An Upper Cretaceous biolithitic complex exposed at Donje Orešje on Mt. Medvednica, Yugoslavia (northern Croatia), is composed of a central portion designated as a barrier-reef made up of rudistid and coral bioherms. Associated with the barrier-reef are fore-reef and peri-reefal breccias (reef talus) that originated from destruction of the reef-front. This breccia is unsorted, and contains the rubble of reef building organisms and biocalcarenite fragments. The breccia grades into detrital limestones, and more distally, the sediments are basinal hemipelagic and pelagic limestones with conspicuous turbidity features (“Scaglia”-beds). The backreef area contains nerineid (gastropod) biostromes and detrital limestone. The lagoon is mainly represented by clastic terrigenous deposits, through some rudist patch-reefs (“Gösau”-beds) occur rarely. Sporadically, the lagoon also exhibits fresh water characteristics. The reef-flat was successively contaminated by terrigenous clastic material transported from the lagoon, e.g., material derived from erosion on an island arc. The porous reef-frame acted as a barrier, preventing transport of clastic material toward the open sea, but trapping it in the lagoon. Areas with no barrier-reefs, or the areas where the reef was breached or destroyed, turbidity currents could and did deposit flysch-type sediments into the basin. Numerous fossils (rudists, corals, benthic and pelagic foraminifers, and nannoplankton) indicate that the Donje Orešje biolithitic complex is Upper Cretaceous in age (Santonian to lower Campanian).
Barrier-reefs of the Donje Orešje type developed in the Upper Cretaceous and lower Paleogene of the Inner Dinarides on the slopes of island arcs. These island arcs and correlative trenches, as well as the inter-arc basins, were formed in a subduction zone of Tethyan oceanic crust beneath the Panonian, e.g., Rhodope Plate. The subduction zone is characterized by the occurrence of ophiolites and mélanges. The Adriatic Plate (the Outer Dinarides) is marked by uniform shallow water carbonate sedimentation (“Carbonate platform”). During its east-ward motion this plate was uplifted at the end of Cretaceous time (“Laramian Orogeny”), and accordingly sedi-mentation reflects regression and bauxite deposition. However, a continuous depositional sequence between Cre-taceous and Paleogene time has been determined for the Inner Dinarides. Apparently, there was not one simple slope between the Outer and Inner Dinarides, with transition from shallow water to basin sediments in the sense of a “classical géosynclinal concept,” but shallow water and basinal facies were successively reestablished due to the influence of island arcs.
Distribution of the biolithitic complexes and other Upper Cretaceous facies in regular bands demonstrates that since Cretaceous time there has not been any regionally extended nappes. Such overthrustings would certainly have dislocated the original paleogeographical distribution of Upper Cretaceous facies belts. The main Dinarides “orogenic phase” occurred by the end of the Eocene, when intensive northward and northeastward movement of the African Craton caused a maximal compression and junction of the Adriatic and Panonian Plates. As a consequence, tectonic movements on the plates, as well as in the area of the “oceanic suture” of the Inner Dinarides, brought about the uplift of the Dinarides.
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The voluminous amount of information presented in this Special Publication not only fills a gap in understanding the European approach to reef studies but also provides the necessary data base to allow us (in particular the North American geologist) to incorporate this information in our overall interpretive studies. These studies should serve as an impetus for new investigations and will broaden our understanding of the complex interrelationships that operate in the reef environment.