Depositional architecture of the late Quaternary lower Tiber River’s valley infill (central Italy): implication for geohazard assessment in Rome

It is widely accepted that deposition in lower reaches of river systems are primarily controlled by changes in relative sea level. Responses of fluvial systems to fluctuating s.l. are complex in terms of stratigraphic and depositional architectures, with resultant peculiar stacking patterns of facies and geometries of component elements, such as channel bodies and complexes, bars, and floodplain deposits. Changes in fluvial styles and facies are mainly studied in the frame of hydrocarbon exploration and aquifer characterization. Less studied are the relations between fluvial architecture and geotechnical setting of the subsoil for geohazards assessment. It is here presented a case study aimed at identifying and modelling highly compressible floodplain-related deposits, i.e. organic mud and peat layers, in the subsoil of Rome along the Tiber River alluvial plain. A large dataset of more than 2000 boreholes allows this fluvial system to be reconstructed in 3D, and to be directly correlated with laterally contiguous deltaic deposits, where lowstand, transgressive, and highstand systems tracts and related flooding surfaces are recognised. The lower reach of the Tiber system corresponds to a strongly laterally confined, upper Pleistocene-Holocene valley infill, up to 40 km long, 2 km wide, and 70 m deep, the base of which was carved in the substratum after the last glacio-eustatic sea level fall and lowstand (between 116 and 18 ka). The valley infill was developed in response to the postglacial sea level's rise and highstand (from 18 ka to Present). The fill is composed of three distinct lithofacies assemblages: 1) basal gravels and sands, up to 15 m thick, resting on the basal unconformity and interpreted as braided river sediments deposited in the early stage of TST; 2) silty sands and sands, that are arranged in vertically stacked channel-levee complexes (up to 200 m wide and up to 10 m thick), which increasingly widen in the intermediate-upper portion of the infill; 3) encasing floodplain muddy deposits, up to 40 m thick, and commonly rich in organic matter and peat. The assemblage 1 and 2, in particular, are referred to a meandering fluvial system and are correlated to TST and HST. Compressible organic mud and peat layers represents ancient palustrine environments and are concentrated in peculiar settings within the floodplain: 1) at the transition between fluvial and deltaic systems, in the TST; 2) close to the confluence of small tributaries with the main trunk, at the concave bank of the meanders, both in the TST and HST. In the first case the marshy deposits border the inner edge of a transgressive lagoon. In the second case organic rich deposits are interpreted to develop because of damming of tributaries from rapidly aggrading levees of the main river. In the city of Rome high levels of geohazard related to subsidence and differential settlements are located right at the confluences between tributaries and Tiber River, and are therefore referable to compaction of compressible deposits. These areas are highly urbanized and tributary streams are often channelled in sewer pipes and no longer observable. Unfortunately, the presence of the buried valleys is still testified by the differential settlements that affect buildings. An attempt to define a settlement susceptibility index was carried out based on the detailed stratigraphic-sedimentological reconstructions and on the large geotechnical dataset available for the alluvial deposits. Preliminary results are encouraging since they are in good agreement with field observations and deformation velocity maps from SAR interferometry