the Ironton Member, the Galesville Member is quartzose, white, fne to medium grained, rounded to subrounded, well sorted, thick bedded, cross bedded, poorly cemented, and 5 to 18 m (60 ft) thick with individual bedding units 3 to 5 m (10 to 16 ft) thick (Mudrey and others, 1987). Wherever the sandstone of the Galesville is differentiated as a member, it is the prospective frac sand source within the Wonewoc Formation. Although the Wonewoc Formation is a highly pure silica sand, it is fner in average grain size than the Van Oser Member of the Jor- dan Formation, so it is relatively less suitable as a frac sand (Brown, 2014). Despite the generally fner grain size, the Wonewoc can be mined for multiple markets that serve non-frac uses for the fner fraction as well as the frac sand market for the smaller proportion of coarser grained fraction (Brown, 2014). The extensive surface exposure of the Wonewoc is encouraging the development of new frac sand mines that target the Wonewoc sand in Trempealeau, Dunn, Buffalo, Jackson, and Monroe Counties, Wisconsin (Brown, 2014). The Chapel Rock Member of the correlative Munising For- mation in Upper Peninsula, Michigan, has been considered for use as a potential glass sand because of its >98 percent quartz content (Heinrich, 2001), although it has not been named as a potential source for frac sand. Mount Simon Formation The Upper Cambrian Mount Simon Formation of the Elk Mound Group (Figure 3) has been identifed in southern Wisconsin, Illinois, Indiana, Kentucky, Ohio, Minnesota, and Iowa (Clayton and Attig, 1990). The formation is observed at the surface in Minnesota, Wis- consin, and Iowa (Figure 7) (Plate 1); but it, too, is mapped as Cam- brian "undivided." From its erosional boundary in northern Wiscon- sin, the Mount Simon Formation thickens southward to maximums of 2,600 ft. (790 m) in northeastern Illinois and more than 2,000 ft. (610 m) in central and north-central Iowa (Young, 1992). The unit is recognized only in the subsurface of the Michigan Basin (Cotting- ham, 1990) where it reaches a thickness of more than 1,000 ft. (305 m) along the basin's western fank (Catacosinos, 1973). The unit is in the subsurface of the Illinois Basin. The Mount Simon For- mation is also known in the subsurface of northern Missouri where it grades southward into the Lamotte Sandstone as it emerges along the fanks the Ozark uplift (Young, 1992). The Mount Simon For- mation extends into the subsurface as far east as western Ohio where it is described as tan, friable, moderately sorted, rounded, coarse- to very coarse-grained, siliceous quartz arenite with minor heterolithic sandstone-mudstone couplets (rhythmites) and a quartz granule conglomerate (Saeed and Evans, 2012). In the northwest quadrant of Wisconsin, the Mount Simon For- mation contains three informal quartzose sandstone units (Mudrey and others, 1987). The uppermost sandstone is quartzose, feld- spar bearing, white to light gray to pale brown, medium to coarse grained, angular, medium bedded, locally lenticular bedded, and at least 52 m (170 ft) thick (Mudrey and others, 1987). Beneath this unit is the second sandstone that is quartzose, pale yellow or- ange to pale gray orange, very fne grained, thin to medium bedded, angular, limonite cemented, and 38 m (125 ft) thick (Mudrey and others, 1987). This unit is underlain by an 18 m (60 ft) thick, gray to pale orange, silty shale (Mudrey and others, 1987). The basal sand- stone unit is quartzose, very pale orange, very fne to fne grained, subangular to subrounded, and at least 35 m (115 ft) thick; but known only in the subsurface in northwestern Wisconsin (Mudrey and others, 1987). Farther eastward in Wisconsin, the Mount Simon is near the surface in Clark, Wood, and northern Jackson, and Monroe Coun- ties (Brown, 2014). Much of the sandstone of the Mount Simon in this area has been reworked and deposited as alluvial sand that is mined as a byproduct of cranberry bog construction (Brown, 2014). In the Wisconsin Dells area in Sauk County (south-central Wis- consin), the Mount Simon Formation is generally described as a me- dium-grained sandstone that also contains considerable amounts of coarse-grained sand and a smaller amount of fne-grained sand, the coarser grains especially having undergone considerable rounding and consisting largely of quartz (Clayton and Attig, 1990).