Facies pattern of western Tethyan Middle Triassic black carbonates: The example of Gutenstein Formation in Silica Nappe, Carpathians, Hungary, and its correlation to formations of adjoining areas

Abstract

During the Middle Triassic, a broad carbonate ramp developed at the western end of the Tethys Ocean. In the early phase of the ramp evolution dark grey or black, finely crystalline carbonates were deposited over a considerable part of the ramp. In the Aggtelek Karst, northeastern Hungary, carbonates of the Aggtelek facies of the Silica Nappe are exposed. In the latest Early Triassic, bioturbated mudstones–wackestones and occasionally packstones were formed in the outer ramp zone below the storm wave-base. During the Early Anisian, oxygen-depleted bottom conditions evolved. Gradual changes took place in the sedimentary features of mud-dominated deposits that is attributed to increasing oxygen depletion as a result of density stratification of seawater in the deeper ramp areas. Remarkable ecological and sedimentary changes began with the colonisation by sponges and microbes in the Aggtelek facies area that occurred coevally with a relative sea-level fall. The appearance of the foraminifer Glomospira densa points to late Early Anisian age for these changes. This unit is very variable in lithology and facies compared to the monotonous development of the lower units. Massive limestones and dolomites of microbial origin are ubiquitous; additionally, oolites with micro-oncoids, cross-bedded and cross-laminated bioclastic, peloidal packstones–grainstones, thin-bedded bioclastic wackestones–mudstones, laminated and brecciated dolomites with evaporite pseudomorphs, and dolocretes are also present. Specific microfacies types, i.e. automicrite (varicoloured micrite–microspar groundmass) and cuneiform spongy microfabric preserved by cryptic microbial films, are recognised and interpreted as sponge–microbe mud-mound deposits. The microfacies of pilot samples from Northern Calcareous Alps, Inner Western Carpathians and Dinarides exhibit similar sedimentary features. Tectonic setting, biota evolution after the end-Permian mass extinction, relative sea-level changes and hypersalinity may have been the most important controlling factors of the non-skeletal carbonate factory, which produced dark grey mud-dominated deposits.