Summary:
The silica sand at the Sumner Property is hosted within the Cambrian Wonewoc Formation. At Sumner, the Wonewoc Formation sandstone varies in thickness from 65.6 to 131.2 feet (20 to 40 m) and is principally medium to coarse grained quartzose sandstone. The Wonewoc Formation is overlain by very fine to fine grained sandstone, siltstone and mudstone of the Tunnel City Group, and thin and discontinuous Pleistocene surficial deposits (slightly gravelly sandy loam till).
Silica (frac) sand mining activity in the northwestern part of Wisconsin, primarily in Barron and Chippewa counties, has concentrated on mining the Wonewoc and Jordan formations from silica sand strata that is situated on lower hillsides and hilltops, respectively. At the Sumner Property, the regional geological map of Mudrey et al. (1987) effectively shows that the Wonewoc Formation is the primary silica (frac) sand mining target as the sandstone unit is generally situated right through the middle of the Property.
As you approach the Sumner Property from west to east, the topography rises along an elongated roughly north-south ridge, and at the Sumner Property, the lower-benched slopes are composed of Wonewoc Formation that is either exposed or overlain by variable thicknesses of Tunnel City Group and/or Pleistocene till and surficial deposits. These topographic ‘benches’ are thought to represent paleoshorelines formed during fluctuating sea level conditions.
In general, the upper Wonewoc contact(s) is sharp and easily distinguished. The mine process confidently strips off the Pleistocene surficial deposits (overburden) and/or the Tunnel City Group to access the Wonewoc Formation silica sand. The overburden consists of dark grey to reddish dark grey, clay-rich sandy till with abundant pebbles and minor cobbles; a thin (<1') iron-stained regolith occurs at the base of the overburden. The basal portion of the Tunnel City Group consisted of fine-grained sandstone and siltstone with a higher component of mudstone in comparison to the underlying Wonewoc Formation. It is evident that there is ‘regional’, and even ‘local’ variation associated with the thickness of the overburden and Tunnel City Group overlying the Wonewoc Formation. Strip ratio is an important criterion for mining, and it is imperative that SES maps out local variations of thickness of the units overlying the Wonewoc Formation to direct the progressive direction of the open pit mine.
The Wonewoc Formation is dominated by white to iron-stained, medium to coarse grained quartzose sandstone. The overall observation of the mine pit face(s) is that the Wonewoc is stratigraphically continuous, and uniformly, is composed of clean, white silica sand. The stratigraphy can be traced laterally with the aid of minor, thin, continuous claymudstone bands that are likely associated with inter-tidal and/or transitions in marine, near-marine and non-marine deposition. In general, the mudstone and/or mudstonesandstone intercalated horizons appear to be thin less than one foot (<31 cm) in thickness.
Fresh Wonewoc Formation silica sand was observed from an auger that was actively drilling a blast grid adjacent to the mine face; here the auger return material was composed of white to slightly iron-stained medium to coarse grained silica sand. The driller noted that drilling was remarkably consistent apart from a narrow zone of clay about halfway through the Wonewoc section.
The basal portion and lowermost contact of the Wonewoc Formation is not as clearly understood as the upper contact. This is because most of the auger drilling to date, including deeper groundwater monitoring holes, has defined a water table elevation of approximately 1140 feet (347.5 m) above mean sea level (amsl) on the Property. This groundwater elevation establishes the lowermost mining extents as defined by the Development Agreement, which states that mining operations shall maintain a minimum of five feet (1.5 m) separation from groundwater table. Accordingly, SES has not drill tested below this depth with regularity to define the detailed extent of the lowermost Wonewoc Formation.
Mineralization
The Paleozoic age bedrock layers of quartzose sandstone in the central mid-continent of North America are known as some of the most mineralogically pure sandstone on Earth with greater than 95% of the sand grains consisting of silicon dioxide (SiO2). Whole rock chemical analysis (x-ray fluorescence) of the Wonewoc Formation sandstone, which was conducted by the Department of Natural Resources (Brown, 2012), shows that the Wonewoc silica sand consists of: Silicon dioxide (SiO2; 99.20-99.70%); Aluminum oxide (Al2O3; 0.10-0.19%); Calcium oxide (CaO; 0.08-0.21%); Iron oxide (Fe2O3; 0.06-0.03%; Potassium oxide (K2O; 0.05-0.14%); Sodium oxide (Na2O; 0.002-0.003%); Magnesium oxide (MgO; 0.01-0.02%); and Titanium oxide (TiO2; <0.01%).
In addition to being composed mostly of quartz, a mineral known for being of high strength and relatively inert, the grains are especially well-rounded, well-sorted, coarsegrained, and poorly cemented. The advanced level of textural maturity in Cambrian quartz grains, including the Wonewoc Formation, remains more uncertain, but is believed to be related to chemical weathering that may have preferentially dissolved plagioclase and similarly unstable minerals, and a long history of abrasion in marine conditions and wind abrasion (Morey, 1972; Odom, 1975, 1978; Dott et al., 1986; Runkel, 1998; Dott, 2003).
Lastly, grain size is an important factor in determining the value of a silica sand deposit because, for example, the 20/40 mesh sand fraction typically has a relatively high value because of its demand for specific hydrofracturing procedures, and the 20/40 fraction is relatively scarce in silica sand deposits elsewhere on the continent (Beckwith, 2011). Runkel and Steenberg (2012) synthesized grain size data from Ostrom (1971) and Thiel (1957) for the Jordan, Wonewoc, Mt. Simon and St. Peter formations from throughout Wisconsin (Figure 7.6); the histogram shows that:
• St. Peter sandstone has a relatively small percentage of 20-40 mesh sand and contains the highest proportion of sand finer than 100 mesh.
• The Wonewoc and Mt. Simon sandstones generally have a diminished coarser fraction compared to the Jordan.
• The St. Peter, Jordan and Wonewoc have similar 40/70 mesh contents.
Despite the relatively finer grain size in comparison to the Jordan Formation, the Wonewoc sandstone can be mined for multiple markets including those oil and gas hydrofracking plays that are asking for a smaller proportion of coarser grained silica sand (Brown, 2014).