The sedimentary architecture of a Holocene barrier spit (Sylt, German Bight): Swash-bar accretion and storm erosion

The southern German North Sea coast was shaped by the last post-glacial sea-level rise that caused a significant retreat of the coastline. Approximately 5 ka BP, the rate of sea-level rise decreased, providing space for the formation of the Frisian Islands which line the coast. This study is focused on the island of Sylt, located at the German North Sea coast close to the Danish border. Here, two sandy spit systems developed during the Holocene. Combining ground-penetrating radar studies and sedimentological investigations based on shallow cores, new aspects on the sedimentary history of this Holocene spit system have been revealed. The data indicate that strong erosional events alternated with phases of progradation and growth. The welding of swash bars is shown to be the predominant process during progradational phases of the spit system. During these periods, progradation was not restricted to linear growth along the spit axis, but also included a seaward-directed component. Major erosion surfaces, which delimit progradational sediment packages, are interpreted to reflect exceptionally severe storms. The fossilization potential of this sedimentary record was controlled by a positive net long-term sediment balance and the position of the ground-water table which controlled eolian deflation. Only with a stable or rising sea level and associated ground-water table position are sediments protected from deflation. The proposed sedimentary model may serve as a template for interpretation of comparable settings in the geological record.