Geochemical properties of terrigenous rocks as indicators of sedimentation conditions and tool for stratigraphic correlation on the example of Achimov clinoform formation (Neocom of West Siberia)

In past few years, two models of clinoform sequence forming are developing: cross-stratification and avalanche-stratification models. Since these formations potentially contain oil and have complex composition/structure, it is necessary to separate/classify them. The conventional methods such as seismic facies analysis and biostratigraphy cannot always give a necessary result. Therefore, in order to define the indicators for such classification we have offered using lithospheric geochemical methods on the example of the Achimovskaya bed (West Siberia). Objects of the research were 52 argillite samples from the units AC3-BU12, AC1-BU12, AC-BU12, AC2-BU11, AC-BU10, ACBU9, which were analyzed by XFA and ICP-MS. Based on the research, studied rocks were classified in two geochemical groups. The first group includes argillites of the units ACBU12, AC2-BU11, which are characterized by low REE accumulation (102-204 ppm) at La/Yb varying from 10 to 14, high Eu/Eu* content (0,58-0,66), and low Ce/Ce* content (0,9-1,07). These values indicate deep-sea regime/setting of the sedimentation. The second group includes the units AC3BU12, AC1-BU12 with higher REE concentration (154-262 ppm), increased Ce/Ce* (1,08-1,58) and La/Yb (10,1-24,07) ratios, and decreased Eu/Eu* content (0,48-0,59). At the same time Fe/Mn, Sr/Ba ranges in cross sections of AC3-BU12, AC1-BU12 demonstrate increase in paleosalinity of the basin (Sr/Ba 0,44-0,56) and effect of chemogenic sedimentation (Mn/Fe 0,007-0,01) in upper levels. It indicates transgressive regime of sediment formation. For the units AC-BU10 and AC-BU9, opposite relationship is observed, where Sr/Ba (0,3-0,44) and Mn/Fe (0,0076-0,011) in lower levels are higher than ones in levels near crust (Sr/Ba=0,33- 0.35. Mn/Fe=0,77-0,92), thus indicating their regression regime. Thereby, implemented studies allow us to define polygenetic sedimentation regime/setting of the clinoform complexes of Achimovskaya bed. It is established a sequential change from northwestern transgression (AC-BU12, AC2-BU11) to deep sea sedimentation (AC-BU12, AC2-BU11) followed by southeastern regression of sea basin (AC-BU10, AC-BU9). These facies reconstructions can be used for classification/correlation of units in a bed's cross section.

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