The Middle Valley of Tiber River, central Italy: Plio-Pleistocene fluvial and coastal sedimentation, extensional tectonics and volcanism.

The Middle Valley of the Tiber River, Italy (MVT), corresponds to a NNW-SSE trending extensional basin that has developed since the middle Pliocene along the western flank of the central Apennines. Stratigraphical, sedimentological, palaeontological and Sr isotope analyses have been conducted to detail the stratigraphy of the MVT and to reconstruct the history of Pliocene-Quaternary basin filling. Most of the basin-fill is composed of fluvial and deltaic deposits that are chronologically constrained by biostratigraphical data and Sr isotopes from marine deposits, and through relationships with volcanic and volcaniclastic units with K/Ar and Ar/Ar radiometric ages. This paper focuses on relationships between sedimentary phases, long-term (> 1 Myr) tectonic movements, shorter term (100 kyr) climatic and eustatic changes, and volcanism, within the overall extensional tectonic context of the MVT. Two main tectonic phases are recognized in the MVT, each recorded by the responses of the mostly gravel-dominated fluvial and deltaic systems. The first phase encompasses the middle Pliocene to earliest Early Pleistocene, and was dominated by rapid subsidence. This phase was characterized by transverse rivers that fed cyclically prograding and retrograding fluvial-deltaic wedges, with interfingering marine deposits, and within an overall aggradational context. Superimposed on this overall aggradational trend is an inferred 4th order cyclicity that is interpreted to reflect eustatic and climatic changes. The second phase began in the late Early Pleistocene and extends to the present, and is linked to uplift of the Apennines. This phase was characterized by complete emergence of the MVT, the occurrence of volcanism, development of the Tiber River system, and the initiation of a long-term trend of uplift-driven net valley incision. Alternating aggradational and degradational phases are superimposed on this longer term trend, and are thought to be linked to Late Quaternary climatic forcing, with 100 kyr glacial-interglacial cycles important in the upper reaches of the fluvial system, and the related effects of glacio-eustasy important farther downstream.