On the Origin of the Joints in the Futaba Area of the Joban Coal-Field, Fukushima Prefecture

On

the Origin of the Joints iri the Futaba

Area

of the Joban

Coal-Field, Fukushima

Prefecture

Shinobu MiTSUI*、、Kazuo HOSHINO**、Tomomasa Hashimoto**          Kazuo Inami** and Akira ＳＡＧＡｗＡ＊＊

     ＊Ｉ、lｓ￡itｕｔｅ ｏｆ Ｇｅｏｌｏｇ:ｙ、Ｆａｃｕlりof Li tｅｒａｔｕｒ･ｅａ、ｉｄＳｃｉｅｎｃｅ、        ＊＊Ｇｅｏｌｏｇｉｃａｌ Ｓｏｃｉｅりｏf Ｊａｐａｎ

 Abstract

・ The

purpose of the present paper is to interpret the origin and period of the

joints developed in the Cretaceous and Tertiary systems distributed in the Futaba

Area

of

the Joban coal-field,Fukushima

Prefecture｡

 The

joints in the investigated area were classifiedinto four stages,

1) joints with strikes

oiｗNW-ESE

to WSW-ENE

in the Futaba-, Shiramizu-,

and Yunagaya

groups are the

oldest and were formed by the structural control of the basement rocks ； 2) dominant joints

with NNW-SSE

to ＮＮＥ-ＳＳぺＶ

strikein the Yunagaya

Group are closely related to the Futaba

thrust fault that was formed

prior to the sedimentation of the Izumi

Group,

and probably

by the same

stress as the thrust fault； 3) predominant

NNW-SSE

to NNE-SSW

trending

joints in the Izumi Group

are tension joints formed

under

the stress fields inwhich

the

maximum

tensil principal stress lies with about horizontal angle towards the east or west

when

the ｍａχimum compressive principal stress was released after the reaction of the Futaba

thrust fault after the deposition of the Izumi

Group ； and,

4) joints with

WNW-ESE

to

WSW-ENE

strike in the Izumi Group

and Yamadahama

Formation

are the youngest‘tension

joints, and were formed after the development

of the joints with NNW-SSE

to NNE-SSW

strike in the Izumi

Group.

INTRODUCTION

AND

ACKNOWLEDGMENTS

 In general, faults are defined as fractures in rocks in which the opposite walls have moved past each other to produce ａ displacement, whereas joints are defined as fractures along which there is little or no displacement (Badgley, 1965 ； Billings, 1966). Usually joints occur in sets, although the mean densities and patterns of the joints are considerably variable in different rock materials. Concerning the origin, joints are classified into tension joints parallel to ａ maximum compressive principal stress, and shear joints oblique to the stress･ Generally, they are related to the regional geological structures. Also, joints are differentiated into systematic joints and non-systematic joints CBadgley, 1965). The systematic joints are generally related with and applied in interpretation of the geological structure, from the following reasons ； 1）Ｔｈｅ non-systematic joints are formed by tectonic origin and are due to physical and chemical causes. 2）The systematic joints commonly possess planar surfaces, whereas the non-systematic joints show random curvilinear pattern, and this brings about difficulty in their application｡

 Because joints are closely related to the geological structure as the faults, many authors have reported on the joints associated with the regional geological structures (Hoshino, 1965 ；

２ _{高知大学学術研究報告  第21巻  自然科学  第１号}

／丿

０ _{10  20 KM}

L＝∃＝＝1

Ｎ

心十ト

０

エＦＴト

５ _{10 KM} →一一→犬Ｔ祠

ｊｍノ 心

﹃／

I ; i ･ ・ d ，   ● . ． ぷ ●   i . ヽ ‘

       Fig. 1. Index map of the investigated area.

