SEDIMENT AND ACOUSTIC RECORDS OF SURFACE WATER- GROUNDWATER EXCHANGE THROUGH SINKHOLE LAKES

This study examines acoustic and sedimentary records from two sinkhole lakes (Balboa and Ponce de Leon) in south Georgia to infer changes in water level that relate to climatically controlled exchanges between surface water and groundwater. We analyze >17 km of 3.5 kHz acoustic profiling data and 4 sediment cores (to 6.2 m in length) and find evidence of 15 m fluctuation in lake level within a >25 m thick elongate sediment plug beneath Lake Balboa. Four stratigraphic units are present. Basal Unit I (unsampled) consists of <16.5 m of acoustically transparent sediment overlying faintly stratified and faulted, unconsolidated Miocene/Pliocene overburden and Eocene limestone. Above a strong basal reflector, Unit II (1.9 m thick) consists of (a) thin rthymically bedded peaty sediment indicative of slow sedimentation in a shallow swamp, (b) desiccated and over-consolidated clay that may indicate subaerial exposure of the lake bed, and (c) an acoustically hard silty-sand capping layer likely associated with slumping. Unit III (2.5 m thick) is composed of dark brown lacustrine gyttja with dispersed water lily rootlets indicative of a deepening lake while uppermost Unit IV consists of 2.4 m of black gyttja associated with modern lake levels.

Unsampled sediments within Unit I are undoubtedly pre-Holocene; they likely record evidence of initial sinkhole collapse, and probably contain paleoenvironmental records for a period of time not well represented in the southeastern United States. A shallow, periodically desiccated swamp with water levels 15 m below present existed by 10 910 ± 50 BP. By 8,150 ± 40 BP, water levels rose 6 m so that only the littoral zone supported water lilies and a wavecut shoreline is evident in neighboring Lake Ponce de Leon. Further deepening submerged shorelines and increased water volume in the lake, increasing autochthonous sediment production. Organic rich, probably annual, rhythmites occur throughout these sediments and, on average, doubled in thickness as water level rose, suggesting increased sedimentation rates and/or reduced rates of organic decomposition contemporaneous with deepening water.