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New magnetostratigraphic data from the Upper Volgian of the Moscow region
Alexei G. Manikin, Mikhail A. Rogov, Vladimir A. Grischenko, Ramir S. Dakirov

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 12:21 PM – 12:24 PM
Last modified: 2018-11-24

Abstract


Strong biogeographical segregation near to the Jurassic/Cretaceous boundary leads to impossibility of direct Boreal-Tethyan correlation by biostratigraphy, thus providing magnetostratigraphy as a key approach. However, magnetostratigraphic data from the Boreal section are very scarce, so far the only Nordvik section (northern Siberia) is characterized by succession which permits direct correlation with Tethysian regions (cf. Houša et al., 2007; Schnabl et al., 2015), but ammonite findings in the Upper Volgian of this section are uncommon and position of zonal boundaries remains tentative (Rogov et al., 2015). Here we are representing the first preliminary results of magnetostratigraphic study of the condensed Upper Volgian succession in the Eganovo sand pit near Moscow (N 55°32'0.16"; E 38° 3'28.28"). This section is well-characterized by ammonites (Rogov, 2017), and containing nearly full succession of ammonite biohorizons except the uppermost Middle Volgian Nikitini Zone, with a gap corresponding to its upper subzone. Uppermost part of the Volgian here is lacking any macrofossils and cannot be referred to any ammonite zone. Totally 12 palaeomagnetic samples were collected in the ~3,8 m interval. Each sample was prepared as several cubic specimens. The magnetic susceptibility (K) and anisotropy of magnetic susceptibility (AMS) were measured using MFK1-FB kappabridge, while the natural remanent magnetization (NRM) was measured using a 2G Enterprises. Alternating field demagnetization was carried out from 3 to 80 mT with 3 mT step. As revealed by magneto-mineralogical study, main sources of natural remanent magnetization were magnetite and closely related minerals, but some samples from member 2 are characterized by presence of iron hydroxydes. In the studied section different lithologies are characterized by different NRM and K values. AMS of samples from the Member 2 (beds E2-E9) is characterized by nearly equal distribution of K1 axis suggesting weak hydrodynamics with only a little influence of bottom currents from NW to SE, but some samples are showing anomalous distribution of axis suggesting active hydrodynamics. Palaeomagnetic data are relatively poor, as in other strongly condensed successions. As follow from Zijderveld diagrams, nearly all samples are characterized by presence of low coercitivity remanent magnetization and characteristic remanent magnetization (ChRM); the latter was used for determination of magnetic polarity. ChRM was recognized in 7 samples only, but even such a preliminary data permits to recognize succession of magnetic polarity zones, which could be correlated with the both Nordvik succession and Standard succession. Two reverse polarity zones, which are corresponding to lower part of the Middle Volgian Nikitini Zone and upper Fulgens to Catenulatum Upper Volgian zones could be ascribed to M20r and M19r respectively. These results fit well with biostratigraphic correlation of Eganovo and Nodrvik successions.