Geomorphology and Internal Sedimentary Structure of a Landward Migrating Barrier Spit (Southern Sylt/German Bight): Insights from GPR Surveys

Abstract

Barriers comprise approximately 15 % of the world’s coastlines and are formed due to the combined action of wind, waves, and longshore currents. In this study ground-penetrating radar data (GPR) of different antenna frequencies and sedimentological data were combined to reveal the sedimentary structure and architecture of the southern barrier island spit of the North-Frisian island of Sylt, to define different coastal environments and to set up a barrier island stratigraphy. Based on these data, a sedimentological model has been generated for Southern Sylt which describes the interaction between extreme events, coastal processes and sedimentary development and elucidates the major episodes of barrier island evolution. The 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 involving an interplay of barrier retreat and washover associated with accumulation of sediment in a backbarrier environment as a result of storm surges. With the exception of the uppermost dune facies the spit add-on zone reveals a transgressive coarsening upward sequence starting with sandy mud flat deposits at the bottom which grade into coarser sandy tidal flat deposits toward the top. Sandy tidal flat deposits are overlain by washover sheet and washover fan deposits. Eroded sediment was transported along the west coast of Sylt by longshore drift and was added to the southern spit-end during fair weather conditions.