GYPSUM KARST IS A MAJOR FACTOR IN SITING OF A PROPOSED DAM IN SOUTHWESTERN OKLAHOMA

Engineering-geology assessment of two proposed damsites in an area of gypsum karst in southwestern Oklahoma has shown that the first site should be abandoned, and the second site is acceptable, but with limitations. Both sites are on Salt Fork Red River, just upstream from the town of Mangum. The Upper Mangum Damsite, the first proposed site, has been studied since 1937 as a potential location for a compacted, earth-fill dam. Abutments for this dam would be in the Permian Blaine Formation, consisting here of 60 m of gypsum, with thin interbeds of dolomite and shale. The Blaine Formation locally has abundant gypsum-karst features, such as caves, sinkholes, disappearing streams, and springs. A final assessment, made in 1999, of the abutments at this site showed the following: open cavities, clay-filled cavities, and other karst features are abundant in and near the abutments; and fluid losses (per 3-m intervals) ranged from 60–250 L/min in most borehole pressure-tests, and in one borehole the losses were 1,600–5,300 L/min. As a result, the first site was abandoned, and a new study of the Lower Mangum Damsite was launched in 2002. This newly proposed damsite is 6 miles farther downstream on Salt Fork Red River, where the foundation and abutments would be in the thick Flowerpot Shale. Based upon geologic and hydrogeologic field studies, along with core study, pressure tests, and laboratory tests of cores, the foundation conditions at this damsite appear favorable. However, owing to the presence of the karstic Blaine Formation in the upper reaches of the reservoir that would be formed at this new site, there will be limitations on the lake level, size, and storage capacity of the Lower Mangum Reservoir. If the conservation-pool level here is too high, it will cause excess water to escape from the lake in the upper reaches; the water will enter a subsurface gypsum-karst conduit and flow as ground water into a different watershed. Investigations at these two sites demonstrate the importance of hydrogeological studies in areas of evaporite karst in order to plan location and size of potential damsites and reservoirs.