Region 4: The Great Plains

The near-surface geology and mineral resources of the Great Plains region result from a complex suite of factors (Figure 5.17). Marine and terrestrial Cretaceous deposits indicate that several periods of sea level rise and fall were associated with the expansion and contraction of the Western Interior Seaway, which extended from what is now western Illinois to central Utah. Deposition ended with the final retreat of the sea and the uplift of the Rocky Mountains. Uplift in Colorado and New Mexico was renewed in the Miocene and Pliocene, gently tilting the underlying strata eastward. This tilting is more pronounced in the Texas panhandle than it is in western Kansas. Mountain streams transported and deposited large volumes of eroded sediment onto the plains, resulting in a thick blanket of sand, gravel, silt, and clay on top of eroded Mesozoic and Permian strata throughout the region. The sands and gravels here are rich in quartz and feldspar from the weathering and erosion of igneous and metamorphic rocks in the Rocky Mountains to the west. Sand, gravel, limestone, and other construction materials are mined throughout the Great Plains, and building stone is quarried from rocks near the Llano Uplift in central Texas.

Figure 5.17: Principal mineral resources of the Great Plains.

Cyclical changes in climate during the Quaternary and Holocene initiated episodes of stream erosion, uncovering underlying Permian rocks containing layers of halite and gypsum. In and near river valleys, the movement of groundwater dissolved these soluble minerals, further accelerating the pace of erosion. Sediments were carried and deposited by the wind during drought periods and by streams during wet periods, leading to the development of soil horizons rich in deposits of caliche (Figure 5.18). Caliche forms when water infiltrates the soil, dissolves soluble material, and evaporates, leaving behind precipitated minerals in the pore space between soil grains. A zone of cemented material forms within the soil if this happens repeatedly. Layers of caliche accumulate to tens of feet in some locations, and multiple layers are commonly found throughout the Great Plains. Caliche is commonly collected for use as an additive in cement.

Figure 5.18: A shelf of caliche in central Texas.

Beginning in the Miocene, episodes of volcanism in the Western and Southwestern US produced widespread ashfalls that covered much of the Great Plains. Deposits of silicate volcanic ash, as well as sands and gravels derived from the erosion of basaltic lavas, are present in the sediments at many locations in the Great Plains, such as the Pearlette Ash Bed in Kansas (Figure 5.19). This volcanic ash was mined between the 1930s and 1950s for use in concrete, abrasives, and as a cleaning material.

Figure 5.19: The Pearlette Ash deposit in Rice County, Kansas.

Widespread fossil fuel resources in the Great Plains have led to the recovery of several associated elements that are often found alongside gas and oil. Oklahoma is the nation’s sole producer of iodine, extracted from deeply buried gas brines that occur in the Woodward Trench in northwest Oklahoma. Helium and sulfur are recovered from the Hugoton Gas Field in southwestern Kansas and the panhandles of Oklahoma and Texas. This area contains the largest reserve of helium in the United States; helium collected here is piped to the National Helium Reserve in Amarillo, Texas, for safekeeping and storage (Figure 5.20).

Potash is mined commercially from Permian deposits in west Texas. Potash is a name used for a variety of salts containing potassium, with mined potash being primarily potassium chloride. The majority of potash is used as fertilizer, but an increasing amount is being used in a variety of other ways: water softening, snow melting, a variety of industrial processes, as a medicine, and to produce potassium carbonate.

Figure 5.20: The Crude Helium Enrichment Unit located in the Cliffside Gas Field outside Amarillo, Texas.