Prolific development of pachythecaliines in Late Barremian, Bulgaria: coral taxonomy and sedimentary environment

Abstract

Diversified and abundant corals of the suborder Pachythecaliina (order Hexanthiniaria) are described from Upper Barremian, biostromal reefs of the Emen Formation, Lovech Urgonian Group, north central Bulgaria. The corals are mostly of the phaceloid growth form and represent 14 species (six new), 12 genera (three new), belonging to five families. Pachythecaliines occur with the small, monopleurid cylindrical rudist Mathesia darderi. The rudists frequently are densely clustered, occur between coral branches or are in contact with them. Other corals, with the exception of the phaceloid Calamophylliopsis, and other rudists, are rare. Non-laminated microbialite crusts provided additional, structural support for bioconstruction development. Microbialites (automicrites) can be interpreted as a product of microbial activity, or alternatively, as a result of carbonate precipitation, brought about by non-living organic substrates (organomineralization s.s.). In addition to microbialites, metazoans are encrusted by heterotrophic skeletal microorganisms, while photophilic and oligotrophic microencrusters, usually common in other coral-bearing limestones of the Emen Formation, are very rare. The section at the Rusalya Quarry (NW of Veliko Tarnovo), about 42 m thick, provides the sedimentary and environmental context for the reefal biostromes. The vertical biotic and sedimentary succession displays a general shallowing trend: from the outer carbonate platform with bioclastic limestones containing small boundstone patches (corals, but not pachythecaliines, Lithocodium aggregatum), to the inner platform with rudist biostromes. The pachythecaliine-rich biostromes, 2.5 m thick, were developed in a low-energy environment, referred to the distal part of the rudist-dominated area of the platform. The development of microbialites was facilitated by a low sedimentation rate, and possibly by increased nutrient level. Only poorly diversified and non-phaceloid pachythecaliines occur in other coral-rich limestones and marls of the Urgonian complex in Bulgaria. The assemblage described is the most remarkable, Early Cretaceous coral community worldwide, with regard to pachythecaliines. Phaceloid pachythecaliines are only more common in the Upper Jurassic rocks, being particularly diversified in the Tithonian–Lower Berriasian Štramberk Limestone (Czech Republic) and its equivalent in the Polish Outer Carpathians. However, their sedimentary context differs from that described for the corals of the Emen Formation.