MICROFOSSILS AS INDICATORS FOR PALEO-MARSH ELEVATION IN THE TIDAL TRIBUTARIES OF THE POTOMAC RIVER (CHESAPEAKE BAY)

Marsh cores from Wilkerson’s Creek (Colonial Beach, Virginia) were analyzed for agglutinated foraminifera to evaluate sea level histories in tidal tributaries of the Potomac River. Paleo-Marsh Elevation (PME) was established by tracking the stratigraphic distribution of taxa that have known vertical ranges in modern salt marshes of the Chesapeake Bay region.

A 600 cm core recovered from Wilkerson Creek yielded two primary lithofacies that include a basal grey clay facies (600-255 cm) and an upper organic-rich clay and peat facies (255-0 cm). Corresponding microfossil assemblages including Ammobaculites dilatatus, A. crassus, and A. exiguus are dominant in the grey clay lithofacies where organic matter is sparse. Miliammina fusca, Ammoastuta salsa, and Tipotrocha comprimata characterize the peat and organic-rich clay facies in the uppermost 255 cm of the core. Agglutinated testate rhizopods (Thecamoebians) are locally abundant in the rooted zones. Uncorrected AMS¹⁴C geochronology indicates a basal age of 860 ±20 YBP (194 cm) in the peat and 3100 ±20 YPB (522 cm) in the grey clay.

The overall trends in the Wilkerson Creek core demonstrate a pattern of initial sea-level rise and estuarine conditions giving rise to increased peat production during warm and humid regional climate associated with the Medieval Warm Period. The dominant Ammobaculites in the grey clay facies is consistent with PME that was more than a meter lower than present during the time prior to the Medieval Warm Period. Ammoastuta salsa and Miliammina fusca giving way to thecamoebans in the peat records late Holocene sea level rise and fresh water ponding in the tidal marshes.