Between Scylla and Charybdis (part 2): The sedimentary dynamics of the ancient, Early Pleistocene Messina Strait (central Mediterranean) based on its modern analogue

The 3-km-wide Messina Strait divides Sicily from the Italian peninsula, representing one of the most impressive examples of tide-dominated marine passageway of the central Mediterranean Sea. The tidal hydrodynamics governing the strait results from a semi-diurnal phase inversion between two interlinked wider basins (i.e., the Tyrrhenian Sea to the north and the Ionian Sea to the south), due to the presence of a nodal tidal point in the centre of the strait. This condition, associated with coastal narrowing, causes continuous water exchanges and a local phenomenon of tidal amplification, producing collinear tidal currents up to 3 m s−1 in velocity in the point of tidal maxima of the strait centre. Currents follow bi-directional patterns across the strait, decelerating towards either opposite peripheries because of the progressive enlargement of the cross-sectional areas. Tides exert a relevant influence on the strait bottom, shaping characteristic erosional features, governing the main sediment transport routes and promoting the migration of distinctive bedforms. The textural and dimensional attributes of these bottom features vary according to the current velocity trends and main directions, indicating a specific partition of the strait in individual sedimentary zones. In the Early Pleistocene, analogous conditions established across the same area, and huge quantities of siliciclastic and bioclastic deposits accumulated in the ancient Messina Strait under a similar tidal hydrodynamics. The 250-m-thick strait-fill succession, which is magnificently exposed across the two opposite margins of the modern passageway (i.e., Scylla and Charybdis), represents a crucial correlative element in supporting the reconstruction of the sedimentary dynamics of the past strait, based on the observation of the modern system. In this work, which represents the second part of a previous article focused on the modern Messina Strait, previous stratigraphic data are reviewed and integrated with new sedimentological observations carried out in both onshore and offshore key areas. In particular, outcrop analysis allows distinguishing a number of facies associations, which are described and interpreted by taking into account the correspondent modern depositional strait settings. The results of this study highlight a mutual correlation between the depositional zones detected in the modern Messina Strait and the individual facies associations recognised in the onshore margins. The strait-centre zone represented a bedload parting sector in the ancient system. This zone, which is one of the most difficult environment to detect in ancient straits due to a condensed sedimentary record, is suggested to coincide with a structural palaeo-high, exposed in the southern sector of the investigated onshore area and continuing offshore in the modern strait. This tectonic horst symmetrically separated two opposite highly-depositional zones to the north and to the south, hosting gravel and sand-size siliciclastic and bioclastic deposits, exhibiting diffuse motifs of large-scale tidal cross stratification and a specific vertical facies stacking. These characteristic tidal deposits, which are believed to record the migration of several generations of tidal dunes forming extensive bedform fields in the strait, reflect a tidal asymmetry between flood and ebb currents. Tidal asymmetry is referable to the oceanographic setting observable nowadays in the Messina Strait and deriving from a marked density-driven, water-mass stratification between the two opposite flows. In the ancient system, tidal dunes transited down-current to highly bioturbated fines, including sporadic small-scale tidal cross strata, representing the distal periphery of the Early Pleistocene strait. Here, bedforms followed migration pathways not always reflecting mutually-evasive net-gross sediment transport routes, due to local conditions of current rotation occurring during each tidal phase inversion. During these stages, which mimic slack-water periods, currents were weak enough to allow fine sediments to be settled from suspension, reproducing a signature typical of other tide-dominated coastal systems (e.g., estuaries, tidal flats, etc.). These sedimentary zones of the ancient Messina Strait were laterally bounded by tectonically-active margins, where fan deltas, aprons and carbonate wedges developed under the constant interplay between mass-transport processes and tidal currents. The resulting architectural and sedimentary features observable in outcrop reveal impressive similarities with the morpho-bathymetric features nowadays documented in the correspondent strait-margin modern environments. These considerations allow deciphering a general palaeo-environmental reconstruction of the ancient strait which, although suffering of limitations due to a partial preservation of the whole rock record, discloses sedimentary processes and depositional environments of the ancient Messina Strait. The process-based analysis of this outcrop case study is crucial in predicting long-term environmental changes and facies migration pathways and improves the state of knowledge on tidal straits, which are the less understood among the most common tidal depositional systems.