Identifying Overwash Deposits in Arid Environments: Towards a Millennial-Scale Record of Cyclones and Makran Trench Tsunamis from Sur Lagoon, Oman

Coastlines along the Arabian Sea are susceptible to marine inundation from Makran Subduction Zone (MSZ) earthquakes and tropical cyclones. Sediments deposited by these forms of inundation can expand the decadal instrumental record of events to include millennial timescales in regions without rich historical records (i.e., Oman). On November 28, 1945 a 8.1 Mw MSZ earthquake generated a tsunami that inundated coastlines of the Arabian sea with wave heights as high as 13m. The stratigraphic, sedimentological, foraminiferal, and geochemical signatures of deposit were examined from a small (12 km2), microtidal lagoon in Sur, Oman. The 1945 tsunami deposit contained distinctive taphonomic assemblages of foraminifera and bivalves. Below the 41cm thick 1945 shell-rich deposit at Sur Lagoon, seven additional anomalous sand (mean grain size of 3.73f ± 1.66; very fine sand) layers, ranging in thickness from 7 to 32cm, were found preserved within fine-grained lagoonal sediment (mean grain size of 4.44f ± 1.66; very coarse silt). The seven inferred overwash layers have features consistent with the 1945 tsunami deposit such as fining upward trends, marine foraminifera (e.g., Amphistegina spp., planktics) and increased concentrations of calcium and strontium. By contrast, the surrounding lagoon deposits contain finer grain sizes, intertidal and nearshore foraminifera (e.g., Ammonia tepida, miliolids), and increased concentrations of titanium and magnesium. Based on these data, the seven overwash layers found below the 1945 tsunami deposit have been attributed to marine inundation. Radiocarbon dating indicated an age of 3127 to 2515 cal yr BP for the deepest stratigraphic unit