Fluvial to bay sequence stratigraphy and seismic facies of the Cretaceous to Paleogene successions in the MITI Sanriku‐oki well and the vicinities, the Sanriku‐oki forearc basin, northeast Japan

This paper describes the significant depositional setting information derived from well and seismic survey data for the Upper Cretaceous to Lower Eocene forearc basin sediments in the central part of the Sanriku‐oki basin, which is regarded as a key area for elucidating the plate tectonic history of the Northeast Japan Arc. According to the results of well facies analysis utilizing cores, well logs and borehole images, the major depositional environments were of braided and meandering fluvial environments with sporadically intercalated marine incursion beds. Seismic facies, reflection terminations and isopach information provide the actual spatial distributions of fluvial channel zones flowing in a north–south trending direction. The transgression and regression cycles indicate that the Upper Cretaceous to Lower Eocene successions can be divided into thirteen depositional sequences (Sequences SrCr‐0 to SrCr‐5, and SrPg‐1 to SrPg‐7). These depositional sequences demonstrate three types of stacking patterns: Types A to C, each of which shows a succession mainly comprising a meandering fluvial system, a braided fluvial system with minor meandering aspects in the upper part, and major marine incursion beds in the middle part, respectively, although all show an overall transgressive to regressive succession. The Type C marine incursion beds characteristically comprise bay center and tidal‐dominated bay margin facies. Basin‐transecting long seismic sections demonstrate a roll up structure on the trench slope break (TSB) side of the basin. These facts suggest that during the Cretaceous to Eocene periods, the studied fluvial‐dominated forearc basin was sheltered by the uplifted TSB. The selective occurrences of the Type C sequences suggest that when a longer‐scale transgression occurred, especially in Santonian and early Campanian periods, a large bay basin was developed, creating accommodation space, which induced the deposition of the Cretaceous Kuji Group along the arc‐side basin margin. 三陸沖前弧堆積盆中央部で掘削された基礎試錐「三陸沖」の坑井データ(コア, 検層ログ, 比抵抗イメージ)および周辺部において取得された三次元地震探査データ(震探相, 反射波列端)の堆積学的・シーケンス層序学的解析の結果, 三陸沖堆積盆の上部白亜系〜下部始新統はおもに網状〜挟炭蛇行河川システムからなること, 堆積盆中央部では河川流路がおおよそ南北方向に伸長すること, 合計13の堆積シーケンスに区分され, 最大海氾濫面層準付近では内湾環境の泥岩が挟在することが明らかになった. 白亜紀〜始新世の三陸沖前弧堆積盆は外縁隆起帯がリッジを造る河川システム主体の堆積盆であったと考えられ, 大規模海進時には全体が内湾環境になった. この海進時には火山弧側堆積盆縁辺部にも堆積空間が生成され, 久慈層群の浅海〜河川相が堆積したと考えられる. Fluvial sequence stratigraphy and seismic facies were analysed for the MITI Sanriku‐oki well and the vicinity in the