      ・       l「      .      1

Hirabayashi, 1964 ； Murai, 1965, a, b, 1966, . 1967 ; Yamaguchi, 1965 ； Hiranoレ1969;

Kitamura', 1962 ； Mitsui, 1971).       ・I･      二

 The purpose of the present paper is to interpret the origin and period of the joint develoびed

in the cover rocks (Cretaceous and Tertiary sedimentary rocks) distributed in the Futaba

      １

Area (Fig. 1) of the Joban coal-field. Fukushima Prefecture, and to describe the relationship

between the joints and the Futaba thrust fault, which is the most remarkable major fault in

φｅ Futaba Area and was named previously as the Hisanohama-Iwanuma dislocation line by

Yabe and Aoki (1924).

 In addition to this study. experiments under high confining pressure undertaken for the

rocks of this region by the present authors. Detailed discussion on the results and geological

      ，  ●applications will be reserved for another opportunity･      .，

 Acknowlegments are due to Prof. Nobu Kitamura of the Institute of Geology and

Paleontology, Faculty of Science, Tohoku University, and Jun-ichi Iwai, Professor

Emeritus of the Tohoku University, for directing the field and laboratory works. Deep

appreciation is due to Professor Kotora Hatai of the Institute of Geology and Paleontology,

Tohoku University for his kind guidance and critical reading of the manuscript, and to ＤｒS･

Masatora Kawai, Toshihiro Kakimi and Masatoshi Sogabe, all of the Geological survey

of Japan, for their suggestions on stratigraphic and geostructural problems relevant to the

present study. Thanks are also due to Professors Takeo Sawamura and Tiro Katto both of

the Institute of Geology, Faculty of Literature and Science, Kochi University, for their

igin of the joints in､the. Futaba Area, of｡the Joban Goal-Field、 Fukushima Pref. 5       ㎜㎜■㎜

       STRATIGRAPHY ＼

 ･The stratigraphic classification in the Futaba Area is shown in Pig:. 2， and briefly outlined helo'w and the geological map is shown in Fig. ろ.

 The basement rocks of the cover rocks (Cretaceous and Tertiary sediments) in the surveyed area are the Abukuma metamorphic rocks, the Paleozoic rocks and pre-Tertiary granitic rocks. The Paleozoic rocks are Permian in age and called the Takakurayama Group (Yanagi-sawa, 1967). The group consists of black slate, sandstone conglomerate and phyllite, etc.,

AGE GR. FORMATION MEMBER THICK. COLUMN ROCK FACIES

    U 」     Z     L よ 」     U     Ｏ     −     ＿ Ｊ     Q 。   L J   Z   U 』   ○   ○   山     山     Ｚ Ｚ ． 山     Ｕ     Ｏ   ・ ← = 噂     Σ     Z     L J     U 訴 追 ' I - t j     ○     山     t A     ⊃     Ｏ     a i     U     μ     山     c 「     U ヘ ノ 心 ￥ ｜ ≪     ﾄ 戸 隠

？

μ

YAMADAYAMA      Ｆ． 匹  SEKINOUE Ｆ． 四  NAKAYAMA Ｆ． 四 ⇒ 匹  MISAWA  SANDSTONE    M. づご  MUDSTONE    Ｍ．   ４１ｍ 心   １６４ｍ ｍ   ２０ｍ 四  250 m

d(1｢k olive mica, siltstone･with pumice and carbonate fragments

yel【ow to brown mica, sandstone

 ●●●●●●● ぇ。乙ぶ

dark olive mica, siltstone with sand-stone thin layers

一一一一一 ﾌ匹匹 ＯＯＯＯＯＯＯ Ｊり､ﾀヽSもむ収Jり､S ●●●●●・● ::ﾆ::::::::::: ごﾆ:::::::::: ごﾆごご ら  a.  ⊃  Ｏ  Q］  ○  g  ≪  ○  ≪  Z  ⊃

 ＞-sandstone, tu↑f,conglomerate

TAIRA F.

yellowish to reddish brown granule to m惣昌司笥7(北畠訟尚

135 m

こ

dj忿r?egrQy to dark olive tuffaceous

￡一一一一

KAMENO-0 Ｆ． 135 m

蔚昌ｍご昌t次次ぷ‰

Å

MIZUNOYA F. 80m ｀       ゞ dark olive micaceous mudstone   ''

GOYASU Ｆ． ７０ｍ

● ● ・ ● ● ● ●

brown color fine grained sandstone reddish brown gr. to eb. conalomerafe bearing arkose c.s(ln孟tone wiw4   P lignite layers ● ● ● ● ● ● ● 〃 O O B ≫ ● 申 ● ● ● ・ ・   ＠ ● ＠ ａ ａ Ｏ Ｏ ● ● ● ● ● ● ●   ○ ● ａ ● ● ● ● ● ● ●  KUNUGIDAIRA      Ｆ． 心 SHIRASAKA F.

