Ostracod zonation of the Givetian-Frasnian boundary interval in the Timan-North Urals Region

Abstract

Research subject.Ostracods and Givetian-Frasnian boundary deposits of the Timan-North Urals Region.Materials and methods.The work was based on the results of studying ostracods from the stratotype sections of the Southern Timan, which are considered referential for regional and global correlations. The analysis of the distribution intervals of the most significant taxa was based on the results of previous studies, both published and archival materials on the Timan-Urals Region. Ostracods were considered in association with conodonts, which is an important criterion for applying a reasonable biostratigraphic subdivision. The use of different-scale regional cyclicity of sedimentation and individual “marking” levels of manifestation of global events were shown.Results.The ostracod distribution in the Givetian-Frasnian boundary interval of the Timan-Northern Urals region was analyzed. The following most significant taxa were identified: Cavellina devoniana, Cavellina uchtensis, Cavellina batalinae, Cavellina chvorostanensis, Nodella faceta, Nodella ex gr. hamata. The correlation of established assemblages with ostracods from the Franco-Belgian basin was shown. The difference consists in the absence of a polyzygid assemblage with Polyzygia beckmanni beckmanni, which was established at the base of the Nismes Formation of Belgium near the historical boundary between the Givetian and Frasnian stages. The determined relationship of biostratigraphic data in combination with the transgressive-regressive sequence of sedimentation shows a more reasonable solution to the problem of the position of the lower boundary of the Frasnian stage of the Upper Devonian in the Timan-Northern Urals Region.Conclusions.An updated version of the scheme of zonal subdivision of the Givetian-Franian boundary interval in the Timan- Northern Urals region has been developed, taking into account the ostracod distribution of the Cavellinidae and Nodellidae families, as well as the discrepancies in the levels of the first appearance of index species and the boundaries of zones in the regional stratigraphic scheme. The correlation of ostracod zones relative to the conodont scale is shown.