ま　　と　　め
　以上に述べた，荒川河床に露出する楊井層（中新統）の層相解析の結果は，次のようにまとめられる．
　1．楊井層は，扇状地堆積物と潮間帯堆積物とが入り組んでおり，ファンデルタ堆積物とみられる．
　2．泥岩層（堆積相Fsc−1）や炭層（堆積相C）の上面−ときに礫岩層（堆積相Gm，Gt）や砂岩層（堆積層St-2）の下面一でかぎられる単層（厚さ2〜20m）が認められる．
　3．単層内では，上下方向に第6図のような上方に細粒化する累積順序が一つ以上認められる．これを小サイクルと名づける．各小サイクルは，単層内で，横方向，上下方向に入り組んでいるが，これは扇状地やファンデルタ堆積物の特徴の一つとみられる．
　4．小サイクルの集合よりなる中サイクルが認められ，上方細粒化の傾向がみられる．中サイクルは横方向によくつらなることや，各中サイクルの厚さがほぼ同じことなどからみて，地殻の運動が大きく関与していると思われる．
　5．楊井層全体としては，上方粗粒化の傾向を示し（大サイクルの可能性がある），ファンデルタ内での堆積環境の変化や，後背地の活発な上昇などに関連すると思われる．
　6．楊井層堆積時には，南側の滑川帯と関東山地から粗粒砕屑物が供給されたとみられ，滑川帯北縁の大谷断層が当時活動していた可能性が強い．
　7．ファンデルタ堆積物は，堆積盆地の早期だけでなく，楊井層の場合のように，盆地の消滅期にも認められる．

第7図　楊井層のファンデルタの堆積モデル．（中サイクル�Uの部分にもとづく）　記号は本文および第1表参照．

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Cyclic sedimentation in the fan-delta deposits of the Miocene Yagii
Formation, northeastern margin of the Kanto Mountains, Japan

Kensaku TAKEl
Kosei Gakuen Senior High School, Wada, Suginami-ku, Tokyo 166 Japan.
Tomoko KAKINUMA
Onishikita Elemerltary School, Onishi-machi, Gumma Pref., 370-14 Japan.
Kazuko NAKAMURA
Maebashi City Daiichi.Junior High School, Maebashi, Gumma Pref. 371 Japan.
Toshiyuki KAKINUMA
Shimosano-machi, Takasaki, Gumma Pref., 370 Japan.

Abstract The Yagii Formation is the uppermost one of the Miocene formations of the Hiki Hills, northeastern margin of the Kanto Mountains. The Yagii Formation is composed of interbedded conglomerate, sandstone and mudstone, and, to less degree, of tuff and coal. Fossils of plant leaves, tree stumps and rhinoceros have been found from the formation.
　　A middle part of the Yagii Formation was once largely exposed in channel floor of the Ara-kawa (river) near Hirakata, Kawamoto-machi, for the improvement of the Aketo Dam.
　　A facies analysis of this part of the Yagii Formation was attempted by the writers. The vertical profile of the studied part of the Yagii Formation is given in Fig. 4. The results are summarized as follows.
　　1. The Yagii Formation comprises sediments characteristic to alluvial fan (facies codes of river deposits follow Miall (1977, 1978), and are shown in Tab. 1) in intimate association with that of intertidal zone (facies IT), and therefore, can be regarded as fan-delta origin.
　　2. Individual beds (2-20m thick) of the Yagii Formation are bounded by the upper sufacc of mudstone layer (facies Fsc-1) and/or coal layer (facies C) ------sometimes by the lower surface of conglomerate layer (facies Gm, Gt) or sandstone layer (facies St-2).
　　3. Fining-upward sequences of small scale (microcycle) are attributable to waning current velocities as a channel is gradually filled, and are principally confined within each beds. Art absence of extensive lateral continuity of cyclic sequences within a bed seems to be a characteristic of fan-delta sediments.
　　4. Middle cycle (mesocycle) consists of a series of small cycle, and shows fining-upward trend. It is quite likely that the cycle has greater lateral extension more than 7km, and reflect the crustal movement, such as decreasing upheaval of source area in the course of deposition.
　　5. The Yagii Formation shows, as a whole, coarsening-upward trend. This may be related to active uplift of the source area, as well as change of depositional environment within the fan-delta.
　　6. Clastics of the Yagii Formation were mainly supplied from the southern region, i. e. Namekawa Zone and Kanto Mountains. The Oya Fault, northern boundary fault of the Namekawa Zone, should have been in activity during the deposition of the Yagii Formation.
　　7. Fan-delta sediments are said to be typically formed in the youthfull stages of basin development, however, also seen in the later stages as in the Yagii Formation.