Geological and geomorphological conditions in the area of Rjecina river and Dubracina river cathcments (Primorsko-goranska county, Croatia)

In this paper geological settings and geomorphological processes at the areas of two river catchments basins in Primorsko-Goranska County (in the NE Adriatic coastal area of Croatia) are presented: the Rjecina river catchment and the Dubracina river catchment. Both areas are a part of a unique morphostructural unit Rjecina valley-Bakar Bay-Vinodol valley in which karstified carbonate rocks prevail. Paleogene siliciclastic sedimentary rock complex, i.e. flysch, has a form of squeezed synciline between carbonate rocks. Geological contacts between carbonate and flysch rock mass are tectonic reverse faults, but with different mechanisms of origin. Neotectonic and recent tectonic movements caused irregular subsidence of flysch areas and the uplifting of the surrounding karstic terrain. Consequently, carbonate rock mass is placed on the top of the slopes, while the flysch crops out at the lower parts. Weathering processes cause disintegration of carbonate rock mass, separation of blocks and gravitational movement of rock blocks and debris over the flysch bedrock, as well as the accumulation of talus in the toe of rock scarps. Unlike carbonate rocks, flysch rock mass is more subjected to weathering with final product of soil type material. During the time, rock blocks and coarse grained fragments originating from the rockfalls were mixed with soil-like material from flysch weathered zone. These geomorphological processes produced potentially unstable slope deposits that are, in some areas, more than ten meters thick. Rockfalls from disintegrated carbonate scarps and sliding of talus are permanent geodyinamic phenomena. Energy of the relief was changing due to tectonic movements, as well as changes of local erosion base and the position of local riverbeds. Strong earthquakes could be also one of important triggering factors, because the investigated area is seismotectonicaly very active. The comparison of morphological evolution, as well as intensity and dimension of instabilities, shows a distinction between Rjecina valley and Vinodol valley, although both areas are a part of the same morphostructural unit. The recent rockfalls and rock sliding phenomena are frequent and intensive in the Rjecina valley. Some of these mass movements have huge dimensions. These instabilities are atypical for Adriatic area but also frequent in the Alps. Active talus is visible on the northwestern slope of the Vinodol valley. Extensive erosion processes are clearly visible in the central part of Vinodol valley: around of tributary streams of Dubracina river: Slani potok, Mala Dubracina and Kucina. The active and dormant landslides are situated in alteration with relatively stable part of slopes.