四

 110 m ぺ  100 m - 一 一 一 一

sandstone and mudstone grayish white rhyolitic tuff and rh olite lava

pelgble to cobble size conglomerate

● ● ● ● ● ・ / ＼ / S / X / 's. / Sv y V / V  ＱＯＯＯＯＯＤ  ＯＯＯＯＯＯＯ  Ｏ○ＯＱＯＯＯ り゛Jふ&4λJりヽ   Q .   ⊃   Ｏ   Q こ   ○   ⊃   N   −   Σ   ≪   Q こ   一   工   C / 7 へ ／ 心   ○   ≪   f f l   ≪

dark gray shaly mudstone or shale

･ASAGAI F. 80m ●●●●●●● ：：：：：：Ｍ

3穴ぬ≒否認回流かe

stratifi- IWAKI F. 四  TAMAYAMA F.

匹

 110 m 四  80m ・ 申   ● ● ●   ● ● a a o o a o O Ｏ ａ Ｏ ● Ｏ Ｏ ａ ● ● ● ● ● ● ●

m‰旨認諾恰U匹昌穴み

;‰。t。cobb【e

size con lomerate

･･

with sandstone and coal

lyers

  0 0 O 0 0 O 0   0 0 0 0 0 0 0 応 ､ 7 . ､ O 0 O O 0 coal

arkose coarse grained sandstone

        ● ● ● ● ● ● ● ● ● ● ・

KASAMATSU F. _{160 m} ● ● ● ● 嘩 ● ● ● ● ● ● ● ● ●

coal

arkose c. sandstone with mudstone ? ASHIZAWA F. 訃竺七三 BASEMENT ROCKS

W

 80m

W

 ●●●●●●●

薗

fine to medium

grained sandstone

granodiorite, amphvbolite, schist, slate limestone, sandstone, ets.

Ｇ･，ＧＲ･．：ＧＲＯりP,Iz.:IZUMI, THICK.:THICKNESS

       Fig. 2. Generalized stratigraphic sequence of the Futaba Area of the Joban

 ４      高知大学学術研究報告  第21巻， 自然科学  第１号

with intercalated small fossiliferous limestone lensesン

 The cover rocks are composed of the Cretaceous and Tertiary sediments.

The Cretaceous Futaba Group overlies the basement rocks with unconformity. and is superposed by the Tertiary with unconformity. The Futaba Group is subdivided into the Ashizawa-, Kasamatsu-, and Tamayama ｆｏｒ皿ations. Ｔｈｅ･Ashizawa Formation is Early Cenomanian in age and consists generally of massive fine grained sandstone attaining about 100 to 200 meters in thickness. The Kasamatsu Formation, about 140 meters in thickness, consists mainly of coarse grained arkose sandstone and is intercalated with layers of mudstone and lignite. The Tamayama Formation, 60 t0 120 meters in thickness, is composed mainly of coarse grained arkose sandstone with conglomerates and crosりamination in part and with intercalated lignite layers in the upper part.    ，'゛

 In this area the Tertiary sedimentary rocks ａ邨 classified, from the older to the yotinger, into the Shiramizu Group, Yunagaya Group, Nakayama Formation, Izumi Group and the

Yamadahama Formation. The Izumi Group and Yamadahama Formation are correlated with the Taga Group by Sugai and Matsui C1957) and Iwaりand Matsui (1961).

