Organic geochemistry of Oxfordian carbon-rich sedimentary rocks of the Russian plate

The lithology and geochemistry of upper Oxfordian sedimentary rocks enriched in marine organic matter (OM) have been studied. These rocks occur as a persistent unit of Upper Jurassic rocks exposed on the right bank of the Unzha River in the Kostroma district. The OM was investigated in detail in both the carbon-rich rocks and their hosts. It was established that the OMs from the Oxfordian rocks are characterized by a low degree of thermal (catagenetic) maturity and their geochemical signature reflects specific features of synsedimentary and early diagenetic processes. Kerogen in the carbon-rich sedimentary rocks is markedly enriched in S_org, and its formation was related to the early diagenetic sulfate reduction (sulfurization of the lipid fraction of the initial OM). The composition of kerogen from the host clay is sharply distinct in many parameters. No derivatives of isorenieratene were revealed in the aromatic fraction of bitumen in the carbon-rich rocks. The Oxfordian carbonaceous rocks are distinguished by slightly enriched in S, Mo, V, and Ni. Anoxic conditions were unstable in the water column during the deposition of carbon-rich sediments (such conditions were probably episodic). The C_org-rich unit formed due to a short-term abrupt increase in the productivity of phytoplankton related to eutrophication of water, probably, as a result of the recycling and redistribution of biophile elements.