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Sand compositional changes as a key for sequence-stratigraphic interpretation: the Pleistocene Tiber River delta succession
ABSTRACT SAND COMPOSITIONAL CHANGES AS A KEY FOR SEQUENCE-STRATIGRAPHIC INTERPRETATION: THE PLEISTOCENE TIBER RIVER DELTAIC SUCCESSION (ITALY) By Daniel Tentori Master of Science in Geology Sand samples collected from the Tiber River of central Italy reflect sediment input from the main tributaries and various source rock lithologies distributed across the drainage basin. Monomineralic quartz and feldspar grains, and siliciclastic sedimentary lithic fragments are supplied by Oligocene to Miocene siliciclastic units that crop out in the upstream part of the drainage basin. Carbonate lithic fragments, very common throughout the Tiber River, are supplied by Triassic to Miocene carbonate successions that crop out in the upper and middle parts of the drainage basin. Volcaniclastic rock fragments and monomineralic pyroxene and feldspathoid grains derived from lavas and pyroclastic flow deposits of the Quaternary Roman Comagmatic Province are concentrated in the lower reaches of the Tiber River, where these volcanic units crop out. The dominant components of Tiber deltaic coastal sand are monomineralic grains of quartz, and pyroxene, and carbonate lithic fragments, suggesting that there is some compositional modification by wave reworking. Sand samples collected from late Quaternary, wave-dominated Tiber-delta deposits are similar in composition to the modern Tiber River, suggesting similar provenance for the paleo- and modern Tiber River. However, lower percentages of siliciclastic sedimentary lithic fragments indicate that the clastic sedimentary succession now exposed in the upper reaches of the Tiber River, may have not been part of the paleo-Tiber drainage basin at ~860-240 Ka. Results show that tectonism following the ~840-240 Ka volcanic activity in the Sabatini, Cimini, Vulsini and Albani volcanic centers 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 lavas and pyroclastic flow deposits. In the Tiber River-Deltaic system, it appears that tectonic signatures override glacio-eustatic sea-level fluctuations (e.g., volume of sediment produced vs. sea-level changes), and that volcanism, which supplied volcanic lithics and associated phenocrysts, and paleosol development associated with thermal uplift controlled the overall composition of the high-rank (3rd-order) Ponte Galeria depositional sequence. Sedimentary processes (e.g., quartz/feldspar and quartz/lithic ratios), post-depositional weathering processes (e.g., amount of carbonate grains), and deposition from coeval and instantaneous pyroclastic events (e.g. volcaniclastic-rich sand) are the dominant processes that produce compositional changes in lower-rank (4th-order) depositional sequences. This work tests the effectiveness of using variation in sand composition as a tool in sequence stratigraphy.