Devonian-Carboniferous boundary beds in the Kuz-Yama section (Northern Cis-Urals) - Conodonts, ostracods, and carbon isotope record

Abstract

The Devonian-Carboniferous boundary interval is characterized by biotic crisis, global transgressive-regressive sequences and carbon cycle change, probably caused by climatic changes and volcanic activity. This interval attracts the attention of researchers worldwide in relation to selecting a new Devonian-Carboniferous boundary level. However the Northern Cis-Urals region remains poorly studied in this respect. This study focuses on the Kuz-Yama key section (Northern Cis-Urals) and contributes to our knowledge of the regional expression of the Devonian-Carboniferous boundary in northeastern Europe. The study of the Kuz-Yama section (Verkhnyaya Pechora Trough) involved an examination of the calciturbidites, hemipelagic shales and carbonates that constitute the poorly constrained uppermost Famennian to lower Tournaisian deep-water succession. The conodont and ostracod associations are poorly diversified. The presence of Polygnathus zikmundovae, in association with a positive δ¹³C_carb excursion (up to 3.9 ‰), marks the base of the Late praesulcata conodont Zone (=base of the Protognathodus kockeli conodont Zone) in the lower part of the succession. The Devonian-Carboniferous boundary is indicated by the appearance of Siphonodella sulcata, that coincides with a positive shift in the decoupling of δ¹³C_carb and δ¹³C_con values, probably caused by a global decrease in CO₂ associated with the regional crisis within pelagic ecosystems. The sulcata conodont Zone, bounded by the first occurrence of Siphonodella sulcata at the bottom and Siphonodella bransoni at the top of the section, contains Tournaisian ostracods. The ostracods compose mixed associations of the shallow-water Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris ostracod Zone and the deep-water Armilla uralica ostracode Zone. The co-occurrence of shallow-water and deep-water conodont and ostracod taxa in the Tournaisian part of the section enhances the correlation of the shallow-water and deep-water conodont and ostracod zonations. The base of the shallow-water Siphonodella ludmilae conodont Zone roughly correlates with the base of the deep-water bransoni Zone. The base of Armilla uralica deep-water ostracod Zone in the Kuz-Yama section lies within the Pseudoleperditia venulosa-Coryellina alba-Cribroconcha primaris shallow-water ostracod Zone. The following sequence of events can be recognised in the Kuz-Yama section: first occurrence of Polygnathus zikmundovae Zhuravlev (approximately corresponding to the base of the Protognathodus kockeli Zone), positive shift in the inorganic δ¹³C (= Hangenberg carbon isotope excursion), latest Famennian regression, negative shift in the δ¹³C of conodont elements just below the base of the sulcata Zone, entry of the Tournaisian transgression, positive shift in decoupled inorganic and conodont δ¹³C near the base of sulcata Zone, positive shift in inorganic δ¹³C in the sulcata Zone, appearance of the Tournaisian ostracods, first occurrences of the conodonts Siphonodella bransoni Ji and S. ludmilae Zhuravlev & Plotitsyn (= base of the bransoni Zone). In general, this sequence corresponds to that identified around the Devonian-Carboniferous boundary in other regions.