Sr and Nd isotopic analysis of phosphorite sedimentation through one Miocene high-frequency depositional cycle on the North Carolina continental shelf

Abstract

A well-developed seismic-, litho-, and biostratigraphic data base with good models for the formation and deposition of phosphate-rich sediments exist for the Upper Cenozoic sediments on the North Carolina continental margin. This provides a unique opportunity to evaluate the potential of utilizing strontium and neodymium isotopic analyses to help unravel a complex genetic history of phosphate formation and deposition. Thirty one samples from three vibracores in Onslow Bay supplied hand-picked concentrate subsamples of five phosphate grain types and thirty one concentrate subsamples of phosphate peloids for stratigraphic analysis. All subsamples were analyzed for their Sr isotope composition, while only thirteen were analyzed for Nd isotopes. The 31 peloid samples represent three different stratigraphic units and include: (1) Holocene surficial sands with highly variable concentrations and types of reworked phosphate; (2) Pleistocene moldic carbonates with minor concentrations of black peloidal phosphate; and (3) Miocene (mid-Burdigalian), Frying Pan Sequence unit 1 (FPS-1) of the Pungo River Formation with very high concentrations of multiple types of phosphate grains that decrease upsection and change grain types with changing lithofacies. The latter unit represents deposition during one high-frequency sea-level cycle. Stratigraphic analysis utilized one grain type, phosphate peloids, for comparative purposes with the depositional model of Riggs and Mallette (1990). The Sr and Nd data cluster into significant categories that are coincident with these three major stratigraphic units.Sr and Nd isotope compositions of selected phosphate grains from three phosphate-rich depositional sequences are utilized to: (1) date the initial Miocene transgression of unit FPS-1 of the Pungo River Formation; (2) decipher processes of formation and deposition of major phosphate grain types through one Miocene sea-level cycle including the processes of in situ formation versus reworking; (3) develop a Sr and Nd chronostratigraphy for evaluating changing patterns of continental margin sedimentation through high-frequency depositional cycles; (4) determine effects of reworking and weathering on the isotope signals between Miocene, Pleistocene, and Holocene sediments; (5) evaluate the suitability of utilizing Sr and Nd isotopes for detailed chronostratigraphic analysis of phosphorite sedimentation; and (6) refine our understanding of the Upper Cenozoic evolutionary history of North Carolina's continental margin.