[S-GL19] Frontier research on chronostratigraphic unit boundaries

[SGL19-P01] Correlation of the Lower Cretaceous Weissert Event in NW Pacific.

*Taiga Tomaru¹, Reishi Takashima², Yuji Orihashi³, Toshiro Yamanaka⁴, Hisao Ando⁵, Yoshihiro Asahara⁶, Hiroshi Nishi⁷, Azumi Kuroyanagi² (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.Department of Earth Science, Graduate School of Science and Engineering, Ibaraki University, 6.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, 7.Institute of Dinosaur Research , Fukui Prefectural University)

Keywords:Soma-Nakamura Group, Lower Cretaceous, Carbon-isotope stratigraphy, U-Pb radiometric age, Organic elemental analysis

The Cretaceous period is characterized by frequent occurrence of carbon isotope perturbation event, such as Ocean Anoxic Events (OAEs). The Weissert Event, occurred during Late Valanginian-Early Hauterivian, has been regarded as the oldest carbon perturbation event during Cretaceous (Erba et al., 2004). Although this event is marked by a pronounced positive excursion of δ13C, the strata of this event lacks black shales in Tethys, Boreal and Atlantic areas, which is entirely different character from other OAEs. At present, black shales of this event have been reported from only two sections of the Central Pacific and Weddell Sea (Erba et al., 2004; Cavalheiro et.al., 2021). Several studies claim that positive carbon isotope excursion during this event was not caused by expansion of anoxic water but world-wide deposition of coal on continents (Westermann et al., 2010). Therefore, it is important to clarify the distribution of black shale in Panthalassa because it was the largest ocean during the Early Cretaceous period. Regardless of its importance, little is known about the paleo-oceanographic conditions in Panthalassa during the Weissert Event. In the viewpoint of Early Cretaceous age model, no radiometric ages have been obtained from the Valanginian strata because no tuffs are interbedded in Tethys and Atlantic sections (Litter et al., 2011; Martinez et al., 2023).
The Koyamada Formation of the Soma-Nakamura Group, exposed in the Pacific coastal region of Fukushima, is a continuous Lower Cretaceous terrigenous sequence accumulated on the Asian continental margin in northwest Panthalassa. In order to elucidate paleo-environmental changes in NW Panthalassa during the Weissert Event and radiometric ages of the Weissert Event, we studied zircon U-Pb radiometric ages of tuff and carbon isotope stratigraphy and CHNS analysis of mudstone in the Koyamada Formation. Our result shows a pronounced (2‰) positive carbon isotope excursion, correlative with the Weissert Event, was identified in the middle to upper part of the Koyamada Formation. The intercalated tuff ages in the lower, middle, and upper part of the Koyamada Formation exhibit 135.6±0.8 Ma, 134.6±0.9 Ma, 133.9±1.2 Ma,133.3±1.8 Ma, respectively. These results are overall consistent with the recently proposed astronomical age model of the Weissert Event, although a slight difference of 60 k.y. is identified around the first build-up phase of the event. In addition, our CHNS analysis shows that oxic environment prevailed throughout the Weissert Event in the Northwest Panthallasa.

Reference
Erba et al., 2004, Geology, 32.2, 149-152.
Westermann et al., 2010, EPSL, 290(1-2), 118-131.
Martinez et al, 2023, Earth-Science Reviews, 104356.