DECIPHERING THE ORIGINS OF COASTAL SEDIMENTS IN THE OUTER BANKS (VA, NC) THROUGH DETRITAL ZIRCON SIGNATURES

The U-Pb age distributions of detrital zircon (DZ UPb) in siliciclastic sands are frequently used to better understand the origins and transport pathways of sediments in the geologic record. Utilizing this method for paleoenvironmental reconstruction often relies on the assumption that DZ UPb age signatures in marine environments directly reflect fluvial inputs and the geology of their associated drainages. However, little empirical information exists on how accurately these provenance determinations represent paleodrainage pathways, particularly for marine strata. One way to address this issue is through the study of analogous modern environments, where regional geographic, geologic, and oceanographic context are relatively well-understood. By precisely tying modern coastal sediments to their catchment areas through their DZ UPb age spectra, a complete source-to-sink picture of the depositional system can be constructed, and these results can be applied to similar settings preserved in the geologic record.

Here, we apply this methodology to the Outer Banks coastal region of Virginia and North Carolina. DZ UPb age spectra of foreshore sediments have a dominantly Grenvillian (900-1200 Ma) signature that is significantly different from the most adjacent Appalachian fluvial sediment inputs, but more closely resembles the signatures of other rivers further to the north. Our work leverages these distinctive signatures to evaluate source-to-sink mixing between endmembers. By applying Monte Carlo mixture modeling to the DZ UPb signatures of coastal plain, foreshore, and Appalachian river sands, the likely sources of Outer Banks sediment (and the pathway by which it arrived) were determined statistically. The results suggest the Outer Banks are the product of longshore drift over hundreds of km, and that the barrier island system effectively sequesters sediment into the back-bays. By integrating sediment sources with oceanographic, estuarine, and fluvial processes, this study has implications for how sediment is commingled and routed to barrier islands during times of sea level highstand and transgression in both modern and ancient settings.