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48 | Frac Sand Insider May 2015 www.rockproducts.com Geology Although the highest quality frac sand owes its origin to multi-cyclical deposition or post-depositional diagenetic processes, mineralogic and texturally mature quartz-rich sands can also form in humid climates through intense single-cycle weathering of underly- ing crystalline basement rock (Dott, 2003). DEPOSITIONAL ENVIRONMENTS FAVORING ACCUMULATION OF FRAC SAND Among the most prospective settings for the accumulation of quartz arenites that produce frac sand are marine shoreline and coastal aeolian environments (Mazzullo and Ehrlich, 1983; Winfree, 1983; Dott and others, 1986; Dott, 2003). In addition to marine shoreface environments, marine intertidal and deltaic settings are considered prospective environments for the generation of frac sand in Canada (Hickin and others, 2010). The perfect example of the ideal combination of frac sand-producing environments is the high-quality frac sand of the Ordovician St. Peter Sandstone of the midwestern United States that formed as coastal aeolian deposits (Mazzullo and Ehrlich, 1983) and marine offshore and shoreface deposits that were reworked by aeolian processes (Winfree, 1983). Settings that produce less ideal sands that may be processed for use as proppant include younger aeolian, glaciodeltaic, and gla- ciofuvial environments (Hickin and others, 2010). Examples of un- consolidated frac sand from glaciofuvial deltaic and aeolian settings are known from northeastern British Columbia, Canada (Hickin and others, 2010). Also identifed as potential unconsolidated sources are Quaternary sand dune deposits derived from older glaciofu- vial or sandy glaciolacustrine sediments and (or) sandy bedrock units that have been reworked along major rivers; and paleobeach ridges and dunes (Levson and others, 2012). In the United States, near Genoa, Nebraska, sand is mined for use as seed sand for resin coating from the highly mature, fuvially deposited, aeolian-re- worked, glacial outwash-derived, unconsolidated Holocene Loup River deposits (Epley, 2014). POST-DEPOSITIONAL PROCESSES AFFECTING NEAR-SURFACE ACCESS TO FRAC SAND Certain post-depositional processes have enhanced surface access to the targeted frac sand deposits. Fluvial erosion by major river systems has exposed Paleozoic frac sand units to the surface. Examples of this are seen along the course of the Mississippi River from Minnesota to Arkansas. As well, tectonic uplift has enhanced the fuvial exposure of Paleozoic frac sand units in areas border- ing the Mississippi River Valley, such as to the east of the Ozark Plateau along the White River in northern Arkansas, where the St. Peter crops out. Complete removal of the units at erosional uncon- formities has left, at the margins, exposures of truncated layers. Tectonism in the Llano uplift of Texas has prevented deep burial and cementation of the Hickory Sandstone Member and has resulted in the faulted patchwork outcrop pattern that guides the location of "Brown" frac sand mines. Much of the frac sand mining in the Midwest is from near-sur- face Paleozoic sandstones of west-central and southwestern Wis- consin; and portions of southeastern Minnesota, northeastern Iowa, and northwestern Illinois referred to as the "driftless area" (Syverson and Colgan, 2004) (Figure 7) (Plate 1). The "driftless area" has long been defned as an area untouched by the advance of the last of the Wisconsinan (35,000 to 10,000 years B.P. [age in time "before pres- ent"]) ice sheets (which includes those of the "last glacial maximum" that occurred 25,000 to 21,000 years ago) or by earlier Wisconsinan (prior to 35,000 years B.P.) ice sheets (Syverson and Colgan, 2004; Syverson and others, 2011). As such, the area was neither stripped of the near-surface Paleozoic strata (as in northern Wisconsin) nor deeply buried beneath glacial till (as in eastern Wisconsin). Over the past ap- proximately 2 million years, large volumes of glacial meltwater drained into the Mississippi River system as the Wisconsinan ice sheets reced- ed, deeply eroding and exposing the nearly fat-lying Ordovician and Cambrian strata in the incised terrain (Runkel and Steenberg, 2012). This has resulted in the exposure of the frac sand source units in river bluffs and hillsides, and in the near surface. A combination of downwarping and burial by glacial sediments has rendered the frac sand source units in the area of southeastern Minnesota westward of Olmsted County nearly inaccessible and, therefore, not economically recoverable at current prices. In this area, subtle downwarping of the Paleozoic beds in the Hollandale embayment, which extends southward into parts of northeastern Iowa, buried and preserved the frac sand source units (Runkel and Steenberg, 2012). These Paleozoic units were later covered with a veneer of unconsolidated glacially derived sediments from tens to hundreds of feet thick, making access to the frac sand units unfeasible in that area, except within major river valleys such as the Minnesota River in Minnesota, where the Paleozoic units that rise toward the eastern fank of the Transcontinental arch have been exposed by fuvial erosion (Runkel and Steenberg, 2012). Post-glacial depositional processes have infuenced the distribu- tion and thickness of overburden on the frac sand source units of the Midwest. Although the glacial outwash drainage system in the driftless area cut deeply into the Ordovician and Cambrian beds of western Wisconsin and extreme eastern Minnesota, removing major thickness- es of overburden from most of the frac sand mining area, relict glacial outwash deposits obscure portions of the river valleys (Runkel and Steenberg, 2012). Quaternary loess deposits also form several meters of cover on upland divides in the driftless area of western Wisconsin (Syverson and others, 2011). Mineability of the frac sand units may not be locally feasible because of the thickness and nature of the overbur- den (Runkel and Steenberg, 2012). POST-DEPOSITIONAL PROCESSES AFFECTING THE QUALITY OF A POTENTIAL FRAC SAND SOURCE Post-depositional processes during burial and diagenesis can alter the sphericity and roundness of grains and the friability of the sandstone unit. Such alterations are not uncommon in many of the frac sand source units. Despite the overall desirability of St. Peter Sandstone as a frac sand source, there are local areas in which portions of the unit natu- rally exhibit characteristics that are less optimal for use as frac sand.