Summary:
The silica sand at the Blair Property is hosted within the Cambrian Wonewoc Formation. At Blair, the Wonewoc Formation sandstone varies in thickness from 40 to 130 feet (12 to 39 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).
Bedrock underlying Trempealeau County consists of Cambrian sandstone, shale, and sandy dolomite, overlain by Ordovician dolomite and sandstone. Cambrian rock units include Elk Mound Group (Mount Simon, Eau Claire and Wonewoc formations), the Tunnel City Group (undifferentiated) and the Trempealeau Group (St. Lawrence and Jordan formations).
The lower Eau Claire-Wonewoc (Galesville Member) contact marks the end of one transgressive/regressive sequence and the beginning of a major transgressive sequence associated with the Wonewoc Formation. The Wonewoc transgression is defined by highenergy conditions with a noticeable lack of clay, silt, very fine sand, and a total lack of fossils. Above the Galesville Member, the Wonewoc (Ironton Member) formed in an alternating high and low energy environment seaward of the beach front. Ironton Member sandstone is well sorted, clean, medium- to coarse-grained quartz-arenite. The uppermost Ireton Member ends in sharp contact with the Lone Rock Formation of the Tunnel City Group. The lithology of the Lone Rock Formation is easily distinguished from the underlying Wonewoc Formation in that the Lone Rock comprises fine-grained glauconitic, thin bedded shaly units.
The position of the Wonewoc Formation is consistent with this unit being situated within topographic ridges of a rugged landscape that is associated with the Driftless Area of southwestern Wisconsin.
At the Blair Property, the Wonewoc Formation is situated between a topographic low (the Trempealeau River) and a ridge to the west.
The ‘floor’ portion of the Blair Property, in which the Wet-Processing Plant is situated, occurs within the Wonewoc Formation at an elevation of approximately 930 feet above sea level (284 m asl).
The Wonewoc Formation within the open pit excavation (Blair Property) is dominated by white to iron-stained, medium to coarse grained quartzose sandstone.
The overall observation of the open pit face(s) at the Blair Property is that the Wonewoc is stratigraphically continuous, and uniformly, is composed of clean, white silica sand. Intercalated mudstone-sandstone horizons appear to be thin less than one foot (<15 cm) in thickness.
The overburden consists of dark grey to reddish dark grey, clay-rich sandy till with abundant pebbles and minor cobbles; the basal portion of the Tunnel City Group was not seen either Property but does occur on the elevated portions of the ridges where it consists of fine-grained sandstone and siltstone with a higher component of mudstone in comparison to the underlying Wonewoc.
The Wonewoc Formation silica sand was also observed in archived auger clippings from Blair; here the auger return material was composed of white to slightly iron-stained medium to coarse grained silica sand.
The basal portion and lowermost contact of the Wonewoc Formation was not observed (in outcrop or in auger return material).
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 99.20-99.70% silicon dioxide (SiO2). 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, coarse-grained, and poorly cemented.
A photomicrograph of a composite 20/40 fraction of Wonewoc Formation sand from the Blair Property shows mineralization at the Property comprises a high degree of roundness and high percentage of quartz.
The size of the sand grains is also an important factor in determining the value of a silica sand deposit. For example, the 20/40 mesh sand fraction typically has a high value because of its demand in 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; the histogram shows that:
• St. Peter sandstone has a relatively small percentage of <40 mesh sand and contains a higher 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 markets that request a finer proportion of silica sand for their specific operations (Brown, 2014).