Pyritization of Rocks in Black Shale/Host Rock Transition Zones: Evidence from the Bazhenov Formation, Western Siberia

A complex of lithological-geochemical studies was carried out in rocks of the Upper Jurassic–Lower Cretaceous Bazhenov Formation and their transition zones to host rocks: composition of rocks, ratio of organic carbon and sulfide sulfur (C/S values), and distribution of several redox indicators (degree of pyritization, content of the authigenic uranium, and values of Mn/Al and Mo/Mn ratios). Morphology of pyrite was examined with a scanning electron microscope. Two main types of pyrite were revealed: (a) framboidal pyrite formed during early diagenesis with the participation of bacterial activity; (b) cryptocrystalline pyrite formed during diagenesis (early stage included) and, presumably, mesocatagenesis, i.e., medium substage of catagenesis. It was established that some part of the cryptocrystalline pyrite predated the framboidal variety. Pyritization of rocks with the cryptocrystalline pyrite formation took place at the lower and upper boundaries of the Bazhenov Formation in the over- and underlying rocks, where the low-carbon pyritic rocks are formed. Intense pyritization is also observed in the high-carbon rocks near the top of the Bazhenov Formation, where the pyrite–kerogen rocks are formed. Both low-carbon pyritic and pyrite–kerogen rocks are characterized by C/S < 1.5; rocks of the Bazhenov Formation, by C/S > 2. The low-carbon pyritic and pyrite–kerogen rocks occur near boundaries of the lithologically different members (mainly biogenic rocks in the Bazhenov Formation and terrigenous varieties in the host rocks) that were deposited under different redox conditions existing in the water column and near the marine paleobasin floor. Boundaries of such members could serve as geochemical redox barriers at late stages of lithogenesis. The pyritic and pyrite–kerogen rocks were likely pyritized during diagenesis because of the deposition of pyrite on the geochemical redox barriers from fluids that contained iron sulfides and migrated from the high-carbon Bazhenov sequence. Presumably, pyritization continued during the catagenesis owing to thermogeochemical processes of the organic matter transformation.