퇴적학/층서학/고생물학/석유지질학 10월29일(금)
Provenance of the Middle Jurassic to Early Cretaceous Tetori Group, Inner Zone of Southwest Japan and its paleogeographic implication
The Tetori Group is a representative Middle Jurassic to Lower Cretaceous deposit in the northern central parts of Honshu, Japan. The Tetori Group is divided into the Kuzuryu, Itoshiro and Akaiwa subgroups with decreasing age. The basement of the Tetori Group is granite and gneissose rocks of the Hida metamorphic Belt. For the provenance study, the framework composition of sandstones and chemical composition of mudrocks were analyzed.
Tetori Group sandstones are characterized by feldspar and quartz dominance in framework composition, and are classified as arkose clan. However, Tetori Group sandstones show compositional variation in each subgroup; feldspathic in the Kuzuryu, lithic in the Itoshiro and quartzose composition in the Akaiwa subgroups. From geochemical compositions of Tetori mudrocks, source of the Tetori Group is interpreted that initially granitic gneiss/
metabasitic-mixed sources changed to granitic gneiss and sedimentary sources with time.
Granitic gneiss/ metabasitic-mixed source mainly represent basement of the Tetori Basin.
Granitic gneiss and sedimentary sources can be related with provenance interpretation of orthoquartzite-dominant type conglomerate. Orthoquartzite clasts occur from the Itoshiro to Akaiwa subgroups, in which sandstones show a increasing trend of detrital quartz contents upwards. Recent studies regard these quartzite pebbles having been derived from Precambrian rocks now distributed in East Asian continent before opening of the East Sea.
Accordingly, more quartzose and more fractionated sediment source supplied from East Asian continent, which is also supported by paleocurrent data showing flows toward the southeast or south. In addition, radiolarian chert fragments are found from pebbles of the lower conglomerate member of the Itoshiro Subgroup, in which paleocurrents were generally flowing toward north. It is believed that radiolarian chert bearing sediments were derived from the Jurassic accretionary complex of Mino-Tamba Terrane. Conclusively, possible sediment sources changed from Hida metamorphic belt, Mino-Tamba Terrane and East Asian continent with time.
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