No 5 (2025)

We have compared a mean ariphmetic chemical composition of different Pleistocene sediment types for Indian, Atlantic and Pacific oceans based on reports of DSDP, ODP, IODP phases of the International deep-sea drilling project and other references. Besides of it, the comparative analysis of mean chemical composition of Pleistocene straton for all three main oceanic basins also took place. Comparison of meanweighted chemical composition revealed the leading role of dry sediment matter masses. The evaluation of mass accumulation rates for oxides of petrogenic elements in Pleistocene sediments of all oceans also has been done. We have considered data about the modern factors influenced the oceanic sedimentation, for example, ratio of watershed areas to areas of accumulation basins, composition of source provinces, continent’s climate, areas of sea floor above and under the critical depth surface, primary production of the ocean. These data have been compared with our results concerning values of oxides of petrogenic elements in Pleistocene sediments of all oceans. We have revealed that dominate influence for the mean chemical composition of the ocean sediments belongs to the level of critical depth surface and degree of geochemical differentiation.

Based on the analysis of the bulk chemical composition of sandstones that make up the Proterozoic sedimentary sequences that unconformably overlie the basement (Ai and Prikamsk formations, Mukun and Valdai groups, Kerur Formation, Gwalior, Athabasca, Libby Creek, Ufoshan, Birim groups, etc.), it has been shown that the content of the main rock-forming oxides and trace elements in them in the overwhelming majority of cases differs significantly from the chemical composition of the average Proterozoic cratonic sandstone. Their composition corresponds mainly to litharenites, sublitharenites, arkoses and subarkoses according to the classifications of F.J. Pettijohn et al. and M. Hirron. Most of the sandstones from the aggregate analyzed by us belong to rocks containing a significant or predominant share of the lithogenic component. The sources of the detrital material composing the sandstones were fairly mature substrates in which the role of the basic igneous and metamorphic (?) rocks was insignificant. In terms of paleogeodynamics, these substrates can be considered to have formed as a result of various orogenic/collisional and riftogenic events. As with the fine-grained clastic rocks present in the Proterozoic sedimentary sequences we studied, it is clear that not all of the discriminant diagrams used in this study yield consistent results.

During the study of the internal structure of the recently described new species of sponge Gzhelistella cornigera [Davydov et al., 2023] and fusulinids from the Gzhelian stage of the Moscow region, numerous polymineral biomorphoses were discovered, composed of mixed-layer clay minerals, goethite, chalcedony and sanidine. A detailed structural and crystallochemical study of clay minerals from biomorphoses and the host rock has revealed for the first time the structural heterogeneity of these mixed-layer minerals. Using the diffraction pattern modeling method, it has been established that the clay material in the studied samples is represented by two authigenic mixed-layer phases, glauconite-nontronite, with a contrasting relationship between different layers content and/or order in their alternation, but identical structural and crystallochemical characteristics of the crystallites. It has been shown that such formations should be considered as a single heterogeneous mixed-layer structure, in which the relationship and/or order in the alternation of different layer types in crystals vary within certain limits. It has been established that the structural and crystallochemical characteristics of mixed-layer minerals from brown biomorphs and from host rocks are almost indistinguishable, whereas for green biomorphs these parameters are significantly different. It is assumed that newly formed clay minerals from host rocks, like their brown analogs, were formed under similar physicochemical conditions. It is also obvious that during the formation of mixed-layer phases, brown biomorphoses were already deprived of biogenic organic matter, while green biomorphoses retained it in sufficient quantity to locally change the environmental conditions locally within themselves.