Hicks Dome, Illinois USA
by: Charles O’Dale
Intrusive Breccias at Hicks Dome, Hardin County, Illinois
Abstract: 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 (Bradbury 1992).
There is evidence that Hicks Dome is volcanic in origin. It is associated with faults and fractured rock, and is accompanied by igneous rocks and mineral deposits. Hicks Dome is a structural dome which has its central Devonian core displaced upward some 4,000 feet in relation to the surrounding strata. The dome has small associated igneous dikes around its flanks.
Explosion pipe in test well on Hicks Dome, Hardin County, Illinois
Abstract: In 1952, St. Joseph Lead Company drilled a well on the apex of Hicks Dome in Hardin County, Illinois, primarily to explore for oil or gas, and with the objective of testing the St. Peter sand horizon, which had not been reached in a previous well on the flank of the dome. A normal sequence of formations was encountered down to 1,600 feet, but at about that depth the drill entered a confused brecciated zone, which persisted to the bottom of the hole at 2,944 feet. This is interpreted as one of the explosion type breccias, or diatremes, common in this Illinois-Kentucky area, as well as in nearby Missouri. A correlation of formations between this and the earlier Fricker well is presented. It is suggested that Hicks Dome is an incipient or uncompleted cryptovolcanic structure.Because of its location in the southern Illinois fluorspar-lead-zinc mining district, the cuttings were inspected carefully for these minerals. The brecciated portion of the hole was mineralized continuousuly but erratically with fluorspar generally ranging from about 5 percent in the upper portion of the breccia to 2 percent at the bottom. This is much deeper than previously known in such amounts in the area. Parallel with the fluorspar content and similarly diminishing downward is an abnormal radioactivity characterized by 0.029 percent eU in the upper part of the breccia zone. The possibility is suggested that this could have been picked up from the formerly overlying New Albany, or Chattanooga, shale (Brown 1954).
J.C. Bradbury and J.W. Baxter; Intrusive Breccias at Hicks Dome, Hardin County, Illinois, A technical report, ISGS Circular 550, , , 1992,
J.S. Brown, J.A. Emery, P.A. Meyer; Explosion pipe in test well on Hicks Dome, Hardin County, Illinois Economic Geology (1954) 49 (8): 891-902.