Late Quaternary multiple incised-valley systems buried beneath the Pisa Coastal Plain (Tuscany, Italy)

Based on integrated sedimentological, micropaleontological (benthic foraminifers, ostracods) and chronological (14C and ESR) data, an alternation of Late Quaternary continental and coastal-shallow marine deposits is recognized in the uppermost 100 m of the Pisa Coastal Plain (Tuscany, Italy). Two incised-valley systems (IVSs), 5-8 km wide and more than 30 meters deep, represent the most prominent stratigraphic feature. The uppermost IVS started to form during the Last Glacial Maximum (MIS 2) close to the modern Arno River course and partially cut the lowermost one, which developed instead during the mid Pleistocene in southern position. The incised-valley fill (IVF) sequences developed during two interglacial phases (MIS 1 and MIS 7, respectively), and display similar facies architecture. Each IVF exhibits a fining-upward tendency, with a basal lag of fluvial gravels sharply overlain by transgressive, mud-dominated coastal plain and estuarine deposits that progressively onlap onto the valley flanks. This overall depositional trend is punctuated by small-scale parasequences. Within the Lateglacial-Holocene IVF, these cycles show a millennial-scale climatic signature. Whereas the inundation of the interfluves preceded maximum marine ingression in both IVSs, a continuous record of the highstand succession is observed uniquely above the uppermost valley fill, due to the erosive processes that probably followed the MIS 7 interglacial. The overall stratigraphic architecture shows that the study area experienced multiple cycles of fluvial incision and subsequent valley filling, mainly in response to sea-level and climatic fluctuations at Milankovich and sub-Milankovich scales. The local effect of tectonics, however, cannot be ruled out as a possible, additional controlling factor.