Biochemical limestones and dolomites on a nanometer scale: significance for the geological record

Electron microscopic, isotopic, X-ray diffraction and spectroscopic studies to clarify the genesis of crypto- and micro-grained limestones of the Upper Ordovician in the Northern Urals and fine- and micro-grained dolomites of the Wenlockian in the Chernyshev Uplift revealed their bio-chemogenic nature. According to the data of the unit cell parameters, dolomites are of primary sedimentary nature. The revealed phases of different ordering of carbonaceous matter in calcite (chlorite stage of metamorphism) and dolomite (debituminated OM) reflect a different degree of their post-sedimentation transformation as a result of processes of different intensity of tectonic events on rocks. The isotopic data of primary sedimentary dolomites in this case actually reflect the primary isotopic ratios d13С (-3.5 ... -2.8 ‰), d18O (22.4-24.1 ‰), characteristic of the offshore somewhat desalinated back-reef lagoon. The wide distribution of fossilized biofilms (glycocalyx), framboidal pyrites, and the presence of bacteriomorphic structures indicate the active participation of microbial organisms in the genesis of the studied microgranular limestones and dolomites.