Lithofacial analysis and paleogeographic reconstruction of Upper Cenomanian deposits of horizon PC1-2 (Pokurskaya suite, Van-Yogan field, Western Siberia)

This work is devoted to the reconstruction of the sedimentary environment of the Upper Cenomanian deposits of the Pokurskaya suite from the Van-Yogan deposit. The main diagnostic instuments were the lithofacies, petrographic, thermal and X-ray diffraction analyses. Using the structural-textural reconstruction of the facial mode of sedimentation of the upper Cenomanian sediments within the Van-Yogan deposit allowed to distinguish three groups of facies corresponding to river (channel, deltaic), coastal-marine (tidal zones, tidal channels, accumulative forms) and shallow-marine (slow-moving shallow water, bays and lagoons (limans)) conditions. This suggests that the formation of the studied section occurred in the coastal-marine environment of the wellhead area of a large river flow. The lithological features and mineral composition of the studied sediments emphasize the substantial variability of the terrigenous component and sorting sandstones, reflecting the lateral and vertical variability of the hydrodynamic activity of the sedimentation basin. The contrasting composition of authigenic assemblages expressed in the development of kaolinite-montmorillonite, chlorite-kaolinite-hydromicaceous, carbonate-clayed cements and scattered occurences of sideritic, sulphide and phosphate mineralization indicates the convergence of the features of continental and marine sedimentation. It emphasizes the variable activity of the river-sea system. The analysis of the lateral and vertical distribution of sediments which formed in various facies conditions showed that the accumulation of the Upper Cenomanian sediments within the Van-Yogan deposit reflects the development of the estuarine basin against transgression. In this case the selected stratigraphic horizons record the main phases of the evolution of the estuary. So the accumulation of the sediments of the lower layer (Saccammina micra) determines its inception in the southern part of the area and occurs against the compression of the hydrodynamic activity of the river by streams. The clay bridge of the horizon (Gaudryinopsis nanushukensis elongatus) was formed directly in the estuarine mode. The upper horizon (Trochammina wetteri tumida) is characterized by the active dynamics of the marine flow, which fixes the coastal marine deposit environment.

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