Abstract

Using archeological evidence, radiocarbon data, well logs, and high-resolution seismic profiles, we applied sequence-stratigraphic analysis to sediments deposited during the last glacio-eustatic cycle on the Tyrrhenian continental margin embracing the Tiber River months. These sediments form a high-frequency sequence of fifth order that has developed over the last 20,000 yr since the last lowstand, hereafter referred to as the Tiber Depositional Sequence (TDS). The TDS is still evolving and lies on a Type 1 sequence boundary that has been identified both on the shelf and below the present delta plain. This sequence consists of lowstand, transgressive, and highstand systems tracts. The lowstand deposits are not present on the shelf but are found on the continental slope, where they constitute the last progradation set of the continental margin. A network of gullies, active when the river mouth was at the shelf break, is present within the lowstand deposits. The transgressive-systems-tract (TST) deposits lie directly on the basal unconformity. Landwards the TST is represented by a retrogradational parasequence set of fluvial, coastal barrier-lagoon, and lagoonal delta facies. Seaward, on the shelf, these parasequences merge into a seismically transparent deposit that forms a condensed section. The flooding surfaces that bound the parasequences are marked by peat layers that were dated by 14 C. The radiometric ages were used to draw a carve of sea-level rise for the last 11,000 yr. The top of the transgressive systems tract (maximum flooding surface) is represented by the last peat layer, which was formed about 4700-5000 yr BP and buried 4-5 m below the present upper delta plain. The highstand systems tract is made up of deposits of the present Tiber wave-dominated delta, i.e., upper and lower delta plain, delta front, and prodelta slope. The aligned beach ridges mark the position of the prograding coastline, which for the last 2500 yr can be dated by archeological and historical data. The recent very fast progradation of the Tiber delta, during the last 500 years, is consistent with historical data on floods of the city of Rome and the suspended load of the Tiber River.

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