* Institute of Geoscience (Institute of Geology and Geoinformation at present)
** Graduate School of Science and Technology, Niigata University
*** Department of Geology, Faculty of Science, Niigata University
**** Geological Museum, Public Relation Department
（Written in 2003）
The Suhara District is situated in central Honshu and occupies an area between longitude 139°0′ E and 139°15′ E and between latitude 37°10′N and 37°20′N (138°59′48.4″E and 139°14′48.3″E, 37°10′11″N and 37°20′10.9″N, refered to the International Terrestrial Reference Frame, ITRF). Topographically, most of this district is located in the Echigo Mountains and northwest rim is in the Muikamachi Basin, separated by the Shibata-Koide Tectonic Line. In this area, the Mizunashigawa Metamorphic Rocks of the Joetsu Belt, Permian rocks of unidentified structural province, Jurassic accretionary complex of the Ashio Belt, Late Cretaceous to Paleogene plutonic rocks, Neogene strata, volcanic rocks of the Asakusa Volcano and Quaternary deposits are distributed.
Mizunashigawa Metamorphic Rocks
The Mizunashigawa Metamorphic Rocks of the Joetsu Belt are low-grade polymetamorphosed rocks, which suffered greenschist facies regional metamorphism and contact metamorphism by the Late Cretaceous to Paleogene granites. They are mainly pelitic schists, including exotic blocks of greenstones, ultramafic rocks. chert, limestone and sandstone. The age and type of the regional metamorphism are not yet confirmed.
Fig. 1 Summary of geology of the Suhara District
Permian rocks of unidentified tectonic province
Permian rocks of unidentified tectonic province are distributed in the southeast part of this district along the Tadamigawa River and classified into the Otori, Takizawa, Maruyama and Sodesawa Formations. These formations are mostly composed of mixed rocks, which include blocks of ultramafic rock, gabbro, basalt, basaltic tuff, granite, rhyolite tuff, conglomerate, sandstone, limestone, chert and others of various scales in the pelitic matrix.
The Otori Formation consists of mixed rock, which includes large-sized blocks of rhyolite tuff, alternating bed of rhyolite tuff and mudstone, and granite. The Takizawa Formation is composed of alternating beds of sandstone and mudstone. The Maruyama Formation is composed of mixed rock, which includes blocks of ultramafic rock and gabbro of large size in the pelitic matrix. The Sodesawa Formation is composed of mixed rock, which includes blocks of basalt, basaltic tuff, tuffaceous sandstone and sandstone. Permian brachiopods and crinoids fossils are present in an unsorted bed, and in-situ basalt is also present in the Sodesawa Formation.
Jurassic accretionary complex of the Ashio Belt
The Jurassic accretionary complex of the Ashio Belt in the Suhara District is classified into the Oshirakawa, Kuromatagawa and Kamigongendoyama Complexes, from the upper to lower structural level, based on the lithology. These complexes are intruded by Late Cretaceous granites and suffered contact metamorphism.
The Oshirakawa Complex is mainly composed of mixed rock, with blocks of chert, greenrock and limestone ranging from 50 to 300 m in thickness. The Kuromatagawa Complex is composed of interbedded sandstone and shale, and blocks of chert and greenrock ranging from 300 to 500m and over in thickness. The Kamigongendoyama Complex is composed of mixed rock accompanying chert, greenrock, sandstone, interbedded sandstone and shale, limestone and conglomerate, chert blocks of various size in thickness and greenrock ranging from 50 to 300 m in thickness.
The accretionary ages based on radiolarian fossils from shale are Early Jurassic for the Oshirakawa Complex and after Middle Jurassic for the Kuromatagawa and Kamigongendoyama Complexes.
Late Cretaceous to Paleogene plutonic rocks
The plutonic rocks in the Suhara District are mostly Late Cretaceous to Paleogene granitoids with small amount of gabbro. These are hornblende gabbro, Oyu Granite, Kuromatagawa Granite, granite porphyry, aplite, silexite and quartz diorite.