 The Shiramizu Group is subdivided into the !waki･Formation, the Asagai Formation and the Shirasaka Formation in upward succession 呻d represents a transgressive facies. The Iwaki Formation lies on the Cretaceous Futaba Group with vmconformity and is about 100

meters thick. It is composed mainly of pebble to cobble size conglomerates intercalated with layers of coarse grained conglomerate bearing sandstone, fine to coarse grained sandstone, mudstone and coal. This Formation shows some cyclic sedimentation, comprising mainly conglomerate CEguchi et al.バ954). The Asagai Ｆ(μ･matioh consists of fine grained sandstone. but with discontinuous conglomerate beds near the boundary with the Iwaki Formation in the vicinity of Shinyashiki, Hirono･cho and on the south side of the Kido River. The Shirasaka Formation is composed of massive shaly mudstone showing good development of

joints･

 The Yunagaya Group lies on the Shiramizu ｗitトunconformity and is superposed by the Nakayama Formation with unconformity. This Group is subdivided into the Kunugidaira-, the Goyasu-, the Mizunoya-, Kameno-0-, and the Taira formations. The latter is subdivided into the Honya Mudstone and the Misawa Sandstone members.

 The lower part of the Kunugidaira Formation consists mainly of conglomerate with con-glomerate-bearing sandstone and the upper of rhyolitic tuff intercalated with rhyolite lava. The uppermost part is of tuffaceous shale and coarse grained sandstone. The Goyasu Formation consists mainly of granule to pebble size conglomeratebearing medium to coarse grained arkose sandstone and is intercalated with layers of lignite (10 to ３０cm thick) and pebble size conglomerate in the lower part. The Mizuncが１ Formation is about 80 meters in thickness and composed mainly of tｕ任aceous mudstone intercalated with the layers of sandstone near the boundary with the Goyasu Formation. In this Area this formation does not form ａ large scale alternation of mudstone and sandstone as in the Iwaki North and Iwaki South areas (Mitsui, 1971). The Kameno-0 Formation is composed of purplish gray stratified and laminated shale with the remains of molluscs and intercalated with layers of medium to coarse grained sandstone in the middle and upper part, which form the

alter-On the Oriein of the Joints in the Futada Area of the Joban Coal- Field) Fukushima Pref. 5

nation. In the alternation layers this formation shows interformational abnormal

deposition

(Iwai, 1953). The Honya

Mudstone

Member

interfingerswith the Misawa

Sandstone Member

and attainsａ thickness of 100−150 meters. The lower part of this Member

is composed

of

ah alternation of mudstone

(dominant)

and sandstone.

The

middle to upper parts consists

of massive shaly mudstone with molluscan and foraminiferan remains. The Misawa

Sandstone

Member

is mainly of granule to pebble size conglomerate-bearing medium

to coarse grained

arkose sandstone showing

the good development

of cross-bedding.

 The

Nakayama

Formation is distributed between

Yusuji

and Hisanohama

with strike of

NNW-SSE.

It is composed

of basal conglomerate

which

comprises gravels of andesite and

basalt, etc.

 The

Izumi

Group

CMitsui, 1971) lies with unconformity

upon the Nakayama

Formation

and consists only of the Sekinoue Formation (Mitsui, 1971) in the Futaba Area. The Sekinoue

Formation is composed

mainly of dark olive siltstoneintercalated with thin layers of medium

to coarse grained sandstone and yields fossilmolluscs, foraminifers and diatoms. From

the

molluscan

and foraminiferan fossilsthe formation is Early Pliocene in age.

 The

Yamadahama

Formation

(Mitsui, 1971) ovelies the Sekinoue Formation

Formation

with unconformity.

The lower part of the formation is composed

mainly of fine to coarse

grained sandstone

with abundant

molluscs and

the upper part is of dark

olive massive

siltstonewith thin layers of sandstone.       ，.

GEOLOGICAL

STRUCTURE

 In the Futaba Area, the major geological structures are the Futaba fold structure. the Futaba thrust fault, and the unconformity between the Nakayama Formation and the Izumi Group･

 The Futaba fold structure with N10°−20°w strike extends northwards from Yusuji to the north of Iriumi spa and consists mainly of one syncline and one anticline, which shows isoclinal folding with westward dip.

