LAUSR

[Conceptual stratigraphic section of the Po di Pila clinothem throughout the 4‐years bathymetric surveys. Bottom, year 2013: the Po di Pila shows compound progradation in the central sector, with a subaqueous Rollover Point (RP) at 8 m, whereas in the northern sector a single RP develops at an average of 3 m water depth. Center, year 2014: the Po di Pila shows compound progradation in both (central and northern) sectors, but a destruction phase in the central sector (eroded topset and RPs separated by a collapse depression) and a constructional phase in the northern sector (normal regression with compound clinothems) occur simultaneously. Top, year 2016: the Po di Pila shows a subaqueous RP at an average 5 m of water depth of the transverse bar in the central sector, whereas compound progradation is recorded in the northern sector. Note that in a subaerial delta, such as the modern Po Delta, subaerial RPs correspond to breaks in slope associated with the presence of mouth bars, whereas subaqueous nearshore RPs correspond to breaks in slope due to the presence of transverse bars and delta lobes. , Abstract Reconstructions of ancient delta systems rely typically on a two‐dimensional (2D) view of prograding clinothems but may miss their three‐dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high‐resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high‐resolution multi‐beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth‐bar) and muddy seaward‐dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high‐resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one‐year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut‐and‐fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.]