桂根相の堆積環境  [in Japanese] Depositional environments of the Upper Miocene to Pliocene Katsurane Facies, Akita Prefecture, Northeast Japan  [in Japanese]

秋田県の陸域から海域にかけての坑井でみられる桂根相をNo. 3 G. inflata bed基底を境にして, 下部粗粒相と上部粗粒相とに分け, それらの分布を明らかにした。桂根相はそれぞれの時期に3つの地域に分かれて堆積しており, 堆積の中心ではより厚く, より粗粒な堆積物からなり, 中心から離れるにしたがい地層は薄くなり, より細粒になる。その堆積環境は底生有孔虫化石から上部漸深海から中部漸深海と考えられる。また, 古流向データは, 砕屑物が東からの側方流でもたらされ, 軸流により南北にひろがったことを示す。その堆積様式としては, トラフ充填型タービダイトの性格を強くもっている海底扇状地が考えられる。桂根相堆積時は出羽変動の主動期であり, 構造運動によって生じた古地形が桂根相の堆積に密接に関係していたものと考えられる。

The Katsurane Facies, distributed locally in coastal and offshore areas of Akita Prefecture, is a coarse clastic facies in the Lower Tentokuji Formation and one of the main targets for petroleum exploration. A real extent of the Katsurane Facies in the subsurface has been determined on the basis of well data using planktonic foraminiferal fossils as datum levels. The Katsurane Facies is divided into "lower and upper sequences" based on changes of lithofacies and seismic profile patterns. The boundary of both sequences is defined as the base of No. 3 Globorotalia in flatabed. Three submarine fans in each sequence has been reconstructed on the basis of lithology and thickness. Assemblages of benthonic foraminifera indicate that the Katsurane Facies was formed in a middle to upper bathyal environment with a regression stage. Paleocurrent data indicate that the coarse clastics were transported predominantly from the east by latteral currents and extended to the north and south by longitudinal currents. The turbidites were deposited in N-S trending basin troughs generated by tectonic movement related to the Dewa Tectonic Disturbance. In the Akita offshore region, the "lower sequence" was deposited in a trough on the western flank of the Akitaoki-Hokubu Structure, while the "upper sequence" was deposited in a trough on the eastern flank.