CARBONATE FACIES EVOLUTION FROM THE LATE ALBIAN TO MIDDLE CENOMANIAN IN SOUTHERN ISTRIA (CROATIA) - INFLUENCE OF SYNSEDIMENTARY TECTONICS AND EXTENSIVE ORGANIC CARBONATE PRODUCTION

During the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (present-day Istria) specific depositi onal systems characterised by frequent lateral and vertical facies var iations were established within a formerly homogeneous area, ranging f rom peritidal and barrier bars to the offshore-transition zone. In sou thern Istria this period is represented by the following succession: t hin-bedded peritidal peloidal and stromatolitic limestones (Upper Albi an); well-bedded foreshore to shoreface packstones/grainstones with sy nsedimentary sliding and slumping (Vraconian - lowermost Cenomanian); shoreface to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shoreface carbonate sand bodies (Lower Cenomanian ); prograding rudist bioclastic subaqueous dunes (Lower to Middle Ceno manian); rudist biostromes (Lower to Middle Cenomanian), and high-ener gy rudist and ostreid coquina beds within skeletal wackestones/packsto nes (Middle Cenomanian). Rapid changes of depositional systems near th e Albian/Cenomanian transition in Istria are mainly the result of syns edimentary tectonics and the establishment of extensive rudist colonie s producing enormous quantities of bioclastic material rather than the influence of eustatic changes. Tectonism is evidenced by the occurren ce of sliding scars, slumps, small-scale synsedimentary faults and con spicuous bathymetric changes in formerly corresponding environments. C onsequently, during the Early Cenomanian in the region of southern Ist ria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were characterised by high-energy environments with ex tensive rudist colonies, and high organic production leading to the pr ogradation of bioclastic subaqueous dunes. This resulted in numerous s hallowing- and coarsening-upwards clinostratified sequences completely infilling formerly deeper environments, and the final re-establishmen t of the shallow-water environments over the entire area during the Mi ddle Cenomanian.