Progradational gravel beach sequences in a moderate‐ to high‐energy, microtidal marine environment

Abstract

ABSTRACTThe anatomy of progradational gravel beaches was analysed in two different tectonic settings: the strongly subsiding Messinian foreland basin of the eastern Southern Alps and the rising Pleistocene marine terraces of the outer margin of the Apenninic chain. Progradation took place near the mouths of fan‐delta or braid‐delta distributaries debouching in microtidal landlocked basins (palaeo‐Adriatic and palaeo‐Ionian seas). A wind‐wave climate, comparable to that of the present‐day, was characterized by periodic intense storm activity. Most of the beachface progradation is thought to have taken place during the post‐storm recovery and fair‐weather stages, whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly‐sorted and coarse‐grained gravels on the shoreface by storm‐intensified seaward‐trending flows and behind the highest berms by washover processes. Analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here. The characteristics of the Apenninic sequences suggest a strongly variable wind‐wave climate, with periodic changes from a relatively dissipative barred morphology during storms, accompanied by strong longshore currents and rip development, to a more reflective state, typical of recovery and fair‐weather stages. The Southern Alpine sequences, on the other hand, are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear‐ and gravity‐sorting in the lower‐beachface gravels. These features indicate a comparatively more reflective average state, due to location of beaches along the deeply embayed head of the palaeo‐Adriatic, and the very narrow directional width of the incoming wave spectrum, which was almost shore‐normal most of the time. In addition to well‐developed shape zonation of gravels, the common presence of wave‐generated gravel megaripples in the Southern Alpine sequences indicates the greater influence of long‐period waves, due to the greater available fetch distances. In both settings the cyclicity is thought to be genetically related to minor changes of sea‐level.