WARP AND WEFT OF THE OZARKS: STRUCTURAL ANALYSIS OF TOP OF THE ROCK SINKHOLE, RIDGEDALE, MISSOURI

On May 22, 2015, a sinkhole collapsed below a 3.1-acre (1.25 ha) lake at Johnny Morris' Top of the Rock Golf Course at Big Cedar Lodge and Ozarks Heritage Preserve. The Sinkhole (36° 32' 29.75" N, 93° 14' 59.41" W) is located on the Table Rock Dam and Hollister 7.5-minute quadrangles. Since the collapse, crews have continued to excavate the sinkhole to reveal a spectacular cathedral-like topography developed in the walls of this unique karst feature. On March 17, 2023, our team measured and described a stratigraphic section and logged a gamma-ray profile in the sinkhole. The sinkhole exposes a continuous succession of the upper approximately 30 ft. (10 m) of lower Ordovician Cotter Dolomite through lower part of the mid-Mississippian Elsey Formation. Typically, such stratigraphy is observed only in drill core. In contrast, sinkhole walls provide three-dimensional, 100% exposed strata, vertically and laterally.

The sub-Mississippian unconformity is about 30 feet above the sinkhole floor. The Bachelor Formation, marking the base of the Mississippian, is a thin sandstone and shale unit that includes chert pebbles eroded from the Cotter. The Compton Limestone, mostly lime mudstone, overlies the Bachelor; it is 10 ft. (3 m) thick. The Northview Formation is 7 ft. thick with interbedded argillaceous limestone and red and light gray-green shale beds. The brick-red argillaceous lime mudstone of the Baird Mountain Limestone Member of the Northview is at least 2 ft. (0.6 m) thick. The Pierson Limestone, brachiopod-crinoid wackestone to packstone is 76 ft. (23.2 m) thick. Dark cherty limestone of the Reeds Spring Formation, 32 ft. (9.8 m) and at least 16 ft. (4.9 m) of the Elsey, interbedded lime mudstone to crinoid packstone, caps the succession.

The sinkhole is 4 miles (6.4 km) southwest of the trace of the Ten O'Clock Run Fault, and 2.5 miles (4 km) northeast of Beardsley Branch fault and monocline. These structures parallel one another, trending roughly 322°. Nearly perpendicular to the structures, fractures cut though the sinkhole area, trending 56°. Another set of fractures trends parallel to the bounding structures. This pattern implies only modest counterclockwise rotation between the Ten O'clock Run and Lampe fault systems with deep-seated basement fault movements.