Depositional architecture and external controls on Quaternary fluvial incised valleys in the subsoil of Rome (Italy)

The depositional architecture recorded within fluvial incised valley fills provide valuable information on external controls on sedimentation of river systems. It is here presented the case of a stacked multi-valley complex recording the Middle Pleistocene to Holocene evolution of the Tiber River system in Rome (Italy), as reconstructed from a dense borehole data and key outcrops along a 20 km-long transect. This complex is composed of innermost fluvial portions of five low rank/high frequency depositional sequences interbedded with pyroclastics, sourced from the nearby Albani Hills and Sabatini Mts Volcanic Districts. In the study area the sequences filled in valley incisions entrenched into a substrate of Pliocene-Early Pleistocene marine clays. Owing to radiometric dating of pyroclastics, a robust correlation exists between valley incisions and infills to odd and even Marine Isotopes Stages (MIS 16-1), allowing the link of the valleys' development to sea level changes. The investigated valleys have widths in the range of 0.5-2 km and depth/thickness of up to 60 m and a recurrent tripartite pattern of their fills including: i) an up to 10 m thick and laterally extensive basal unit of dominantly gravelly-sandy deposits; ii) a middle unit (thickness in the range of 20-30 m) composed of vertically stacked sandy channel bodies (width in the range of 200-400 m) sided by floodplain muds; iii) a top unit composed of laterally stacked channel sands and pedogenised floodplain muds, which result in extensive tabular sand bodies. Compositionally, the fluvial sediments derive from the erosion of older carbonate and siliciclastic successions of the Apennines and from coeval pyroclastic products, with the notable exception of widespread freshwater tufas filling the MIS 14-13 paleo-valley and connected to hydrothermal, carbonate-enriched waters. Correlation to MIS suggests that the fill of the studied incised valleys reflects an early fluvial deposition under low accommodation conditions, forming a braided channel belts during the early transgressive phases, followed by deposition in meandering-style channel belts during the late transgressive phase. Highstand deposits are generally not preserved due to the erosion induced by the relative sea-level falls. In turn, successive incised valleys are stacked in an entrenching and westward shifting pattern, as evidenced by detailed palaeogeographic reconstructions, which is interpreted to reflect superimposition of regional uplift, at 0.1-0.2 mm/y rate, topographic confinement from pyroclastic flows and glacio-eustasy.