北部フォッサマグナ小谷-笹ケ峰地域の新第三系堆積岩類の後背地と堆積盆地  [in Japanese] Provenance and basin morphology of Neogene sediments in the Otari-Sasagamine area, northern Fossa Magna, Japan  [in Japanese]

北部フォッサマグナ西縁部に位置する小谷-笹ケ峰地域には, 前沢層・真川層・乙見山層などの新第三系が分布する。南西部の小谷地域と北東部の笹ケ峰地域とでは, 北〜北北西向きの古斜面, 南〜南西からの古流向を示す別々のトラフが存在していたと考えられる。砂岩のモード組成および乙見山層の礫組成からは, 両地域とも花崗岩類が伴う古期堆積岩類が一つの主要な供給源であったことを示す。砂岩に含まれる重鉱物は, 青色角閃石が高圧型変成岩起源, 単斜輝石が斑レイ岩起源と考えられる。ザクロ石は小谷地域では高圧型変成帯および高温型変成帯起源, 笹ケ峰地域ではおもに高温型変成帯起源と考えられる。以上のことから, 小谷地域の砕屑岩は飛騨帯・飛騨外縁帯・美濃帯から, 笹ケ峰地域では美濃帯・領家帯・三波川帯からの供給が考えられる。従って, 本地域の二つのトラフは近接しているが, 異なる供給系から運ばれた堆積物からなることが示される。

The Otari-Sasagamine area is located in the westernmost part of the northern Fossa Magna in the central part of Honshu, Japan. The Miocene Maezawa, Shinkawa, and Otomiyama Formations in the area were studied to clarify the petroprovenance and the Miocene basin morphology. The Neogene system of the area is composed mainly of turbidites and their associated deposits. The distribution of thick-bedded sandstones suggest that the area is divided in to the Otari area to the southwest and the Sasagamine area to the northeast. The paleoslope and paleocurrents indicate that most of the sediments were supplied by lateral currents from the south and southwest. Petrographic studies of sandstones and conglomerates of the area were made mainly by examining heavy minerals such as zircon, blue amphibole, clino-pyroxine, and garnet. As a result, the petroprovenance of the sediments in the Otari area were found to be the Hida, Hida Marginal and Mino Belts to the west and southwest, and those of the Sasagamine area to be the Mino, Ryoke, and Sanbagawa Belts to the southwest. It is concluded that during the Miocene there were two separated troughs both elongate from northeast to southwest, and the sediments of the troughs were supplied from the different provenances.