Holocene stratigraphy and land subsidence: a key lecture from the Volturno River alluvial plain (southern Italy)

Most of the world’s major river deltas and related alluvial coastal plain are affected by subsidence. Studies were carried to figure out the potential drivers of subsidence, along with tectonics, reduced aggradation, volcanism, fluid extraction, as well as natural compaction that can drive subsidence of several mm for years, especially in coastal organic-rich deposits. In the Mediterranean, several alluvial coastal plains developed after the Holocene transgression are affected by subsidence. The Volturno alluvial-coastal plain, along the eastern Tyrrhenian Sea (southern Italy) is characterized by subsidence rates determined through InSAR data analysis and ranging between 0 and <-20 mm/yr in an area of about 750 kmq. In this study we focused on the investigation of relationships between the subsidence process and geological and geotechnical features of the plain. To understand the driving mechanisms of this process, a lithostratigraphic reconstruction was provided focusing on the spatial distribution of the horizons considered weak by a geotechnical point of view; then, the subsidence map was overlain spatially with geological data. The spatial analysis highlighted the major ground deformation occurring within the outer boundary of the incised paleo-valley, corresponding to the Holocene alluvial/transitional filling that overlies a volcanic compaction-free Pleistocene basement. Inside this general trend, differential compaction was detected corresponding to thick occurrence of clay and peat deposits, suggesting that the subsidence rate are due in part to the consolidation of primary settlements of soft and compressible soils that characterize the subsoil of these areas, and in large part to the secondary consolidation settlements.