On the possibility of inferring paleoflow conditions from turbidite deposits: an example from the Laga Formation

The possibility of inferring paleoflow conditions of the Messinian turbidites from the Laga Formation (Central Apennines, Italy) is here analyzed by solving an inverse problem. This inverse problem is solved for elucidating the condition for flow ignition for current velocity, height, and sediment concentration by comparing field data with an analytical solution for the change in bed elevation, related to net-depositional turbidity currents. The obtained layer-averaged values are 2-5 m/s for current velocities, 40-100 m for current thicknesses, and 0.1-2% for sediment volume concentrations. Moreover, the duration time of these flows has been also estimated. Results show values from 1 to 15 hours, depending on the energy of the turbidity currents. This variability in the hydrodynamic conditions also provides some insight into the high-order cyclicity observed into the Laga turbidite deposits. A brief discussion on the vertical structure of a net depositional turbidity current is finally presented. The hydrodynamic conditions of a three-layer turbidity current are inferred from classical tank experiments. The behavior of each layer during the net-depositional phase of a realistic turbidity current is consequently described by using layer-averaged results of the inverse problem, as a function of the Richardson number and the Reynolds number. Results confirm subcritical turbulent behavior of the thin basal layer, a middle layer with a reduced turbulence, and an upper layer with a turbulent cloud where the mean kinetic energy is smaller than the turbulent kinetic energy.