Sedimentary architecture and depositional controls of a Holocene wave-dominated barrier-island system

Barrier-island system evolution is controlled by internal and external forcing mechanisms, and temporal changes in these mechanisms may be recorded in the sedimentary architecture. However, the precise role of individual forcing mechanisms is rarely well understood due to limited chronological control. This study investigates the relative role of forcing conditions, such as antecedent topography, sea-level rise, sediment supply, storms and climate changes, on the evolution of a Holocene wave-dominated barrier-island system. This article presents temporal reconstruction of the depositional history of the barrier-island system of Rømø in the Wadden Sea in unprecedented detail, based on ground-penetrating radar profiles, sediment cores, high-resolution dating and palynological investigations, and shows that ca 8000 years ago the barrier island formed on a Pleistocene topographic high. During the initial phase of barrier evolution, the long-term sea-level rise was relatively rapid (ca 9 mm year ⁻¹) and the barrier was narrow and frequently overwashed. Sediment supply kept pace with sea-level rise, and the barrier-island system mainly aggraded through the deposition of a ca 7 m thick stack of overwash fans. Aggradation continued for ca 1700 years until sea-level rise had decreased to <2 mm year ⁻¹. In the last ca 6000 years, the barrier prograded 4 to 5 km through deposition of a 10 to 15 m thick beach and shoreface unit, despite a long-term sea-level rise of 1 to 2 mm year ⁻¹. The long-term progradation was, however, interrupted by a transgression between 4000 years and 1700 years ago. These results demonstrate that the large-scale morphology of the Danish Wadden Sea shoreline influences the longshore sediment transport flux and the millennial-scale dispersal of sediment along the shoreline. On decadal to centennial timescales, major storms induced intense beach and shoreface erosion followed by rapid recovery and progradation which resulted in a highly punctuated beach and shoreface record. Major storms contributed towards a positive sediment budget, and the sustained surplus of sediment was, and still is, instrumental in maintaining the aggradational–progradational state of the barrier island.