 Field observations suggest that the Futaba fold structure was formed under the stress field where 03 the maximum compressive principal stress, lies nearly in east or west direction with horizontal angle (Fig. 4). Whereas, the cover rocks were deposited successively in upward sequence from the west towards the east. But, the dips of each formation are not equal. Namely, when the dips of each formation west of the Futaba thrust are observed, the Futaba Group and the Iwaki Formation dips at 10°−15°E, the Asagai and Shirasaka formations at 20°−40°Ｅ，and the Yunagaya Group and the Nakayama Formations show dips of 50°−80°Ｅ or W. Whereas, in the east of the Futaba thrust, the Izumi Group dips at" 10°E, and already reported by Ninagawa (1954) shows that the formations older than        ＝

the Izumi Group were not folded and dip eastwards gently. Also ａ remarkable angular unconformity is developed between the Yunagaya Group or Nakayama Formation and the Izumi Group (Fig.ろ), indicating ａ considerable tectonic movement between the two groups.

Hence, this fold structure was originally ａ ｎｅχualfold which subsequently transformed into the present structure by the horizontal stress of the tectonic ｍＯvement, before the deposition

１ ｒ ゝ ６ _{高知大学学術研究報告  第21巻.･ 自然科学  第１号}

Ｎ

H-2

Ｎ

Ｈ-３

Fig. 4. Principa】stress field along the Futaba thrust fault.

of the Izumi Group.

 The

Futaba thrust fault extends with N-S

to NNW-SSE

trend along the western margin

of distributionof the Izumi Group‘from Yusuji

Hirono･cho

to the vicinity of Iwanuma-cho,

Miyagi

Prefecture and is equivalent to the“Hisanohama-Iwai!uma

dislocation line” of Yabe

and Aoki (1924). It is considered that the thrust was formed before deposition of the Izumi

Group

but again reacted after development

of the Izumi Group,

because the thrust fault cuts

the Izumi

Group

and is covered in part by the sedi皿entary rocks of the Izumi Group.

The

movement･ of the reactive faultis considered to have been slight from

the following reasons,

that is, ' 1) the shear along this thrustis small.

2) the pumice

grains contained in the

Izumi

Group

adjacent to the thrust are littledeformed

and 3) the thrust is discontinuous

(Tsuneishi,

1966).

       METHOD OF ＪＯ]:NT STUDY

 The method of investigation of the joints with' the geological structure were according to Ｍｕr尽i'ｓreport C1965)｡

 In order to observe the state of development of the joints in detail, it is advisable to measure the joints of different rock materials. Because each formation in the investigated area is classified, on the whole, by the rock facies, it does not contradict to consider that 出ｅ measurement of the joints of each formation, agrees with that of each different kind of rock materials. Accordingly, the joints in each:formation are measured in this paper. The stastical method was done in the following order･; at (きach locality the strike and dip of   ■    l      i        1

30 t0 50 joint planes were measured, and a distribution dli昭ram was made by plotting the poles･ perpendicular to the joint plane on the lower hemisphere of the Schmidt equal-area

net, :and then by drawing

the ｃ(χitour

lines in the net.

The

predominant

direction of the

observed joints are shown

for ･each group in Figs. アー9.

       MEAN･ DENSITY OF THE JOINTS

 For the analysis of the joints, the mean density of the joints at each locality was determi一 所d because of its usefulness to interpretate the development of the joints. Here, the mean ｄｅｎがtyiｓ｡defined as the ratio of the number of occurrence of the joints to the interval in

meters (number / meter). The mean density of the joints in the Futaba Area is shown in Fig. 5. Also, the density contour map of the joints is shown in Fig. 6.  :

