北部北上山地,上部白亜系-古第三系種市層の砕屑岩組成と堆積相  [in Japanese] Composition of clastics and sedimentary facies of the Upper Cretaceous Paleogene Taneichi Formation, northern Kitakami Mountains  [in Japanese]

種市層は, 北部三陸海岸に南北15 km, 東西1 kmにわたって分布する。本層は, 主に細粒〜粗粒砂岩からなり, 礫岩を伴っている。砂岩は, チャート, ホルンフェルス, 火山岩類, 石英および長石などの粒子で構成されている。礫岩は, チャート, ホルンフェルス, 花崗岩および火山岩類等の礫からなる。斜層理の測定から推定された古流向は, N→S, W→E, S→Nが優勢である。種市層下部層と中部層の主要な後背地は, 堆積盆地西方の北上山地である。一方, 種市層上部層の堆積物は, 北側の火山岩類からなる陸地より供給された。種市層から, 7つの堆積相が区分され, そのうちの1つは2つの亜相に分けられた。これらの堆積相から推定される堆積環境は次のようである。種市層下部層の堆積は, 河川・沼沢環境から始まり, 中部層堆積期には海進を被り, 後浜〜下部外浜環境下となった。海退後, 削剥期を経て上部層堆積期には河川〜上部外浜環境へと変遷した。

The Taneichi Formation is exposed along the northern Sanriku Coast, extending about 15 km in a north-south direction and 1 km to the east-west. The formation is divided into lower, middle and upper members. Fossil evidence indicates that the middle member and the upper member are middle Santonian and Paleogene in age, respectively. The middle member is inferred to be overlain unconformably by the upper member. The Taneichi Formation comprises fine to coarse grained sandstone associated with conglomerate, mudstone and tuff. The sandstone is composed mainly of fragments of chert, hornfels, volcanic rock, quartz and feldspar. The conglomerate is characterized by fragments of chert, hornfels, granite and igneous rocks. One hundred and fifty measurements made on diagonal bedding in the Taneichi Formation indicate N→S, W→E, S→N paleocurrent directions. The main source area for the lower and middle members of the Taneichi Formation was the Kitakami Mountains to the west of the sedimentary basin. On the other hand, the sediments of the upper member of the Taneichi Formation were derived from a land constructed of volcanic rocks to the north of the sedimentary basin. Seven sedimentary facies, one of which includes two sub-facies, are recognized in the surveyed area. Late Cretaceous sedimentation of the Taneichi Formation commenced in fluvial and swamp environments, then changed into backshore to shoreface environments due to the Urakawa transgression. The upper member of the Taneichi Formation has accumulated as fluvial to shoreface deposits following an erosional stage.