Multiple valley fills from the Late Quaternary record of the Tuscan coastal plain ( Italy)

Three consecutive, time-equivalent valley fills of Lateglacial-Holocene age (13 to 8 cal kyr BP) were identified in the Tuscan coastal plain, across a 30 km long transect approximately parallel to the present Tyrrhenian Sea shoreline. From South to North, the Arno valley fill is 35-40 m thick and 5-7 km wide, the Serchio valley fill is 15-20 m thick and 7-8 km wide, while the Camaiore valley fill is 20-30 m thick and 8-10 km wide. The three valley fills display remarkably similar facies architecture, with thin basal fluvial deposits overlain by a variety of transgressive, mud-dominated coastal plain and estuarine deposits. The valley bodies deeply incise and crosscut sand and gravel-prone fluvial-channel deposits assigned to the Last Glacial Maximum (LGM), and are capped by a sheet-like, late transgressive succession of shoreline sands grading vertically into offshore clays. Beneath present Arno River, the valley body consists internally of three superposed millennial-scale cycles bounded by flooding surfaces (parasequences). These cycles display a similar facies architecture, with sharp-based central/outer estuarine clays grading upward into inner-estuary, bay-head delta and coastal-plain deposits. Overall, the three parasequences display an aggradational, rather than retrogradational stacking pattern, which is interpreted to reflect the complex relation between eustatic sea-level rise and sediment supply during the post-LGM transgression. Diagnostic changes in vegetation patterns, driven by opposite climate conditions, enable documentation of parasequence development as a function of climate change. Pollen spectra invariably show expansions of broad-leaved forests at parasequence boundaries, suggesting that rapid shifts to warmer climate conditions accompanied episodes of sea-level rise. In contrast, stillstand phases saw the development of cold-temperate communities, suggesting transition to temporary colder climate conditions. Stratigraphic architecture beneath the Lateglacial-Holocene valley bodies shows that the coastal system of Tuscany experienced multiple cycles of fluvial incision and subsequent valley filling in response to late Quaternary sea-level fluctuations. Below the LGM fluvial deposits, an older set of estuarine clays is observed at depths ranging from 40 to 90 m. Based upon physical correlations with nearshore deposits, these valley fills are likely to record an older phase of generalized fluvial incision, which is tentatively assigned to the MIS 6-5 transition.