The Futaba Group consists mainly of coarse grained sandstone and the mean density values are from １.0 to 2.1 numbers / meter. The value is below 1.0 in the Iwaki Forma-tion which is composed mainly of conglomerate and conglomerate-bearing coarse grained

sandstone. In the fine grained sandstone of the Asagai Formation and mudstone of the S!lirasaka Formation the mean density ranges from ３.8 t0 5.0 and the development of the joint is good.. In the conglomerate and conglomerate-bearing coarse grained sandstone of the Ｋyｕ!ugidaira Formation and conglomerate-bearing coarse grained sandstone of the Goyasu Formation the, mean density values are below 1.0. The Miziinoya and Kameno-0 forma-tions ａｎｄ‘theHonya Mudstone Member of the Taira Formation consist of muddy rocks that    ’j

have been influenced strongly by the movement of the Futaba disturbed zone and the mean density values are 3.0-5.0. The value is below 1.0 in the Misawa Sandstone Member of        Φ    1       1

the Taira Formation which is composed mainly of conglomerate-bearing coarse grained  ｜

sandstone. In the Izumi Group and the Yamadahama Formation both composed mainly of tuffaceous muds tone, the mean density value is less than 1.5 and the development of joints

  ’７ ＣＤ Ｅこωｏｃ ５ ４ 0 ０ Ｃ Ｍ １ ０

      Fig. 5. Mean density of jointsin the Futaba Area.

ＦｕこFutaba Group, Iw ：Iwaki Formation, As ：･Asagai Formation Si ： Shirasaka Formation, Ku ： Kunugidaira Formation, Ｇｏ'：Goyasu Formation Mz ： Mizunoya Formation i   ．， Ｋｍ： Kameno･o Formation, Ho ｚ Honya Mudstone Member, Ｍｉ：Misawa- Sandstone Member, Iz ： Izumi Group, Ya ： Yaraadahama Formation

８

Ｎ丿千丁ト

高知大学学術研究報告  第21巻  自然科学  第１号 o KIT ３

喩暫り

ｆ

Fig. 6. Density contour map of joints in

 the Futaba Area.

is bad. Even in the neighbourhood of the ’Futaba thrust fault, the mean density values of the Izumi Group do not change so much.

  Based on the results on the mean density values of the joints described above, the following is considered ；  0 The mean density of the joints  becomes larger in the order of, muddy  rocks as in the Shirasaka Formation

 吋

 sandstone as in the Asagai Formation,  coarse grained sandstone as in the  Futaba Group and conglomerate and  conglomerate-bearing sandstone as in  the Iwaki-, Kunugidaira- and the  Goyasu formations and the Misawa  Sandstone Member, and the joints are  developed as good as in the former  rock materials. This ｐｒｏ▽esthat it is  as ductile as in the latter materials  andas brittle as in the former materials  during the formation of the joints. and  itdoes riot contradict the result of the  high pressure tests on the sedimentary  rocks CHoshino, 1967; Mitsui, 1971).  2) The mean density values ranges  from ３.0 t0 5.0 in the muddy rocks  of the Yunagaya Group and is below ユ：5 in that of the Izumi Group, that

 is, a large difference is recognized  between the mean density values of  .bothgroups. This proves that ａ large  difference exists between the tectonic  movements to which the Yunagaya and  Iztuni groups were subjected. This is  also upheld by the remarkable angular  unconformity between the Yunagaya  and lｚｕ皿igroups. The Futaba thrust  fault occurs at the borderline between  both groups. Accordingly, from the  （!ifferenceof the mean density values,

On the Origin of the Jointsin the･Futaba Area of the Joban Ｃｏａ】-Fie】d,Fukushiraa Pref. 9

it is considered that the Futaba thrust fault was formed before deposition of the Izumi Group,

and that the thrust strongly affected the Yiuiagaya Group.

3）Ｔｈｅmean

density values in

tｈ（きmuddyrocks of the Izumi

Group

are below 1.5, and this value is not very large even

near to the Futaba thrust. The

Futaba

thrust fault cuts the sedimentary rocks of the Izumi

Group

and is also covered by the group. Hence,

from the small value and invariability of

the mean

density of the Izumi

Group,

it is considered that the reacted Futaba thrust fault

was not large so much

in scale and had only littleinfluence on the Iziimi Group.

This is

also upheld by that the pumice

grains contained in the Izumi Group adjacent to the thrust

are littledeformed.

