FRICTION- AND INERTIA-DOMINATED EFFLUENTS IN A LACUSTRINE, RIVER-DOMINATED DELTAIC SUCCESSION (PLIOCENE UPPER VALDARNO BASIN, ITALY)

Sediment transport and deposition at river-dominated river mouths are controlled by: i) outflow inertia, ii) turbulent bed friction, or iii) outflow buoyancy. In mouth-bar-type deltas, which typically form in shallow-water settings, friction-dominated effluents are prevailing, although inertia-dominated ones may develop depending on: i) bedload at the river mouth and ii) water depth immediately downstream of the outlet. This paper deals with lacustrine river-dominated deltaic sediments deposited in a Pliocene, small, and protected lake, where wave reworking was limited to absent, allowing the preservation of the primary sedimentary features and stratal architecture developed by river-mouth sedimentary dynamics. This study focuses on sedimentary facies and stratal architecture of these deltaic deposits and aims at detecting the role of river floods in shaping and distributing sediment in shoalwater deltaic bodies. A detailed facies analysis resulted in the identification of sedimentary bodies related to friction- and inertia-dominated effluents, which appear to be genetically linked. High-discharge events promoted development of jet-flow deposits (i.e., from an inertia-dominated effluent) at the outlets of the main distributary channels, whereas during lower-discharge events minor channels bifurcated around the jet lobe feeding friction-dominated effluents giving rise to low-relief lobes. Coexistence of different types of jet deposits is also linked to deposition at different stages within a single flood event, with friction-and inertia-dominated bars accumulated during low-and high-stage flood discharge, respectively.