Coral reefs near the Eocene-Oligocene boundary in the northern Transylvanian Basin, Romania: Composition and paleoenvironmental interpretation

Eocene-Oligocene reefs have been reported in Europe largely from the circum-Mediterranean region. In this paper, small coral reefs from the northwestern Transylvanian Basin (Romania) are described for the first time. They developed near the Eocene-Oligocene boundary, most probably during the Priabonian, and belong to northernmost outposts of the reef belt developed during this time in Europe. The studied sedimentary successions in Letca and Bbeni-Cuciulat (Slaj County), up to 55m thick, belong to the Cozla Formation. The reefs occur within a shallow-water succession composed mostly of bedded limestones, dominated by bioclastic (coralline) packstones. Low-relief (constratal) reefs, locally up to 10-15m in thickness, are spaced cluster (matrix-supported) reefs. Scleractinian corals are common but poorly diversified (10 species and 8 genera). Branching ramose colonies, branching low-integrated phaceloid, and sheet-like (foliaceous) corals dominate. Neither lateral zonation nor vertical succession of reefs was recognized. Corals co-occur with encrusting and geniculate red algae, but they are of subordinate significance for a nonrigid reef framework. Branching corals baffled or trapped suspended carbonate mud that contributed to the reef growth and ongoing development of topographic relief. Associated fossils are of low to moderate diversity. A relatively low-energy environment, moderate to high sedimentation rate, and increased turbidity are inferred from carbonate muddy and fine-grained matrix, dominance of sediment-resistant corals, their morphology, common occurrence in growth position, as well as low to moderate degree of bioerosion and encrustation. Transylvanian reefs in terms of poor coral diversity, matrix-supported texture, and turbid-water sedimentary setting show similarities with many coeval reefs from the circum-Tethyan area.