STRATIGRAPHY, EVOLUTION, AND CONTROLS OF A HOLOCENE TRANSGRESSIVE-REGRESSIVE BARRIER ISLAND UNDER CHANGING SEA LEVEL: DANISH NORTH SEA COAST

摘要

This study provides a detailed reconstruction of the formation of a wave-dominated barrier island and assesses the sedimentological and morphological effects of sea-level changes on barrier evolution. Sedimentological and stratigraphic characteristics of the Holocene deposits are resolved by percussion cores and an extensive ground-penetrating-radar survey. A high-resolution chronology of the cored barrier island deposits is constructed by optically stimulated luminescence dating. This approach facilitates a high spatio-temporal resolution of the island's morphological and depositional evolution. The results show that the barrier island experienced multiple phases of transgressions and regressions during the mid-and late Holocene and that these changes were driven primarily by changes in rates of sea-level rise, sediment supply and the impact of storms. Due to the postglacial sea-level rise, the seaward part of the study area was transgressed by the retreating mainland shoreline, forming a back-barrier basin. At the time of the initial transgression, sea level was rising by more than 4.0 mm yr(-1). As sea-level rise decreased to less than 2 mm yr(-1) the back-barrier basin rapidly started to fill with sediments before being transgressed by the still retreating open-ocean shoreline. These events were associated with periods of very rapid landward displacement of up to 10 m yr(-1) of the mainland shoreline. After stabilization of the barrier island in its most eastward (i.e., landward) location, the open-ocean shoreline prograded about 3 km seaward at a rate of > 3 m yr(-1) through the deposition of a 7-m-thick sandy beach and shoreface succession. The progradation occurred despite a sea-level rise of about 1.8 mm yr(-1). After the regressive period the barrier island once again became transgressive before shifting back to its current regressive stage. The study shows how comparable rates of sea-level rise can result in very different morphological responses for a single barrier island since sediment supply apparently has varied significantly through time. It also appears that a transgressive situation is reached for this particular barrier island for a sea-level rise higher than about 2 mm yr(-1). This gives reason for concern since a number of sea-level change scenarios indicate that such a rate will be reached within a few decades.