Graptolite biostratigraphy and pecularities of depositional environments during Late Llandovery and Wenlock according to geochemical data from Kurtuvenai-161 borehole (NW Lithuania)

The graptolite biostratigraphy and pecularities of depositional environments during the Late Llandovery and Wenlock according to geochemical data were studied in the Kurtuvėnai-161 borehole (NW Lithuania) (Fig. 1). A detailed description of the graptolite zones is provided. The biostratigraphical studies enabled to establish the following graptolite biozones: spiralis-wimani and lapworthi (Upper Llandovery), centrifugus, murchisoni, riccartonensis, antenularius, flexilis, perneri, radians, lundgreni, parvus, nasa and virbalensis with praedeubeli, deubeli subzones (Wenlock) (Fig. 2). The new parvus, praedeubeli biozones and the deubeli subzone in the Kurtuvėnai-161 borehole are described as well. Trace element ratios Zr/Rb, Th/U, Sr(Zr + Th) and total organic carbon (TOC) were examined in the sedimentary rocks of the Kurtuvėnai-161 borehole (Table). The study showed that the mentioned trace element ratios could be used as indicators of the redox conditions (Th/U), grain size variation (Zr/Rb) and the siliclastics versus carbonate contents variation ((Zr + Rb)/Sr). An overall decrease in grain size according to Zr/Rb of the basin was observed from the lapworthi to flexilis times (Fig. 3). Also, in the same span of time, an increase of total organic carbon could be interpreted as an overall deepening of the basin. The more redox depositional environment in terms of Th/U data variation support this presumtion. The combination of palaeontological and geochemical results could be used not only in the further graptolite ecological investigations, but also could provide information about the varying depositional conditions in the basin.