STRATIGRAPHIC CONSIDERATIONS IN DEEPENING THE WILMINGTON SHIP CHANNEL, BALDHEAD SHOALS TO WILMINGTON, CAPE FEAR RIVER, NORTH CAROLINA

Deepening the Wilmington ship channel between mile -6.7 in Long Bay (terminus of a realigned Baldhead Shoals channel) and mile 27.2 on the Cape Fear River (at Wilmington) to –43.0’ or –45.0’ mean low lower water (dredging prism) began in fall 2000 by the U.S. Army Corps of Engineers. To provide information on the potential engineering problems associated with deepening the ship channel, over 200, 2" (NW) and 4" (4" x 5 1/2") core holes were drilled and almost 250 miles of seismic "boomer" data collected.

In Long Bay, the channel is developed in the Paleocene Yaupon Beach Formation, Bald Head Shoals Formation and Eocene Castle Hayne Limestone. Along the Cape Fear River, the channel is developed in the Castle Hayne Limestone, Oligocene River Bend Formation and Cretaceous Peedee Formation (Rocky Point and Island Creek Members). The occurrence, distribution and thickness of lithostratigraphic units along the channel within the permitted dredging prism govern the method to be employed for channel deepening. Where rock is encountered, drilling and blasting are necessary to deepen the channel and pose some potential for environmental harm. In areas where rock is thin or non-existent, rock-cutter-head type dredge, dipper dredge, hydraulic hammer or pick can be used to achieve deepening to the permitted depth with less environmental harm. Rock is most extensive and best developed in Baldhead Shoals (Bald Head Shoals Formation and Castle Hayne Limestone), Big Island-Keg Island (Rocky Point Member and Castle Hayne Limestone), and the Anchorage Basin (Rocky Point Member). In these areas, rock induration is related to the unit present, the original sediment type, and the occurrence of unconformities. Generally below unconformities, rocks are well indurated, and when the section is thick and joint frequency minimal, blasting will be required. Where the Peedee Formation, Yaupon Beach Formation and River Bend Formation occur within the dredging prism, rock is absent or thin, allowing for other deepening techniques. In these areas, channel deepening techniques pose less potential damage to the environment.