Southeastern Section - 57th Annual Meeting (10–11 April 2008)

Paper No. 2
Presentation Time: 2:00 PM


WHITTECAR, G. Richard, Ocean Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529, DANIELS, W. Lee, Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061 and CARTER III, Charles H., 501 Shirley Plantation Road, Charles City, VA 23030,

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.