SEDIMENTARY FACIES, BOUNDING SURFACES, AND THE POST-CRETACEOUS, SHALLOW AQUIFER SYSTEM, LOWER NEUSE RIVER BASIN, CENTRAL COASTAL PLAIN CAPACITY USE AREA, NORTH CAROLINA

North Carolina's Central Coastal Plain Capacity Use Area (NC-CCPCUA) includes 15 counties that are legislatively mandated to reduce their use of Cretaceous aquifers (up to 75%), and find alternative water sources by the year 2018. Overuse is depleting the resource, causing subsidence, threatening the sustainability of the water supply, and enhancing salt water intrusion. Shallow, post-Cretaceous, aquifer facies are potential targets for alternative, shallow water supplies.

The U.S. Environmental Protection Agency, through the NC Division of Water Quality's (DWQ) 319h Non-Point Source Grant Program provided funds to characterize the shallow aquifer system in the lower Neuse River Basin at the subbasin scale. This area includes the Surry, Walterboro and Suffolk Scarps, and the Wicomico, Talbot and Pamlico Terraces. A dip-parallel cross section follows the basin's south border; a strike-parallel section extends from this border north to the Neuse River, near the Suffolk Scarp.

The shallow aquifer system is defined here as the sedimentary package that overlies the Peedee Confining Unit – i.e., the Cretaceous Maastrichtian Peedee Formation. Cross sections that show sedimentary architecture, facies, bounding surfaces, and potential shallow aquifers, confining units, and pathways, are based on ~920 m of borehole data (core and gamma logs), collected by the NC-DWQ and the U.S. Geological Survey. Using bioclast content, strata are divided into siliciclastic (<30%), mixed siliciclastic/bioclastic (30-70%), and bioclastic (>70%) facies assemblages. Potential aquifers and confining units are classified using composition, texture, cementation index, dissolution fabric, porosity type, and borehole log response patterns.

Strata encountered include the Paleocene Beaufort, the Eocene Castle Hayne, and the Oligocene Riverbend/Belgrade Formations. Paleocene and Eocene facies include bioclastic to mixed grainstones/packstones. The Oligocene is more diverse. Basal updip siliciclastic shelf muds and sands are overlain downdip by younger, mixed to bioclastic facies assemblages. Neogene deposits are mostly Pleistocene and siliciclastic. Sequence boundaries are marked by cemented, phosphatic, mixed to bioclastic “caprock” gravels and hardgrounds; leached boundaries exhibit dissolution fabrics.