Interplay of tidal and fluvial processes in an early Pleistocene, delta-fed, strait margin (Calabria, Southern Italy)

The architecture and morphodynamics of modern and ancient tidal straits and in particular the deposits of strait-margin zones, have been significantly understudied compared to other marginal marine settings, even though many reservoirs in the North Sea and the Norwegian Continental Shelf are developed in narrow grabens or seaways. This paper presents a detailed sedimentological and stratigraphic analysis of an early Pleistocene marginal-marine succession deposited along the northern margin of the Siderno paleostrait (southern Italy). This area represents an excellent case study of sedimentation along a tidal strait margin, interpreted to record the interaction of fluvial and tidal processes. Here, syn-depositional tectonics produced a complex coastal morphology, significantly influencing sedimentation and hydrodynamic processes. Along the strait margin, the emplacement of an isolated tectonic high (Piano Fossati) created a ca. 3.5 km-wide local passageway. This morpho-structural element induced interplays between fluvio-deltaic processes (fed from the northern strait margin) and tidal current reworking (active within the marine strait).

The field-based facies analysis reported here documents an initial stage of non-tidal shallow-marine sedimentation across the strait. A subsequent regression caused river-generated hyperpycnal flows and the transfer of large volumes of pebbly and shelly sandstones into deeper water. Tidal currents became amplified in the strait, and, in the delta front area, they were able to rework river-derived sediments generating large dune fields. Following the local tidal transport pathway, strong tidal currents skewed the delta front (causing it to be asymmetrical) and elongated sand bodies in a direction parallel to the marine strait axis. Differently from the classical tide-influenced deltas in which onshore-offshore tidal flow predominates, coast-parallel deflection and strong asymmetry of delta-front deposits is a typical feature of deltas entering tide-dominated seaways and straits, where strong tidal currents are capable of dispersing large volumes of sand for significant distances along the coast and along the strait axis. This process became progressively enhanced during the following transgression, when tide-modulated currents reworked biocalcarenitic sands over the previous delta deposits, generating southeasterly migrating dunes. At the end of the transgression, strandplain progradation caused the closure of this marginal branch of the Siderno Stait. This last stage of sedimentation was followed by a dramatic regional-scale structural uplift, which ended any marine circulation within the strait. This work provides new insights on sedimentation in a tide-dominated strait, and helps to predict sandbody distribution along the strait margin and axis. These findings can be applied to any other setting characterized by a narrow (possibly structurally-confined) basin dominated by tidal currents.