The Origin and Importance of the Dolomite-Limestone Breccia Between the Lower and Upper Cretaceous Deposits of the Adriatic Carbonate Platform: An Example from Ćićarija Mt. (Istria, Croatia)

Abstract

On the NE slopes of Ćićarija Mt. (N Istria) a 120-150 m thick complex composed of dolomite-limestone breccia crops out between the Lower and Upper Cretaceous deposits. This studied breccia sequence is of post-sedimentary, tectogenic-diagenetic origin. It was formed by polyphase tectonic fracture of the Upper Albian to Lower Cenomanian early- and late-diagenetic dolomite succession with relics of recrystallized limestone, which enabled very important subsequent diagenetic alteration. This included partial dissolution, dedolomitization, recrystallization and calcitization of the fine-grained, crushed dolomite matrix, and centripetal dissolution of dolomite fragments and their cementation by calcite and ferroan calcite cements, as well as the partial collapse of fragments from the roofs of dissolution cavities and limited late-diagenetic silicification (the silica surplus  originating from layers of diagenetic quartz from underlying Upper Albian deposits). Such a complex pattern of different events resulted in the high variability of breccia characteristics over relatively small distances, especially near more intensively tectonized zones. The contemporaneous stratigraphic level (Lower to Upper Cretaceous transition) in other parts of the Adriatic Carbonate Platform is also characterised by predominantly late-diagenetic dolomites with relics of limestones (including local occurrences of early- diagenetic dolomites) which are, in more tectonized areas, late-diagenetically altered into tectogenic-diagenetic breccias.