Abstract
The Seychelles, an archipelago in the Indian Ocean at a distance of 4,500–5,000 km from the west coast of Sumatra, were severely affected by the December 26, 2004 tsunami with wave heights up to 4 m. Since the tsunami history of small islands often remains unclear due to a young historical record, it is important to study the geological traces of high energy events preserved along their coasts. We conducted a survey of the impact of the 2004 Indian Ocean tsunami on the inner Seychelles islands. In detail we studied onshore tsunami deposits in the mangrove forest at Old Turtle Pond in the Curieuse Marine National Park on the east coast of Curieuse Island. It is thus protected from anthropogenic interference. Towards the sea it was shielded until the tsunami in 2004 by a 500 m long and 1.5 m high causeway which was set up in 1909 as a sediment trap and assuring a low energetic hydrodynamic environment for the protection of the mangroves. The causeway was destroyed by the 2004 Indian Ocean Tsunami. The tsunami caused a change of habitat by the sedimentation of sand lobes in the mangrove forest. The dark organic rich mangrove soil (1.9 Φ) was covered by bimodal fine to medium carbonate sand (1.7–2.2 Φ) containing coarser carbonate shell fragments and debris. Intertidal sediments and the mangrove soil acted as sources of the lobe deposits. The sand sheet deposited by the tsunami is organized into different lobes. They extend landwards to different inundation distances as a function of the morphology of the onshore area. The maximum extent of 180 m from the shoreline indicates the minimum inundation distance to the tsunami. The top parts of the sand lobes cover the pneumatophores of the mangroves. There is no landward fining trend along the sand lobes and normal grading of the deposits is rare, occurring only in 1 of 7 sites. The sand lobe deposits also lack sedimentary structures. On the surface of the sand lobes numerous mostly fragmented shells of bivalves and molluscs were distributed up to 150 m from the coastline. Intact bivalve shells were mostly found positioned with the convex side upwards. On small ledges of a granitic body at 130–150 m from the shore mostly fragmented and gravel sized shells were deposited at different elevations up to 4 m above sea level. This implies a run up height of at least 4 m above sea level up to 150 m from the present shoreline.
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Acknowledgments
This study was funded by grant BA 1011/38-1 of the German Research Foundation (DFG). We thank the Government of the Republic of Seychelles for permitting and supporting our research project. Especially we thank Mr. Didier Dogley, the Department of Environment, Mr. Rodney Quatre, National Park Authorities, and Mr. Glenny Savy, Island Development Company (IDC), for their cooperation and support in the organisation of our field work on the Seychelles. Christian Drewer and Laura Czaya (University of Muenster, Germany) assisted with the field work. We thank Patrick Samson and Eddie Belle (Seychelles Petroleum Company) for discussions and for granting access to reports and literature. For archive research on former tsunami impacts on the Seychelles we acknowledge the employees of the Seychelles National Archives and Jean-Claude Mahoune, anthropologist, historian and keeper of the folk memory. This paper benefited from discussions with Michaela Spiske (University of Muenster). Susanne Klaus (University of Muenster) helped with the translation of archive documents. We appreciate the constructive reviews by A. Dawson (University of Aberdeen, UK) and an anonymous reviewer.
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Nentwig, V., Bahlburg, H. & Monthy, D. Sedimentology of Coastal Deposits in the Seychelles Islands—Evidence of the Indian Ocean Tsunami 2004. Pure Appl. Geophys. 172, 641–656 (2015). https://doi.org/10.1007/s00024-014-0990-9
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DOI: https://doi.org/10.1007/s00024-014-0990-9