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.

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