手取層群の主要分布地域の堆積環境の復元と堆積盆地の発達

抄録

 The Tetori Group comprises significant Mesozoic (middle Jurassic–early Cretaceous) marine and terrestrial strata in East Asia. A facies analysis of the group is conducted to reveal the development of the Tetori sedimentary basin. The Tetori Group in the Mt. Hakusan Region is mainly distributed in three areas: the Kuzuryugawa Area in Fukui Prefecture and the Shiramine and Shokawa districts in the Hakusan Area in Ishikawa and Gifu prefectures. Seven lithofacies associations are recognized, which represent the deposition in talus and proximal alluvial fan, gravelly braided river and alluvial fan, sandy braided river, lacustrine delta, estuarine, shoreface, and inner shelf environments. Based on the characters and spatio-temporal distribution of these lithofacies associations across the basin in the three areas, the group is interpreted to have developed in four stages. Stage 1 is represented by the lower part of the Tetori Group in the Kuzuryugawa Area in the southern part of the basin, and shows, in ascending order, talus and proximal alluvial fans, inner shelf, shoreface, and alluvial fan facies. Stage 2 represents the lower middle part of the group in the Shokawa District in the northeastern part of the basin, and shows a change from estuarine, shoreface to inner shelf, and back to shoreface facies. Stage 3 is recognized in the middle part of the group in both the Shiramine and Shokawa districts in the northwestern and northeastern parts of the basin, respectively. Stage 3 was initially formed as talus and proximal alluvial fan, gravelly braided river and alluvial fan, and sandy braided river facies, and was later changed to lacustrine delta, sandy braided river, and gravelly braided river and alluvial fan facies, and back to lacustrine delta and sandy braided river facies in ascending order in the Shiramine District, and was initially formed as estuary and shoreface facies, and was later changed to estuary, lacustrine delta and sandy braided river facies in ascending order in the Shokawa District. Stage 4 is represented by the upper part of the group in all three areas, and shows talus and alluvial fan, gravelly braided river and alluvial fan, and sandy braided river facies. The Tetori basin reflects an upheaval of the basin forming an inter-mountain basin. This supports the hypothesis of a juxtaposition of late Jurassic to earliest Cretaceous accretionary complexes along the eastern margin of the Asia continent during the Hauterivian (Early Cretaceous).