Carbon isotope stratigraphy for regional correlation of the Upper Jurassic–Lower Cretaceous Tetori Group in Japan

Regional correlation of non-marine strata is challenging despite its crucial importance in understanding terrestrial environmental transitions. This study applied carbon isotope stratigraphy for preliminary regional correlation of the Tetori Group, an Upper Jurassic to Lower Cretaceous shallow marine and non-marine siliciclastic succession in west-central Japan. To strengthen credibility for this correlation, new U–Pb zircon dating on a tuff from the Itsuki Formation provides a weighted average date of 128.8 ± 1.4 Ma (Hauterivian). Three separate samples were collected from several stratigraphic horizons within the Tetori Group. Their organic carbon isotope ratio (δ¹³C_org) was determined to assess the horizontal variation of the values. The stratigraphic fluctuation of organic carbon within the Tamodani and Itoshirogawa logging road sections ranged within ca. −25.5 to −19.5 ‰. A notable character in the obtained δ¹³C_org curves is the large values exceeding ca. −22 to −20 ‰ in the upper parts of both sections, coupled with underlying low values of ca. −25 ‰ in the middle parts of both sections. The Hauterivian long-term negative excursion and the positive excursion associated with OAE1a in the Aptian are plausible events potentially correlated to these sections. These isotopic curves were compared with those from other regions of Asia, and it is noted that the −20 to −22 ‰ positive excursion related to the OAE1a can be correlated directly with other Asian sections. This study serves as a benchmark, showcasing the potential for regional correlation within the Tetori Group using carbon isotope stratigraphy.