Abstract

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.