Failed rift complex in early to middle Cambrian, followed by subsidence throughout the Paleozoic. Major deformation occurred during the Ouachita and Allegheny orogenies, leading to formation of many major structures and fault systems.
GEOGRAPHY
The Illinois Basin is an interior cratonic basin covering approximately 60,000 square miles (155,000 km²) in central and southern Illinois, southwestern Indiana, and western Kentucky.
MAJOR STRUCTURAL FEATURES
The most significant structures within the Illinois Basin are the LaSalle anticlinal belt, the DuQuoin monocline, and the Cottage Grove-Rough Creek-Shawneetown fault system.
STRATIGRAPHY
The Basin contains primarily sedimentary rocks ranging from Cambrian to Pennsylvanian in age. A small section of Permian sediments are preserved in a graben in Union County, Kentucky. Limited areas of Cretaceous and Tertiary deposits occur in western and southern Illinois. Most of Illinois and Indiana are covered by Pleistocene glacial drift. Major unconformities are Precambrian-Cambrian, middle Ordovician, middle Devonian, Mississippian-Pennsylvanian, and Pennsylvanian-Pleistocene.
THICKNESS
The deepest point occurs in the area of Union and Webster Counties, Kentucky, where the Paleozoic sediments reach a thickness of over 20,000 feet.
ECONOMIC GEOLOGY
Products include oil and natural gas, coal, lead, zinc, limestone, gravel, fluorite, and gypsum.
OIL AND GAS HISTORY
SOURCE OF OIL PRODUCTION
THE ILLINOIS BASIN
