Formation of chenier plain of the Doñana marshland (SW Spain): Observations and geomorphic model

Abstract

The Guadalquivir estuary is one of the most important in Europe. It is isolated from the sea by two important littoral spits (Doñana and La Algaida). Its sedimentary filling occurred as a result of deltaic contributions of the Guadalquivir River, giving rise to marshes primarily composed of mud deposits, among which a succession of sandy and shelly ridges can be found. The aim of this paper is to explain the genesis and evolution of these sandy and shelly ridges clarifying what types of ridges are found and their spatial distribution, identifying the processes responsible for their formation. These ridges extend in the form of long strands along the estuary, influencing its evolution during the Late Holocene. The geomorphologic and lithostratigraphic features of these ridges allow them to be defined as transgressive cheniers, regressive cheniers and laterally accreted ridges (spits), as components of the Guadalquivir estuary chenier plain complex. This chenier plain has the peculiarity of developing on a semi-enclosed shoreline, contrary to other chenier plains in the world, which develop on open coasts. In the Guadalquivir estuary the chenier plain formation is related to two relatively higher sea level (4200–2800 yr BP and 1400–1000 yr BP) during which high energy events occur (storms and tsunamis), that transgressed the large littoral spits. The Carrizosa (∼ 4000 yr BP) and Vetalengua (∼ 1400–1300 yr BP) spits have been formed across the inlet, and prograded landwards of the estuary. From here, several sequences of cheniers (mainly transgressive cheniers) associated with these events were formed. These cheniers form two groups: Las Nuevas (∼ 1100–1000 yr BP) and Marilópez (∼ 3000 yr BP), composed mainly of shells and shell fragments. These cheniers were constructed by waves which reworked the shells above estuarine levees, and led to their intense landward migration, thus explaining the peculiar, high inland position of the cheniers.