1989 Volume 43 Issue 6 Pages 428-442
The Median Tectonic Line borders between the south foot of the Asan ranges and the Yoshinogawa valley in northeast Shikoku. A new description is given on the stratigraphy and geological age of the deposits in the Yoshinogawa valley and in the north foot of the Asan ranges. The newly defined Moriyama formation in the southside of Yoshinogawa valley floor includes the tuff layer 2.3±Ma Fission truck age in the lower part and the coarse gravel from Asan ranges of Izumi Group (the Cretaceous sedimentary rocks) that suggests the north wall of the tectonic valley had developped by the Late Pliocene. The schist gravels in the Plio-Pleistocene Mitoyo Group in the north foot of the Asan ranges have been belived to be derived from the Outer zone mountains south of the MTL before the time of uplift of the Asan ranges and the depression of Yoshinogawa valley. But our detailed lithologic analysis suggests that the schist gravels in Mitoyo formation originated from the west of the area along the Setouchi inland sea, because the arkose sand matrix and the small number of granite gravel were also included in Mitoyo Group in the foot of Asan ranges. The granitic materials might be from the Ryoke zone in the north. The FT dating made possible the correlation between the Mitoyo Group (Late Pliocene to Early Pleistocene by plant pollen fossil and FT age of tuff) in the north of the Asan range and the Moriyama formation in the south of the Asan ranges. The result suggests that the Asan ranges had already uplifted during late Pliocene. The same paleo-geomorphologic condition is presumed in the west part of Izumi ranges where the Plio-Pleistocene Osaka Group and Shobudani Group deposited. The thick fanglomerate called as "the Dochu Gravel Bed" develops in the north side of the Yoshinogawa valley. The age of the "Dochu Gravel Bed" have been thought to be the last glacial age based on about-30000-y.B.P. radiocarbon age. But the tuff layers 1± to 0.5±Ma FT age and the horizons of various pollen assemblage were newly found, and the new data indicate the age of Dochu Gravel Bed is the late of Early Pleistocene to middle Pleistocene. The revised stratigraphy suggests that the middle to later Quaternary tectonic movement may have been noticed too excessively to explain the differentiation of the Asan ranges and the Yoshinogawa valley. The occurence of Dochu Gravel Bed Group and some faults cutting and up-thrusting to the Dochu Gravel Bed suggests the tectonic movement of MTL had continued during middle and late Quaternary.
