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

On the NE slopes of Ćićarija Mt. (N Istria) a 120-150 m thick complexcomposed of dolomite-limestone breccia crops out between theLower and Upper Cretaceous deposits. This studied breccia sequenceis of post-sedimentary, tectogenic-diagenetic origin. It was formed bypolyphase tectonic fracture of the Upper Albian to Lower Cenomanianearly- and late-diagenetic dolomite succession with relics ofrecrystallized limestone, which enabled very important subsequentdiagenetic alteration. This included partial dissolution, dedolomitization,recrystallization and calcitization of the fine-grained, crusheddolomite matrix, and centripetal dissolution of dolomite fragmentsand their cementation by calcite and ferroan calcite cements, as wellas the partial collapse of fragments from the roofs of dissolution cavitiesand limited late-diagenetic silicification (the silica surplus originatingfrom layers of diagenetic quartz from underlying Upper Albiandeposits). Such a complex pattern of different events resulted in thehigh variability of breccia characteristics over relatively small distances,especially near more intensively tectonized zones.The contemporaneous stratigraphic level (Lower to Upper Cretaceoustransition) in other parts of the Adriatic Carbonate Platform isalso characterised by predominantly late-diagenetic dolomites withrelics of limestones (including local occurrences of early-diageneticdolomites) which are, in more tectonized areas, late-diageneticallyaltered into tectogenic-diagenetic breccias.