9:45 AM - 10:00 AM
[SGL28-04] Late Cretaceous sea-level changes and tectonic history recorded in the forearc sedimentary successions of the Northeast Japan
Keywords:Late Cretaceous, Northeast Japan, forearc basin, Kuji Group, Sequence stratigraphy, tectonics
The Kuji Group is distributed along the northeastern coastal area of the Iwate prefecture. The group mainly comprises sandy siltstone, coaly mudstone, sandstone and conglomerate, with intercalated coal and felsic tuff beds. The total thickness is over 700 m. It unconformably overlies the pre-Cretaceous accretionary complex and lower Cretaceous granitoids, and is unconformably overlain by the Paleogene siliciclastic succession. The Kuji Group is divided into three lithostratigraphic units: the Tamagawa (marine and non-marine), Kunitan (marine), and Sawayama (non-marine) formations, in ascending order. The overall succession consists of at least four largely upward-fining sequences. Previous studies on sedimentary facies analysis divided them into five depositional sequences derived from the third-order sea level change (DS1 to DS5) (Ref. 1).
The newly established age model, based on carbon isotope stratigraphy and U−Pb ages, indicates that the depositional sequences in the Kuji Group is correlated with the middle Turonian to middle Campanian stages. The DS2 to DS4 in the Kuji group are likely coeval with the depositional sequences of the adjacent Futaba Group (the Coniacian to Santonian stages) (Ref. 2). Relative sea-level changes recognized in the DS1 to DS4 are mostly consistent with the second- and third-order global eustatcy (Ref. 3). In contrast, the DS5, equivalent to the Sawayama Formation of the Kuji Group, represents lowstand systems tract (non-marine fluvial facies) with maximum thickness of 200 m. Similarly, development of shallower facies is implied in the Hakobuchi Formation (the Campanian stage; Ref. 4) of the Yezo Group, accumulated in the northern sub-basin. These coeval relative sea-level falls are apparently inconsistent with contemporaneous global transgressive trend, and thus are interpreted to reflect basin-wide uplift of the forearc region in the early Campanian. Here we provide a new constraint for the age of the major tectonic event occurred in the Late Cretaceous East Asian continental margin, possibly related to the change in subducting oceanic plate motion (e.g. Ref. 5).
*1. Mitsuzuka, S., Ando, H., 2016, 123th Ann. Meet. Geol. Soc. Japan, Abstr., 205. *2. Ando, H. et al., 1995, Jour. Tokyo Geogr. Soc. 104, 284–303. *3. Haq, B., 2014, Glob. Planet. Change 113, 44–58. *4. Tsujino, Y., 2009, Jour. Geol. Soc. Japan 115, 122–129. *5. Ando, H., 2003, Jour. Asian Earth Sci. 21, 921–935.