Late-glacial to Holocene depositional architecture of the Ombrone paleovalley system (Southern Tuscany, Italy): sea-level, climate and local control in valley-fill variability

The Ombrone palaeovalley was incised during the last glacial sea-level fall and was infilled during the subsequent Late-glacial to Holocene transgression. A detailed sedimentological and stratigraphic study of two cores along the palaeovalley axis led to reconstruction of the post-Last Glacial Maximum valley-fill history. Stratigraphic correlations show remarkable similarity in the Late-glacial to early-Holocene succession, but discrepancy in the Holocene portion of the valley fill. Above the palaeovalley floor, about 60 m below sea-level, Late-glacial sedimentation is recorded by an unusually thick alluvial succession dated back to ca 18 cal kyr BP. The Holocene onset was followed by the retrogradational shift from alluvial to coastal facies. In seaward core OM1, the transition from inner to outer estuarine environments marks the maximum deepening of the system. By comparison, in landward core OM2, the emplacement of estuarine conditions was interrupted by renewed continental sedimentation. Swamp to lacustrine facies, stratigraphically equivalent to the fully estuarine facies of core OM1, represent the proximal expression of the maximum flooding zone. This succession reflects location in a confined segment of the valley, just landward of the confluence with a tributary valley. It is likely that sudden sediment input from the tributary produced a topographic threshold, damming the main valley course and isolating its landward segment from the sea. The seaward portion of the Ombrone palaeovalley presents the typical estuarine backfilling succession of allogenically controlled incised valleys. In contrast, in the landward portion of the system, local dynamics completely overwhelmed the sea-level signal, following marine ingression. This study highlights the complexity of palaeovalley systems, where local morphologies, changes in catchment areas, drainage systems and tributary valleys may produce facies patterns significantly different from the general stratigraphic organization depicted by traditional sequence-stratigraphic models.