ABSTRACT
Lithologic and stratigraphic data from rocks dredged from the continental shelf off Onslow Bay, North Carolina, provide surface control for seismic studies of the southeastern United States continental margin and help to explain the distribution of potentially economic phosphate-rich sediments on this shelf. Outcropping Miocene rocks in this area indicate that the region has long been a positive geologic feature and has received relatively little Pliocene and Pleistocene sedimentation. Leached, molluscan-moldic calcareous quartz sandstones of late Oligocene to early Miocene age (Belgrade Formation) and middle to late Miocene age (Pungo River Formation) crop out in southwestern and northeastern Onslow Bay, respectively. These two areas border a band of highly phosphatic surficial sediments probably derived from unlithified, phosphatic units of the Pungo River Formation. Lower Pleistocene calcarenites that correlate with the Waccamaw Formation crop out on the sea floor near Cape Fear, which bounds Onslow Bay on the south. A core on Frying Pan Shoals off Cape Fear, after passing through Pleistocene coquina, calcareous quartz sandstone, and oolitic sand, penetrated upper Pliocene calcarenites of the Bear Bluff Formation and middle Miocene phosphatic argillaceous sandstones of the Pungo River Formation. Samples from this core hole show that phosphate increases toward the top of the unindurated Pungo River Formation section, indicating that this formation is probably the source of the high phosphate concentrations in the surficial sediments near Cape Fear, where the Bear Bluff calcarenites also crop out. Upper Pleistocene units include oolitic limestones adjacent to the outer shelf, calcareous quartz sandstones around Cape Fear, and molluscan coquinas near Cape Lookout which bounds Onslow Bay on the north. The outer shelf is blanketed by submarine lithified algal limestones and sandstones of Holocene age.
