Spits and bars

Abstract

The Piedras Estuary is one of the most significative estuarine systems on the mesotidal Huelva Coast, in the Northwestern portion of the Cadix Gulf. The river mouth is presently an estuarine lagoon partially closed by a large spit constructed from an old barrier island system. This estuary is in an advanced state of infilling and its tidal prism has decreased during the Holocene causing instability and clogging of old inlets and transforming the barrier island chain into a spit. Sedimentation is controlled by the interaction of ebb tide currents and the prevailing SW waves. The main sediment supply is provided by an intensive West-to-East longshore current, transporting sand material from Portuguese cliffs and the Guadiana River. Tidal range is mesotidal (2.0 m) and the mean significant wave height is 0.6 m with an average period of 3.6 s.A boxcore study allowed five depositional facies to be distinguished in the Piedras Estuary mouth: (1) main ebb channels; (2) marginal flood channels; (3) ebb-tidal delta lobes; (4) marginal levees; and (5) curved spits. The recent evolution studied in this area suggests a cyclic evolutionary model for the ebb-tidal delta system. The architectural facies relations shown by the vibracore/boxcore study confirm that the apical growth of the spit occurred over the innermost of these ebb-tidal deltas. Consequently the preserved sequence shows the ebb-tidal delta facies under the spit facies.