Sedimentology of a transgressive middle-upper Miocene succession filling a tectonically confined, current dominated seaway (the Logudoro Basin, northern Sardinia, Italy)

The sedimentary dynamics of modern current dominated seaways are well known, based on the monitoring of movement of large sand bodies over short time-scales and the geophysical reconstruction of millennial time-scale processes from recent subsurface successions. Analogues from the ancient record are less common and the longer-term (geological time-scale) sedimentary dynamics of such systems is still not properly assessed or understood. In this work, we review the interpretation of a Serravallian sand-rich stratigraphic interval (namely, the ‘Sabbie di Florinas’ Fm), variously exposed in the Logudoro Basin, a Miocene 10 km-wide and 20 km-long, tectonically-confined extensional depression, developed as a segment of the wider Sardinian Graben System, Italy. The basin includes three main depositional sequences separated by regional-scale unconformities and encompassing the middle Burdigalian to the late Messinian. The intermediate sequence, Serravallian-Tortonian in age, is the one that records the onset of the marine connection between two small basins (i.e., the Logudoro to the south and the Porto Torres to the north), which were previously isolated, resembling endorheic conditions. The oceanographic opening of a seaway, at the beginning of a regional-scale marine transgression, is firstly suggested by facies indicating a strong current reworking of sediment transiting along submerged deltaic platforms, where cross-bedded vertically stacked m-thick sandstone intervals prevail. These deposits correspond with large-scale cross-bedded sand bodies in the axial part of the seaway, where energetic currents promoted the migration of bedforms (i.e., bars and subaqueous dunes). The overlying deposits, accumulated during a late stage of transgression, show an increased maturity of textural and grain size features and exhibit herringbone cross bedding, reactivation surfaces and lamina bundles indicative of a clear tidal signature. At this stage, the seaway experienced a more mature oceanographic setting, governed by bi-directional and rotatory current patterns oriented roughly parallel to the elongation axis. Carbonate-rich large-scale bedforms (i.e., ridges) developed during this late stage of open-marine circulation, inferring possible conditions for sediment starvation in the basin. This work reconstructs the depositional scenario of a segment of the wider Sardinian Seaway, which intermittently connected isolated basins along the Sardinian Graben System. Moreover, the data discussed in this paper help in unravelling the complex sedimentary dynamics of ancient seaways and highlight their relevance for even more accurate palaeogeographic reconstructions.