The hornblende gabbro appears as small xenoblocks in the Oyu Granite. The Oyu Granite is a medium-grained leucocratic biotite granite and granodiorite. It intrudes into the Kuromatagawa and Kamigongendoyama Complexes and caused contact metamorphism. The Kuromatagawa Granite, a coarse-grained biotite granite, intrudes into the Kuromatagawa and Kamigongendoyama Complexes and caused contact metamorphism. It also intrudes into the Oyu Granite. The granite porphyry is a small stock intruding into the Oyu Granite. The aplite and silexite are dykes intruding into the Kuromatagawa Granite. The quartz diorite is intruded by rhyolite and basalt and appears as several small bodies.
Neogene
Neogene strata lying in the Suhara District are the Okura Formation of the Jonai Group and the Matsukawa, Yuzawa, Nishimyo, Toyagamine, Wanazu and Uonuma Formations.
The Okura Formation of the Jonai Group and the Matsukawa Formation are composed mainly of andesitic rocks, which lies on the ridge and foot of mountains. They occupy the lowest part of the strata in the Niigata Sedimentary Basin. The Yuzawa Formation is composed mainly of andesite volcaniclastic rocks, conglomerate and rhyolite pumice tuff, partly with andesite lava. Many rhyolite dykes with basalt rarely andesite, diorite porphyry and granodiorite porphyry dykes intrude into the Okura, Matsukawa and Yuzawa Formations. The Nishimyo Formation is mainly composed of rhyolite pumice tuff and lies in the northwest side of this area. The Toyagamine Formation is composed of andesite lava of Middle Miocene, lying in the northwest rim of this area. The Wanazu Formation lies in the northwest rim of this area and composed of dacite volcanic breccias. The Uonuma Formation is composed of partly fluvial riverbed sediments of Late Pliocene to Early Pleistocene in the Niigata Sedimentary Basin, whose maximum thickness is 3,000 m.
Asakusa Volcano
Products of the Asakusa Volcano in this district are composed of the Tadamisawa Lavas and the Onigatsura Volcanic Rocks. The Tadamisawa Lavas consist of two-pyroxene andesite with or without megacrysts of hornblende, and are composed of lava and intrusive rocks partly with volcaniclastic rocks. The Onigatsura Volcanic Rocks consist of olivine-two pyroxene andesite and composed of lava and volcaniclastic rocks. The Asakusa Volcano was active at approximately 1.6 Ma ago in Early Pleistocene.
Quaternary
The Quaternary deposits of this district are limited and composed of Pleistocene landslide deposits, older colluvial deposits, terrace deposits, colluvial deposits and flood plain and river bed deposits.
The Pleistocene landslide deposits lie in the western side of the Aburuma River, northwestern part of this district. It is partly still active and composed of debris and colluvial soil with hummocky hills that originated from the Nishimyo Formation. The older colluvial deposits are composed of debris and colluvial soil with andesite blocks in the northwest rim of this district. These deposits are the Late Pleistocene covered with tephra. The terrace deposits are located along main rivers in the western part of this district. Along the Aburumagawa River, two terrace plains are distinguished. The older t1 plain is covered with Late Pleistocene tephra, and the younger t2 plain is distributed along the Aburumagawa, Hanegawa and Sanashigawa Rivers. These terrace deposits are composed of mainly gravel and sand. The colluvial deposits are located on the foot of the Asakusa Volcano, northeast part of this district. The flood plain and riverbed deposits are located along the main river and dam lakes of the Tadamigawa and Kuromatagawa Rivers.
