Sedimentary features influencing the occurrence and spatial variability of seismites (late Messinian, Gargano Promontory, southern Italy)

Seventeen layers characterized by soft-sediment deformation structures (SSDS) were identified within the “calcari di Fiumicello”, an upper Messinian (Miocene) stratigraphic unit (30 m thick), cropping out in the northern sector of the Gargano Promontory (Apulia, southern Italy). Facies analysis was performed on the whole outcrop and detailed sedimentological investigations were carried out on the deformed beds, in order to interpret the deformation mechanism, the driving mechanism and the possible trigger agent. Deformed layers occur in some thin-bedded ooidal limestones, skeletal calcarenite, as well as in some pebble-size conglomerate, alternated with marls, deposited in a protected embayment or barrier-island-lagoon system, possibly characterized by high salinity, and tidal influx. SSDS can be classified as load- and slump/slide structures. The continuous exposures allow us to follow a single deformed layer along tens of meters, hence several types of lateral variations were observed that can be summarised as follows: (1) SSDS disappear within a few meters (with a decreasing pattern of their deformation or in an abrupt way); (2) deformed layers laterally change in thickness and morphology; and (3) a single deformed bed can laterally correspond to two deformed beds. Most of the soft sediment deformation features were identified as liquefaction and/or fluidization features related to seismic shocks (seismites). Seismites are often used as an indicator of seismic events, especially along small outcrops, trench excavation and core analysis. This study highlights the value of the sedimentological analysis for paleoseismic investigations, with the aim of improving criteria for identifying seismites in the sedimentary record, and their suitability as marker of seismic events.