Coalescent valley fills from the late Quaternary record of Tuscany (Italy)

Three prominent incised-valley fills of post-Last Glacial Maximum age are described from the northern Tuscan coast of Italy. Stratigraphic correlation of core data along a cross-section transversal to the present fluvial-channel axes enables identification of a suite of genetically related valley bodies that fill the incisions made up by Arno, Serchio and CamaioreeStiava rivers during the last glacial/interglacial cycle. The valley fills display different shapes and size, but remarkably similar facies architecture. Valley bodies range between 5 and 10 km in width, and between 30 and 45 m in thickness, with width/thickness ratios of about 100-300. Above a lowstand (and early transgressive?) gravel fluvial deposit, the three incised-valley fills display a distinctive succession of coastal plain to estuarine facies, dated to about 13-8 cal ka BP. Radiocarbon dates document that the three valleys were active simultaneously. Accommodation space was rapidly created during transgression and then filled under conditions of very high sediment accumulation (about 1 cm/y). In contrast to the more common deepening-upward trend recorded by the latest Pleistocene-early Holocene valley-fill successions worldwide, sedimentation in the Tuscan valleys equalled, or even exceeded, the rate at which accommodation was created, thus leading to an aggradational, rather than backstepping, stacking pattern of high-frequency (millennial-scale) parasequences. Above the valley fills and on the interfluves, a thin deepening-upward succession of nearshore deposits marks the rapid change from aggradational to retrogradational depositional style. This is invariably overlain by a characteristic shallowing-upward motif of prograding deltaic and coastal facies. The maximum flooding surface, which can be tracked across thewhole study area on the basis of subtle palaeontologic indicators, is shown to represent an almost isochronous surface, dated to about 7.8 cal ka BP. Through examination of large-scale geometry and facies attributes of the valley fills, including the relationships between valley bodies and interfluves, this study represents an example of how adjacent river systems with significantly different characteristics may respond simultaneously in a consistent manner to rapid changes in sea-level and climate conditions.