Holocene paleo tidal inlets as markers of the rising sea: a case study from the northern Adriatic shelf (Italy)

We recently detected and characterized a series of abandoned and filled incised channels documented in the north-western Adriatic shelf. About 100 of them have been recognized as remnants of paleo tidal inlets which formed during the post-LGM transgression that led to the submersion of the shelf. These features were identified through the re-analysis of about 7000 km of high-resolution geophysical surveys (CHIRP-sonar profiles) and tens of stratigraphic cores carried out in the area during the last 30 years. The preservation potential of paleo tidal inlets is usually low, as such features are often almost completely erased by the wave ravinement processes. As a matter of fact, only few examples are known in literature and they are usually confined to low-gradients shelves. Despite their rarity, paleo tidal inlets are ideal markers for the reconstruction of the timing and impact of the sea-level rise on the transgressed coastal plain. A wealth of information can be obtained by analyzing their buried morphology, stratigraphic position and infilling sediments, such as the location of the paleo coast lines, the dimensions of the paleo lagoon systems and, in some conditions, the relative paleo sea-level. These features can therefore represent valid means to reconstruct the last marine transgression. The paleo tidal inlets identified in the shelf of the northern Adriatic are characterized by a wide range of thickness, spanning from few to almost 20 m, and by the absence of any evidence of lateral or landward migration. These characteristics, along with the great number of recognized features, suggests the recurrent formation and overstepping of large lagoon systems during the last marine transgression. In particular, these features can be subdivided into clusters based on the depth of their top, thus allowing to infer the position of a series of paleo coastlines and suggesting the occurrence of periods of stasis of the relative sea-level rise, which allowed the formation of such inlets. By coupling the ages obtained for the infilling materials and the stratigraphic relations intercurring among the analyzed features, it was possible to reconstruct some of these moments and, in particular, a still stand phase of the sea level was identified around 9.5 ka cal BP, as also confirmed by other indicators available for the Adriatic Sea. This research provides new insights on two main topics: i) it improves our knowledge on the post-LGM marine transgression, therefore contributing to reconstruct the history of sea-level rise and to constrain the modelling of future behavior; ii) it contributes to understand the evolution of tidal inlets and lagoon-barrier island systems under the forcing of high rates of sea-level rise.