APPLICATION OF CARBON ISOTOPE STRATIGRAPHY FOR REGIONAL CORRELATION OF NON-MARINE JURASSIC-CRETACEOUS TETORI GROUP IN JAPAN

Regional correlation of non-marine strata is much challenging comparing with marine one due to lithological lateral variations and the absence of reliable index fossils despite its crucial importance to understand terrestrial environmental transitions. In this study, we applied stable carbon isotope (δ¹³C) stratigraphy for preliminary regional correlation of the Tetori Group, a Late Jurassic to Early Cretaceous shallow marine and non-marine siliciclastic succession in Japan. Three sections of the Tetori Group were examined: Tamodani (26 horizons), Shokawa (47 horizons), and Itoshirogawa Logging Road (13 horizons).

δ¹³C curve was constructed for each section. Three samples per horizon were collected and analyzed from the Tamodani and Itoshirogawa Logging Road sections to realize horizontal variation in the δ¹³C values. The stratigraphic fluctuation of δ¹³C in the Tamodani section ranged within ~3.8‰, while the Shokawa section exhibited ~2.5‰. The Itoshirogawa Logging Road section displayed a fluctuation range as large as ~4.0‰. Majority of the values fell within the range between -23‰ and -24‰ for each section.

A notable character in δ¹³C curves was the high values exceeding -22‰ in the upper parts of the Tamodani and Itoshirogawa Logging Road sections coupled with underlying low values of -25‰ in the middle parts of the Tamodani and Itoshirogawa Logging Road sections. The upper part of the Shokawa section also showed similar low values.

After this study, we learned following issues are fundamental for further study of non-marine sequence including correlation with δ¹³C curve: (1) Collecting at least three samples per horizon to evaluate horizontal variation and track stratigraphic fluctuation of δ¹³C values; (2) Selecting multiple sections encompassing longer time-stratigraphic interval would be beneficial for capturing global δ¹³C trends; (3) Identifying conspicuous δ¹³C events as correlation markers would be crucial. We perceived the importance of information from other correlation techniques (i.e. radiometric dating and paleomagnetism) for international correlation and age identification. The application of δ¹³C stratigraphy to non-marine sequences in other regions, particularly continental East Asia, is eagerly awaited for regional and global correlation.