The complex geomorphology of a barrier spit prograding into deep water, Hel Peninsula, Poland

The Hel Peninsula (the Southern Baltic, Poland) is an unusual example of a barrier spit that enters and progrades into unusually deep waters. Here, we categorize and examine the underwater geomorphology of the barrier spit terminus using a combination of bathymetry, side-scan sonar, seismic profiling, and grab and core sampling. The morphology of the spit is characterized by a sigmoid profile, with four distinct physiographic segments corresponding to topset, offlap break, foreset, and bottomset features. These features are connected to changes in subsurface structure and the sediment type. The influence of gravity-driven processes on the formation of the offlap break, foreset, and the distribution of the deposits was identified. In addition, wave-dominated processes of shoreline re-organization and channeling of storm return flows to form ridges along the topset margins were observed. The mass wasting processes occurring on the underwater slopes of deep-water barrier spits have a significant influence on the distribution of deposits with sediment sourced across the entire spit terminus accumulating along the foreset and bottomset. These gravity-driven depositional processes may accelerate the sediment deposition in deeper parts of the slope and consequently may accelerate the progradation of its base. Such examples of deep-water prograding barrier spits are rare, and consider that the Hel Peninsula belongs to an end member in a continuum of spit development, with increasing morphological complexity occurring with greater antecedent water depths.