Rocks of the Inland Basin

Overview

The Inland Basin is a large geophysical province that extends over much of the central and Southeastern U.S. and includes includes all of West Virginia, much of Kentucky and Tennessee, and portions of northern Georgia and Alabama. Inland from the mountain-building events that occurred throughout the Paleozoic, the Earth’s crust was buckled (downwarped) into a series of depressions called "basins" that give the region its name

There are two major basins in the Inland Basin region—the Appalachian and Illinois basins—separated by the Cincinnati Arch and its branches. Other, smaller basins have existed throughout the region at various times through geologic history. One notable area of deposition is the Black Warrior Basin of northern Alabama and Mississippi (at the southern tip of the Appalachian Basin), which is a particularly important area for fossil fuel production.

Since the Inland Basin was not at the center of the tectonic collisions that occurred during the Paleozoic, there are almost no igneous intrusions exposed at the surface, and the rocks here were not metamorphosed as they were in the Blue Ridge and Piedmont region. The easternmost section of this region, however, called the Valley and Ridge, was squeezed into tight folds during the Taconic, Acadian, and Alleghanian orogenies. The Appalachian Plateau, the central section of the Inland Basin, was broadly folded as the effects of mountain building decreased to the west (away from the collision). In contrast, the westernmost section of the Inland Basin, known as the Interior Low Plateaus, was minimally affected by orogenesis during the Paleozoic.

The Inland Basin is dominated by sedimentary rock thanks to its low topographic relief; basins are naturally excellent places for the preservation of thick sediment layers because they easily collect sediment and often subside from its weight. The rocks of the Inland Basin, including conglomerate, sandstone, siltstone, shale, limestone, and dolostone, reveal the changing depositional environments of the inland sea as it advanced and retreated repeatedly throughout geologic time.

Early Paleozoic Rocks

Following the Precambrian Grenville Orogeny, global sea level began to rise, until most of North America was covered by a shallow inland sea. A period of erosion gradually wore down the Grenville Mountains, and their weathered sediments were carried westward and deposited into the Inland Basin. As the sea widened, sand and mud were deposited near shore, while organically derived carbonates including limestone and dolostone formed in deeper water. Gradually, the amount of sediment settling into the basin declined as the mountains were weathered down. Sea level remained high through the Ordovician, but the reduced sediment supply resulted in the formation of more limestone and dolostone, which are common in warm, shallow, sediment-starved seas. These widespread carbonate rocks are thousands of meters (feet) thick in Kentucky and Tennessee.

As sea level dropped later in the Ordovician, the carbonate rocks were exposed to intense erosion, and many layers of sediment were removed. The eroded layers represent an unconformity, a gap in the geological record where stratified layers have been interrupted or destroyed due to erosion or deformation. A large, regional unconformity occurs at the top of the Ordovician Knox Group—a formation of dolomite and limestone that stretches across eastern Tennessee, northwestern Georgia, western North Carolina, and southwestern Virginia—where several hundred meters (feet) of sediment may have been eroded away.

Toward the later half of the Ordovician, the Iapetus rocks (including the Taconic volcanic islands, Piedmont Terrane, and associated marine sediment) collided with the margin of North America, forming the Taconic Mountains. The Appalachian Basin, created as a result of this collision, was submerged under an inland ocean.

Layers of bentonite clay, altered volcanic ash from volcanic activity during the collision, were deposited in the inland ocean and subsequently preserved within the region’s limestone and shale. In parts of Tennessee, reefs developed along the Appalachian Basin's shallow margin, resulting in the formation of coarsely crystalline limestone that is now referred to as "Tennessee marble." While not technically a true marble, this stone gets its moniker from the fact that it polishes to an attractive and architecturally sound dimension stone. Many prominent structures throughout the US were built using Tennessee marble; these include the National Air and Space Museum, the National Gallery of Art, and the Taft Memorial in Washington DC, as well as the Tennessee Supreme Court Building in Nashville and the historic Knoxville Post Office.

A deltaic wedge of sediment formed on either side of the Taconic Mountains as they eroded. Conglomerates formed close to the highlands, while streams brought sandy, muddy sediment to floodplains, lakes, estuaries, beaches, and into the inland ocean to form sandstone, siltstone, and shale. Sediment from the Taconic highlands spread as far south as northern Alabama and as far west as central Tennessee, but was concentrated mainly in Virginia and West Virginia. Farther away from the highlands, carbonate rocks continued to form, along with sandstone and shale. Thanks to folds, faults, and erosion, Ordovician rocks are exposed in the Valley and Ridge section of the Inland Basin, and along the Cincinnati Arch in Kentucky and Tennessee.