北部フォッサマグナ堆積盆浅海相前進ステージにおけるデルタ〜陸棚システムと堆積シーケンス : 東頚城地域鮮新統東川累層・奈良立累層の例  [in Japanese] Dclta to shelf systems and depositional sequences of thcPliocene Higashigawa and Naradate formations, deposited during the prograding stage of the Northern Fossa Magna Basin, central Japan  [in Japanese]

北部フォッサマグナ堆積盆の北部 (東頚城地域) に分布する鮮新統東川・奈良立累層の堆積相とその累乗様式の検討を行った結果, 北東〜北北東方向へ前進するデルタ〜陸棚システムが復元されること, およびこれらの地層には13の第5次オーダーの堆積シーケンスが認められることが明らかになった。第5次オーダーの堆積シーケンスは層厚約20〜50 mで, 基底部に海進礫岩とコンデンスセクションを付随する上方浅海化サクセッションによって構成される。これらの地層は, 北部フォッサマグナ堆積盆第3ステージに発生した東頚城から西山中央油帯への浅海相の前進的堆積作用の一部をなすことから, この前進的堆積作用が相対的海水準変動の影響を受けながら進んだものと推測される。

The Pliocene Higashigawa and Naradate formations, distributed in the Naradate Syncline Area of the Higashikubiki District in the northern part of the Northern Fossa Magna Basin, central Japan, are studied, and following twenty two sedimentary facies and three facies associations are recognized on the basis of facies analysis. Facies association PL, formed of sedimentary facies PL1, PL2 and PL3, indicates delta plain environment. Facies association D, made up of sedimentary facies D1, D2, D3, D4-a, D4-b, D5-a, D5-b, D6, D7 and D8, indicates delta front environment. Facies association PD, composed of sedimentary facies PD1, PD2, PD3, PD4, PD5 and PD6, indicates prodelta to outer shelf environment. Spatial distribution of these facies associations and paleocurrent directions strongly suggest the delta to shelf systems prograded northeastward during the deposition of the formations. Thirteen 5th order depositional sequences are recognized in these strata. Each depositional sequence shows shallowing-upward succession of 20 to 50 m in thickness, accompanied with transgressive lag and condensed section at the base. This succession represents a cycle of transgression and regression caused by relative sea level changes. The analysis of stacking patterns of the 5th order depositional sequences and the spatial distribution of these facies associations results into two stages, i.e. aggradational and following progradational stages, probably corresponding to the 4th order cycles. As these delta to shelf deposits comprise a part of a larger-scale progradational unit distributed from Higashikubiki to Nishiyama/Chuo Oil Field in the Late Pliocene age, the sedimentation of the strata of this area also seems to have been strongly controlled by the relative sea level changes of various orders.