Dynamics of latest Albian-Cenomanian sedimentary environments in the NW part of the Adriatic Carbonate Platform (Istria and northern Adriatic Islands, Croatia)

During the work on the Geological Map of the Republicof Croatia several teams investigated Uppermost Albian - Cenomanian deposits in the area of Istria and the northern Adriatic islands, an area comprising part of the NW section of the large Mesozoic Adriatic Carbonate Platform (AdCP). This work represents a synthesis of the results achieved through several years of study. The investigated Uppermost Albian -Cenomanian carbonate sequences of the AdCP, the Albian deposits are similar -predominantly composed of thin-bedded alternating peritidal and/or lagoonal mud-and grain-supported lithotypes (skeletal- peloid wackestones/packstones, rarely grainstones). The transition from the Albian to the Cenomanian was characterized by formation of diverse environments in different parts of the investigated area.In southern Istria the Uppermost Albian and Lower Cenomanian deposits indicate evolution bioturbated carbonate sand bars and subaqueous dunes composed of rudist and chondrodont debris, and finally back to the shallow environments with rudist biostromes. The coeval succession of northern Istria records three different depositional settings: the western part indicates the continuation of peritidal deposition, in the central part large-scale foreshore-shoreface carbonate sand bodies developed, while the eastern part was characterized by a deeper lagoonal environment with rare tempestites and intercalations of kerogenous limestones. The Albian-Cenomanian transition in other parts of the study area is characterized by a thick sequence of late-diagenetic dolomites (with rare relics of peritidal early-diagenetic dolomite), some crystalline limestones and tectogenic-diagenetic breccia. A similar transition characterizes the larger part of the AdCP, and indicates very shallow depositional environments. In the NE part of the area investigated the aforementioned succession is overlain by peritidal cycles (Cicarija, northern Cres), while in its SE part deepening tendencies were recorded (S Cres, NW Krk, Losinj, Unije, Ilovik), resulting in partial platform drowning and deposition of micritic limestones with pithonellid calcispheres and globotruncanids, gradually thickening southwards (from 10 cm ->130 m). Different environments established in the NW part of the AdCP during the latest Albian and Early Cenomanian, were, in the late Middle Cenomanian, gradually replaced by uniform shallow-marine environments over the entire area, mostly characterized by shallowing-upward cycles. The Latest Cenomanian was characterized by the second important change in the palaeogeography of the studied area.A significant part was influenced by drowning -S Istria andCicarija (NE Istria), and islands: southern Cres, Losinj, Susak and Ilovik -as a reflection of the regionally important transgressive event near the Cenomanian-Turonian boundary. All the aforementioned areas are therefore characterized by prolonged deposition into the younger Late Cretaceous.In contrast, N Istria and the northern part of the island of Cres were completely emerged in the Late Cenomanian, and their palaeokarstified relief, in many places filled with bauxite deposits, was subsequently overlain by Lower Eocene Foraminiferal limestones.Hence, the insular area that existed in the central part of Istria from the Lower Cretaceous, or even latest Jurassic, Cenomanian.The transition is characterized by continuous shallow-water deposition (including only very rare and thin interbeds of pithonellid limestones), as the youngest Upper Cretaceous deposits. This facies variability described over a relatively small area of the NW part of the ACP clearly indicates palaeogeographical differentiation of the study area during the Latest Albian -Cenomanian.Despite some apparent evidence of eustatic sea-level were very active on this part of the AdCP during the entire Cretaceous.Generally, two events can be separated: (1)A transient Albian-Cenomanian event, resulting in the formation of laterally very different environments. During the late Middle Cenomanian these areas were gradually merged into a large area again characterized by uniform, typical shallow-marine carbonate deposition. (2)A latest Cenomanian event with the opposite tendencies: platform drowning and coeval emergence of some areas. The significant influence of the described synsedimentary tectonics was a consequence of the specific position of the study area near the NW margin of the ACP, and probably represented an introduction to subsequent events, when drifting of the Adria microplate towards the European continent resulted in significant deformation during the Palaeogene and Neogene - the formation of flysch troughs, major compression and the final uplift of the Dinaride mountain chain.