HOLOCENE SEDIMENTS AT BROWNS POND, A VERNAL POOL LOCATED WITHIN A KARST SINKHOLE IN THE SILURIAN TONOLOWAY LIMESTONE, BATH COUNTY, VIRGINIA

In Bath and Highland Counties, Virginia, many karst sinkholes in the Silurian Tonoloway Limestone are blocked with sediment and organic debris, becoming local sites of sediment accumulation. Some of these sinkholes form geographically separated, perennially ponded wetlands (vernal pools) that develop their own insular ecosystems with unique rare species. These vernal pools thus contribute greatly to regional biodiversity despite constituting a disproportionately small percentage of geographic area. One such vernal pool is Browns Pond, located at 631 m above sea level on the east side of Tower Hill Mountain within the George Washington National Forest in northern Bath County, Virginia. Water flows off this mountain (a northeast-trending anticline) over Silurian sandstone and shale until it encounters the Tonoloway Limestone, where it enters the subsurface and forms sinkholes roughly oriented along the regional strike. Previous studies of Browns Pond (Kneller and Peteet, 1999) revealed a >7 m-thick accumulation of sediments with radiocarbon ages ranging back to ~17,000 years Before Present (BP). In July 2019, the U.S. Geological Survey took two new sediment cores from Browns Pond. Core #1, which reached a total depth (TD) of 2.54 m, consists of a >0.03 m-thick unit of gray clay (2.51 to 2.54 m depth) overlain by a 0.4 m-thick unit of brown organic matter (2.11 to 2.51 m depth). This unit of organic matter is overlain by a 0.62 m-thick unit of gray clay (1.49 to 2.11 m depth), which is overlain by a 1.46 m-thick unit of brown organic matter (0.03 to 1.49 m depth). Two samples of plant material from the upper clay unit (158.5 cm and 146.5 cm depths) of Core #1 yielded radiocarbon ages of 7,930 ± 30 years BP (OS-151858) and 7,860 ± 30 years BP (OS-151748), respectively. Core #2, which reached a TD of 1.19 m, consists of a unit of brown organic matter that correlates with the upper unit of Core #1. One sample of charcoal at 88.3 cm depth in Core #2 yielded a radiocarbon age of 6,070 ± 30 years BP (OS-151747). The clay units are interpreted as accumulation via surface runoff when nearby karst sinkholes were blocked and surface water transported clay to Browns Pond, whereas the organic units are interpreted as accumulation when nearby karst sinkholes were open and surface water (transporting clay) was diverted from Browns Pond into nearby sinkholes.