A multidisciplinary approach for the investigation of coastal boulder deposits in southern Istria (north Croatia)

This paper provides a detailed study of coastal boulder deposits (CBD's) that were recently discovered along the southern Istrian coasts at Premantura, and on the nearby islet of Fenoliga. Additional observations have also identified CBD's at the Brijuni archipelago 20 km's to the northwest. The northern Adriatic Sea is a semi-enclosed basin, limiting the number of storm wave capable of the detachment, transport, and deposition of large boulders. However, despite this constraint extensive CBD's are evident. A multidisciplinary approach was used to investigate the sites including geological and geomorphological surveys, together with the use of an Uncrewed Aerial Vehicle (UAV), digital photogrammetric analysis and swim surveys. Measurements of boulder position, elevation, size, shape and density were carried out recently at the two sites. We recognized and mapped approximately 950 clasts at Premantura and 592 clasts at Fenoliga. At Brijuni several observations have identified that some blocks periodically appear and disappear following severe storm events. Furthermore, we carried out multitemporal monitoring activities at the Premantura test site, identifying the movement of a dozen of blocks primarily during the extreme low pressure Mediterranean storm Vaia in 2018. Biogenic marine carbonate encrustations observed on 14 boulders in Premantura suggest the infra- and sublittoral zones as source areas, while for other boulders a subaerial origin is hypothesised. Local topography, together with the stratified limestone bedding planes and dense joint pattern constitute the predisposing factors for boulder size and detachment