[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-P04] Integrated stratigraphy of the lower Cretaceous (Berriasian–Valanginian–Hauterivian) sequences in the Northwest Panthalassa, Northeast Japan

*Reishi Takashima¹, Takumi Otsubo¹, Taiga Tomaru¹, Masayuki Ehiro¹, Toshiro Yamanaka², Yuji Orihashi³, Yuki Asahara⁴, Azumi Kuroyanagi¹, Hiroshi Nishi⁵ (1.Tohoku University, 2.Tokyo University of Marine Science and Technology, 3.Hirosaki University, 4.Nagoya University, 5.Fukui Prefectural University)

Keywords:Karakuwa Group, Oshima Group, Lower Cretaceous, Kitakami Massif, Valanginian, Weissert Event

The Valanginian period is characterized by the oldest LIPs emplacement and carbon cycle perturbation events in the Cretaceous. In recent years, the GSSPs for the Valanginian and Hauterivian stages have been ratified, and stratigraphic and palaeoenvironmental studies of the Valanginian are becoming more active. On the other hand, there is little paleoceanographic information on the Early Cretaceous Panthalassa which was the largest ocean at the time. The Berriasian–Hauterivian shallow-marine to hemipelagic terrigenous sequences are sporadically exposed in Northeast Japan. These sequences were accumulated in the active Eurasian continental margin in the northwestern Panthalassa Ocean. Many biostratigraphic studies have long been conducted for these sequences based on ammonoid and radiolarian fossils, while no calcareous nannofossils and calpionellids have ever been reported. For ammonoids, these sequences do not yield the same species as the index fossils in the Tethyan region, although endemic species of the same genus with the Tethyan index fossils sporadically occur. In this context, it is difficult to make detailed international stratigraphic correlations between Tethyan sections including GSSPs and Japan, and to identify the individual stage boundaries in the Berriasian–Hauterivian sequences in Japan.
In this study, we have attempted detailed international stratigraphic correlation based on zircon U-Pb ages of tuff as well as carbon isotopic stratigraphy for the Lower Cretaceous of the Karakuwa and Oshima Groups exposed in Northeast Japan. As a result, the Berriasian/Valanginian boundary and the Weissert Event were successfully identified at the base and middle part of the Isokusa Formation, respectively. In addition, an approximate stratigraphic level for the Valanginian/Hauterivian boundary was recognized, although it was not conclusive. The Weissert Event interval of the studied sequences does not show any significant lithological changes, such as the intercalation of black shales as well as decrease in bioturbattion. On the other hand, lithological change reflected regional tectonics of large-scale sinistral strike-slip movement and subsequent ridge subduction along the active continental margin of Eurasia.