Calcimicrobial stromatolites at the Permian–Triassic boundary in a western Tethyan section, Bükk Mountains, Hungary

Abstract

Basal Triassic carbonates of the Gerennavár Formation (Bükk Mountains, Northern Hungary) were studied. The succession was deposited in an open marine deep ramp environment of the western Tethys, and can be divided into three parts on the basis of the dominant microfacies. The lowermost 0.5 m-thick interval is made up by thin-bedded and bioturbated mudstones which contain ostracods and Earlandia along with relict skeletal fragments. The successive 8 m-thick deposits consist of planar stromatolites with well-preserved microfabrics. The most characteristic microfabric elements are sphere clusters in a dense micrite groundmass, bushy aggregates of micrite clots, bundles of prostrate micrite threads, and peloids. The formation of the spheres and the surrounding dense micrite is attributed to calcification of a coccoid–cyanobacteria dominated mat community. The tufted micrite clot clusters crudely resemble Angusticellularia. A filamentous microbial origin is likely for the thread bundles. The peloids are interpreted either as reworked pieces of calcified mat, or rolled microbial clumps. The reduced preservation of microfabric components in the upper unit refers to less favourable condition for mat development which is probable due to deepening of the depositional environment. The overlying mudstone unit is very poor in fossils. The preservation of microbial microfabrics in the stromatolites indicates that early lithification by carbonate precipitation was widespread and intense even in open marine environments following the end-Permian boundary events. This supports the assumption that significant changes took place in geochemistry of seawater during the Late Permian–Early Triassic time interval.