[S-GL22] Frontier research on chronostratigraphic unit boundaries

[SGL22-03] The Weissert Event record and radiometric ages of Soma Nakamura Group

*Taiga Tomaru¹, Reishi Takashima², Yuji Orihashi³, Toshiro Yamanaka⁴, Hiroshi Nishi⁵, Azumi Kuroyanagi², Yoshihiro Asahara⁶ (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.The Center for Academic Resources and Archives Tohoku University Museum , 3.Global Environment and Disaster Prevention Sciences, Graduate School of Science and Technology, Hirosaki University, 4. School of Marine Resource and Environment, Tokyo University of Marine Science and Technology, 5.Institute of Dinosaur Research , Fukui Prefectural University, 6.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University)

Keywords:Early Cretaceous, Somanakamura Group, Carbon-isotope stratigraphy

The Oceanic Anoxic Events (OAEs), frequently occurred during Cretaceous, are characterized by widespread distribution of black shale and positive δ¹³C excursion in the stratigraphic record. Major OAEs were accompanied with emplacement of Large Igneous Provinces (LIPs) before the onset of events. The Weissert Event, occurred Late Valanginian-Early Hauterivian, is represented by a prolonged δ¹³C positive excursion, and LIPs of Panana-Etendeka flood basalt erupted contemporaneously with the event. Consequently, Weissert Event was previously regarded as one of the global OAEs. However, due to limited distribution of black shale, there are no consensus about the trigger of perturbation of carbon cycle and environmental change during the Weissert Event.
Global biostratigraphic correlation of Lower Cretaceous is problematic because strengthened faunal and floral provincialism during Early Cretaceous. On the other hand, a distinct positive isotope peak in δ¹³C of the Weissert Event is useful chemostratigraphic marker for international stratigraphic correlation. The positive δ¹³C peak of Weissert Event have been reported from various sections in Europe while few studies were performed for the Weissert Event in circum-Pacific sections. In order to discover positive δ¹³C excursion of the Weissert Event in the northwest Pacific and to estimate the numerical age of the event based on radiometric dating of the tuffs, we studied stable carbon isotope stratigraphy and U-Pb dating of tuffs in the Upper Jurassic-Lower Cretaceous Somanakamura Group, exposed eastern Fukushima prefecture, Japan. We identified a prominent positive carbon isotope excursion from the uppermost formation (Koyamada Formation) of the Somanakamura Group. U-Pb zircon ages of the tuffs exhibit 135.59±0.77, 133.9±1.2, 133.22±0.60 Ma. The former age was obtained from the horizon below the positive δ¹³C peak while the latter two were from above the positive δ¹³C peak. These line of evidence suggests that the positive δ¹³C peak in the uppermost Somanakamura Group corresponds to the Weissert Event, and radiometric age of the Weissert Event accord well with the calculated astronomical age by Martinez et al., 2015.