The 20,000-years-long sedimentary record of the Lesina coastal system (southern Italy): From alluvial, to tidal, to wave process regime change

Abstract

The present-day Lesina area (Adriatic coast of southern Italy) preserves in the subsurface the stratigraphic signature of a recent sedimentary process regime change, which was responsible for the conversion of a former alluvial plain into a back-barrier tidal flat and, finally, into the modern barrier island. Facies-based analyses of the first 55 m of the upper Pleistocene–Holocene stratigraphic record, integrated with biostratigraphic sampling, radiocarbon data, and aerial observations of some diagnostic relict morphologies, allowed us to reconstruct the history of the last 20,000 years of this area. The succession investigated is adjacent to a salt dome, which uplifted in recent times, forming the easternmost boundary of the present-day Lesina lagoon. Three main stratigraphic intervals were detected in the subsurface: the lowermost unit is made up of conglomerates, sandstones, and mudstones of terrestrial origin, belonging to a complex system of alluvial plain filling a pre-existing Last Glacial Maximum (LGM) topography. The second interval consists of sands and muds, with subordinate conglomerates of brackish and marine origin, and lies on the previous one through a wide ravinement. Its composing lithofacies exhibit a strong tidal signature preserved in tidal rhythmites belonging to a net of tidal channels, associated with marshes, mud flat, and lagoonal deposits. These sediments record the emplacement of a back-barrier tidal flat, which developed under the strong influence of a tidal influx enhanced by the late post-LGM transgression. The third uppermost interval resulted from the deposition of coastal-marine sands and gravels accumulated during the ensuing modern normal regression, under the dominance of a wave-dominated coastal dynamics, which was responsible for the progradation of the present-day beach barrier and the closure of the Lesina lagoon. The paleogeography of the back-barrier tidal flat preceding the onset of the modern barrier island is thus reconstructed based on the results of our facies analysis, biostratigraphy, and AMS dating. Many of the elements composing this mid-Holocene tide-influenced system were also interpreted from the aerial-photograph observation of several relict morphologies, which are still preserved in many parts of the modern Lesina barrier island. We propose some new interpretation on the origin some of these elements, which possibly developed under sedimentary process regimes different from the modern ones, including some flood-tidal deltas, previously interpreted as tsunami-derived washover fans.