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).
The Wonewoc Formation and other Cambrian-aged silica sand units in the midcontinental United States formed during a major transgressive event where shallow ocean currents formed a texturally graded shelf. The advanced level of textural maturity in Cambrian quartz grains is thought to be related to a long history of weathering that included wind and marine shoreface abrasion and chemical weathering that preferentially dissolved plagioclase and similarly unstable minerals.
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.
During the personal inspection, the senior author observed two distinct mine faces: 1) an ‘older’ mine workings featuring a mine face with an 80 foot (24 m) wide stratigraphic section; and 2) a 40’ section (12 m) mine face in an area of mining. The mine walls provided a visual view of the stratigraphic uniformity of the Wonewoc Formation at this locale, and of the geological contacts between the Wonewoc Formation and the overlying Pleistocene surficial deposits and/or the Tunnel City Group.
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.
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.
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%).