Winnowed gravel lag deposits between sandbanks in the German North Sea

The "Sylter Outer Reef" is a special protected area in the German North Sea. The area is characterized by patches of coarse sediments compared to the otherwise sandy North Sea. Sediments coarser than sand size originate from the last glacial period or postglacial sorting processes. One special feature is winnowed gravel lag deposits, which are very rich in species. The features have a sickle-like to elongated shape with a distinct boundary between fine sand and gravel on one side, and a fuzzy boundary with gravel to fine sand on the other side. The sharp boundary indicates an abrupt shift in morphology. Gravel deposits are observed to be morphologically deeper than the surrounding finer sediments. In addition, three soft-bottom communities can be associated with this feature: Tellina fabula on fine to medium sediments; Goniadella–Spisula on medium to coarse sediments; and Goniadella–Spisula on coarse sediments.