Summary:
The Ottawa Operation’s target silica bearing formation is the St. Peter Sandstone, which is a massive formation in areal extent and thickness. Aerially, it extends from Minnesota to Arkansas and from Illinois into Nebraska and South Dakota. On a regional basis, the St. Peter Sandstone ranges in thickness from a few feet to over 1,200 ft, with a general thickness of 100 ft to 200 ft. In northern Illinois, the thickness can be over 300 ft thick.
The St. Peter Sandstone is the only strata of economic interest at the Ottawa Operation, and is very uniform in depositional nature and continuity throughout much of the surrounding region. BOYD considers the subject silica sand deposit to be of low geologic complexity. Furthermore, the geology of the St. Peter Sandstone is well understood after a lengthy history of commercial operations at Ottawa.
Within the defined resource boundaries, the St. Peter Sandstone exhibits: (1) low depth of cover, (2) lateral continuity, (3) a minimum thickness of 70 ft, (4) gentle dipping, and (5) minimal faulting.
The two members of the St. Peter Sandstone—the coarser-grained upper Starved Rock and finer-grained lower Tonti members— are easily identifiable and separable during mining and can be blended as required to meet product specifications. Deleterious materials such as iron (which manifests as orange staining) are easily removed during mining and processing. The sandstone unit is covered by a thin layer of overburden that is generally less than 20 ft thick.
The Ottawa Operation’s sands are generally characterized by a high silica content, high roundness and sphericity, white coloration, and lack of deleterious material. The sandstone is very weakly cemented, allowing it to be mined hydraulically without the need for crushing, which retains the well-rounded grain shape. Because of the monocrystalline structure, these sands have superior grain strength when compared to other silica sands and are suitable for pressure applications generally up to the 9,000-pounds per square inch (psi) range. These characteristics are responsible for the market popularity of the Ottawa Operation’s silica sand products.
Grain size distribution and iron-staining drives the mine planning. Iron tends to be concentrated near the surface and is visible in orange staining. Iron also increases at the bottom sandstone contact, occurring mostly as pyrite. The deposit is coarser in its top half. Where the upper part of the formation is eroded, multiple mining faces must operate to ensure adequate sand is available to meet product specifications.
In the defined resource areas, the St. Peter Sandstone is overlain by a thin layer of overburden material consisting of clay, sandy gravel, peat, and limestone cap rock.