Evolution of and processes acting on inner continental shelf areas, resolved with hydroacoustic and sedimentological methods: Case studies from the Baltic Sea and the Andaman Sea.

Feldens, Peter (2011) Evolution of and processes acting on inner continental shelf areas, resolved with hydroacoustic and sedimentological methods: Case studies from the Baltic Sea and the Andaman Sea. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 144 pp.

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Abstract

Shallow seas are influenced by a magnitude of processes, acting on- and offshore. Therefore, most shallow sea areas have a unique appearance, preserving information both about their geologic history and processes acting on them. For this study, extensive hydroacoustic surveys, including side scan sonar, multibeam echo sounder, and reflection seismic, as well as sedimentological analysis were carried out in the Fehmarn Belt (SW Baltic Sea) and offshore Khao Lak (Andaman Sea, Thailand) between 2007 and 2011. The geologic evolution since the last glacial as well as recent sediment dynamics were investigated in Fehmarn Belt, while offshore impacts of the 2004 Indian Ocean Tsunami where evaluated off Khao Lak. The development of the Baltic Sea since the last glacial period was controlled by several regression and transgression events during the phases Baltic Ice Lake, Yoldia Sea, Ancylus Lake and Littorina Sea. Many details regarding the development of these stages are unsolved, one question being whether, and to what extent, the regression of the Ancylus Lake at 9,200 to 9,000 14C yr BP took place over the Darss Sill. Next to the general geological evolution of Fehmarn Belt since the last glacial, this study addresses the question whether a drowned river system in Fehmarn Belt (SW Baltic Sea) can be related to the drainage of the Ancylus Lake. The river channel is cut into glacial till in the western Fehmarn Belt, reaching an incision depth of up to 12 m at a base level of 40 m b.s.l. (below sea level). Its continuation towards west can be recognized in bathymetric data, while the channel is buried towards Mecklenburg Bay. According to seismic images, it is rapidly widening from several hundred meters to more than 1 kilometre and seems to fade towards east. Sediment thickness above the glacial till can exceed 30 m in the buried section of the channel. It is proposed that the channel was mainly shaped as part of a glacial meltwater system at a water level of 30 m b.s.l., although it was eventually incised subglacially. During the lowstand of the Baltic Ice Lake, local, shallow water bodies covered the study area and calm conditions prevailed. A previously reported westward directed drainage of a lake in the eastern Fehmarn Belt could be restricted to a time interval following the highstand of the Ancylus Lake, and prior to the Littorina transgression. Timing, water level and potential water discharge of this event suggest its connection to the partial drainage of the Ancylus Lake over the Darss Sill. Subsequently to the regression, cliffs and lake deposits point to a local water level between 24 to 26 m b.s.l. However, finding of a channel system filled with sediment deposited during the early Littorina Transgression might indicate a short phase with a water level down to 30 m b.s.l. With rising water level during the Littorina transgression, a large subaqueous dune field was formed in the central Fehmarn Belt. It is situated in water depths between 11 and 25 m, with an extension of about 8.1 km E – W and 1.8 km N - S. It consists of asymmetric dunes, indicating a W to E directed current, with crest heights of up to 2.5 m. The dunes are composed of allochthonous, well-sorted medium to coarse sand. Sand ribbons, connected to the subaqueous dune-field and protruding towards the southwest, are supposed to be sediment-conduits. Only minor movements of the field over the annual cycle could be observed, but comparisons with older maps show an increase in spatial extension over the course of decades. It is assumed that sediment movements in the subaqueous dune field occur mainly during west-storm conditions, when salt water infrequently intrudes from the North Sea into the Baltic Sea. The aim of the second case study was to resolve impact of the 2004 Indian Ocean Tsunami on offshore areas. The coastal area of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Meanwhile, its impact on offshore areas is mostly unknown, although offshore tsunami deposits were speculated to be widespread in the geological record. The 2004 tsunami offered the unique opportunity to catalogue its offshore effects from a well-recorded series of events. In Thailand, reported onshore tsunami deposits, containing marine sediments, as well as satellite images, showing large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. Offshore Khao Lak, paleoreefs with associated boulder fields and sandy sediments dominate the inner continental shelf. Patches of fine-grained (silt to fine sand) sediments exist in water depths of less than 15 m. The sediment distribution pattern is stable between 2007 and 2010, apart from small shifts regarding the boundaries of the fine-grained sediment patches. In sediment cores and grab samples an event layer was documented, situated below a cover of modern sediments that is only a few cm thick. The event-layer can be securely traced down to 18 m water depth. It consists mostly of sand, including coral fragments, but contains compounds of terrigenous origin as well. It is interpreted as a 2004 Indian Ocean tsunami deposit, which was the last major event in the area. Beneath 18 m water depth, indications of potential tsunami influence on a system of sand ridges are found, including erosion of app. 1 m deep channels at the NW-flank of the sand ridges and the deposition of silty material sandwiched by sandy sediments. On wide areas of the study-site an impact of the tsunami is hardly identifiable by seafloor morphology or sediment distribution five year after the event, pointing towards a tsunami impact focussed to some areas and a rapid return of the seafloor to equilibrium conditions.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Stattegger, Karl and Krastel-Gudegast, Sebastian
Additional Information: Als Print-on-Demand erschienen: Südwestdeutscher Verlag für Hochschulschriften. 144p. ISBN: 978-3-8381-3328-7
Keywords: 2004 Indian Ocean Tsunami Tsunami - Offshore Tsunami Impact - Baltic Sea Evolution
Research affiliation: Kiel University
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Date Deposited: 11 Dec 2012 10:52
Last Modified: 19 Apr 2013 12:29
URI: http://oceanrep.geomar.de/id/eprint/19618

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