An early Middle Anisian (Middle Triassic) Tubiphytes and cement crusts-dominated reef from North Dobrogea (Romania): Facies, depositional environment and diagenesis

Abstract

A well-developed Triassic carbonate platform is exposed in the eastern part of the Tulcea Unit, in the Cimmerian North Dobrogean Orogen, southeastern Romania. Facies analysis of the 200 m thick succession of lower Middle Anisian limestones exposed in a large limestone quarry south of the village of Mahmudia suggests a transition from upper slope towards toe-of-slope carbonate facies, reflecting sea-level fluctuations and tectonic tilting. The slope is dominated by in situ microbialites in the upper portion, consisting of reefal boundstone facies, and by molluscan coquina and cement boundstones. A key role is played by the cosmopolitan micro-encruster Tubiphytes, which became common in the aftermath of the mass extinction at the Permian/Triassic boundary, and by autochthonous micrite and synsedimentary marine cement. The absence of metazoan reef builders, such as sponges and corals, reflects the fact that microbes were the first organisms to recover after the Permian/Triassic crisis under unusual marine conditions and that their main role in reef formation was sediment stabilization along the upper slopes. The lower slope is mostly detrital, being dominated by platform-derived bioclastic rudstones and crinoidal floatstones, which are interbedded with basinal carbonate hemipelagics. The toe-of-slope is composed of pelagic wackestones framed by thin tongues of intraclast breccia. All these observations are in agreement with the slopeshedding model described for the Pennsylvanian microbial margin in Asturias (northern Spain) and the Anisian-Ladinian flat-topped, steep-rimmed Latemar platform (Dolomites, Italy). As most of the Anisian reefs were described from western and eastern Tethys (Southern Alps, Hungary, China), the occurrence of the early Middle Anisian Tubiphytes-reef from North Dobrogea (Romania) contributes to resolving the puzzle of the geographic distribution of reef recovery in the Middle Triassic.