Clay minerals and detrital material in Paleocene–Eocene biogenic siliceous rocks (Sw western Siberia): Implications for volcanic and depositional environment record

The paper presents the results of a study on clay minerals and detrital material of biosiliceous rocks (Paleocene–Eocene) from three sections in the Transuralian region. The authigenic processes in sediments resulted in the formation of dioctahedral clay minerals (illite, smectite) and insignificant amounts of sulfide phases (pyrite, hydrotroillite). Detrital minerals from the studied diatomites and diatomaceous clays often have a subangular and semi-rounded habit that is evidence of a low degree alteration of the sedimentary material in the provenance areas. The high degree of preservation of the bioclastic debris and the transformation of the limited volcanogenic substratum in clay minerals apparently was possible by initial burial diagenesis. The morphology of kaolinite and illite suggests that these mineral formations were caused by diagenesis with feldspars and smectites as a substrate for their formation. The smectite zone of weathering crust that developed on the adjacent land could have also served as a significant source of smectites entering the sea basin. The association with smectite in aggregates of mixed clayey composition indicates a sequential smectite-to-illite reaction via mixed-layered minerals. Such minerals as amphiboles, pyroxenes, and olivines, semi-stable to transportation and genetically associated with ultramafic rocks, form a significant part of the clastic fraction of the rock, indicating the proximity of provenance areas. This is the evident reason that the provenance areas made of mafic and ultramafic rocks played an essential role.