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In the previous paper (ARAI et al., 1963), we made clear that the Atokura Klippen consist of the Atokura formation (Senonian) and the older rocks covered unconformably by it and that the Klippen were thrust up from the north. Then, searching for the root of the Klippen, we surveyed the Shimonita district to the north of the Atokura district, southern Gunma Prefecture, Central Japan. This paper is the result of a collaboration study on the root problem and a few subjects arose newly in the course of the study. In the Shimonita district, a characteristic zone of E-W trend, 3km wide, i.e., the Shimonita Tectonic Zone, is developed along the northern margin of the Kanto mountainland which is composed of the Paleozoic and Mesozoic formations as well as of the Sanbagawa crystalline schists. The characteristic rocks of the Shimonita Tectonic Zone, from the older to the younger, are as follows. The Nanjai formation is composed mainly of mudstone and sandstone, with thin beds of intercalating acid tuff. The rocks are suffered generally from shearing movement and also from contact metamorphism near quartz diorite mentioned below. Some of the hornfels are quite similar to those of the Atokura Klippen. In spite of the remarkable minor deformation, the formation, as a whole, gently undulating folds. Geologic age of this formation is unknown but is surely presenonian as mentioned later. Probably, it is early Cretaceous or Cenomanian-Turonian. The Name granite occupies the northern part of the tectonic zone. It is composed of medium to coarse-grained leucocratic granite rarely containing biotite. The effect of shearing movement is remarkable in this granite and it results, in some cases, in mylonite. Several small masses of this granite are found also as sheets in the Nanjai formation. Quartz diorite, which is also intruded into the Nanjai formation, is quite identical with the Kawaiyama quartz diorite, one of the constituents of the Atokura Klippen. The Kanohara conglomerate formation, that has long been regarded as the basal conglomerate of the Miocene, was proved to be much older. It is composed mainly of boulder conglomerate in which boulders attain a colossal dimension of 2m across. Most of the boulders are represented by granite porphyry and welded tuff, and rarely by granite. Matrix is represented generally by coarse sand of acid volcanics, but tuffaceous matrix and thin beds of acid tuff are also found in some parts. Granite porphyry, which is identical with the boulders of the Kanohara conglomerate formation, crops out in the southern part of the tectonic zone, and is intruded into the Nanjai formation. Moreover, the conglomerate formation covers unconformably the quartz diorite. Therefore, the Kanohara conglomerate formation is unquestionably in an unconformable relation to the Nanjai formation. An extinct formation of acid welded tuff, which had been thoroughly eroded out, is inferred from the boulders of the Kanohara conglomerate formation. The Kotsutateyama acid tuff formation covers the Kanohara conformably. Some of the rocks of this formation are also welded, but welding is less conspicuous than in the welded tuff boulders of the Kanohara conglomerate formation. The Kanohara and the Kotsutateyama formations seem to maintain a gentle geologic structure, excepting the narrow belts where they are steeply inclined along high-angled faults. They are more than 150 m and 100 m thick, respectively. Prior to and posterior to the Kanohara conglomerate formation, two cycles of acid volcanic activity are recognised in this tectonic zone, so far as the mapped area is concerned. These activities suggest the first stage of the tectonic development of this zone. The Miocene sediments with many Molluscan and Foraminiferan fossils rest on these older rocks with remarkable unconformity. The basal conglomerates are variable in thickness from 0 to 30m. A questionable instance is known where the thickness is estimated at about 100m. The lithologic feature, in this case, is similar to that of the Kanohara conglomerate. Except the questionable instance, the Tertiary conglomerates are distinct and the unconformable contact to the older rocks is detected at many exposures. The Miocene sediments are cut by several highangled faults, among which the southernmost one, the Okitano-Iwayama Line, represents the southern boundary of the Shimonita Tectonic Zone. A faulting movement is also discriminated in the pre-Miocene time in connection with the acid volcanic activities. The important contributions of our study are the following two points ; 1 Some of the hornfels of the Nanjai formation and quartz diorite are quite identical with the older rocks covered unconformably by the fossil-bearing Senonian Atokura formation in the Atokura Klippen. Therefore, the Nanjai formation and quartz diorite are considered to be the root of the Atokura Klippen and to be pre-Senonian in geologic age. On the other hand, as the Kanohara conglomerate formation, which is not participated in the movement of the Atokura Klippen, is pre-Miocene, the movement of the Atokura Klippen occurred in the age from post-Senonian to Paleogene. We call this movement the Atokura revolution. 2 The acid volcanic activities prior to and posterior to the Kanohara conglomerate formation, especially the older one, are correlated to the latest Mesozoic (or the earliest Cenozoic) volcanic activities in Southwest Japan, represented by the Nohi rhyolites, not only in their geologic age but also in their lithologic characters. It shows that the area of this tectonic zone is an extention of the inner zone of Southwest Japan, and that the Okitano-Iwayama Line corresponds roughly to the Median Tectonic Line of Southwest Japan. But, as the Ryoke metamorphic belt is not found in the present area, its strict discussion must be withheld.
地学団体研究会の論文
1966-03-30

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