Microbioerosion from the Rhaetian Limestone and Adnet Formation in the Tannscharte and Fahrenberg sections (Northern Calcareous Alps, Late Triassic to Early Jurassic) – Evidence for accumulations and emergent facies dependency

Microborings are the most common evidence of microbial activity in the ancient oceans, contributing to the destruction of carbonate rocks by returning their material to the biogeochemical cycle. The distribution of microboring communities is controlled by facies and, moreover, exhibits characteristic patterns reflecting the time available for colonization. We studied microbioerosion in petrographic slides of five facies from two sections spanning from the Upper Rhaetian to the Sinemurian in the Northern Calcareous Alps (NCA). Microboring structures were commonly found in various fragments of echinoderms and molluscan shells, foraminifera and in other smaller carbonate grains colonized by fossil microorganisms. Very rare microborings were recognized at the top of the Rhaetian Limestone, in the Brachiopod Beds and in the beds with high terrigenous input. Red limestones of the Adnet Formation contain a redeposited assemblage of microborings from the primary environment with prolonged intervals of exposure of grains on the sea floor prior to burial. The shapes of the microborings in intraclasts of the younger Crinoidal Limestone indicate colonization during shallow-water sedimentation with distinct branched and unbranched tubular borings and camerate dendritic cavities. The composition of the identified microborings reflects facies shifts at the onset of an extensive marine transgression.