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Marine and terrestrial records of the Holocene geological history of Sylt and the adjacent Wadden Sea (SE North Sea)

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Citation:
Hass, H. C. , Bartholdy, J. , Betzler, C. , Boldreel, L. O. , Fürstenau, J. , Kuijpers, A. , Lindhorst, S. and Madsen, E. B. (2008): Marine and terrestrial records of the Holocene geological history of Sylt and the adjacent Wadden Sea (SE North Sea) , 33rd International Geological Congress, Oslo, Norway, August 6-14 .
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Abstract:

Sylt is a barrier island at the German-Danish border in the SE North Sea. It consists of a core of Miocene to Pliocene marine and riverine sediments in the central part which forms the source sediment for two spit systems that extend more than 15 km to the south and the north. These protect the backside tidal basins and the mainland against the direct influence of the North Sea. Presently, Sylt is strongly eroded (about 1 m land loss per year at its west coast). Goal of the investigations presented here is to reconstruct the Holocene geological history of the area and to provide data instrumental to build up a scenario for the future development of the area in the light of climate change and sea-level rise.We use the sea-floor classification system RoxAnn to characterize the present state of the sublitoral sedimentary environments, and a parametric echosounder as well as sparker/boomer shallow seismics to reconstruct the Holocene development of the sedimentary environments offshore. Onshore, ground-penetrating radar (GPR), long sediment cores, and surface samples are used to reconstruct the transition from marine to terrestrial environments. West of Sylt and Rømø Sparker data reveal an upper unit with north-directed foreset structures overlying more horizontally bedded sediments. The foreset structures are likely related to the List Deep tidal delta situated between Sylt and Rømø. Under the immediate influence of the recent tidal currents large sandwaves (crest hight up to 7 m) locally occur on the seafloor. Onshore, GPR data reveal alternating processes of swash-bar accretion and severe-storm erosion at the west coast and longshore sediment transport with beach-ridge accretion at the north-facing part of the northern spit. Sediment core granulometric data show the transition from marine via beaches to dune environments. AMS radiocarbon age determinations allow to reconstruct the growth of the spit system whose youngest recurved part became permanently terrestrial about 1000 years ago. Acoustic and sedimentologic data from the recent seafloor west of the coastline show extended alternating zones of coarser and finer sediments that stretch perpendicular to the coastline. Depressions within the otherwise sandy area are being filled with muddy sediments. We will show how the different marine and terrestrial depositional environments are linked to each other and how sediment transport is likely governed. It becomes clear that Sylt is not the result of a retreating coastline as there is clear evidence for periods of extended seaward island growth during the late Holocene. Erosive and accumulative phases of the island appear to be the result of atmospheric forcing and the fluctuating sea level which suggests implications with regard to the future development of this and similar areas.

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