Geometría y edad de los depósitos aluviales de los ríos Agrio y Guadiamar entre Aznalcóllar y las marismas del Guadalquivir (Huelva-Sevilla)

The main geomorphological features of the Agrio and Guadiamar alluvial deposits, at the Bajo Guadalquivir basin, are described from 94 hydrogeologic boreholes and field observations. Three terraces, aligned from north to south, have been distinguished (T3, T2 and T l). Each one is made up of a lower level o f gravels and an upper level o f sand and silt. These terraces display an assymetric arrangement caused by the shifting of the valley axis to the east. The present day river forms a fourth terrace (TO) inserted on the T l. We have obtained a 14C age of 6285 years BP for the top o f the T2 terrace, and an age o f 5000 and 300 years BP respectively for the bottom and top of the Tl terrace. Therefore, alluvial deposits o f the Guadiamar river have been developed mainly during the holocene time, possibly following the movements of a sinsedimentary N-S fault in the tertiary deposits below the river, which caused the terraces shiftig.