From Siliciclastics to Carbonates and Black shales: Deciphering Sedimentary Continuity and Discontinuity in the Devonian Landscapes of the Volga-Ural Petroleum Province

Analyses of the three predominant types of sedimentation – siliciclastic, carbonate and black shale – revealed their discontinuity/continuity across various facies environments within the Middle and Late Devonian of the central part of the Volga-Urals petroleum province. These environments include condensed and non-condensed depressions, slopes of depressions and shallow water settings located on tectonic arches or local biohermal uplifts. The regional zonation of conodonts, correlated with the International chronostratigraphic chart, confirms the general stratigraphic completeness of the Middle-Upper Devonian geological record in this area. Meanwhile, sedimentation exhibited a discontinuous pattern, which enables to speak about the socalled “discontinuous continuity”. In particular, the sedimentation of organic-rich black shales, traditionally considered as oil-source rocks, lasted more than 29 million years, from the Late Eifelian up to the Devoniancarboniferous boundary. The longest hiatus (ca. 2.5 Ma) in the accumulation of black shales (as well as carbonates) took place at the end of the Givetian. Notably, this interval contains the most productive siliciclastic reservoirs. During the Early Eifelian to Early Frasnian (ca. 17 Ma), siliciclastic sediments demonstrate the highest accumulation rates and the maximum discontinuity caused by short marine transgressive episodes. Simultaneously, the most complete sequences were deposited in the depressions where carbonate and black shale sediments accumulated. During the Famennian (ca. 12 Ma), siliciclastic sedimentation completely ceased, leading to the stable accumulation of carbonate sediments on the slopes of depressions and in shallow waters; the deposition of organic-rich black shales continued in the deep axial settings of the troughs. Our results highlight the complexity of the spatial relationship between various coexisting sedimentation types and the incompleteness of the geological record in different environments.