上越帯は，黒田（1963）による東北日本の地体構造区分試論の中で提唱され，フォッサマグナ東縁と足尾帯の間を占める藍閃石片岩の出現で特徴づけられる変成作用を受けた地質帯とされた．その後，Hayama et al.（1969）は，上越帯に点々と分布する変成岩類を検討し，これらはかつて連続した変成岩帯として存在したものと想定し，上越帯を上越変成帯と呼び，飛騨外縁帯の延長と考えた．一方，茅原・小松（1982）は，上越帯を蛇紋岩と若干の結晶片岩の分布する谷川岳帯と，変玄武岩・変斑れい岩―閃緑岩複合岩体と中生代の陸棚堆積物（上部三畳系奥利根層群，下部ジュラ系岩室層，下部白亜系戸倉沢層）で特徴づけられる片品帯に区分することを提案し，谷川岳帯を飛騨外縁帯に，片品帯を舞鶴帯に対比した．また，1983～1985年に行われた総研「上越帯・足尾帯」（研究代表者 茅原一也）により，上越帯と足尾帯の境界地域の地質と構造の解明が進み，足尾帯西縁部において変苦鉄質岩が卓越する地帯は足尾帯西帯として区別された（茅原，1984；茅原・小松，1992）．
水無川変成岩類は，南隣八海山地域内の水無川流域を模式地とする片状構造の発達した低変成度の変成岩類であり，原岩堆積後に緑色片岩相の変成作用を受けた後，花崗岩のへい入に伴う接触変成作用を被った複変成岩である（Hayama et al.，1969）．水無川変成岩類の原岩は泥質岩を主体とし，緑色岩類・超苦鉄質岩・チャート・石灰岩・砂岩の岩塊を含む（竹之内，1988）．花崗岩類による接触変成作用を被る前の初生的な変成作用の性格，変成年代等は今のところよく分かっていない．本報告では，山元ほか（2000）を踏襲し，水無川変成岩類を上越帯のメンバーとして扱う．
新潟県南東部，越後山脈の中核部を占める越後三山（駒ヶ岳，中の岳，八海山）地域には，水無川変成岩類（茅原・西田，1968）と呼ばれる低変成度の変成岩類が露出している（第2.2図）．この水無川変成岩類は谷川岳山頂付近に露出する結晶片岩及び群馬県沼田市北方に露出する川場変成岩類とともに上越変成帯の一部を構成していたものと考えられた（Hayama et al.，1969）．一方，小松（1980），茅原・小松（1982）らは，上越帯を構成岩石の特徴から，変成オフィオライトと中生代の陸棚堆積物すなわち，奥利根層群（上部三畳系）・岩室層（下部ジュラ系）・戸倉沢層（下部白亜系）からなる片品帯と，結晶片岩・蛇紋岩からなる谷川岳帯とに区分し，水無川変成岩類はその分布位置から片品帯に含められた．その後，竹之内（1988）は，水無川変成岩類の詳細な野外調査を行い，その岩相や原岩構成が周辺の足尾帯構成岩類と連続することから，水無川変成岩類を足尾帯の構成要素であるとした．また，竹之内（2000）では，水無川変成岩類の変形相解析を行い，2つの変形時相を識別し，水無川変成岩類の構造運動を論じた．さらに，Takenouchi and Takahashi（2002）では，川場変成岩類と水無川変成岩類の岩相及び変形史が共通することから，上越地域の中・古生界中に断続的に分布する低変成度変成岩体は，かつて存在していた低変成度変成岩帯を構成していたものであると考えた．川場変成岩類は足尾帯の分布域とは地理的に離れた，上越帯の中核部を構成する地質体であり（第2.2図），川場変成岩類と岩相，変形史の共通する水無川変成岩類は上越帯のメンバーとみなすのが妥当と考える．本報告では前述のように山元ほか（2000）を踏襲し，水無川変成岩類を上越帯の構成要素として扱う．
付加コンプレックスにおいて，岩相とその堆積年代から復元される初生的な岩相層序は，一般に下位より，海洋地殻や海山を構成する緑色岩と石灰岩，遠洋性堆積物のチャート，半遠洋性堆積物の珪質泥岩，更に陸源性砕屑物の泥岩・砂岩が累重する特徴を持つ．これは海洋プレートが中央海嶺で生成されてから，プレートの移動に伴い海溝に近づき，大陸縁辺に沈み込むまでの堆積環境の変遷を示している．そのためこの岩相層序は，海洋プレート層序と呼ばれている（Taira et al.，1989；Isozaki et al.，1990）．一般的に海洋プレート層序では，陸源性砕屑物の堆積年代の上限を，堆積物が付加した年代，すなわち付加コンプレックスの形成年代としている．
