Methods of foraminiferal zonal schemes development

The Upper Cretaceous and Paleogene stratigraphy of Western Siberia is based on establishing zonal subunits determined by foraminiferas. Four methods have been applied for this purpose: 1) the analysis of foraminiferal assemblages; 2) the construction of phylogenetic schemes for rapidly evolving taxa; 3) the paleozoogeographic research with establishing subordinated biochores; 4) the establishment of rhythmostratons. The first method is the leading one, as it is based on the investigation of foraminiferal assemblages. The second method accounts for the ontogenesis and phylogenesis of foraminiferal groups with accelerated development. The third method includes establishing paleozoogeographic subunits. The compared foraminiferal assemblages of Western Siberia and other provinces are assigned to the same paleogeographic area. Within Western Siberia, the districts have been established, according to the relation of the species compositions and structures of foraminiferal assemblages. These districts are necessary for the comparison of assemblages by the lateral and for the construction of zonal schemes. The fourth method consists in distinguishing rhythmostratons. The study of the rhythmicity in the distribution of foraminifera within the Upper Cretaceous and Paleogene section (the systematic and numeric composition) taking into consideration lithology has enabled constructing the scheme of rhythmostratigraphy with recognising subordinated rhythmostratons - rhythmothems, rhythmotherms and rhythmolithes; the latter corresponding to the zonal subdivisions based on foraminiferas.

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