The Toyora Group is one of the Mesozoic strata in Japan, and was originally named Toyoura Series (or Formation) by Hisakatsu Yabe in 1920.Yabe H., 1920, On the so-called Mesozoic formations in the Kiku Peninsula, Buzen. Jour. Geogr., 32, pp.513-520. (in Japanese) The present name was defined by Tatsuro Matsumoto in 1949.Matsumoto, T., 1949, The Late Mesozoic geological history in the Nagato Province, southwest Japan, Japan. Jour. Geol. Geogr., 21, pp. 235-243.
It is the Lower-Upper Jurassic sediments in the East Asian continental margin that distributes in the eastern part of the Shimonoseki, Yamaguchi Prefecture, southwest Japan. The distribution of the Toyora Group extends north and south, and has been separated between north and south districts by the Tabe Basin and the Kikugawa Fault that is an active left-lateral strike-slip fault.Arkell, 1956, Japan and Korea. Jurassic Geology of the World. Oliver&Boyd, London and Edinburgh, pp. 418-430. Sato, T., 1967, Jurassic-Geological history of Jurassic Period in Japan. Historical geology, 2nd volume. Asakura Publ., Tokyo, pp. 386-407. (in Japanese) Kiminami, K., 2009, Jurassic System-Toyora Group. In Geological Society of Japan ed., Regional Geology of Japan 6-Chugoku district. Asakura Publ., Tokyo, pp. 95-97. (in Japanese)
Geology The Toyora Group is thick, and divided into the Higashinagano, Nishinakayama, Utano, and Ohchi Formations in ascending order. The group rests unconformably on the tilted Paleozoic Renge Metamorphic Rocks and Toyohigashi Group, formations assigned to the Akiyoshi Belt. The Toyora Group is separated by a parallel or locally angular unconformity with the Latest Jurassic to Early Cretaceous Toyonishi Group.
The Toyora Group sediments are composed mainly of black mudstone, sandy mudstone, sandstone, conglomerate that deposited in a shallow marine embayment.Kawamura, H., 2010, Stratigraphic revision of the Jurassic Toyora Group of the southern part of the Tabe basin, Yamaguchi Prefecture, southwest Japan. Jour. Geol. Soc. Japan, 116, pp. 27-44. (in Japanese with English abstract) doi: 10.5575/geosoc.116.27 The mudstone and very fine-grained sandstone beds are often bioturbated by an ichnogenus Phycosiphon.
See also Toarcian turnover Toarcian formations Fernie Formation, Alberta and British Columbia Whiteaves Formation, British Columbia Navajo Sandstone, Utah Whitby Mudstone, England Posidonia Shale, Lagerstätte in Germany Ciechocinek Formation, Germany and Poland Lava Formation, Lithuania Marne di Monte Serrone, Italy Calcare di Sogno Toundoute Continental Series, North Africa Los Molles Formation, Argentina Mawson Formation, Antarctica Kandreho Formation, Madagascar Kota Formation, India Cattamarra Coal Measures, Australia
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