Field mapping in the Abbott Formation and examination of topographic lineaments in the Creal Springs, Stonefort, Eddyville, and Harrisburg Quadrangles (southeastern Illinois) reveal the New Burnside “anticline” and its northeastern extension, the Stonefort “anticline” to be a single, extensively faulted structure. Interpretation of this evidence also leads to the conclusion that this is a fault-block structure rather than an anticline. Trending northeast-southwest, the structure seems to be the northwesternmost extent of the Fluorspar Area fault complex. We found evidence for two episodes of faulting. The first involved northeast-trending, high-angle faults similar to those in the known Fluorspar complex to the southeast. Faults on the northeast (Stonefort “anticline”) step down toward the center of the structure, forming a graben. Vertical movement also occurred to the southwest (New Burnside “anticline”), but the structure in this vicinity is a horst with some blocks tilted. As with other faults in the Fluorspar complex, horizontal slickensides are present locally. The second episode of movement occurred along northwestsoutheast-trending strike-slip faults that offset the northeast-trending high-angle faults. This second phase of faulting may correspond with previously reported reactivation of northwest-trending faults elsewhere in the Fluorspar Area fault complex.

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