Why is barrier spit´s accretion not a simple process? Insights from GPR-surveys of Northern Amrum (North Sea/German Bight)

Barrier spits are geological young, highly dynamic and represent a complex coastal system that includes a number of different but closely related sedimentary depositional environments and geomorphologic elements of varying origin, genesis and evolution. The stratification of coastal barriers is mainly investigated by using borehole data. Therefore, the processes of evolution and the internal structure are often unknown. The classical barrier spit development dominated by storm surge erosion, longshore drift and accretion is typical for a lot of spit systems of the Southern North Sea. A similar development of the northern barrier spit of the North-Frisian Island of Amrum seems to be most likely and has been supposed until now. In this study ground-penetrating radar (GPR) data and sedimentological data were combined to reveal the sedimentary architecture of the northern spit of Amrum. Unexpectedly, remnants of an underlying older barrier spit have been detected in the underground. This fossil barrier spit shows a different direction of progradation and is overlain by Holocene sandy tidal flat deposits. The study discusses a theory of the fossil barrier´s origin. Especially GPR data show that barrier spit´s accretion is much more complex than previously thought.