Drowned Foredune Ridges as Evidence of Pre- Historical Barrier-Island State Changes Between Migration and Progradation

Abstract

The processes driving barrier-island state changes between erosion/migration and growth/progradation are poorly understood. Stratigraphic and chronologic data are used to infer past state changes of Cedar Island, VA, USA. These data indicate that Cedar Island formed seaward of its present position approximately 5000-6000 years ago. Following a period of net landward migration, the north-central section of the island was breached prior to ca. 450 years ago. During this time, an extensive flood tidal delta was deposited in the backbarrier. After inlet closure, the island migrated landward atop these flood delta deposits, and aggraded, eventually entering a phase of net progradation. From 1852 to present the island has eroded and migrated landward ~1 km. The modern barrier is located stratigraphically above a shallow antecedent high (-5.5 m MSL). The state changes and antecedent geology observed here are discussed and used to infer potential future sediment sources to the island.

Can Barrier Islands Survive Sea‐Level Rise? Quantifying the Relative Role of Tidal Inlets and Overwash Deposition

“…Despite an improved representation of coastal processes such as inlet dynamics, BRIE does not yet aim to simulate the dynamics of any particular barrier system. To do that, we would need to incorporate additional factors such as variations in the coastal lithology and topography (Cooper et al, ; Schwab et al, ; Shawler et al, ; Swanson et al, ), human influence (Oost et al, ), and/or the extent of the marsh platform (Lorenzo‐Trueba & Mariotti, ), which can potentially lead to complex dynamics in the barrier system. For example, paleovalleys can constrain inlet migration rates, and even anchor inlets in place over geological timescales (Beets & van der Spek, ; Mallinson et al, ).…”

Quantifying thresholds of barrier geomorphic change in a cross-shore sediment-partitioning model

“…This is exemplified by formerly and presently wide barrier islands such as Cedar and Parramore islands in Virginiaboth of which were historically around 4 times wider than the 400 m wide barrier initialized in our model investigation. Despite experiencing an acceleration in relative SLR of 3 to 4 mm yr −1 over the last century (Boon and Mitchell, 2015), these islands have recently or historically sustained kilometer-scale landward shoreline migration over decadal to centennial timescales (McBride et al, 2015;Deaton et al, 2017;Shawler et al, 2019). Similar longer-term and sustained narrowing of previously wide barriers has also been inferred at Bogue Banks, North Carolina, a system of formerly progradational islands that began to undergo net shoreline erosion at approximately 1 ka (Timmons et al, 2010).…”

“…While the latter is a cross-shore construct based on wave energy, Hallermeier (1978) describes the littoral zone as being significantly shaped by alongshore transport, a process which at least one study has shown can cause upper-shoreface profiles to depart significantly from cross-shore equilibrium predicted by the Bruun rule (List et al, 1997). Similarly, historical and geological investigations indicate that barriers subjected to kilometer-scale shoreline movement over multidecadal timescales are strongly influenced by spatiotemporal variations in alongshore sediment delivery (Brooke et al, 2008;Lindhorst et al, 2010;Oliver et al, 2017;Raff et al, 2018;Shawler et al, 2019).…”

“…It is well documented at places like Fire Island, New York, that backbarrier strata beneath the modern island contain preserved beach, overwash, and marsh facies (Sirkin, 1972;Leatherman, 1985), indicating the barrier has both migrated and changed its width through time. Additionally, Raff et al (2018) and Shawler et al (2019) document marsh migration onto the bayside and low-lying interior of formerly progradational barrier islands, indicating a relationship between barrier width and backbarrier drowning, such that increasing island width restricts the flow of sediment to the backbarrier, thereby increasing passive interior flooding from sea-level rise. This suggests that comparing the modeled backbarrier-marsh interface through time to stratigraphic information from real-world barriers could yield insights into the sediment flux conditions at the shoreline, even if past shoreline geometries are not fully recorded in the geological record.…”

“…Combinations of these influences are possible, which suggests the baseline sensitivity of barriers to sediment input and loss magnitudes varies considerably. For example, modern barrier islands along the US Gulf and Atlantic coasts feature accommodation depths on the order of 4-6 m (e.g., Parramore and Cedar Islands in Virginia - Shawler et al, 2019, Raff et al, 2018Brigantine Island, New Jersey -Shawler et al, 2021;Fire Island, New York -Schubert, 2010, Leatherman, 1985 to less than 3 m (western central Florida - Locker et al, 2001Locker et al, , 2002a. Interestingly, the barrier islands in western central Florida are considered to be among the most morphologically variable in the world and are prone to rapid change (Davis and Barnard, 2003).…”