Depositional history of estuarine infill during the last postglacial transgression (Gulf of Cadiz, Southern Spain)

The Late Pleistocene and Holocene evolution of the estuaries in the Gulf of Cadiz is interpreted for the first time using drill cores, logs, trenches, and 38 new radiocarbon data, and the results compared with the shelf The Odiel, Tinto and Guadalete Rivers deposited conglomerates during a highstand that did not reach the present sea level dated at ca. 25-30 ka (Isotopic Stage (IS) 3), corresponding to a relatively humid period in the area. Rivers incised these coarse-grained deposits during the last main lowstand at ca. 18 ka, when sea level dropped to - 120 m and the coastline lay 14 km seawards from the present. The erosional surface is a sequence boundary and the flooding surface of the postglacial eustatic rise, overlain by the valley fill deposits of the transgressive and highstand phases of the last fourth- and fifth-order depositional sequences recognised in the shelf. The first marine influence in the estuaries during the transgression occurs at - 25/ - 30 m at ca. 10,000 years BP. According to fossil assemblages, the transgressed basins changed from brackish to more open marine as the sea rose until ca. 6500 years BP, when it reached the maximum flooding and the sandy estuarine barriers ceased to retrograde toward the muddy central basins. Then, the rate of eustatic rise decreased drastically, and the estuarine filling followed a two-fold pattern governed by the progressive change from vertical accretion to lateral (centripetal) progradation. At ca. 4000 years BP the fluvial input surpassed the already negligible rate of rise, causing partial emergence of tidal flats and spit barriers in the largely filled estuarine basins. Prevalence of coastal progradation upon vertical accretion at ca. 2400 years BP caused accelerated expansion of tidal flats and rapid growth of the sandy barriers. Further changes since the 16th century reflect widespread anthropic impacts. (C) 2000 Elsevier Science B.V. All rights reserved.