Sedimentation and hydrodynamic processes associated with the tsunami generated by the 1755 Lisbon earthquake

An unusual sedimentary horizon, believed to have been deposited by the tsunami associated with the 1755 Lisbon earthquake, has been identified at Boca do Rio in southern Portugal. This horizon marks a distinct sedimentological and micropalaeontological break from the deposits enclosing it. The sedimentary characteristics of the horizon, which consists of several distinct sub-units, vary greatly in both vertical and horizontal section. Detailed textural analysis through vertical sections of these sub-units shows abrupt changes in particle size characteristics. The highly variable nature of the horizon appears to be due to rapid variation in the hydrodynamic characteristics of the depositional event. The lowermost section of the deposit appears to have been deposited from a highly turbulent water mass which was able to transport gravel sized clasts as well as gravel sized mud balls eroded from underlying material. The water mass rapidly lost energy, leading to the deposition of predominantly sand, silt and clay particles. Sharp breaks in textural characteristics are likely to be due to changes in current direction caused by wave reflection or swash-backwash interference. The main sources of sand sized material within the deposit are the seaward barrier (beach-foredune system). The tsunami deposit at Boca do Rio shares many of the sedimentary characteristics of tsunami deposits from other sires around the world. (C) 1999 INQUA/Elsevier Science Ltd. All rights reserved.