Submarine slope–fan sedimentation in an ancient forearc related to contemporaneous magmatism: The Upper Cretaceous Izumi Group, southwestern Japan

The Izumi Group in southwestern Japan is considered to represent deposits in a forearc basin along an active volcanic arc during the late Late Cretaceous. The group consists mainly of felsic volcanic and plutonic detritus, and overlies a Lower to Upper Cretaceous plutono‐metamorphic complex (the Ryoke complex). In order to reconstruct the depositional environments and constrain the age of deposition, sedimentary facies and U–Pb dating of zircon grains in tuff were studied for a drilled core obtained from the basal part of the Izumi Group. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone‐dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone‐dominated unit; tuff unit; tuffaceous sandstone–mudstone unit; sandstone–mudstone unit; and sandstone‐dominated unit. This succession suggests that the depositional system changed from non‐volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U–Pb age (82.7 ±0.5 Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt. 後期白亜紀における活動的島弧の前縁に堆積した和泉層群は, 珪長質な火山岩や深成岩の砕屑物を主体とし, 前期〜後期白亜紀の深成変成コンプレックス(領家コンプレックス)を不整合に覆う. この和泉層群の堆積環境の復元と堆積年代の制約を目的として, 和泉層群基底部を貫くボーリングコア試料の堆積相解析と挟在する凝灰岩に含まれるジルコン粒子のU−Pb年代を測定した. コア観察による岩相組合 せに基づき, コア試料は下位より, 1) 花崗閃緑岩との不整合直上の泥岩優勢相, 2) 凝灰質泥岩相, 3) 凝灰岩相, 4) 凝灰質砂岩泥岩相, 5) 砂岩泥岩相, 6) 砂岩優勢相の6つのユニットからなる. このことは, 非火山砕屑性泥質堆積物を主体とする前弧斜面が火山砕屑性砂質堆積物を主体とする海底扇状地へと遷 移したことを示している. 周辺地域に分布する火成岩類の既存放射年代と比較すると, U−Pb年代の測定値(82.7 ±0.5 Ma)は当時の活動的な火山フロントであった山陽帯の珪長質火山岩や火山砕屑岩の放射年代と一致する. A schematic model of suggested depositional environment at the coring site. The basement rock was nonconformably covered by mass‐transport deposits on an inner forearc slope or basin floor. Increase of sediment supply from penecontemporaneous volcanic activity in the arc led to progradation of the submarine fan in the forearc basin.