A Source-to-Sink Compositional Model of a Present Highstand: An Example in the Low-Rank Tiber Depositional Sequence (Latium Tyrrhenian Margin, Italy)

Our source-to-sink compositional model of sediment distribution in the present highstand of the Tiber Depositional Sequence has important implications for sediment source lithology, generation, transport, and deposition. In the Tiber River system, sand compositional trends reflect provenance mixing, anthropic intervention, and the effects of local autogenic factors in continental to marine depositional environments. Fluvial feldspatho-quartzo-lithic sand results from the contribution of the major tributaries of the Tiber drainage basin, which can be divided into two sub-basins separated by the Corbara dam built in the 1950s. Upstream-drainage-basin sand has a siliciclastic sedimentary lithic signature, whereas carbonate and volcanic lithic fragments dominate downstream-drainage-basin sand. Downstream sand is further modified in the coastal environment by hydraulic sorting, and mixed with detritus derived from the recycling of coastal dune sand and altered volcaniclastic paleosols. The latter are associated with lower-course floodplain and channel deposits, which produce a quartzo-feldspatho-lithic sand. Sediment grain-size zonation in the marine environment is influenced by coastal hydrodynamics, which, in turn, produce two detrital populations with distinct compositional signatures and hydraulic behavior. Coarser minerals (essentially pyroxene phenocrysts) and finer feldspar grains are progressively concentrated in higher-energy and lower-energy environments, respectively. Biogenic continental shelf and slope deposits off the Tiber River mouth show very little riverine influence. This study of the last highstand system tract in the Tiber Depositional Sequence clarifies the effects of hydraulic sorting, grain-size dependence, and sediment reworking in controlling the final composition of modern sand. This knowledge can be used to better understand detrital modes of ancient sedimentary successions.