Building the Bedrock
Through several billion years of geologic time, the geology of the region that includes what is now called Illinois underwent many changes (see figure 1). The oldest rocks in the state belong to the Precambrian basement complex. Little is known about these rocks because they are not exposed at the surface anywhere in Illinois and can't be seen directly. Only a few drill holes have reached deep enough to sample these igneous and metamorphic rocks.
FIGURE 1: Generalized geologic column showing the succession of rocks in Illinois (thick black line indicates rocks with known oil reserves).
These rocks were once exposed at the land surface, from about 1 billion to 0.6 billion years ago. From the time of their formation through the middle Cambrian Period, they were deeply weathered and eroded. During this time, tectonic forces in the midcontinent region began to rip apart the North American continent, forming rifts—long, narrow, continental troughs.
From about 543 to 323 million years ago, during the Cambrian through Mississippian Periods, the rifting stopped, and the region's landscape began to sink slowly, allowing the invasion of a shallow, tropical ocean from the south and southwest. Illinois was immersed in this shallow sea. The sands deposited in those oceans became sandstone, and the formation of carbonate mud and the deposition of billions of marine organisms, such as shells, algae, and corals, formed limestone. The extensive beds of the Mississippian Period are more than 3,000 feet thick in some parts of Illinois and contain limestone deposits that hold large amounts of oil and fluorspar.
FIGURE 2: Location of the Illinois Basin
From about 323 to 290 million years ago, during the Pennsylvanian Period, the oceans advanced and retreated many times. During retreats, large river systems buried the shallow ocean bed under a series of deltas. Vast swamps grew on the ancient deltas and formed thick deposits of peat. Eventually these peat deposits were buried and compressed over millions of years to form the coal beds found in the Pennsylvanian rocks. All of the coal in Illinois comes from this time period. The Pennsylvanian rock, which is widespread in Illinois, also contains important deposits of limestone, shale, clay, sandstone, and some oil and gas.
The deltas of the Pennsylvanian Period later covered the entire state, building up great thicknesses of sediment as the land underneath gradually subsided over millions of years. These sediments compacted to form layers of rock that are over 2,000 feet thick in southern Illinois. During this time of oceans and deltas, as tectonic forces gently folded the bedrock of Illinois, a large basin was formed centered in the southeastern part of the state. This basin is known today as the Illinois Basin. Because of the basin shape, the Paleozoic Era rocks are much thicker in southern than in northern Illinois (figures 2 and 3).
During the last part of the Paleozoic Era (Permian Period, 290 to 248 million years ago), throughout the Mesozoic Era (248 to 65 million years ago), and through most of the Cenozoic Era (65 to 1.8 million years ago), Illinois had a rolling topography. With time, water carved valleys into the exposed and weathered rocks of the ancient ocean and deltas, and great rivers flowed through Illinois removing sediment, much like today. The combination of flowing waters, blowing wind, and the freeze-thaw cycles eroded the bedrock and transported the sediments, thus creating an eroded land surface.
FIGURE 3: Stylized north-south cross section shows the structure of the Illinois Basin. To show detail, the thickness of the sedimentary rocks has been greatly exaggerated, and younger, glacial deposits have been eliminated. The oldest rocks are Precambrian (Pre-∈) granites. They form a depression filled with layers of sedimentary rocks of various ages, primarily Cambrian (∈), Ordovician (O), Silurian (S), Devonian (D), Mississippian (M), Pennsylvanian (P), Cretaceous (K), and Tertiary (T). Scale is approximate.