海洋プレートが大陸プレート縁辺に沈み込む際，地層に平行な大規模な滑り面（デコルマ）とそれから派生する衝上断層群が発生する．海洋プレート上の堆積物は，デコルマを境に下部は沈み込み，上部は海洋プレートから剥ぎ取られ，大陸プレート前面に付加していく（加賀美ほか，1983など）．付加された堆積物は，衝上断層群によって，覆瓦構造を発達させる．また付加体の深部では，デコルマが下方に転移し，デュープレックス構造を形成しながら付加が進行する（加賀美ほか，1983；Sample and Moore，1987など）．このように形成年代の新しい地質体が順次付加することで，下位に向かい構造層序単元の形成年代が若くなる年代極性が生じる．
足尾帯ジュラ紀付加コンプレックスは，北部フォッサマグナ東縁部と棚倉構造線に挟まれた広範囲に点在して分布する．このうち，足尾山地と八溝山地の足尾帯ジュラ紀付加コンプレックスは，80～90年代に微化石層序学及び付加体地質学の調査・研究手法が導入され，ジュラ紀に形成された付加コンプレックスとしての特徴が明らかにされた．（Kamata，1996；鎌田，2000；指田・堀，2000など）．一方，新潟県から福島県にかけた地域に分布する足尾帯ジュラ紀付加コンプレックスは，白亜紀以降の深成岩類や火山岩類によって，その分布は断片的となっている．そのため目立った研究は少なく，散点的な放散虫化石の報告のみである（Mizutani et al.，1984；黒川ほか，1987；川端・伊藤，1993；松本ほか，2001；原・柏木，2004など）．須原地域は，足尾帯ジュラ紀付加コンプレックス分布域の北西部に相当する．
段丘堆積物は，本図幅地域西部の主要な河川である破間川及びその支流の松川川や，佐梨川・羽根川沿いに分布する．本地域の段丘堆積物については高浜（1982a），柳沢ほか（1986），荒川ほか（1992）などの研究があるものの，テフラに基づいた詳しい研究は信濃川流域に比べて遅れていた．Choi et al. (2002)は，破間川沿いの段丘堆積物とそれを覆うテフラについて研究し，3つの広域テフラDKP（大山倉吉軽石，5万年前），AT（姶良丹沢火山灰，2万5千年前），As-K（浅間草津火山灰，1万2千年前）を含む4層のテフラを見い出し，段丘面をAb-IからAb-VIの6面に区分した．本図幅地域にはChoi et al.（2002）の区分のうちAb-IとAb-IVの2つの段丘堆積物が分布し，本報告ではこれらをそれぞれt1段丘・t2段丘とした．
t1段丘堆積物は守門村須原に分布する．段丘面の現河床からの比高は約30mで，堆積物は厚さ約6mの礫層及び砂層からなり，上位に広域テフラDKPを含む厚さ1mのテフラが載る（Choi et al., 2002）．
t2段丘堆積物は守門村須原に破間川に沿って分布する．段丘面の現河床からの比高は約20mで，堆積物は厚さ1.5～4mの礫とその上位に厚さ50cm以下の砂からなる（Choi et al., 2002）．上位にはAs-K以下のテフラは欠如しているが，表土中により新期のテフラAb-t1が見い出されている（Choi et al., 2002）．t2段丘堆積物は従来の区分との比較では，柳沢ほか（1986）のT9段丘，高浜（1982a）の段丘IV群堆積物（T4）に相当する．本報告では佐梨川・羽根川沿いに分布する段丘堆積物も現河床からの比高からt2段丘堆積物に相当するものとみなした．
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