UNEXPECTED HYDROSTRATIGRAPHIC COMPLEXITY IN TWO COASTAL PLAIN TERRACES, CHARLES CITY COUNTY, VIRGINIA

Analysis of borehole logs, ground-penetrating radar, and topography surrounding two sediment disposal basins reveal unexpectedly high levels of hydrostratigraphic complexity within a 1.5 km² study area. Constructed adjacent to the James River with the intent of returning dredged sediments to a beneficial upland use, the basins sit on adjacent Pleistocene terraces with significant compositional and textural differences. One sediment disposal site lies on a terrace mapped as the Shirley Formation. The surface deposit, a 3m-thick clay loam with appreciable amounts of expansive clay, overlies coarse sand with lenses of both silt-clay and fine gravel. Although very similar to the Shirley Formation sediments described at the type section on a neighboring farm, this deposit appears to be younger, lying on a 10m-elevation terrace surface slightly lower than most of the Shirley Formation. Below this younger deposit lies 5m-thick organic-rich silty clay topped by in situ cypress stumps. This unit, in turn, lies next to, and seems be younger than, a sand-and-gravel unit that closely resembles the rest of the Shirley Formation. The Tabb Formation which carpets the lower terrace consists of a mud-capped fining-upward fluvial deposit floored by high permeability cobble-to-boulder gravels. Erosional remnants of Shirley Formation gravelly sand and fine-grained valley-fill fluvial packages that post-date the dominant Tabb Formation lithology alter the permeability pattern across this terrace surface. Most stratigraphic complexity in this study area stems from the history of stream incision and valley-infilling caused by Pleistocene sea level fluctuations but the cut-and-fill history of sand-and-gravel mining also produced significant hydrostratigraphic units. Analyses of water-level fluctuations in monitoring wells and groundwater flow patterns suggest that at least eight different geologic units influence shallow groundwater flow in this small study area.