The calcareous nannofossils and magnetostratigraphic results from the Upper Tithonian–Berriasian of Feodosiya region (Eastern Crimea)

Abstract

This article is concerned with nannofossil study of Tithonian–Berriasian sediments of Eastern Crimea. The NJT 16, NJT 17a, NJT 17b, NKT, and NK 1 nannofossil zones were determined. The occurrence of Nannoconus kamptneri minor, one of the potential marker-types of the Tithonian–Berriasian boundary (the base of the NKT Zone) of the Tethyan sequence in the Feodosiyan section is assigned here to the Pseudosubplanites grandis ammonite Subzone and the magnetic Chron M18n. The base of the NKT Zone is closer to the Grandis Subzone base than to the base of the Jacobi Subzone. Contradictions in the interpretation of magnetic chrons obtained by the present authors (Arkadiev et al. 2018) and by Bakhmutov et al. (2018) might be caused by mistakes admitted in the latter work on the compiled section.

Further observations on the bio- and magnetostratigraphy of the J/K boundary interval in southern Ukraine

“…Here we con sider stratigraphically in dic a tive nannofossil FOs and not nannofossil biozones, as these are in a state of flux and have not yet sta bi lised: zones have some times been ap plied in pub li ca tions with out tak ing into ac count pub lished changes in the po si tions of marker spe cies, e.g. in Cri mea, Arkadiev et al (2019), cit ing and ap ply ing Bralower (1989) and Casellato (2010). Fig ure 8 shows how sev eral taxa used as zonal in di ces now have very dif fer ent po si tions and ages com pared to those that were firstly, and sub se quently, at trib uted to them.…”

“…How ever, the ev i dence pro vided by nannofossil taxa gives age in di ca tions for the two, and this sug gests that the west ern sec tion is slightly lower in the Tithonian than the east ern out crop (which is only just be low the base of the Berriasian). Arkadiev et al (2019), in the 80 metres be low the same brec cia (their out crop 2922), found no stratigraphically in dic ative nannofossil. Though the up per 25 metres had yielded to Svobodová et al (2019a) a typ i cal late Tithonian as sem blage of Nannoconus infans, N. puer and N. globulus mi nor, Polycostella sp.…”

“…About 140 metres be low the brec cia bed, Arkadiev et al (2019) re corded iso lated nannofossil finds that they in ter preted as the FOs of Nannoconus compressus and Polycostella senaria. These spe cies are noted in the lit er a ture as hav ing their first oc cur rences well be low both M19r and M19n, in the early Tithonian: re spec tively, in magnetozone M21n (fide Varol and Bow man, 2019) and magnetozone M20r (fide Bralower et al, 1989).…”

“…Next, a lower stratigraphic in ter val some 3 km fur ther west was as sessed (Svobodová et al, 2019a). A com men tary on these stud ies then appeared (Arkadiev et al, 2019), and this was fol lowed by a re ply from our team -tak ing the op por tu nity to com pare re sults from the Ukrai nian suc ces sion with those won at nu mer ous other Tethyan sites (Wimble don et al, 2020a).…”