Formation of Hicks Dome in Southern Illinois
Hicks Dome in Hardin County is a feature that is both a topographic and structural dome forming a bull's-eye pattern on a geologic map. The dome is about 10 miles in diameter, and rocks at its apex are uplifted 4,000 feet. Middle Devonian rocks at the center are surrounded concentrically by younger rocks out to Pennsylvanian on the rim. Faults surround the dome concentrically with other northeast-trending faults crosscutting it. (Source: ISGS publication Geology of Illinois).
Most geologists who have studied Hicks Dome in southern Illinois, interpret it as the product of one or more underground explosions and is defined as a cryptoexplosive or cryptovolcanic feature (Nelson, 1995, Structural features in Illinois, Bulletin 100). Drilling into the core of Hicks Dome reveals greatly shattered sedimentary rocks, intermixed with igneous material. Also, ultramafic igneous dikes, breccias, and diatremes are present near the center of Hicks Dome (Bradbury and Baxter, 1992). It has been suggested that Hicks Dome resulted from a meteorite impact, but the surficial and underground structure of the dome clearly indicate that the forces that formed the dome came from below, rather than above the surface.
A technical report, ISGS Circular 550, Intrusive Breccias at Hicks Dome, Hardin County, Illinois, by J.C. Bradbury and J.W. Baxter, 1992, provides more details and can be purchased from ISGS for $5.00.
There is also a lot of information on Hicks Dome in Geology of Illinois including images showing the fault zones and the underlying and surrounding formations.
For more information purchase Geology of Illinois.