Geomorphology of soft clastic rock coasts in the mid-western Adriatic Sea (Abruzzo, Italy)

We present the first detailed overall study on the rock coasts in the central Adriatic Sea (Abruzzo, Italy), which is one of the few cases of a coast with clastic soft rocks in the entire Mediterranean area. The coast is composed of cliffs with small beaches, coastal slopes with or without a contiguous coastal plain and intervening low-lying coasts. It is developed on the eastern seaward side of a wide plateau and mesa landscape shaped on an Early-Middle Pleistocene clay-sand-sandstone-conglomerate marine sequence covered by Late Pleistocene – Holocene continental deposits.The study focuses on the overall coast and on eight specific sites, combining: (i) DEM (2 m cell, LiDAR-derived) analysis and geological-geomorphological surveys of the emerged and submerged areas, for the cliff/coastal slope characterization, (ii) the results of U/Th, ¹⁴C sediment dating and archaeological and palaeontological attributions, for constraining the coastal evolution, and (iii) aerial photo time-series (1954–2013 time span) analysis, for the recent cliff retreat assessment. The overall features of the cliffs and coastal slopes were defined in terms of their morphology, lithology, tectonic setting, landforms and geomorphological processes, whereas the analysis of Late Pleistocene – Holocene continental deposits and landform distribution defined and constrained the evolution of the coastal system. This enabled us to define eight different types of coastal cliffs/slopes meaningful for clastic soft rock coasts. The coastal types were defined in terms of: (i) lithology (15% cliffs on marine sandstone, 15% cliffs on marine conglomerate, 3% cliffs on continental deposits, 43% coastal slopes on clay-sand-sandstone-conglomerate sequences), (ii) morphostructural setting (i.e. clay/sandstone-conglomerate interface elevation), and (iii) of state of activity (3.8% active cliffs, 13.8% inactive cliffs, 15.9% palaeocliffs, 43% of inactive and locally reactivated landslides on coastal slopes). Variable retreat rates were also defined, from retreat measurements over a 60-year time span (ranging from 0.15 m/yr to ~1 m/yr), and were found to be induced by episodic and localized cliff recession processes and connected to combined wave-cut and gravity-induced slope processes. The assessment of coastal and gravity-induced geomorphological processes, combined with the analysis and dating of the continental deposits, allowed us to define the Late Pleistocene to present evolution and timing and the evolutionary mechanisms of the main coastal types of soft rock coasts. They are connected to wave-cut and gravity-induced erosion cycles on cliffs and to large rotational ancient landsliding with local recent reactivations on coastal slopes. For the different coastal types the expected hazard conditions were outlined.