Evolution and facies architecture of the mesotidal Guadiana River delta (S.W. Spain-Portugal)

Abstract

The Guadiana River estuary and delta is one of the most important mesotidal fluvio-marine systems of the southwestern Iberian Peninsula. The river mouth was formed as a narrow channel excavated by fluvial incision during the Pleistocene lowstand and then flooded 6500 yr ago, during the Flandrian transgression (Holocene). The estuary is in an advanced state of sediment infilling in its proximal part, due to its narrow morphology which prevents passage of sediment through to the open coast. Consequently, sediment is accumulating in the river mouth, causing progradation as response of the interaction between coastal and fluvial processes. Sedimentation is controlled by the interaction of sediment load delivered by the Guadiana River and the active longshore current which also supplies sand eroded from barrier islands and friable cliffs in the Portuguese Algarve to the west. Tidal range is mesotidal (2.0 m) and mean significant wave height is 0.6 m with an average period of 3.6 s.Morphological differences exist between the two sides of the river mouth: the western side has a large spit separated from the mainland by a marsh and the eastern side shows an array of old barrier islands separated by salt marsh areas. A vibracore study allows us to distinguish 12 different depositional facies in the system: fluvial delta, tidal inlet, flood-tidal delta, washover, beach, dune, estuarine channel, lagoon, tidal creek, channel margin, tidal flat and salt marsh. Furthermore, a different architectural scheme of these facies is present on each side. To the east, there is a longitudinal repetition of the sandy facies corresponding to the barrier islands; on the west side, there is a transverse growth of a single spit containing eolian dunes. These differences result from asymmetric interaction between the tidal and fluvial currents at the river's mouth and the waves and tidal currents of the open coastal zone.