Middle Pleistocene fluvial incised valleys from the subsoil of the centre of Rome: facies, stacking pattern and controls on sedimentation

A review and correlation of borehole data, mostly from a recent survey of drilling in Rome (Palatino Hill and Fori Romani), and outcrops studies have allowed to reconstruct the stratigraphic architecture of the subsoil. Fluvial lithofacies of the ancient Tiber River system are recognized, which fill three high relief incised valleys of Middle Pleistocene age within of which well dated pyroclastites occur. These valleys constitute portions of the low rank/high frequency depositional sequences (PG4, PG5 and PG6) forming the Middle Pleistocene to Holocene composite/high rank Ponte Galeria Depositional Sequence (Milli et al., 2016). The latter represents the most recent stratigraphic unit of the Roman Basin. From a lithostratigraphic point of view the low rank sequences have a good correspondence with the Villa Glori, Fosso del Torrino and Quartaccio Synthems (Funiciello & Giordano, 2008) and are correlated to MIS 14-13, 12-11, 10-9 respectively. Valleys are elongated in N-S direction, and each of them has dimensions comparable to the tributaries valleys developing within the low rank Late Pleistocene-Holocene Tiber Depositional Sequence (Milli et al. in press): 1-2 km in width and up to 50 m in thickness. They are well entrenched into the substratum that is composed of Pliocene-Lower Pleistocene marine clay and Lower-Middle Pleistocene fluvial sediments and pyroclastites. The investigated valley segments are located some 20 km upstream from the coeval shorelines, nevertheless they show an internal stacking pattern of facies recording base level changes, in turn related to the Quaternary climatic and high frequency/amplitude sea level fluctuations. Each valley infill shows at the base a 10 m thick and laterally continuous body of amalgamated pebbles and sands forming a braided channel belt, deposited under low accommodation conditions and correlated with late lowstand and early transgressive phases. It follows a 20-30 m thick intermediate portion showing sandy-silty deposits attributed to sinuous channel belt, and laterally confined by muddy floodplain deposits, often rich in organic matter (transgressive phase). On the top, the channel sandy facies tend to widen and grade laterally to pedogenized floodplain mud. The three recognized incised valleys are organized into a compound-composite stack that records their progressive entrenchment and westward migration, in response to regional moderate uplift, to lateral supply from pyroclastic flows sourced by Colli Albani from SE, and to differential erodibility of the substratum.