High-resolution Uniboom, sparker, and 3.5 kHz subbottom seismic profiles, in combination with 9-m vibracores from Onslow Bay, delineate a belt of Neogene sediments which crop out on the North Carolina continental shelf around the east flank of the mid-Carolina platform high. This broad, southeast-trending basement high has controlled the general distribution of Cenzoic sediment sequences within the mid-Atlantic continental margin. The Neogene section includes the Miocene Pungo River Formation, which thickens downdip to the east and southeast off the mid-Carolina platform high. The Pungo River Formation consists of three third-order and at least 21 fourth-order seismic depositional sequences, many of which are abruptly truncated by erosional surfaces and associated channels. These seismic sequences reflect third- and fourth-order eustatic sea-level cycles within the Miocene second-order transgressive sea-level cycle. The fourth-order seismic sequences appear to be equivalent to cyclic lithostratigraphic units characterized by interbedded phosphorite sands, phosphatic foraminiferal muds, calcareous quartz sands, calcarenites, and dolosilts. Some phosphorite sands also occur in the Holocene sand sheet which forms a thin and discontinuous cover on the Pungo River sediments in the Frying Pan phosphate district.Preliminary evaluation of phosphate resources in Onslow Bay has delineated several potential economic prospects. The Pungo River Formation crops out in a northeast-southwest-trending belt which is 150 km long by 25 to 50 km wide and extends into the subsurface to the east and southeast. In two different areas within Onslow Bay, vibracores have penetrated five beds which contain anomalous concentrations of phosphate. Two beds within the Frying Pan phosphate district are estimated to contain 3.75 billion metric tons of phosphate concentrate with an average content of 29.2 percent P 2 O 5 . The other three beds in the northeast Onslow Bay phosphate district are extensive units with low to intermediate phosphate content within the total sediment (up to 9.8% P 2 O 5 . These three "lean" deposits contain at least 780 million metric tons of phosphate concentrate with average P 2 O 5 contents which range from 29.7 to 31 percent. Phosphate resources in the Frying Pan phosphate district have a moderate to high potential for economic development, whereas those in the northeast Onslow Bay phosphate district have a low potential for economic development. This newly discovered continental shelf phosphate province could become economically feasible to mine in the near future when rapidly expanding demand for fertilizers and agricultural products, combined with ever-increasing land-use pressures, produces an unacceptable escalation in the cost of mining land-based reserves.

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