Region 1: The Central Lowland

Mineral resources in the Central Lowland have accumulated primarily due to the deposition of sediment (Figure 5.2). The region’s surface rocks are sedimentary strata from the Pennsylvanian and Permian, covered by glacial, river, and wind-blown deposits from the Quaternary and Holocene. Sources of non-organic sediment (sand and finer-grained materials) are derived from erosion, while organic carbonate sediment accumulated in shallow seas to form limestone. Ancient forests produced layers of organic debris that eventually formed coal. All of these depositional patterns have also been influenced by cyclical fluctuations in sea level, producing cyclothems: repeated sequences of terrestrial shale, sandstone, and coal layered with marine shale and limestone (Figure 5.3). Episodes of Quaternary glaciation and erosion in northeast Kansas and northern Missouri left behind discontinuous patches of glacial deposits consisting of till interspersed with sand and gravel outwash. As a result of all these processes, sand, gravel, stone, limestone, and clay occur abundantly throughout the Central Lowland, and all are quarried for use in construction. Industrial sand is mined at several locations in Oklahoma and Texas, primarily for use in glassmaking.

Figure 5.2: Principal mineral resources of the Central Lowland.

Figure 5.3: An example of a cyclothem.

During and after the formation of Pangaea, the humid and tropical Carboniferous environment transitioned to the hot and arid climate of the Permian. These arid conditions led to the formation of hypersaline, shallow seas with restricted circulation, and layers of evaporite minerals were deposited as these seas evaporated. Today, Permian evaporite beds in Kansas, Oklahoma, and Texas are mined for halite and gypsum.

Halite is mined in two ways. When deposited in thick beds, salt can be excavated by mechanically carving and blasting it out. This method, called “room and pillar” mining, usually requires that pillars of salt be left at regular intervals to prevent the mine from collapsing (Figure 5.4). Another method, called solution mining, involves drilling a well into a layer of salt. In some cases, the salt exists as part of a brine that can then be pumped to the surface and the water then removed, leaving the salt behind. In others, fresh water is pumped down to dissolve the salt, and the solution is brought back to the surface where the salt is removed (Figure 5.5)

Figure 5.4: In pillar and room mining, the mine is divided up into smaller areas called “panels.” Groups of panels are separated from one another by extra-large (barrier) pillars that are designed to prevent total mine collapse in the event of the failure of one or more regular-sized (panel) pillars.

Selenite, a variety of gypsum, is commonly found where Permian rocks appear at the surface, most notably in the Salt Plains of Oklahoma. In many locations, crystals of selenite are impregnated with sand and clay and are often referred to as “sand crystals.” In Salt Plains National Wildlife Refuge, groundwater seeping through salt- and gypsum-saturated sand becomes concentrated with these minerals, spurring the formation of selenite crystals with a distinctive hourglass-shaped sand inclusion (Figure 5.6). The Salt Plains are the only place in the world where this phenomenon occurs. Barite roses can also be found at the surface in central Oklahoma (Figure 5.7). Due to their attractive form, sand crystals and barite roses are often sought after as collectibles.

Figure 5.5: An example of solution mining that involves the pumping of fresh water through a borehole drilled into a subterranean salt deposit.

Figure 5.6: A selenite crystal from Salt Plains National Wildlife Refuge, Oklahoma with distinctive hourglass-shaped sand inclusion. Individual crystals up to 18 centimeters (7 inches) long have been found at this locality.

Figure 5.7: A barite rose from Cleveland County, Oklahoma. Each rosette measures over 4 centimeters (1.6 inches) across.

Igneous activity has also contributed to the formation of minerals found in the Central Lowland. During the late Cretaceous, eastern Kansas experienced episodes of volcanism. Some magma solidified in the necks of erupting volcanoes, eventually becoming kimberlite. These deposits, exposed by erosion at several locations in northeastern Kansas, yield small garnets. In southeastern Kansas, igneous intrusions led to the formation of lamproite sills and pipes. The lamproite contains shiny flakes of mica (Figure 5.8), which, when first observed in the 1870s, led to reports of silver and the formation of a mining town called Silver City. Although the lamproite does not actually contain silver, it is still mined today for its mica (used in polymers, coatings, and construction). In addition, the lamproite itself is ground and used as a mineral supplement in cattle feed.

Figure 5.8: A piece of lamproite containing abundant mica flakes.

Ancient sedimentation patterns and tectonic activity have favored the placement of widespread fossil fuel resources in the Central Lowland. Processing plants in Texas and Kansas recover commercial quantities of helium gas, an important byproduct of natural gas extraction. Non-commercial deposits of asphalt, formed by the breakdown of petroleum in the underlying rock, are also common in eastern Oklahoma.

The Central Lowland does not contain economically viable metal deposits. However, copper-bearing minerals can be found in the Permian rocks of southern Kansas, parts of Oklahoma, and north-central Texas.