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www.rockproducts.com Frac Sand Insider May 2015 | 47 Geology great plainS region Deadwood Formation The Upper Cambrian and Lower Ordovician Deadwood For- mation in South Dakota (Figure 12) (Plate 1) has been described as a variegated, yellow to red, brown, gray, and green, glauconitic, conglomerate, sandstone, shale, dolomitic limestone, and dolo- mite; with a thickness of 4–400 ft. (1–122 m) (Martin and others, 2004). Although studies by Ching (1973) and Huq (1983) indicated that parts of the Deadwood Formation are potential sources for frac sand, the South Dakota Geological Survey reports that the forma- tion is not a prospective frac sand source (Marshall and others, 2014). When compared with API requirements for frac sand, the Deadwood Formation does not consist of >99 percent quartz, has too broad a grain size distribution, has grains that are not the cor- rect shape, and is too tightly cemented (Rapid City Journal, 2014). Despite these differences in assessments of the Deadwood For- mation's frac sand potential, South Dakota Proppants LLC, is cur- rently pursuing permits to develop a mine, processing plant, and transportation hub in an area of the Black Hills National Forest (Hirji, 2014). SouthweSt region White Rim and Cedar Mesa Sandstone Members The Lower Permian White Rim Sandstone and Cedar Mesa Sandstone Members of the Cutler Formation are recognized in the subsurface of the Paradox Basin of Utah (Baker and Reeside, 1929; Steele, 1987). The White Rim Sandstone Member is a quartz sandstone of shallow margin origin that forms the top of the Cut- ler Formation and is overlain by the Triassic Moenkopi Formation (Blakey, 1974). It was deposited in a coastal environment during marine transgression where it was later exposed to aeolian and oth- er nonmarine processes (Steele, 1987). Where the White Rim and Cedar Mesa Sandstone Members are exposed along the fanks of the San Rafael Swell in Emery County, Utah (Figure 11) (Plate 1), they are proposed by Rupke (2014) as potential frac sand sources. White Throne Member The Middle Jurassic White Throne Member of the Temple Cap Formation in Kane and Washington Counties, Utah, (Figure 11) (Plate 1) is proposed by Rupke (2014) as a potential frac sand source. The Temple Cap Formation has limited geographic extent and occurs as the basal sandstone unit of the San Rafael Group that unconformably overlies the Navajo Sandstone in extreme west- ern Kane County and extreme eastern Washington County (Pe- terson and Pipiringos, 1979). At the type section in Zion Canyon, Washington County, the Temple Cap Formation is subdivided (in descending order) into the White Throne Member: a 49.7-m (163-ft) thick, fne-grained, well-sorted, cross-bedded sandstone; and the Sinawava Member: a 6.1-m (20-ft) thick, fat-bedded sandstone, silty sandstone, and mudstone (Peterson and Pipiringos, 1979). The White Throne Member is a cliff-forming unit that is exposed in canyon walls in Johnson Canyon, Mount Carmel Junction, and Zion Canyon, and pinches out westward into a thick deposit of the other- wise underlying Sinawava Member (Peterson and Pipiringos, 1979). Quaternary Aeolian Dune Sands Quaternary aeolian dune sands in Kane and Washington Counties, Utah (Figure 11) (Plate 1), are proposed by Rupke (2014) as potential future frac sand sources. GEOLOGICAL PROCESSES CONTRIBUTING TO FRAC SAND FORMATION The best frac sands are supermature quartz arenites that owe their physical and chemical characteristics to their origin as ma- rine shoreline sands that were multiply reworked by wind and wa- ter (Winfree, 1983), were never deeply buried, or they later under- went diagenesis or intense chemical weathering that reduced or removed cements (Dott and others, 1986; Dott, 2003). Many pure quartz arenites were deposited in non-orogenic basins during early Paleozoic and Proterozoic time (Dott, 2003). The multiple cycles of mechanical reworking contributes to the proper shaping of grains, mineralogical maturity, and grain-size sort- ing (Levson and others, 2012). There are many depositional set- tings in which sand is reworked, but the geologically older the sand deposit, the more chance it has had to undergo multiple cycles of sediment reworking (Levson and others, 2012). Environments that allow for aeolian abrasion produce exceptional rounding of grains (Dott, 2003). It has been observed that sand in aeolian settings ex- periences abrasion from wind that can be 100 to 1,000 times more effective at rounding grains than transport by water (Kuenen, 1959, 1960). Additional characteristics of these sands that are consistent with recycling are mixed sources, upward maturation, association with major unconformities, and an inverse relationship between la- bile (unstable) grain content and grain size (Dott, 2003). The history and provenance of the source quartz sand affects the strength or hardness of the grains. Quartz grains that have un- dergone metamorphism or tectonic shear stress may contain weak planes that may fail under the high pressure conditions to which the sand is exposed during hydrofracking (Zdunczyk, 2007; Levson and others, 2012). Also, single-crystal (monocrystalline) sands have greater compressive strength than do grains consisting of multiple intergrown crystals (polycrystalline) (Roberts, 2009). Post-depositional diagenesis can add textural maturation to multi-cycled sands or can independently form pure quartz arenites (Dott, 2003). Long periods of land stabilization are necessary for intense weathering (Dott, 2003). GEOLOGIC ORIGIN AND PRESERVATION OF FRAC SAND DEPOSITS