Region 1: The Central Lowland

The rocks of the Central Lowland, extending from northern Missouri southward into northern Texas, are primarily marine in nature. These sediments were mainly deposited during the late Paleozoic, as a shallow sea covered most of the area during the Carboniferous and Permian periods (Figure 2.2).

Figure 2.2: Generalized geologic map of the Central Lowland.

Missouri’s Central Lowland is composed almost entirely of Carboniferous-aged deposits, produced from the erosion of eastern mountains formed by the Appalachian Orogeny. Enormous quantities of sediment were transported westward and deposited within the shallow sea that covered the region. The dominant rocks in this area are Pennsylvanian in age, with a thin strip of Mississippian along their eastern margin. In northwestern Missouri, repeating patterns of marine limestones and shales interbedded with non-marine sandstones indi-cate that the shallow sea advanced and retreated many times over this area of the state. The Carboniferous rocks of Missouri are famous for their fossil deposits as well as being important economic sources of lead and zinc. Beneath these deposits lie Precambrian igneous and metamorphic rocks, which form the core of the continent and the basement of the Interior Plains.

Northern Missouri showcases the effects of recent glacial advances in deposits of glacial drift and wind-blown loess. The Loess Hills, found along the Missouri River Valley, extend from Iowa into the northwestern corner of Missouri (Figure 2.3). The area also contains plentiful glacial erratics—these large boulders, transported from as far north as Canada, are most commonly composed of granite and quartzite.

Figure 2.3: Cross-section of the Loess Hills.

Why are there different sedimentary rocks in different environments?

Most sedimentary rock deposited in underwater settings originated from material eroded on land and washed down streams or rivers before settling to the bottom of a body of water. Intuitively, the faster the water is moving, the larger the sediments it may carry. As the water slows down, the size of sediments it can carry decreases. Furthermore, the farther the grains of sediment are carried, the more rounded they become as they are tumbled against each other. In this way, rivers emptying into a sea are effectively able to sort sediment. Near the mouth of the river, the water is still relatively high-energy, dropping only the largest pieces; farther from the shore, the dropped particles get smaller. Therefore, conglomerates and sandstones are interpreted to have been deposited on or near the shore, siltstone farther from the shore, and shale in deep water quite far from shore where currents are slow enough that even very tiny particles may settle.

Increased distance from shore and water depth can also reduce the presence of oxygen in the water, causing organic material to decompose less completely. This causes darker, carbon-rich rocks (including some that contain exploitable fossil fuels) to form in these areas. Limestone is made primarily of calcium carbonate, the components of which are dissolved in the water. Living creatures, like coral and foraminifera, take those components out of the water to make calcium carbonate shells, which, after the creatures die, accumulate to become limestone. These shelled creatures tend to fare better in clear water, so limestone usually forms far from other sources of sediment. While this process happens over much of the seafloor, if more than 50% of the sediment being deposited is from another source, the rock that forms is, by definition, not limestone.

In Kansas, exposed Paleozoic marine rocks decrease in age as one moves westward. The oldest rocks, from the Pennsylvanian, dominate the eastern margin of the state, and chiefly contain sandstones, shales, and limestones. The Cherokee Lowlands, an area of Pennsylvanian strata located in the state’s southeastern corner, are extremely rich in coal; this area is the largest center of coal mining in Kansas. To the west lie the Flint Hills, a band of Permian rocks containing prevalent chert and limestone. Many buildings in this area are constructed out of local limestone (Figure 2.4).

Although most surface rocks in Kansas’ Central Lowland are sedimentary, Woodson and Wilson counties are known for their exposures of lamproite, an intrusive igneous rock that pierced the Paleozoic shales during the Cretaceous. The northeastern corner of the state, while underlain by the same Pennsylvanian and Permian sediments mentioned previously, is covered in glacial drift deposited during the ice age.

Figure 2.4: The Chase County Courthouse in Cottonwood Falls, Kansas, constructed from Flint Hills limestone.

Shallow seas covered Oklahoma during most of the Paleozoic, leading to sedimentary deposits overlying nearly all of the Precambrian igneous and metamorphic bedrock in the Central Lowland. Only a single exposure of Precambrian rocks occurs in the state: the Arbuckle Mountains, which contain a 1.4-billion-year-old core of gneiss and granite capped by Cambrian rhyolite. About 750 million years ago, a rift began to form in what is now southwest Oklahoma. Magma rising to near the surface cooled to form a variety of granitic rocks, which are now the oldest parts of the Wichita Mountains, and the only Oklahoman Cambrian rocks other than those found in the Arbuckles.

During the Carboniferous, rivers flowing through Oklahoma deposited vast deltas, producing a swampy landscape that was periodically flooded by rising sea levels. The alternation of marine and non-marine environments led to the creation of cyclothems (Figure 2.5), alternating sequences of terrestrial and marine sedimentary layers dominated by thick limestones and dolomites. These Pennsylvanian formations cover almost 25% of Oklahoma’s surface.

The most intense period of mountain building in the region also occurred during the Pennsylvanian, with the uplift of the Ouachita, Wichita, and Arbuckle mountains. This orogenic episode downwarped the crust and created basins where thick layers of shale, sandstone, and limestone were deposited. In central Oklahoma, Permian rock predominates; it includes sand and mudstones made up of eroded sediment from the newly-formed Ouachitas, as well as thick layers of salt and gypsum deposited once the sea level began to regress.

In Texas, the Central Lowland is dominated by Permian rock, with a band of Pennsylvanian sediments and a small segment of Cretaceous shoreline sediment found to the east. After the Pennsylvanian rise of the Ouachitas to the north in Oklahoma, shallow seas were filled with sediment. As the inland seas retreated, the area became home to broad evaporite basins, where deposits of salt, gypsum, and red muds formed throughout the hot, arid environments of the late Permian (Figure 2.6). Today, these famous red beds are home to an extensive fauna of fossilized Permian amphibians, reptiles, and synapsids (ancient relatives of mammals).

Figure 2.5: An example of a cyclothem.

Figure 2.6: The Permian-age gypsum and red mudstones of the Quartermaster Formation are exposed in the red cliffs of Caprock Canyons State Park in Briscoe County, Texas.