Rocks and fossils through time

Geologists study the rocks, or strata, that have filled the Illinois Basin to learn about the geologic history, resources, and hazards (e.g., earthquakes) of the Basin. The resources may include water supplies, energy resources, construction materials, and strata that can be utilized for sequestration. Geologists acquire the data for these studies by a variety of methods: examination of rock exposures at the earth's surface and in mines, examination of rock samples from wells drilled from the surface, and examination of geophysical data of the strata (e.g. seismic and well logging data). From this large and diverse database, geologists have been able to construct the geologic history of the strata that were deposited within the Basin. Geologists traditionally present this history in the same manner as biography-starting with the earliest history first. For the Illinois Basin, the Cambrian strata were the first to be deposited.

Cambrian (542-488 million years ago) strata in the Illinois Basin are dominated by very thick sandstones; shale and dolomite beds occur only in the upper part of the stratigraphic section. In the Illinois Basin, only the uppermost Cambrian is exposed in Ogle, Lee, and DeKalb Counties in northern Illinois. During the early part of the Cambrian Period, this area of the Midwest was above sea level and it was not until the later part of this time period that the seas covered the Basin. The Cambrian seas left widespread and thick deposits of mostly coastal and nearshore shallow marine sand. Cambrian deposits are up to 3,500 feet thick in the northeastern part of the Illinois Basin. The dominant fossil found in Cambrian strata worldwide, and in the Basin, is the trilobite.

For the next 100 million years (Ordovician to the mid-Devonian), the Basin was dominated by carbonate deposition. This pattern started near the beginning of Ordovician time (488 to 444 m.y.a.) when the marine waters deepened in the Basin and Illinois was the depositional site for limestone (calcium carbonate), most of which was later altered to dolomite (calcium-magnesium carbonate). The best exposures of Ordovician strata in the Basin are in northwestern Illinois and the area around Starved Rock State Park, LaSalle County. Ordovician deposits reach a thickness of over 5,000 feet in the southern part of the Illinois Basin. Worldwide, Ordovician strata are dominated by brachiopods, bryozoans, trilobites, nautiloid cephalopods, echinoderms, and graptolites. In the Illinois Basin, the dominant fossils include brachiopods, gastropods, bryozoans, crinoids, solitary corals, cephalopods, and benthic marine calcareous algae.

The next oldest rocks, deposited in the Silurian (444-416 m.y.a.), outcrop widely in northeastern Illinois and are dominantly composed of limestone and dolomite. Silurian deposits are over 700-feet thick in the northeastern part of the Illinois Basin. Worldwide, Silurian strata are dominated by brachiopods, bryozoans, colonial corals, echinoderms, and eurypterids. In Illinois, the seas were equatorial, and for the first time large reefs — composed of sponges and colonial corals — flourished, along with brachiopods, bryozoans, trilobites, and crinoids.

Stratigraphic column

The long period of carbonate-dominated sedimentation in the Illinois Basin ended during the Devonian Period (416-359 m.y.a.). These Devonian carbonates are over 1,400 feet thick in the southeastern part of the Basin. The best exposures of these strata are at scattered localities along the Mississippi River on the western edge of the Basin. World wide, Devonian strata are dominated by corals, brachiopods, crinoids, goniatitic ammonites, and jawed fish. In Illinois, the main fossils are colonial corals, trilobites, and brachiopods. After 100 million years of mostly carbonate deposition, the succeeding strata consist of a very widespread and thick black organic shale, the New Albany Shale. This shale was deposited primarily in the Late Devonian Period and attains a thickness of about 400 feet in the southern part of the Illinois Basin. It is best exposed in the southeastern part of the Basin in southern Indiana. The most common fossils in the shale are the microscopic conodonts; in associated strata, ammonites occur.

The next stratigraphic interval is the Mississippian Period (359-318 m.y.a.), which was dominated by the deposition of beds of limestone with intervening fine-grained sandstone and shale beds. Mississippian strata are well exposed in southernmost and southwestern Illinois. Mississippian deposits are over 3,200-feet thick in the southern part of the Illinois Basin. Worldwide, Mississippi strata are dominated by brachiopods, bryozoans, corals, goniatitic cephalopods, and echinoderms. These strata, especially the limestones, are very fossiliferous, containing brachiopods, gastropods, bryozoans, and echinoderms.

The end of the Mississippian Period and the beginning of the Pennsylvanian Period marks another notable change in deposition as, for the first time, deposition of significant terrestrial sediments (e.g., organic material which became coal after compaction and dewatering) occurred along with intervening marine deposits. There are many exposures of these strata within the Basin in the southern two-thirds of Illinois, the southwestern part of Indiana, and in westernmost Kentucky. Pennsylvanian deposits are over 3,000-feet thick in the southeastern part of the Illinois Basin. Worldwide, Pennsylvanian strata are dominated by brachiopods, goniatitic cephalopods, fusulinids, bivalves, and many types of land plants, as well as early reptiles and amphibians. In the Illinois Basin, these "coal measures" contain abundant and diverse plant fossils in the terrestrial deposits and brachiopods, gastropods, corals, bivalves, and crinoids in the marine strata.

Strata of the Cretaceous Period (145-65 m.y.a.) are found in western Illinois and in the southern part of the Basin in southernmost Illinois and western Kentucky. The strata are dominated by sand, silt, clay, and gravel. These deposits are up to 100-feet thick in western Illinois and up to 500-feet thick in the southern part of the Basin. Fossil leaves have been found in these strata. Strata of the Tertiary Period are mostly restricted to the southernmost part of the Basin in southernmost Illinois and western Kentucky and can reach a thickness of 400 feet. These strata are mostly marine sands and clay deposits. Poorly preserved gastropods and bivalves occur in these strata.

Rocks of the third period of the Mesozoic Era, the Cretaceous, are present in small areas of western and southern Illinois. This suggests that during the Cretaceous Period, both erosion and deposition of sediments took place within the Basin. This trend continued during the Tertiary Period as these deposits occur in the extreme southern part of the Basin and in scattered outcrops, mostly in the western part of the Basin.

Much of the soil on the surface area of the Illinois Basin has developed on a widespread blanket deposit of the Pleistocene glacial sediments (drift). These deposits are dominated by till, which is a mixture of gravel, sand, silt, and clay. Other sediments include water-deposited sands and silts, as well as wind-deposited sand and silt (also known as loess). These deposits are on the average about 100-feet thick, but in some places, they are up to 500-feet thick.

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