A facies-based depositional model for ancient and modern, tectonically-confined tidal straits

Modern and ancient tidal straits are the least well understood of all tide-dominated depositional systems. In order to provide an increased understanding of these systems, a facies-based depositional model is assessed by comparing multibeam surveys of three present-day tidally-dominated seaways with a number of superbly exposed Neogene-to-Quaternary strait-fill successions of Calabria (south Italy). The model points out the existence of four depositional zones, laterally-adjacent from the narrowest strait center to its terminations, distributed along symmetrical or asymmetrical seaways. These zones, whose signature is recorded in four facies associations in the Calabrian tidal straits, are: (i) the strait-center zone, associated to the tidal current maxima and where sediments are scarce or absent; (ii) the dune-bedded zone, where sediments form dune complexes due to tidal flow expansion; (iii) the strait-end zone, where currents decelerate accumulating thinly-bedded, fine-grained deposits; (iv) the strait-margin zone, where sediment massflows descend tectonically-active, steep margins towards the strait axis. In ancient, tectonically-confined, narrow seaways, these facies generate a distinctive deepening-upward vertical succession, where tidal currents are the dominant process in the sediment distribution.