Evidence of high-energy events in the geological record:: Mid-holocene evolution of the southwestern Donana National Park (SW Spain)

Four phases are distinguished in the Mid-Holocene evolution of the southwestern Donana National Park (SW Spain), based on the multidisciplinary study of sediments in drill cores. In the oldest phase (> 5500-5400 cal. years BP), a coastal, brackish lagoon occupied the central part: of the study area, partly enclosed by the Donana spit and limited by fluvial levees. An evolution from subtidal to intertidal conditions characterized the areas located close to the inner side of the Donana spit during the same phase. The following phase (similar to 5400-5200 cal. years BP) is characterized by a high-energy event (tsunami?), which caused the breakthrough of the Donana spit and the creation of new littoral strands in the inner areas. In the first period of the third phase (similar to 5200-4200 cal. years BP), estuarine infilling was probably the dominant process, with the accumulation of phyllosilicate-rich clays in the lagoon bed. This was followed by a renewed phase of instability (similar to 4200-4100 cal. years BP) indicated by the presence of fine storm-lain deposits and thicker, probably tsunami-induced shelly deposits. The last phase (similar to 4100-3700 cal. years BP) comprises: a) an infilling period, with a diminution of the marine influence; and b) two new high-energy events, which caused the erosion of the Donana spit and the creation of new cheniers. In the last 7000 years, the comparison of these results with a revision of both geological and historical records has revealed the occurrence of twenty tsunamis (at least) during this period in Portugal and southern Spain. Evidences of these high-energy events are washover fans, bioclastic coarse-sized layers within salt marsh deposits or the presence of giant boulders and pebbles. The determination of the recurrence period is very difficult owing to the very scarce data, although our data and other authors do not discard a possible periodicity of 2000 years for a cycle of two events separated by 300-400 years. (c) 2005 Elsevier B.V. All rights reserved.