Papel de la sedimentación estuarina en el intercambio sedimentario entre el continente y el litoral: el estuario del río Guadiana (SO de España-Portugal)

Abstract

The analysis of the sediments in the Guadiana River estuary (grain size, mineralogy, geochemistry and organic carbon content) allows to distinguish two types of sedimentation: a fluvial-marine sedimentation, favoured by tidal action, and a autochtonous sedimentation related to the water mixing. The net transport of sediments is towards the sea because of the tidal current asymmetry. The autochtonous sedimentation and the extreme fluvial supply that tide are unable to rework are preserved on the meandering convexe margins with low tidal velocities. These areas acquire a lateral tidal bar morphology because the narrow geometry of the estuary inhibits the presence of longitudinal tidal bars; as may be expected in tide-dominated systems. The higher energy zones (deeper zones of the channel) become bypassing channels where the flocculated material cannot be settled on the bottom. Two stages have been differenctiated in the bypassing of fluvial materials to the coast : first, during the tidal flood, the river deposes its sedimentary load in the upper zone of the central domain, whre the river velocity decreases in the confluence between the fluvial and flood currents. In a second stage, during the ebb, the sand fractions are reworked towards the sea, where they arrive after various tidal cycles and are mixed with the marine sediments introduced by the tidal flood into the estuary. This process is constrasted by the microfaunal distribution (foraminifera and ostracods).