Geomorphological and morpho-sedimentary features of a sand barrier in a tectonically asymmetrical estuary during the Late Holocene: La Algaida (SW Spain)

Analysis of a number of drill cores, geomorphic patterns, and radiocarbon assays on mollusk shells from La Algaida spit, in the estuary of the Guadalquivir River (SW Spain), has revealed the genesis and evolution of a sandy barrier located in a rather complex neo-tectonic setting: the geological boundary between the Alpine belt of the Baetic mountain ranges and the Hercynian massif. The development of this sandy barrier during the Late Holocene has been conditioned by the presence, on the eastern bank of the estuary, of a Plio-Pleistocene paleo-relief which forms part of a raised block in a set of reverse-fault systems with SW-NE alignment, the most conspicuous of which is that of the Lower Guadalquivir Fault (LFG). These systems have influenced the morpho-stratigraphic and geomorphological disposition of the Holocene sediments on both sides of the tectonic alignment. The evidence presented here indicates that La Algaida spit is part of this raised tectonic block. As signs of subsidence are negligible, the spit exhibits series of exposed progradation units which started to develop shortly after the spit itself emerged, in the form of a barrier island. By contrast, subsidence processes affecting the Doñana spit, on the western bank of the estuary, are clearly marked, as they were sustained and massive. This asymmetry explains the relatively meager thickness, 22 to 24 m, of the sedimentary formations constituting La Algaida. Deposits at this location began to accumulate in about 6000–5500 cal. BP, originally as part of extensive shoals lying on top of the Plio-Pleistocene paleo-relief. At present, the spit exhibits three exposed progradation units (PS). Punctuated by erosive discontinuities, these units, or phases, succeeded one another until the Roman period (PS1 and PS2); thereafter, a tombolo formed to connect the erstwhile barrier island with the mainland (PS3). The peculiar, tectonically conditioned, active asymmetry geomorphological as well as sedimentary in the Guadalquivir estuary and its environs contradicts received geological understandings of the area, yet helps understand comparable transformations in other coastal areas of the planet.