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

Abstract

ABSTRACT:Popa, L., Panaiotu, C.E. and Grădinaru, E. 2014. An early Middle Anisian (Middle Triassic) Tubiphytes and cement crusts-dominated reef from North Dobrogea (Romania): facies, depositional environment and diagenesis. Acta Geologica Polonica, 64 (2), 189-206. Warszawa.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 AnisianLadinian 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.

Middle Triassic (Anisian, Bithynian) gastropods from North Dobrogea (Romania) and their significance for gastropod recovery from the end‐Permian mass extinction event

“…These species were dwellers of microbial dominated buildups which lack corals or other metazoan frame‐builders and yield only few sponges (Popa et al . ). Thus they may still fall in the category of ‘disaster reef’.…”

“…Its lithofacies types and depositional environment have been described in detail by Popa et al . (). The early middle Anisian age (i.e.…”

“…The middle Anisian Tubiphytes ‐microbial reef in North Dobrogea was first documented by Popa et al . (). It is the only known occurrence of this facies in the western part of the Cimmeride Orogenic System.…”

Numerical analyses of selected microencrusters from the Cipit boulders of the St Cassian Formation (Dolomites, NE Italy): palaeoecological implications

“…For instance, Popa et al . () reported Tubiphytes and cement crusts as the main reef builders from the Anisian of North Dobrogea, Romania. The role of microencrusters during the Triassic continued to take more relevance in the Ladinian‐Carnian [Flügel et al .…”

“…In some cases, microencrusters were ubiquitous components in reefs, and there are even instances at which they were the main component. For instance, Popa et al (2014) reported Tubiphytes and cement crusts as the main reef builders from the Anisian of North Dobrogea, Romania. The role of microencrusters during the Triassic continued to take more relevance in the Ladinian-Carnian [Flügel et al 2002 (and references therein); Flügel 2010;Sánchez-Beristain & Reitner 2012, 2018Sanchez-Beristain et al 2013;Gale et al 2016Gale et al , 2018], until the Carnian extinction event (Benton et al 2018) and up to the Norian-Rhaetian in the Tehyan realm [e.g.…”