Modern Vivianite at the K–Pg Boundary, New Jersey Coastal Plain: A Redox-Driven Proxy at a Legacy Stratigraphic Interface

Vivianite [Fe₃(PO₄)₂·8H₂O] is a rare iron phosphate mineral that typically forms under reducing conditions in the presence of organic matter and iron-bearing minerals. This study documents vivianite occurrences along the Manasquan River in Freehold, New Jersey, where the mineral consistently appears at the lithostratigraphic contact between the Late Cretaceous Tinton Formation and the Paleocene Hornerstown Formation. At this site, vivianite precipitates as thin, laterally extensive crystal aggregates localized along multiple facies boundaries that act as semi-permeable aquitards, focusing porewater migration through more permeable intervening horizons. Additional occurrences are observed within fossil molds and voids, where reducing microenvironments promote authigenic mineralization.The confinement of vivianite to these stratigraphic interfaces suggests that long-standing contrasts in permeability and sediment composition continue to structure redox gradients favorable for phosphate precipitation. The Tinton–Hornerstown boundary, a preserved paleoenvironmental transition characterized by glauconitic sands, phosphatic shell lags, and bioturbation structures, serves as a semi-permeable geochemical barrier. This interface shapes modern porewater chemistry and fosters localized reducing conditions that facilitate the mobilization and concentration of ferrous iron and phosphate in microenvironments such as fossil voids and burrow fills.While grounded in field observations and sample collection, it proposes a framework for future analytical work—including mineralogical, isotopic, and geochemical analyses—to constrain the timing, recurrence, and environmental controls on vivianite formation. These findings highlight vivianite as a modern tracer of redox stratification and illustrate how ancient stratigraphic architecture can remain geochemically active, guiding authigenic mineralization in marginal marine settings of the New Jersey Coastal Plain.