Part 2

Next below the Pennsylvanian are the rocks of the Mississippian System, shown in blue on the map (M¹ and M²). The lower and middle Mississippian rocks (M¹) are largely limestone in and near the areas mapped, but in the central and eastern part of the state where they are buried under Pennsylvanian rocks they contain much siltstone and cherty limestone. The upper Mississippian rocks (M²) consist of a succession of sandstone, shale, and limestone formations.

This system of rocks takes its name from the Mississippi River because there are excellent exposures of these strata along the Mississippi Valley in western Illinois, southeastern Iowa, and eastern Missouri.

The Mississippian rocks are a source of limestone, fluorspar, zinc, and ganister, and are of greatest economic significance in southeastern Illinois where they are the most important of our oil-producing rocks.

The Devonian (D, dark gray on the map), Silurian, (S, violet), Ordovician (O, light pink), and Cambrian (C, deep pink) rocks, in the order named, are older than the Mississippian strata. In general, they include dolomite, limestone, shale, and sandstone. Except for small areas along the Mississippi and Illinois River Valleys, these older rocks are found at the surface only in the northern quarter of the state and locally in Alexander, Hardin, Jackson, Monroe, Pike, and Union Counties. They are nevertheless economically important because they yield limestone, dolomite, silica sand, oil, zinc and lead, tripoli, novaculite, and novaculite gravel.

The rocks of the Cambrian through Pennsylvanian Systems belong to the Paleozoic Era. The Paleozoic rocks overlie crystalline rocks, such as granites, that extend to unknown depths in the crust of the earth. The crystalline rocks are not exposed in Illinois but are encountered in the drilling of some deep wells and may be seen in the nearby Missouri Ozarks and in central Wisconsin.

STRUCTURAL GEOLOGY

The rock formations appear to lie flat in most of Illinois, but they are slightly inclined in most places. In some areas they are down-warped into basins and troughs (synclines), upfolded into domes and arches (anticlines), or broken by faults.

The largest structural feature in Illinois is a great spoon-shaped basin—the Illinois Basin—that extends southeastward into Indiana and Kentucky. The deepest part of the basin is in southeastern Illinois.

Because the entire region, including the basin, has been eroded by rain, ice, wind, and many rivers and streams, the youngest Paleozoic rocks (top-most layers) are preserved only in the middle of the basin, as shown in figures 3, 6, and 7. This is why the map shows the coal-bearing rocks of Pennsylvanian age in the center of the state and the older rocks cropping out in successive bands around the margins of the basin in southern, western, and northern Illinois.

Distribution of bedrock in northern Illinois is influenced by a broad upfold or arch (see figures 4, 6, and 7). The map (plate 1) shows the older rocks (Cambrian and Ordovician) exposed at the center of the arch and surrounded by younger Silurian and Devonian rocks. Another elongated upfold (anticlinal belt) extends from the vicinity of Dixon southeastward into Indiana. The crest of the upfold is indicated on the map by patches of Silurian and Devonian rocks in Champaign and Douglas Counties.

In extreme southern Illinois and in north-central Illinois, the rock layers are broken by great faults (see figures 5 and 6) which displace the layers of rocks by as little as a few inches to as much as 3,000 feet.

The structural map of Illinois (figure 6) shows the position of the major geologic structures. The big basin is indicated by shading, with the darkest pattern showing where the basin is deepest. The same strata that lie at sea level at the basin’s outer edge are downwarped to 6,000 feet below sea level in its deepest part. Axes of some small anticlines and locations of major fault zones also are shown.

KANKAKEE ARCH MISSISSIPPI R. ARCH LA SALLE ANTICLINAL BELT OZARK UPLIFT KEY Position of major anticline Major faults

HISTORICAL GEOLOGY

Reading the history of the earth’s crust is like reading a mystery story. The geologist must examine, or read, each rock layer, from the youngest at the top to the oldest at the bottom, searching for clues to its origin, age, and development from which he can reconstruct the past.

One of the clues to the past is the fossil remains of plants and animals found in the rocks. For instance, if a certain coral is known to have lived only during one span of time, all rocks containing that coral fossil must have been formed within that span.

Records of wells (well logs) and rock cores or samples collected when test holes are drilled into the bedrock also reveal much about the sequence of rock strata beneath the surface. The kinds of rocks encountered tell, in addition, something about ancient geography, for some were formed on land and others were deposited in long-departed seas.

Such knowledge is extremely important in finding and developing mineral resources such as coal and oil.

ECONOMIC GEOLOGY

Minerals produced commercially in Illinois include crude oil, coal, limestone, dolomite, clay, sand, gravel, fluorspar, tripoli, ganister, novaculite gravel, silica sand, and the metals zinc and lead. The distribution of mineral industries, shown in the several maps of figures 8 through 12, is, of course, related to the distribution of the rocks (plate 1). For example, the coal mines are scattered along the margin of the area of Pennsylvanian rocks where the coals are at relatively shallow depths.

The mineral fuels, coal and petroleum, are the leading mineral products of Illinois, making up about 70 percent of the annual value of all minerals produced in the state. For many years coal was at the top of the list, but petroleum now holds first place.

Petroleum

In recent years production of petroleum has averaged about 78 million barrels each year, making Illinois the eighth largest petroleum producing state. The major oil area is the deep part of the Illinois Basin in the southcentral and southeastern parts of the state, but significant discoveries have been made recently in central and western Illinois. There are about 490 oil fields in Illinois, ranging from a few to several thousand acres in size.

Oil has been discovered in rocks of the Pennsylvanian, Mississippian, Devonian, Silurian, and Ordovician Systems, but the Mississippian are the most productive and account for about 76 percent of our total oil production.

KEY SHIPPING MINES Underground Strip Slope LOCAL MINES OVER 25,000 TONS Underground Strip

Coal

Illinois ranks fourth in the nation among states that produce bituminous coal, the type of coal most widely mined in the United States. Coal is used by many manufacturing industries, by public utilities companies that burn coal to generate electricity, and by individuals for heating homes.

All Illinois coal deposits are found in the Pennsylvanian System. Of the many coals, only a few are thick enough to be mined profitably. The coal is of bituminous rank, but it varies in quality. Illinois coal reserves in strata more than 28 inches thick are estimated at 137 billion tons and are larger than those of any other state.