Mode of stacking and internal heterogeneity of multiple incised valleys fills in the subsoil of Rome (Italy): Implications for connectivity of high-permeability geobodies.

Incised valleys and their fill represent important features of fluvial systems and can provide valuable information on external controls on sedimentation. Furthermore, the mode of stacking of multiple incised valleys may result in diverse connectivity of high permeability geobodies, which makes their origin and architecture important to understand for natural resources development. Based on dense borehole data and key outcrops along a 10 km-long transect, this study attempts at delineating the depositional architecture of a stacked multi-valley complex recording the Middle Pleistocene to Holocene evolution of the Tiber River system (Rome, Italy). Such a multi-valley complex is composed of four low rank/high frequency depositional sequences intercalated with pyroclastics, which fill in valley incisions entrenched into a substrate of Pliocene-Early Pleistocene marine clays. Owing to radiometric dating of pyroclastics, a robust correlation exists of valley incision and fill phases to odd and even Marine Isotopes Stages (MIS 14-1), which allows linking incised valley development to sea level changes. Located c. 20 km upstream from coeval shorelines, the investigated valleys have widths in the range of 0.5-2 Km and depth/thickness of up to 60 m and a recurrent tripartite pattern of their fills including: i) an up to 10 m thick and laterally extensive basal unit of dominantly gravelly-sandy deposits; ii) a middle unit (thickness in the range of 20-30 m) composed of vertically stacked sandy channel bodies (width in the range of 200-400 m) sided by floodplain muds; iii) a top unit composed of laterally stacked channel sands and, subordinately, floodplain muds, which result in extensive tabular sand bodies. Correlation to MIS suggests the fill of the studied incised valleys might reflect early deposition under low accommodation conditions within braided channel belts (lowstand and early transgressive phases) followed by deposition in sinuous channel belts (late transgressive and high-stand phases). In turn, successive incised valleys are stacked in an entrenching and westward shifting fashion, which is interpreted to reflect superimposition of regional uplift, topographic confinement from south-easterly sourced pyroclastics of the Albani Hills Volcanic District and glacio-eustasy. The resultant stratigraphic architecture of the Tiber multi-valley complex is such that high-permeability gravelly and sandy deposits are locally connected across successive valley fills. However, the studied example highlights how degree of connectivity of multiple incised valleys fills is ultimately controlled by external forcing factors (e.g. regional uplift and topographic confinement) superimposing to higher frequency changes of base level. It can be speculated that, by forcing lateral shifting of a fluvial stream, superimposition of differential uplift/subsidence to relative sea level changes more likely develops into less connected incised valley fills.