The role of the microencruster-microbial reef-building consortium in organic reefs evolution (Late Jurassic, northern Tethys shelf, southern Poland)

Abstract

The carbonate sediments of the Polish part of Tethys shelf contain Crescentiella-microbial-cement facies. The facies is associated with Middle Oxfordian-Lower Kimmeridgian microbial-grain dominated reefs, microbial-sponge dual hybride reefs, and less frequently sponge/coral-microbial-abiotic triple hybride reefs. The Crescentiella (incertae sedis) and microbialites form numerous cluster bioconstructions with stromatactis-like cavities in which rapid lithification was supported by early marine cementation. However, the factors controlling the development of such bioconstructions and the role of Crescentiella in the development of reef frameworks remain enigmatic. The sediments from five localities were investigated using thin-sections analyses supported by cathodoluminescence observations, stable oxygen and carbon isotope analyses and dispersive X-ray spectrometry. The development of Crescentiella-microbial-cement facies took place in three intervals spanning the Transversarium, Upper Bifurcatus, and Planula zones and coinciding with the period of maximum development of the Late Jurassic reefs in Poland. The reefs contain microencruster Crescentiella which was an important rigid framework constructor among the microbialites while the skeletal metazoans were rare. The inferred palaeosetting of the facies ranges from the seaward steep-fronted outer margin to the upper slope broad reef complex/microplatform where the development of skeletal metazoans was limited and mainly formed stratiform microbialites binding coated grain-bioclastic dominated sediments. The occurrence of Crescentiella-microbial-cement boundstone facies is useful for palaeogeographic, palaeobathymetric, and palaeotectonic investigations.