The origin and dynamics of coastal boulders in a semi-enclosed shallow basin: A northern Adriatic case study

This paper documents the analysis of a coastal boulder deposit that was recently identified along the northern Adriatic coast (Premantura Promontory, Istria, Croatia). Accumulations of large boulders have not previously been reported in the northern Adriatic, which can be viewed as a semi-enclosed basin. A multidisciplinary approach was used to investigate the site including geological and geomorphological surveys, together with the use of an Unmanned Aerial Vehicle (UAV), digital photogrammetric analysis, hydrodynamic modelling and 14C AMS datings. Measurements of boulder position, elevation, size, shape and density were fed into hydrodynamic equations that are used to estimate the minimum storm and tsunami wave height required to enable the accumulation of boulders. Biogenic marine carbonate encrustations observed on 14 boulders suggest the infra- and sublittoral zones as source areas, while for most of the boulders a subaerial origin is hypothesised. The boulder deposit occurs on a flat promontory where the topography, together with the stratified limestone bedding planes and dense joint pattern constitute the predisposing factors for boulder size and detachment. Comparisons between satellite images taken between 2008 and 2017, pictures collected from Internet and a 2012 snorkel survey of the Istrian coast made it possible to highlight the emplacement of a boulder with an estimated weight of 7.65 t during late 2013 or early 2014. The study examines the mechanisms that may be responsible for the detachment and transport of these large limestone rock fragments from the emergent part of the coast and from the sea bed towards inland areas. The results suggest the occurrence of very recent extreme weather conditions as well as multiple historical storm events and exclude a tsunami origin of the boulders.