Late Holocene widening of karst voids by marine processes in partially submerged coastal caves (Northeastern Adriatic Sea)

The coastal scenery of the Northeastern Adriatic sea is widely interested by caves and related coastal features, which are developed in correspondence of geological weaknesses of sea cliffs. We present the preliminary surveying of five partially submerged coastal caves cut in limestone cliffs, relating the dissolutionally widened vadose karst voids and the present- day forms. The analysis pointed out two well-defined morphological zones inside the caves. The boundary between the zones roughly coincides with the mean sea level. The submerged zone is mainly affected by abrasion processes on the bottom and the lateral walls, while the emerged zone is interested by karst processes and collapse of blocks from the roof. Their effects produce a bell-shaped cross-section, in which the submerged part of the caves is significantly larger than the emerged one. Considering the tectonic behaviour of the area inferred from literature the caves were flooded about 6 ka BP, when marine processes started to shape their submerged part. Our results allowed, in particular, to evaluate processes shaping the partially submerged coastal caves in the Northeastern Adriatic Sea after the marine transgression. Considering the very preliminary surveyed data, we suggest that the early phases of cave evolution was mainly dissolutionally-controlled and produced the widening of pre-existing joints or faults, as demonstrated by the occurrence of karst features in the upper part of the caves. Recent evolution is instead marine-controlled and the widening is mainly due to the overlapping of marine processes effects on karst voids, since they are closely related to the Late Holocene sea level rise.