Phosphate Exploration and Resource Potential on the North Carolina Continental Shelf
Authors: Riggs Stanley R. , Hine Albert C. , Snyder Stephen W. , Lewis Don W. , Ellington M. Douglas , Stewart T. Lori 
Published: October 2008

An extensive network of high resolution uniboom/ sparker/3.5 kHz subbottom seismic profiles in combination with 9 mvibracores were obtained across the North Carolina continental shelf in Onslow Bay. The resulting data has delineated a depositional pattern of Miogene sediments around the E flank of the Cape Fear Arch. This broad SE trending structural high, located on top of the older Carolina Platform, has controlled the deposition and distribution of Cenozoic units across the mid-Atlantic shelf. The Neogenes section is dominated by the Miocene Pungo River Formation, a downdip thickening sediment wedge which dips E and SE off the Cape Fear Arch. The Pungo River consists of numerous depositional sequences, each abruptly truncated by an erosional surface and associated channels. The depositional sequences, which may reflect third- and foruth-order cyclical events within the within the mid-Nicone second-order transgressive cycle, are characterized by interbedded lithologies consisting of phosphorite sands, phospatic foraminiferal muds, dolosilts, and calcareous quartz sands. Some phospohorite sands also occur in the Holocene sand sheet which forms a thin discontinuous cover on the Pungo River sediments in the Frying Pan Area.

The preliminary evaluation of phosphate resources in Onslow Bay has delinated several excellent prospects with a vast resource potential. The Pungo River Formation crops out in a NE-SW trending belt which is over 150 km long by 40 km wide and extends into the subsurface to the E and S. Shallow vibracores have penetrated 8 beds which contain significantly anomalous concentrations of phosphate, 5 of which can be considered potential phosphate resources. Based upon the preliminary stratigraphic, seismic, and chemical analyses, these 5 beds are estimated to contact 1.5 billion short tons of phosphate concentrate with grades between 28% and 30% P2O5 and a moderate to high resource potential. The other 3 beds are extensive units which contact minor concentrations of phosphate and have a low resource potential. This new continental shelf phosphate province will become technically available at a time in the near future when the rapidly expanding demand for fertilizers and agricultural products with the ever-increasing land-use and environmental pressure produces an unacceptable escalation in the cost of mining the land-based reserves.

Introduction

The Miocene section of the southeastern Atlantic coastal plain-continental shelf is characterized by abnormally high concentrations of sedimentary phosphate. The stratigraphic section consists mainly of the contemporaneous and contiguous Hawthorn Formation in Florid and South Carolina and the Pungo River Formation in North Carolina. In northern South Caroline and southern North Carolina, this sediment sequence is displaced seaward across the continental shelf to the continental margin through Long and Onslow Bays by the Cape Fear Arch, a per-Tertiary first-order structural high (Riggs, 1980, 1981). The distribution of phosphate-rich sediments within the Hawthorn-Pungo River Formations is not continuous or uniform either laterally or vertically. The strong spatial distribution of phosphate is controlled a) regionally by the tectonic setting and the first-order structural framework; b) locally by the second- and third-order structural elements; and c) temporally by the cyclicity of major paleoanographic events such as sea level oscillations, changing current patterns, and fluctuations in the chemical parameters of the ocean margins (Riggs, 1979; Riggs et al., in press a and b).