Evidence of carbonate rocks formation on geochemical barriers in black shale on the example of the Bazhenov formation of the Western Siberia

A comprehensive lithological-geochemical study of rocks of the Upper Jurassic-Lower Cretaceous blackshale Bazhenov formation showed that most of its carbonatized interlayers to the boundaries of packs of different composition differing in carbonate content, degree of siliceousness or clayiness. At the same time, at the boundaries of the Bazhenov formation with host sediments, where carbonate rocks are often found in association with “pyrite” low carbon rocks according to geochemical parameters (degree of pyritization, Mn/Al, Ua), a change in the redox regime is recorded. In the most studied stratum at intervals of occurrence of carbonates, a change in the redox regime is not observed. It is assumed that they existed alkaline barriers, as evidenced by the change in the composition of rocks. The following geochemical barriers (bottom-up along the section) were identified in the Bazhenov formation and its transition to the enclosing sediments, on which evidence of localization of carbonate minerals was found: redox barrier 1 (lower BF boundary), alkaline barrier 1 (border of mixes of kerogen - clay-siliceous low-carbonate and silicites), alkaline barrier 2 (border of kerogen-clay-siliceous low-carbonate and siliceous-carbonate “coccolith” packs); Redox barrier 2 (the upper limit of the BF).