HOLOCENE BENTHIC FORAMINIFERAL ASSEMBLAGES OF TIDAL-INLET DEPOSITS ALONG CEDAR ISLAND, VA, USA: INSIGHTS INTO STORM IMPACTS, BREACH DYNAMICS, AND INLET EVOLUTION

Cedar Island, VA, is a rapidly transgressing, mixed energy, washover-dominated barrier island that has breached and formed a short-term ephemeral inlet at least three times in the past sixty years. Each wave-dominated ephemeral inlet was only open from five to nine years. Satellite imagery and geomorphic features, including relict flood tidal deltas and recurved-spit ridges, document the former inlets. However, benthic foraminiferal and sedimentological analyses of three vibracores from inlet and flood tidal delta deposits offer novel insights that facilitate high-resolution interpretations of inlet dynamics. Elphidium excavatum completely dominates all non-barren assemblages comprising 54-100% of samples. The benthic foraminifera within the cores record two distinct inlet events, whereas grain-size trends suggest more inlet events. From bottom to top, we observe a deposit consistent with a flood tidal delta/inlet fill environment abundant with a mix of shelf and estuarine foraminifera (Buccella frigida, Ammonia parkinsoniana, Haynesina germanica, and Trochammina inflata) capped by a washover/beach deposit that is rich in shelf species (Buccella frigida, Ammonia parkinsoniana, Elphidium mexicanum). Second, a thick quiescent estuary, barren of foraminifera, is characterized by several thin distal flood tidal delta layers. The most recent inlet is marked by washover, then a tidal-inlet floor environment with low foraminiferal abundancies and the largest proportion of a secondary species (23% Haynesina germanica). Next, flood tidal delta/inlet fill deposit contains coarsening upward packages within an overall fining upward succession, which may indicate two inlet events are reflected in this deposit. Finally, high-energy inlet fill, containing low abundances of shelf species (Elphidium gunteri), are capped by a washover/beach/aeolian environment. The stratigraphic relationship of the benthic foraminiferal biofacies, grain-size trends, and newly available satellite imagery suggest that – contrary to previous thought – ephemeral wave-dominated inlets of Cedar Island do not follow a prescribed lifecycle (i.e., breach, lateral migration to the south, and close) but rather each time an inlet formed, it evolved uniquely rotating, migrating, or remaining relatively stationary.