Delta-drift sedimentation in a strait system: The Early–Middle Miocene Bonifacio Strait, Southern Corsica, Western Mediterranean

Despite significant advances in the study of straits over the past decade, depositional processes governing their geological development remain poorly constrained because of incomplete stratigraphic records, lack of distinguishable deposits, and fossil-poor facies hindering environmental interpretation. This study investigates the initial stages of the opening of the Bonifacio Strait, a current-dominated passageway that connected two adjacent sub-basins of the western Mediterranean during the early–middle Miocene. Field-based observations indicate that the opening of the Bonifacio Strait occurred under high-energy depositional dynamics, likely driven by currents interacting with a topographic sill separating the two sub-basins. The oldest deposits consist of prograding and mounded sedimentary bodies that formed a delta-drift complex. The complex consists of mixed siliciclastic and bioclastic clinoforms, several tens of meters thick, showing variable dip angles. Sediment was exported and redistributed across the delta-drift complex through subaqueous channels, likely incised into the crest of the submerged sill, although no direct outcrop evidence of these feeder conduits is preserved. Clinoform deposits accumulated from underflows and sediment avalanches triggered by progressive slope steepening. Hydraulic jumps and associated supercritical-flow processes produced large scale scour-and-fill structures in this lower interval. Petrographic data indicate that the siliciclastic component originated from subaerial erosion and subaqueous scouring of pre-Miocene substrate. The bioclastic fraction was derived from at least two carbonate producing systems: one adapted to energetic, erosional settings in the central strait, and another established in association with subaqueous dune-bedded deposits. An initial stage of strait infill was followed by a rapid transition to basin-wide, fully marine conditions during a major transgression. The delta drift changed to a current-dominated depositional regime characterized by the accumulation of large subaqueous dunes with abundant bioclastic sediment. The transgression also led to the drowning of the carbonate platform and a full throughgoing connection of the strait. These results i) define an example of a fossil delta-drift system, ii) refine the overall paleogeographic evolution of the Bonifacio Strait and its relationships with global climate changes, and iii) identify key sedimentary zones—erosional and depositional—typical of a strait preserved in the present-day distribution of Bonifacio bedrock and strait-fill deposits.