Bedrock exposures across the majority of Illinois are sedimentary in origin, ranging in age from Cambrian along the LaSalle Anticline near Joliet to scattered outcrops of Cretaceous and Tertiary age at sites along the Mississippi River. Pennsylvanian (Late Carboniferous) rocks blanket more than half the surface, although they are covered by a few tens to hundreds of feet of glacial drift over the northern two thirds. A few igneous dikes of Permian age crop out at Hicks Dome near Rosiclaire, on the Ohio River. These are the only non-sedimentary surface exposures in the state.
Since Precambrian time, the U. S. Midwest has occupied a position on the North American continental shield. The earliest known sedimentary rocks in the region are Cambrian in age, and comprise a transgressive sequence of sandstone, shale, and limestone. The Cambrian Mt. Simon Sandstone filled a surface with moderate topographic relief, and thus is of irregular thickness. The Mt. Simon is thickest in the north, where it exceeds 2500 feet thick; but is locally between 500 and 1000 feet thick in southern Illinois. After deposition of the Mt. Simon, the region formed a vast, stable carbonate platform that underwent millions of years of gentle subsidence. Rocks deposited during this period, the Knox Supergroup, range from a few hundred feet thick in northern Illinois to at least 4000 feet and perhaps as much as 6000 feet at the southernmost tip.
A regional unconformity places Middle Ordovician St. Peter Sandstone directly on Knox and older strata, with locally pronounced karst development on underlying carbonates. Ordovician to early Devonian strata in the area are dominated by carbonates and dark shales. A band of Niagaran-age pinnacle reefs tracks southwestward from Chicago toward St. Louis. East of this trend, thin Silurian strata represent sediment-starved conditions. A second regional unconformity places Late Devonian strata of the New Albany Shale and equivalents atop a surface ranging from Middle Devonian near the Indiana border to Lower Ordovician on the Kankakee and Pascola Arches.
From Mid-Mississippian to Late Pennsylvanian time, a shallow cratonic basin covered the southern half of the state, along with southern Indiana and western Kentucky. This basin is known as the Illinois Basin. A heterolithic mix of thin sedimentary units filled this basin, with deposition keeping apace of gentle subsidence throughout the period. Mississippian and some Pennsylvanian strata form major petroleum reservoirs in the basin. Cyclic Pennsylvanian deposition resulted in widely-distributed coal deposits, making the Illinois Basin the USA’s most important coal province.
The sedimentary section is thickest near the depositional center of the Illinois Basin in southeasternmost Illinois. The deepest exploratory well in the state is the Mary L. Streich #1 (Pope County) at slightly less than 15,000 feet deep. This 1976 dry hole reached total depth of 14,920 feet in the Mt. Simon. Total sediment thickness in this deepest portion of the basin includes more than 3500 feet of Mississippian and Pennsylvanian strata of the most petroleum-productive portion of the stratigraphic column, plus more than a mile of Knox-age carbonates and more than 1000 feet of Mt. Simon. As the well did not reach Precambrian granite before being abandoned, the total thickness of Phanerozoic (Cambrian and younger) sedimentary rocks in the deepest part of the Illinois Basin is likely to exceed 15,000 feet.
There is seismic evidence, albeit limited, that layered rocks of Precambrian age occur within the center of the ancient Reelfoot Rift that defines the trend of the Mississippi River south of its confluence with the Ohio River. The thickness of this package, if indeed it is of sedimentary origin, is unknown. Until more definitive imagery of these rocks is obtained or they are penetrated by a well, the maximum thickness of sedimentary rocks in Illinois can only be said to be at least 14,920 feet; the deepest sedimentary rocks encountered to date.
References:
Illinois State Geological Survey Bedrock Geological Map (http://isgs.illinois.edu/maps-data-pub/pdf-files/bedrock-frnt.pdf)
Midwest Geologic Sequestration Consortium (http://sequestration.org/basin.htm)
American Association of Petroleum Geologists Datapages (http://search.datapages.com/data/doi/10.1306/C1EA43FB-16C9-11D7-8645000102C1865D)
Illinois Oil and Gas Association (http://www.ioga.com/Special/Geohist.htm)
