関東山地北縁部のいわゆる跡倉層の砂岩  [in Japanese] Sandstones of the so-called Atokura Group in the northern area of the Kanto Mountains, central Japan  [in Japanese]

関東山地北縁部に分布する白亜系(いわゆる跡倉層)のうち, 西部の下仁田地域, および東部の寄居〜小川地域の砂岩の生成に関する問題を, 砂粒の鉱物組成を中心にして検討した。白亜系の砕屑物の供給源として花崗岩類, 堆積岩類, 結晶片岩類, および噴出岩類が認められる。このうち, 花崗岩類は, 少なくとも下仁田地域の跡倉層(浦河統)および寄居〜小川地域の栃谷層(上部白亜系)の場合, クリッペをなす花崗岩類に一部は相当するが, おそらく未知の花崗岩類も存在した。結晶片岩類は, とくに下仁田地域の中の萱層(ギリヤーク統)では, 三波川帯のものが多いであろう。跡倉層下部層(浦河統)は, ほとんど花崗岩地域からの供給物で構成されており, 供給源に近い堆積物とみられる。跡倉層と栃谷層は, 砂岩(や礫岩)の組成その他に共通性が多く, しかも原地性とみられる白亜系の砂岩(や礫岩)とは異なっており, 両層は共にクリッペである可能性が高い。

Mesozoic sediments, the so-called Atokura Group, are distributed in the northern area of the Kanto Mountains, and their origin was disputed in the 1940's. Many studies since the 1960's maintained that these Mesozoic sediments were Cretaceous in age, and that many of them were klippes locally associated with granitic rock bodies (Atokura Klippen). Sandstones of the Cretaceous formations of the Shimonita and Yorii-Ogawa areas have been studied petrographically, and their provenance and some problems related to deposition and tectonics are discussed here. The results are as follows. (1) In the sediment source region, granitic rocks, sedimentary rocks, crystalline schists and eruptive rocks existed. Among them, granitic rocks, at least of the Atokura Formation (Urakawan) and the Tochiya Formation (Upper Cretaceous), correspond in part to the granitic rock bodies forming klippe associated with the Cretaceous formations, though unknown granitic rocks may also have been a source of the rocks. Crystalline schists, especially of the Nakanokaya Formation (Gyliakian), perhaps correspond largely to the Sanbagawa metamorphic rocks of the northern Kanto Mountains. (2) In the Lower member of the Atokura Formation of the Shimonita area, sandstones mostly consist of clastics derived from granitic rocks, and might have been deposited close to the source region. (3) The Tochiya Formation of the Yorii-Ogawa area may correlate to the Upper member of the Atokura Formation of the Shimonita area, for they display a close resemblance in mienralogical compositions of sandstones, as well as in the mode of stratification. (4) On the basis of distinctive features of the sandstones of both formations, not only the Atokura Formation of the Shimonita area, but also the Tochiya Formation of the Yorii-Ogawa area probably belong to the Atokura Klippen.