RESEARCH ARTICLE Coral reefs near the Eocene–Oligocene boundary in the northern Transylvanian Basin, Romania: Composition and paleoenvironmental interpretation Jan J. Król 1 | Boguslaw Kolodziej 2 | Ioan I. Bucur 3 1 Institute of Geology, Adam Mickiewicz University, Poznań, Poland 2 Institute of Geological Sciences, Jagiellonian University, Kraków, Poland 3 Department of Geology, Babeş‐Bolyai University, Cluj‐Napoca, Romania Correspondence J. J. Król, Institute of Geology, Adam Mickiewicz University, ul. Krygowskiego 12, 61‐680 Poznań, Poland. Email: jan.jozef.krol@amu.edu.pl Handling editor: X‐D. Wang 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 proba- bly 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 Băbeni–Cuciulat (Sălaj County), up to 55 m 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 10–15 m 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 (folia- ceous) 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 sedi- mentation 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. KEYWORDS carbonates, corals, paleoecology, paleogene, reefs, Romania, Transylvania 1 | INTRODUCTION After the extinction event at the Cretacous–Paleocene boundary, the evolution of coral assemblages in the Paleogene was a complicated process. In some parts of the world, corals did not return to the role of relevant reef builders until the Middle Eocene. In others, they formed large reefal complexes, as early as the Danian (Baceta et al., 2005; Perrin, 2002; Perrin & Kiessling, 2010; Zamagni et al., 2012). Global trends in the development of coral assemblages in the Paleo- gene are believed to result largely from the climate changes. Remark- ably, the abundance of coral reefs decreased instead of increasing in the time of global warming, around the so‐called Paleocene–Eocene Thermal Maximum, and increased during cooling events, such as the Eocene–Oligocene greenhouse–icehouse transition. These trends resulted from other factors connected with climate, such as the changes in nutrient supply as well as global pCO 2 levels, affecting the calcification potential of marine organisms, rather than temperature itself (Perrin & Kiessling, 2010; Zamagni et al., 2012). Coral buildups gained widespread development and distribution during the climate cooling, which started in the Late Eocene. As compared to the Middle Eocene, the Late Eocene coral assemblages from Europe are character- ized by low diversity (Bosellini, 1998). The single exception is the diversified Late Eocene coral fauna from Eisenrichterstein in Bavaria (33 species; Darga, 1990). Late Eocene–Early Oligocene reefs are developed mostly as small reefal structures. Late Oligocene (Chattian) reefs, even though their number is not significantly different from Received: 6 May 2016 Revised: 24 January 2017 Accepted: 24 January 2017 DOI 10.1002/gj.2913 Geological Journal. 2017;1–15. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/gj 1