長野県北部に分布する北部フォッサマグナ中新統砂岩の後背地と分散システム  [in Japanese] Provenance and dispersal system of the Miocene sandstones in the Northern Fossa Magna, Nagano Prefecture, central Japan  [in Japanese]

北部フォサマグナに南北に雁行配列する込地向斜・高府向斜・日影向斜の各地域から代表的なルートを選定し, 岩相の記載, 砂岩のモード組成分析と砕屑性ガーネットの化学組成分析をおこなった.これらと既存の古流向, 礫組成資料, 堆積相解析による堆積システムの復元などと合わせて考察した.その結果, 中期中新世のこの堆積盆地には, 西方ないし南西方の宇奈月帯・飛騨帯・飛騨外縁帯・濃飛流紋岩類分布域・美濃帯と南方の領家帯・三波川帯を後背地として4つの分散システムによって砕屑物がもたらされていた.後期中新世に入ると南部の込地向斜地域から高府向斜地域にファンデルタが前進し, 南方ないし南西方の濃飛流紋岩類分布域・美濃帯・領家帯・三波川帯を後背地とする砕屑物により, 堆積盆地は埋積されていった.一方, 西方の濃飛流紋岩類分布地域や飛騨帯, 飛騨外縁帯なども中新世をとおして堆積物を供給し続けた.

The purpose of this study is to clarify the provenance of the clastic materials and the dispersal systems by the combination of sandstone petrology and sedimentary facies analysis. The Middle to Upper Miocene sandstones in the southern part of the northern Fossa Magna region can be classified into four types, based on their modal composition. The provenance of sandstones of each type is inferred from the sandstone petrology and chemical composition of detrital garnets. In addition, paleocurrents data and compositions of conglomerated also are referred to inference of the provenance.The dispersal systems within the basin are illustrated from the comprehensive analysis of distribution of four types sandstones and depositional systems which have been studied recently. During Middle Miocene time (Aoki stage), the clastic materials in the Hikage Syncline area were derived from the west provenance such as Unazuki, Hida and Hida Marginal Belts, and from the southwest such as Mino Belt and Cretaceous to Paleogene igneous rocks (Nohi-type rhyolites). In the northern part of Takafu Syncline area, the sediments were also supplied from the west and southwest. The materials of the sandstones in the southern part of Takafu Syncline area were derived from the southwest and south provenance such as Mino Belt, Nohi-type ryolites and Ryoke Belt. In the Komiji Syncline area where the southern end of the sedimantary basin, sediments were supplied from south provenance such as Nohi-type ryolites, Mino Belt, Ryoke Belt and Sambagawa Belt. The sediments were supplied to the sedimentary basin by four different dispersal systems in this stage. While during Late Miocene time (Ogawa stage), in the Komiji and Takafu Syncline areas, sediments were transported mainly from south by the northward progradation of fan-delta system. In the northern part of Takafu Syncline, however, some sediments were supplied from the western and southwestern source continuously.