Southeastern Section - 68th Annual Meeting - 2019

Paper No. 6-1
Presentation Time: 8:00 AM-12:00 PM


LANDOLT, Benjamin W., BALASCIO, N.L. and BAILEY, Christopher M., Department of Geology, College of William & Mary, Williamsburg, VA 23187

Sinkhole ponds have developed in several regions along the western Blue Ridge Mountains of Virginia. They form from the dissolution of Cambrian-age carbonate bedrock underlying relict alluvial deposits. The timing of formation of these basins is poorly constrained and detailed characteristics of subsequent sedimentation have received little attention, yet they have the potential to provide paleoclimatic or paleohydrologic information for the region. In this study, we investigate geomorphic and sedimentary characteristics of sinkhole basins in the Maple Flats area near Stuarts Draft, VA. We compiled high-resolution LiDAR data and collected ground-penetrating radar (GPR) profiles across several of the ponds to characterize the geomorphology of the region and general sinkhole geometry. The basins analyzed were approximately 50-70 m in diameter, and GPR data show they generally contain ~2-3 m of unconsolidated sediment. Detailed analysis of sedimentation in one pond, Twin Pond West (TPW) (37o58’58” N, 78o59’47” W), was also conducted by analyzing grain size, magnetic susceptibility, organic content, and organic carbon/nitrogen ratios on a 2.0 m core that extends to the base of the pond. The core reveals two distinct stratigraphic units with the lower sediments consisting of a dark brown sandy silt with high C/N ratios (40-50) and ~6% organic matter that abruptly transitions to a dense, light gray silty clay with much lower C/N ratios (~5) and organic matter that steadily rises from ~1% to ~10%. We interpret the lower unit to have been deposited during and immediately following basin formation, and the upper unit to represent sedimentation in an ephemeral pond similar to modern conditions. Our age model, constructed using six radiocarbon dates, indicates that this transition occurred c. 28 cal ka BP and was followed by relatively continuous sedimentation to present. This date could mark a shift to wetter regional climate conditions or a change in the hydrologic system that would have facilitated an increase in bedrock dissolution. Sedimentological data also show subtle changes over the last c. 28 cal ka BP that we interpret and use to characterize paleoenvironmental conditions around Maple Flats.