Sedimentological Study of the Izumi Group in the Northern Part of Shikoku, Japan
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概要
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The Upper Cretaceous Izumi Group consists of extremely thick sedimentary piles. This group in the Asan Mountain Range, Shikoku has been investigated and its sedimentary environment and sedimentary history are inferred. The Izumi Group in the studied area consists of six formations; the Shiroyama, Hiketa, Takikubo, Horita, Higaidani and Bandodani Formations. The Shiroyama Formation consists of shallow marine deposits and the Hiketa Formation consists of hemipelagic shale. The other four formations are composed of alternating beds of sandstone and shale. Sandstone in the alternating beds is inferred to be turbidites. In this study, vertical and lateral changes of thickness, grain size and sedimentary structure of sandstone beds have been examined. As a result, three kinds of sedimentary cycles are recognized. Thirty-three sedimentary cycles of medium-scale are recognized in the studied area through studies of a vertical transition in the values of entropy and average thickness of sandstone beds. The controlling factor of these sedimentary cycles is thought to be fan progradation and tectonic events. Large-scale sedimentary cycles are recognized after examination of the frequency distribution of thickness of sandstone beds in each medium-scale cycle. Four large-scale cycles are distinguished in the studied area. Large-scale cycles are thought to reflect changes of both the amount of clastics supplied from the source area and the time interval of turbidity currents. Small-scale sedimentary cycles, 20-30 m in thickness, may have been caused by a shift of fan lobe. Radiolarian and foraminiferal fossils have been obtained from shale and calcareous concretions in alternating beds. The Izumi Group in the studied area may be middle or late Campanian to early Maastrichtian in age. The maximum water depth of the sedimentary basin is assumed to have been an intermediate depth (upper bathyal) on the basis of the abundance and composition of radiolarian genera. The facts that more than half of the foraminiferal fossils are planktonic form and abundant microfossils are found suggest the Izumi Group was deposited under open seas.Izumi Groupsedimentary cycleturbiditesedimentary history
- 1986-02-15
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