INFLUENCE OF COASTAL PLAIN CARBONATES ON RESERVOIR DEVELOPMENT: BIENVILLE NATIONAL FOREST, SMITH COUNTY, MISSISSIPPI

A portion of the Bienville National Forest in Smith County, Mississippi is the proposed site for a 1000+ hectare recreational lake in the Oakohay Creek drainage basin. The footprint of the dam and reservoir lie across the outcrop of the carbonate Glendon and Marianna Formations of the Oligocene Vicksburg Group (dipping ~7 m/km to the SSW), raising concerns of karst flow either bypassing the dam, or weakening the dam’s foundations. Stream caves in the Glendon Limestone up to 500 m long are known from the drainage basin to the east. Logs from water wells, lignite assessment boreholes, seismic shot holes, and oil wells in the area reported numerous instances of lost circulation and bit drops (up to 2 m).

Field reconnaissance found that carbonate outcrops are small and infrequent, concentrated primarily along stream banks; surface expressions of karst are rare, with four small cover-collapse sinkholes from a single location being the only obvious karst landforms. Geophysical traverses (primarily GPR) located some anomalies, but targeted drilling did not locate significant voids. Water budget analysis of the Oakohay Creek and its tributaries, at high and low flow, found no evidence of water gain or loss consistent with conduit flow in karst caves. Small tributaries flow over the carbonate outcrop, producing karren, but the main Oakohay Creek has incised below the carbonates. Bit drops and loss of circulation are now attributed to the transition, during drilling, from the dense, hard Glendon Limestone to a soft, friable marl underneath. Construction of cross sections from well log data demonstrates considerable subsurface relief on the top of the carbonates, possibly indicating a paleokarst but more likely faulting. While karst flow is not active today, filling the reservoir will place water with over 20 m of head against carbonate outcrops along the valley wall, with the potential for rejuvenation of abandoned karst flow routes.