On the Atokura Conglomerate in the Shimonita District, Gumma Prefecture

The Atokura conglomerate has long been discussed whether it is sedimentary or igneous origin. That is, in some parts, so severely deformed and altered that some authors have regarded as igneous rocks. The occurrence of the conglomerate and the associated rocks has also been the matter of keen interest among Japanese geologists, because a nappe structure had been proposed for these rocks. The writers' detailed study on these problems has brought the following results. 1. The low angle thrusts and the nappe structure are verified with some revision. The sedimentary formations and igneous or metamorphic masses are overturned, yet the internal structures are left unsolved. 2. Three units i.e. the hornfels member, the Kawaiyama quartz-diorite and the Atokura formation are discriminated in the thrust mass. The Atokura formation is subdivided into the basal conglomerate (the Atokura conglomerate), the lower and the upper members. Several fossils of probable Senonian age are found in this formation. Accordingly the nappe formation is concluded to be post-Senonian, while it must be earlier than the Miocene formations found in the northern area. If the flat-lying Eocene deposits of Shikoku are taken into account, the age of the movements, probably, is considered to be still earlier. The deformation may be the most remarkable phenomenon representing the Laramide phase in Japan. 3. The Atokura conglomerate covers the hornfels member unconformably. But an exposure representing the very point of contact well exhibits quite a continuous transition between the two. No hiatus nor any visible boundary is discernible on this excellent outcrop, as if they were welded into one and continuous body. The unique phenomenon should be called a welded unconformity, though there is no evidence of melting. 4. The Atokura conglomerate is concluded to be sedimentary rocks from various facts. The characters regarded as igneous by some authors are explained by later deformations and alterations. They are insufficient for the igneous hypothsis. 5. Cenomanian-Turonian fossils including Trigoniae and Inocerami are found in the Nakanokaya formation, and an unconformity is found at its base. Probable Lower Cretaceous formation is also found.