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7871294 
Journal Article 
Geomorphology of Tromelin, Indian Ocean 
Marriner, N; Guerout, Max; Romon, T; Dussouillez, P 
2010 
1 
Comptes Rendus Geoscience
ISSN: 1631-0713 
342 
10 
766-777 
WOS:000283470000002 
Tromelin is a small coral reef island (1 km(2)) located in the Indian Ocean, approximately 440 km east of Madagascar and 580 km north of La Reunion. Despite the presence of a permanent Mete France weather station on Tromelin since the 1950 s, a detailed geomorhological study of the island has never been undertaken. In this paper, we describe results from the 2008 field season, which enabled us to map and describe seven geomorphological zones on the atoll. (1) Tromelin"s bathymetry is characterised by a pronounced submarine slope, which attains depths of 1000 m at only 2.5 km from the island"s coastline. This geomorphology is typical of volcanic hotspots. (2) A coral platform borders the emerged crown of the island. This platform is exposed at low tide and has been fashioned in an ancient Pleistocene substratum. The substratum has been eroded by abrasive marine action to its present shape since the stabilisation of relative sea level around 6000 years ago. Erosion of the reef yields most of the biogenic sediment supply to the island. (3) The upper foreshore is characterised by outcrops of beachrock. These formations show a stratfication in slabs and dip gently towards the sea in the same manner as the contemporary clastic sediments. (4) Tromelin"s beaches are divided into three sub-units, which reflect the different energy dynamics around the island, in addition to the transfer of sediment from the windward to the leeward side: (i) the southern storm beaches are characterised by coral blocks reworked during episodic high-energy events; they form boulder ramparts. Four perched rampart ridges have been recorded at similar to 1 m, similar to 3 m, similar to 4.5 m and similar to 7 m above mean sea level; (ii) transitional beaches are observed between the south and the north, constituting storm blocks and coarse sands and gravels; and (iii) sandy beaches are noted on the northern leeward side, comprising rounded gravels and coarse sands. The northern tip of the island is characterised by a sand spit (similar to 125 x similar to 225 m), whose geometry varies based on the seasonal and meteo-marine contexts. (5) On Tromelin, a number of dune formations are observed. These are best developed in the north of the island due to the south-north translation of clastic sediments by aeolian wind action, and the trapping of sands by the well-developed vegetation. The height of the dunes varies between 10 cm (the southern micro-dunes) to 250 cm in the Northeast of Tromelin. (6) Exceptional waves have deposited storm tracts up to 250 m from the foreshore zone, comprising coarse gravels and coral blocks. The storm tract deposits are most prevalent on the windward side of the island, due not only to the high-energy dynamics, but also to the absence of vegetation cover in this area. (7) The southern depression covers an area of similar to 40,000 m(2). At its lowest point it is only 1.2 m above sea level. (C) 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved. 
Coastal geomorphology; Geoarchaeology; Coral reef; Tromelin; Indian Ocean