Depositional Ages, Sequence Stratigraphy and Transition Process of Forearc Setting From Paleogene Restricted Bay/Estuarine to Neogene Open‐Marine Deltaic/Slope Systems in the Sanriku‐Oki Forearc Basin, Northeast Japan

ABSTRACT This paper aims to discuss the transition process of the forearc basin setting along the Northeast Japan arc, based on the results of strontium isotope dating, resistivity image facies analysis, sequence stratigraphic and depositional system interpretation, and seismic facies mapping, mainly using the Site C0020 succession data of the Integrated Ocean Drilling Program (IODP) Expedition 337, off Shimokita Peninsula and surrounding seismic sections. The detailed correlations and strontium isotope ages constrain the geologic ages of Units II, III and IV of the Site C0020 succession as Eocene to Early Miocene. Cores and resistivity image logs show that Units II, III and IV consist of five facies associations, indicating bay, estuarine to fluvial, delta and muddy slope systems and eleven depositional sequences. Plot mapping of these facies associations and seismic facies indicates the drastic changes of the forearc basin setting through four tectonic phases from Eocene to Miocene. During Phase 1 (Eocene to Early Oligocene: Unit IV), the bay‐to‐estuarine system was dominant within a restricted forearc basin by a subaerially uplifted trench slope break. Phase 2 (Early to Late Oligocene: Unit III and the lowermost Unit II) was characterised by further uplift and erosion of the trench slope break, which formed three Oligocene unconformities: Ounc1, Ounc2 and Ounc3. During Phase 3 (Late Oligocene to Early Miocene: Unit II), a large‐scale subsidence of the trench slope break started, possibly related to the onset of tectonic erosion of the subducting plate, and the forearc basin became an open‐marine setting with a prograding delta system. After the formation of Miocene unconformity (Munc), Phase 4 (Middle Miocene‐: Unit I) caused the cessation of the delta system, and the forearc basin became a muddy deep‐water slope system, possibly resulting from the continent‐derived sediment supply decrease due to the backarc opening of the Sea of Japan.