宮城県登米地域の稲井層群の層序 : とくに大沢層の海底地辷り堆積物について

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The Triassic Inai Group in the Toyoma area, is divided into the following four formations in ascending order as in the type area. The Hiraiso Formation consists of a thick conglomerate and sandstone in the lower part, and an alternation of calcareous sandstone and mudstone in the upper part. Along the western margin of the area, the basal part of the formation is intercalated with thick bed of reddish-purple tuffaceous shale and sandstone, and thinning toward northeast. This formation unconformably overlies the Upper Permian Toyoma Formation. The Osawa Formation consists mainly of laminated shale with thin sandstone, and yields fauna of ammonoids, pelecypods, brachiopods, plants and reptiles. These ammonoids show the range from the Columbitan to Prohungaritan of SPATH (1930, 1934). In the western area, submarine slide deposits locally develop in the lower part of the formation. Such coarse elastics as conglomerate and sandstone with penecontemporaneous shale chips, are overlain by the slide deposits consist of rather fine material. Controversial intraformational conglomerate is, in my opinion, belonging to the lower part of the Osawa Formation, and is closely associated with the said submarine slide deposits. The Fukkoshi Formation consists of thickly bedded coarse-grained sandstone intercalated with thin shale and lenticular pebbly conglomerate. The Isatomae Formation consists of a thin alternation of shale and calcareous sandstone, sometimes intercalated with massively bedded shale and thick bedded sandstone. The shale in this formation is characterised by bioturbation. Paleocurrent data and lithologic facies change show that the deposition of Inai Group might have been initiated by sediment supply from the west into shallow marine environment. The basin became a little deeper during the deposition of the Osawa Formation. Coarse-grained sediments in the formation were supplied from the northwest and transported locally by the sediment-gravity flows. Submarine slide deposits are characterized by slump overfolds and dislocated blocks with welded contact to the adjacent rocks. Based on analyses of slump fold and welded contact submarine slide occurred on slopes inclined towards the southeast. These reconstructed paleoslopes are consistent with the paleocurrent data. The slide deposits were closely associated with sandstone and conglomerate directly underneath the deposits. The writer concludes that submarine slide was initiated by the mobilized coarse-grained sediments and fluidization of sand size sediments which are both underlying the slide mass and both locally transported from the source area during the deposition of lower to middle Osawa Formation.
1983-05-25

著者

鎌田耕太郎
北海道大学理学部地質学鉱物学教室

宮城県登米地域の稲井層群の層序 : とくに大沢層の海底地辷り堆積物について

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