Region 3: Rocks of the Inland Basin

Mississippian Rocks

The Inland Basin is a large geophysical province, and only a small portion of it exists in the Midwest in eastern Ohio, southern Indiana, and Illinois. Here, only a relatively narrow slice of time is represented in the rocks. During the Mississippian and Pennsylvanian periods, the limestones, sandstones, and shales of this region formed. Except for a small band of Cretaceous sedimentary rock in extreme southern Illinois, rock from no other period is found here.

Inland sea may sound like a contradiction in terms, but there is a very simple, yet important, distinction that differentiates it from other seas: an inland sea is located on continental crust, while other seas are located on an oceanic crust. An inland sea may or may not be connected to the ocean. For example, Hudson Bay is on the North American plate and connects to the Atlantic and Arctic Oceans, while the Caspian Sea is on the European plate but does not drain into any ocean at all.

During the Mississippian and Pennsylvanian periods, the Inland Basin region was an inland sea environment, with sediment being shed into the basin from the Acadian highlands to the east (Figure 2.8). Gradually, the amount of sediment settling into the basin declined, as the mountains were weathered down. The shoreline of the inland sea moved back and forth across the basin as the sea level rose and fell during this period. The #fluctuating water levels created alternating sequences of marine and non-marine sedimentary rocks, known as cyclothems, that are characterized by their light and dark colors (Figure 2.9). Limestones were also forming in the inland sea in areas receiving very little sediment. The Midwest was still located near the equator at this time, and the warm climate created lush vegetation. Large swamp forests covered the shorelines, and plant remains accumulated as thick piles of peat. Buried by sediment and more vegetation, the peat was compressed. Over time and through continued burial, the peat was transformed into layers of coal. The Pennsylvanian and Mississippian rocks of the Inland Basin region are the result of shorelines advancing and retreating, creating alternating bands of marine sandstone or mudstone and terrestrial coal deposits, respectively.

See Chapter 1: Geologic History for more information on the position of the Midwest during the Paleozoic.

Figure 2.8: Much of the Midwest region was covered by an inland sea during the Mississippian and Pennsylvanian periods, with sediment being brought in from the erosion of the Acadian highlands to the east.

Figure 2.8: Much of the Midwest region was covered by an inland sea during the Mississippian and Pennsylvanian periods, with sediment being brought in from the erosion of the Acadian highlands to the east.

Figure 2.9: An example of a cyclothem.

Figure 2.9: An example of a cyclothem.

As mentioned above, a portion of the Inland Basin in the southern tip of Illinois is made up of the second isolated outcrop of sandstone and mudstone from the Mississippi Embayment.