Submerged tidal notches in the Rijeka Bay NE Adriatic Sea: indicators of relative sea-level change and of recent tectonic movements

Abstract

The Rijeka Bay coast is formed predominately in carbonate rocks. Due to relatively sheltered position, processes of bioerosion dominate, while mechanical wave action has low impact. Biological zone limits in Rijeka Bay are well expressed, and they reflect the local biological mean sea level (BMSL). Therefore, they were used as referent levels for measurement of submerged tidal notch position. These in situ biological indicators could be more useful than sea-level mareograph data. Well developed tidal notches are permanently submerged and quite ubiquitous in the whole Rijeka Bay area; that is, they are situated in the infralittoral zone. They have elongated, asymmetric shape. Inasmuch as the roof top of the notches is well preserved, they were correlated with well defined biological zone limit [upper limit of white (WE) zone; sensu Schneider, J., 1976. Biological and inorganic factor in the destruction of limestone coast. Contributions to Sedimentology 6, 1–11]. Results of survey showed that, in most of the area, tidal notches are located 50–60 cm below BMSL. However, within the Bakar Bay, notches are from 103 to 115 cm below BMSL. Regional occurrence of well preserved tidal notches at 0.5–0.6 m below BMSL indicates their possible coseismic origin (rapid tectonic subsidence). Deeper positions of tidal notches in the northeastern part of Rijeka Bay and within Bakar Bay indicate enhanced subsidence of that zone which is assumed to be the most seismotectonicaly active.