: Sand composition is one of the factors of a stratigraphic succession that best records the interaction between allogenic and autogenic processes. This is particularly true for the Quaternary successions where the effects of this processes are better recognized and differentiated. The Quaternary succession of the Roman Basin and, in particular, the one developed during the late early Pleistocene to Holocene, records a close interaction among tectonic uplift, volcanism, climate, and glacio-eustasy. Such interaction is reflected in a complex stratal pattern and stratigraphic architecture where high-rank and low-rank depositional sequences are developed and where qualitative and quantitative changes in sand composition are recorded in the same systems tracts. The analyzed succession corresponding to the high-rank Ponte Galeria sequence (PGS) was supplied by the Tiber River and its tributaries, which developed along the Latium Tyrrhenian margin; such deposits include sediment derived from carbonate to siliciclastic Meso-Cenozoic rocks and from Pleistocene volcanic complexes of the Roman Magmatic Province. We defined three main sand petrofacies (A, B, C) that have a good correspondence with lowstand (LST), transgressive (TST), and highstand (HST) systems tracts of PGS, which reflect changes in sand composition and sand provenance under the effects of tectonism, volcaniclastic input, sedimentary processes, and relative sea-level variations. Petrofacies A is feldspatho-litho-quartzose to feldspatho-quartzo-lithic in composition. It records the erosion and influx of siliciclastic and carbonate rock detritus without volcanic input into the LST fluvial and coastal sands of the PGS. Petrofacies B is characterized by a modal composition varying from feldspathic to litho-feldspathic and feldspatho-quartzo-lithic. It characterizes the TST of the PGS and reflects the abrupt and rapid introduction of volcaniclastic sediment into the system. Petrofacies C is feldspatho-quartzo-lithic in composition. This petrofacies characterizes the HST of PGS and, with respect to the other two petrofacies, better records the effects of downstream transport and river-mouth sedimentary processes. Sand samples collected from ancient deposits are similar in composition to the modern Tiber River, suggesting provenance from a similar river system. Results show that tectonism during middle–upper Pleistocene volcanic activity in the Sabatini, Cimini, and Vulsini volcanic complexes played a major role controlling stream-network reorganization in the Tiber drainage basin and resulted in enhanced volcaniclastic input from ash fall and recycling of pyroclastic flows. Volcanic input (volcanic lithics and associated phenocrysts) and postdepositional alteration during paleosol development define pre-, syn-, and post-volcanic compositions in the high-rank Ponte Galeria depositional sequence. In low-rank depositional sequences, several processes produced variable quartz/feldspar and quartz/lithic ratios, as well as textural changes; these include hydraulic sorting during fluvial and coastal transport and postdepositional in situ weathering processes. Weathering and pedogenic processes in the source area (catchment) potentially remove provenance information, reducing correlation potential of petrographic signatures of proximal successions in the Tiber River sedimentary basin. This work tests the effectiveness of using variation in sand composition as a tool in sequence stratigraphy.

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.

This study reports a high‐resolution micromorphological characterization of floodplain deposits to investigate the relationships among compositional, textural and geotechnical data, and integrate soil micromorphology with sequence stratigraphy. Compositional and textural characterization of facies associations and soil features are calibrated against geotechnical parameters. The latter, obtained from cone penetration and pocket penetrometer tests from a borehole advanced 60 m into the Tiber channel belt and floodplain, show that depositional features and post‐depositional modifications are intrinsically associated with cone penetration test parameters: cone resistance; sleeve friction; and friction ratio. Petrographic and micromorphological features document pedogenetic modifications across stratigraphic markers evidenced by faunal and plant activity, accumulation of peat, and typified by precipitation of heavy metals, iron oxides and secondary carbonates. All of these features developed in correspondence with alluvial flooding surfaces that are correlated with non‐marine and marine flooding surfaces recognized in the transgressive and highstand coastal and lagoonal deposits of the Tiber Depositional Sequence. These observations may serve as a model to reconstruct the sequence‐stratigraphic evolution of ancient relict soils. Nevertheless, additional criteria (for example, their stratigraphic position and correlative surfaces) are necessary to adequately interpret the genesis of such low‐rank stratigraphic surfaces. This work demonstrates that a combination between sedimentological and stratigraphic observations and soil micromorphology can be critical to supplement field observations and determine the relative effect of pedogenic and depositional processes on the organization, composition and texture, and geotechnical properties of floodplain in urban areas.