GYPSUM KARST LEADS TO ABANDONING THE UPPER MANGUM DAMSITE, SOUTHWESTERN OKLAHOMA

摘要

Gypsum is a highly soluble rock, and projects to be built in or near gypsum require full evaluation of the presence and impact of gypsum karst. Geologic study of the proposed Upper Mangum Damsite, in an area of gypsum karst in southwestern Oklahoma, has shown that the site is unsuitable and should be abandoned. The damsite, located on Salt Fork Red River near the town of Mangum, has been studied since 1937, mainly because of its favorable topography; both proposed abutments are high and form a narrow water gap between them. This early decision did not adequately consider the site's geology, foundation conditions, or water-impoundment capabilities. Abutments would be in the Permian Blaine Formation, here consisting of 200 ft 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. In 1999, a final assessment was made of the surface geology and the results of coring and pressure-testing of boreholes along the proposed dam. The assessment showed: open cavities, clay-filled cavities, and other karst features are abundant in and near the abutments; and fluid losses of 15-62 gpm in most borehole pressure-tests, and in one borehole the losses were 410-1,316 gpm. Engineering measures needed to remediate karstic foundation conditions here would add greatly to the cost of construction, and still would not assure tightness of the reservoir or integrity of the dam. Therefore, it was recommended that this site be abandoned, and that future studies be at the Lower Mangum Damsite, a more favorable site 7 mi downstream on Salt Fork Red River.