Seismic stratigraphy and depositional history of a mixed-energy, accommodation-limited flood-tidal delta along the Eastern Shore of Virginia Conceptual model development and implications

Studies of transgressive tidal-inlet flood-tidal delta (FTD) stratal architecture have tended to focus on barrier island-inlet systems located in wave-dominated settings and in a limited number of mixed-energy settings all with ample accommodation for stacking of depositional units and landward migration. Consequently, the conceptual models that link processes to morphology and subsurface stratigraphy (i.e., depositional history) come from depositional environments unconstrained by lagoon configuration and hypsometry, marshes, and/or tidal flats. Less is known about the stratal architecture and depositional history of FTDs in spatially constrained settings where tidal forcings wield a greater control on deposition-erosion processes over various time scales. This study used ~20 km of seismic transects ground-truthed with sediment cores and surface grab samples to characterize the seismic stratigraphy, stratal architecture, and depositional history of Wachapreague Inlet FTD along Virginias Eastern Shore. These data indicate that high-energy conditions created by strong tidal flow constricted by adjacent marsh and tidal flats (and higher tidal ranges than wave-dominated coasts), in concert with tidal channel cut-and-fill processes, lead to extensive reworking of surficial sediment within the FTD. Internally, reworked units (i.e., discordant to wavy subparallel low amplitude and chaotic internal reflectors) stack and inter-tongue with storm-generated deposits (parallel to landward dipping reflectors). The resulting areal and three-dimensional external FTD geometry is characterized by numerous, segmented lobes built into inlet-proximal backbarrier tidal channels, each containing highly reworked, complex internal depositional patterns. This study provides a detailed stratigraphy of an accommodation-limited FTD which suggests that variations in forcings over multiple time scales create multiple (predictable?) deposition-erosion patterns that develop marsh platforms, sequester backbarrier sand, and influence backbarrier morphodynamics kilometers away from the inlet throat.