Barrier rollover and spit accretion due to the combined action of storm surge induced washover events and progradation: Insights from ground-penetrating radar surveys and sedimentological data

Barrier islands and spits are geological young, highly dynamic and represent a complex coastal system that includes a number of different but closely related sedimentary depositional environments. In this study ground-penetrating radar data of different antenna frequencies and sedimentological data were combined to reveal the sedimentary structure and architecture of the southern barrier island spit of Sylt and to set up a barrier island stratigraphy. Based on these data, two sedimentological models have been generated for Southern Sylt which describes the inter-action between extreme events, coastal processes and sedimentary development and contains the major episodes of barrier island evolution. The first model is concerned with the spit add-on zone where the barrier spit is attached to the central island moraine core and shows a landward migration through barrier rollover affected by an interplay of barrier retreat and washover flooding associated with accumulation of sediment in a backbarrier environment as a result of several storm surges. The spit add-on zone reveals a transgressive coarsening upward sequence starting with sandy mud flat deposits at the bottom which turn into coarser sandy tidal flat deposits toward the top. Sandy tidal flat deposits are overlain by washover sheet and washover fan deposits. The second model demonstrates a barrier spit accretion through southerly directed progradation. Eroded sediment was transported along the west coast of Sylt by longshore drift and was added to the southern spit-end. Progradation and barrier spit accretion were interrupted by severe storm surges. Storm surge generated erosion unconformities in a foreshore to shoreface environment redraw old spit-end positions that represent stages of barrier spit progradation.