Sedimentology and preservation potential of carbonate sand sheets deposited by the December 2004 Indian Ocean tsunami: South Baa Atoll, Maldives

Observations of carbonate sand sheets deposited by the December 2004 Indian Ocean tsunami across island surfaces in the Maldives archipelago provide an opportunity to investigate tsunami flow behaviour across relatively simple coastal forms. This paper presents data on tsunami sand sheet deposits for five islands located along an east-west transect through the central atoll of South Baa. Sedimentary data include: field measurement of sand sheet planform, thickness and volume; descriptions of sedimentary structure and stratigraphic relationships with underlying surfaces; laboratory measurement of grain-size as a proxy of settling velocity; and observations of post-depositional changes to sand sheet characteristics two years following the Indian Ocean tsunami. Results show cross-atoll trends in volume and form of sand sheets that are interpreted as a proxy of waning and scattering of energy, as the wave train interacted with the shallow waters and numerous islands of the atoll. On individual islands, the basin topography resulted in tsunami overwash but no return flow, so that sand sheets are the product of unidirectional flow, with evidence in grain-size trends for minor acceleration across landward slopes. A key finding is the evidence for rapid post-depositional changes to the form and extent of tsunami deposits. Within two years of deposition, reworking of the seaward edge of sand sheets has destroyed the thickest part of the deposit and the thinner landward part of the sand sheets has been reworked by bioturbation. This study concludes that the preservation potential of these tsunami deposits is low to moderate, with the only probable recognizable signature being local variation in the micro-topography of island surfaces.