Hydrothermal-bacterial structures with a brachiopod fauna on the surface of Tessel paleovolcano (South coast of Crimea)

Abstract

   Research subject. For the first time, the results of a study of hydrothermal-bacterial carbonate structures and a shell bank of brachiopods on the surface of the effusive sequence of the Late Triassic Tessel paleovolcano are presented.   Methods. Petrographic thin sections were studied using an Olympus BX 5 microscope with an Olympus DP 12 camera. Geochemical investigations were performed according to the conventional methodology by an inductively coupled plasma mass spectrometer (ICP-MS). The isotopic composition of carbon was determined by isotope ratio mass spectrometry (IRMS) by a Deltaplus Advantage instrument. The mass spectrometer was paired with an EA Flash1112 analyzer and a TC/EA convector. When measuring, the NBS-19, NBS-18, and IAEA-C-3 standards were used.   Results. The data obtained on the geology, geochemistry, and isotopic composition of the studied materials confirm the existence of a symbiosis between brachiopods and protozoa. The primary formation of organic matter and carbonate of bacterial structures occurred due to the processing of paleofluids by the community of bacteria and archaea. These organisms created carbonate, sulfide and organic matter in a suboxide environment of hydrogen sulfide and methane. The biocenosis of the shell bank of brachiopods was characterized by a poor taxonomic composition and a high biological productivity. The formation of a shell bank of brachiopods with bacterial carbonate edifices took place during the period of volcanic activity and continued during breaks in eruptions. The results of geochemistry and the carbon isotopic composition of the carbonate of bacterial buildups confirm the deep nature of the paleofluids. The Norian age of paleovolcanism was determined from finds on the surface of the terrigenous- volcanic sequence of brachiopods Worobiella ex gr. caucasica Dagys and ammonite Megaphyllites insectus (Mojsisovics).   Conclusions. According to geological features, the material of the shell bank of brachiopods with bacterial carbonate structures on the surface of the Tessel paleovolcano belongs to the ancient hydrocarbon seeps-carbonate. Based on their genesis and formation material, these formations should be referred to as Triassic methanoliths. Signs of allocation of similar formations in other time intervals and regions are given. Emissions of fluids with hydrogen sulfide and methane by the Tessel paleovolcano in the Late Triassic may have been related to the causes of the “global extinction”.