中央北海道北部羽幌地域の新第三系の堆積環境とシークェンス層序学  [in Japanese] Sedimentary environments and sequence stratigraphy of the Neogene System in the Haboro area, northern part of the central Hokkaido, Japan  [in Japanese]

羽幌地域の中部中新統から鮮新統は, 築別層・古丹別層・チェポツナイ層(相当層金駒内層・東野層)・遠別層・茂築別層からなる。築別層は, 谷埋め堆積物である河川成層から始まって浅海成層を経て海盆の堆積物に変化し, 急速な海進と堆積盆の深化を示す。 古丹別層は深化した海盆に粗粒タービダイトが堆積することによって形成された。さらに, 堆積盆が埋積されることによってファンデルタ環境へ変化した。このように中期中新世には, 日本海の拡大と関連して急速な堆積盆の沈降とその埋積が起こった。一方, チェポツナイ層以上の地層は, いずれも陸棚から沿岸で堆積したと考えられ, その堆積相の変化は, 汎世界的海水準変動と調和的である。したがって, 中期中新世後期には堆積盆の急速な沈降はほぼ終了し, 海水準変動が堆積相とその累重関係を決定していたと考えられる。このような変動域の堆積盆の解析にもシークェンス層序学の枠組みが適応できる。

The Middle Miocene to Pliocene sedimentary succession in the Haboro area of northern Hokkaido are divided into the Chikubetsu, Kotambetsu, Chepotsunai, Embetsu and Mochikubetsu formations in ascending order. The incised valley fill sediments of the Chikubetsu Formation which consist of fluvial deposits formed before the succeeding transgression overlies the Early Miocene Sankebetsu Formation unconformably in several places. The shallow marine deposits of the Chikubetsu Formation lie on these sediments and grades upward into deep marine mudstones. This succession suggests a rapid transgression foIlowed by basin subsidence. The coarse-grained turbidite of the Kotambetsu Formation was formed after the basin subsidence. The basin became shallower, resulting in a fan-delta front at the depositional time of the upper part of Kotambetsu Formation. These Middle Miocene events are considered to have a relation with the opening of the Japan Sea. The sedimentation of the late Middle Miocene to Pliocene formations took place in a shelf or near shore condition. The stacking pattern of sedimentary systems during this time appears to have been controlled by third order eustatic changes whose duration ranges several million years. The framework of sequence stratigraphy can be applied to the basin analysis of this mobile belt.