T226 Cushman Foundation Symposium: Microfossil Mayhem—Murder, Misfortune, and More

Tuesday, 24 October 2017
1:30 PM - 5:30 PM
Room 611

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

Paper number: T226-14
Program: Technical Sessions
Time: 4:50 PM – 5:05 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-308089

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