SEDIMENTARY CHARACTERISTICS OF THE DECEMBER 2004 INDIAN OCEAN TSUNAMI DEPOSIT AND THE POTENTIAL OF A LONG TSUNAMI RECORD IN THE RIDGE AND SWALE REGION BETWEEN MEULABOH AND CALANG, NORTHERN SUMATRA

The December 2004 Indian Ocean tsunami strongly hit the coastal area between Meulaboh and Calang, northern Sumatra, with wave heights reaching 15 m and inundation distances of several kilometers. In this study, we investigated the tsunami deposit along five transects in the wetlands between Meulaboh and Calang, an area that often shows a characteristic ridge and swale topography. We studied small trenches close to the beach, auger peat cores from the swamps and marshlands, as well as short gravity cores taken from small boats in coastal lakes, dead river arms and lagoons.

At the beach, the tsunami deposit can reach up to 1.40 m thickness, consisting of horizontally layered medium to coarse sand with some dewatering structures. Often massive erosion took place within swampy areas behind the beach, which led to the deposition of up to 40 cm thick dark brown, organic-rich mud with abundant plant debris, which was found as far as 2 km inland. In marshlands with grassy vegetation the deposit was rather sand-dominated (fine to medium sand) with thicknesses of 25 cm closer to the beach to a few centimeters about 1.2 km inland. Here the tsunami deposit overlies dark brown organic-rich mud, which is typically deposited in the marshlands.

Along two transects, coring within swales in these marshlands revealed a second, 7 to 20 cm thick sand layer, that might represent an older tsunami deposit, about 70 cm below the modern surface. Furthermore, an up to 3 mm thin grey layer was found a few centimeters below the second sand layer, probably representing an ash layer. Coring in the ridge and swale region is limited by the presence of old beach sands typically only about 1 to 2 m below the modern surface. However, extension of the tsunami record is possible by going further inland, where subsequently older beach complexes will be reached.