44-2 What the Clay? Using GPR and Vibracores to Investigate the Origin and Extent of a Unique Sediment Layer Found in a Prograding Salt Marsh in Greenbackville, VA.

Abstract:

The Greenbackville salt marsh is located on a promontory point along the western edge of Chincoteague Bay (CB), Eastern Shore, Virginia. Previous work at this site has documented an unusual bay-ward progradation of the saltmarsh over the Holocene. Here we present an analysis of an anomalous clay layer sitting directly below the disconformity interpreted as the Late Pleistocene– Early Holocene transition. 

The blue-grey clay layer is sampled in 7 vibracores extracted from the modern salt marsh beginning at depths of 125-150 cm and thickening toward CB. The clay caps an organic carbon-rich sandy silt layer likely deposited in a lacustrine environment circa (46-28 ka cal bp, Marine Isotope Stage 3 (MIS 3)). The disconformity separates the clay from Late-Holocene to ~1800 y cal bp bedded and massive fining-upward quartz sands deposited in a high energy coastal plain environment. 

XRD analysis indicated that the clay layer consists of predominately kaolinite, montmorillonite, illite, detrital plagioclase and muscovite, and diagenetic pyrite. SEM imaging revealed the clay layer contains fecal pellets, pine pollen, marshmallow pollen, and diatom fragments, including Cyclotella distinguenda, a centric diatom found in freshwater lakes and rivers. The lack of oxidized iron, the presence of freshwater diatoms, pollen, and fecal pellets indicate that the clay layer was likely deposited in a deep pond/lake.  

Taken together, the sediments in the vibracores reveal Pleistocene environmental change in the region, i.e. from a fluvial system to a sand-dominated coastal plain to a muddy marine bay, and the overall bay-ward movement of the salt marsh to its current location. 

GPR radargrams show primarily horizonal reflections correlating with modern marsh facies and sand layers in the cores. More detailed GPR interpretations and correlation with new sediment cores will allow us to map the extent and lateral variations of the clay layer in the subsurface.  Further analysis of microfossils and other mineralogical indicators of the depositional environment’s pH and oxidation state will help more tightly constrain the type of vegetation, climate, and water-level conditions present during the time of environmental transition when the clay was deposited. This transitional clay layer will provide clues to the puzzling bay-ward growth of the Greenbackville salt marsh and add to our record of post-MIS 3 coastal evolution and relative sea level history of the Delmarva. 

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