Linking sand composition and sequence stratigraphy: insights from the Late Lower Pleistocene to Holocene deposits of the Roman Basin (Latium, central Italy)

Sand petrographic changes record the interplay between allogenic (e.g., eustatic, climatic, tectonic) and autogenic (e.g., transport, hydraulic sorting, post-depositional alteration) processes that also govern sequence-stratigraphic architecture. The consequence is that compositional trends vary within the systems tract that forms the framework of the high- and low-rank depositional sequences constituting the late Quaternary successions of the Roman basin, a basin supplied by the Tiber River throughout the Pleistocene. The relationship between sediment supply and sediment composition is well evidenced in the Late Lower Pleistocene to Holocene deposits of the high-rank Ponte Galeria Sequence (PGS), that supplied by the Tiber River and its tributaries records, during the time, the dramatic change in sediment composition due the introduction into the Roman Basin of pyroclastic and volcaniclastic material derived by the Pleistocene volcanic complexes of the Roman Magmatic Province. Three main petrofacies (A-C) were recognized in the PGS that have a strong correspondence with the lowstand (LST), transgressive (TST), and highstand (HST) system tracts deposits of the PGS. Petrofacies A (feldspatho-litho-quartzose to feldspatho-quartzo-lithic) reflects erosion of carbonate and siliciclastic sources with minimal volcanic input during LST. Petrofacies B (feldspathic to litho-feldspathic/feldspatho-quartzo-lithic) captures the abrupt volcaniclastic pulse and recycling associated with Pleistocene volcanism during TST. Petrofacies C (feldspatho-quartzo-lithic) best records downstream reworking and Tiber river-mouth processes during HST. In particular, in the modern highstand, upstream sands show a siliciclastic lithic signature, whereas downstream sands are enriched in carbonate and volcanic lithics; coastal hydrodynamics partition detritus into two populations, concentrating coarser pyroxenes in higher-energy settings and finer feldspars in lower-energy environments, while outer-shelf/slope deposits are largely biogenic and weakly river-influenced. Overall, compositional changes in correspondence with the key stratigraphic surfaces mark paleogeographic reorganizations and changes in sediment pathways. Because multiple drivers often act at once (e.g., tectonism, volcanism, and local autogenic effects), petrographic analysis should be integrated with facies, texture, and process data to isolate the forcing mechanisms better. These results demonstrate that sedimentary petrography can play an important role in the sequence-stratigraphic interpretation of a sedimentary succession, strengthening links between modern routing systems and the ancient record.