JOINT

SYSTEM

OF EACH

FORMATION

(Description)

 In the basement composed of granitic and metamorphic rocks, the predominant joints strike N60°Ｅ一N60°χｖ with dips of 70°−90?Ｎ or S， and strike N100ぺＶ一N30°E with dips of 65°− 90°Ｅ or w, whereas those with N-S strike and with 10ｗ dip are rare. In the Futaba Group, the predominant' ones have N70°Ｅ一N60°ぺＶ strike and dips of 500−９００Ｎor S. The other predominant ones strike N一S and dip at 800−90°Ｅ or Ｗ. In the Iwaki Formation。 the dominant joints strike N55°E-N55°Ｗ with dips of 55°−85°Ｎ or s, and strike N5°Ｅ− N35°Ｗ with dip of 75°−80°Ｅ or Ｗ， whereas those with NW-SE strike are rare. In the Asagai Formation, the most dominant joints strike N60°E-N70°Ｗ with dip of 55°−90°Ｎ or S， and the dominant ones of second rank strike N10°−30°Ｗ with dip of 50°−70°Ｅ or W, whereas those with NVV一SE strike are rare. The dominant joints in the Shirasaka Formation have faces with N75°Ｅ一N70°ぺＶ strike and dip of 55°−85°Ｎ or Ｓ， and Ｎ５°− 20°Ｅ strike and dips 60°−85°Ｅ ｏｒべA^, whereas, those with NIV一SE strike are rare. In the Kunugidaira Formation, the most predominant trends are N75°Ｅ−N60°W (dips 55°−90°Ｎ or S) and the predominant ones of second rank have N10°E-N15°Ｗ (dips 50°−85°Ｅ or Ｗ）， whereas those with NE − sw strike are rare. In the Goyasu Formation the dominant joints strike N60°E-N60°Ｗ with dips of 40°−70°Ｎ or S and strike N15°Ｅ−N30°Ｗ with dips of 65°−８５９ＥｏｒべA'^,whereas those with ＮＥ一SべＶ strike are. rare. In the Miziinoya For-mation the most dominant joints have faces with N15°Ｅ一N30°w strike and dips of 40°− 85°Ｅ or w and the dominant ones of the second rank are with N55°Ｅ一N65°w strike and dips of 65°−85°Ｎ or Ｓ， whereas those of NE-SW strike are rare. In the Kameno-0 For-mation the predominant ones have N20°Ｅ−N30°Ｗ strike and dips of 70°−90°Ｅ or ぺN. The other predominant ones strike N65°Ｅ一N60°ぺＶ and dip at 50°−90°Ｎ or S， whereas those with Nχ,V-SE and ＮＥ一sw strikes are rare. In the Honya Mudstone Member of the Taira Formation the dominant joints strike N85°Ｅ一N70°Ｗ with dips of 40°−90°Ｎ or S and strike N25°E-NIO°Ｗ with dips of 70°−90°Ｅ or Ｗ， whereas those of NW-SE strike are rare. In the Misawa Sandstone Member of the Taira Formation the dominant trends are WNW-ESE to ＷＳＷ一ENE and NNW-SSE. In the Izumi Group bordering the Futaba thrust

fault, the predominant joints have faces with N30°Ｅ−N30°Ｗ strike and dips of 35°−85°Ｅ or W, and the predominant ones of the second rank have N70°Ｅ一N60°べＶ strike and dips of 40°−85°Ｎ or S, whereas those of NE-SW and NW-SE strike are ａ

10

Ｎノ詣丁ト

矢

高知大学学術研究報告  第21巻． 自然科学  第１号

六千上作

羊 十

０  １  ２、３  ４  １ ｔ KIDO一一RIVER

Fig. 7. Joint pattern in the Shiramizu Group

little dominant as compared with the

pre-Izun!i Group. In the Izumi Group

and the Yamadahama Formation at a

distance from the Futaba thrust fault。t屈耳lost predominant joints strik N

60°Ｅ−N60°w with dips of 60°−90°Ｎ

or S and the predominant joints of

the second rank strike N30°Ｅ−N30°w

with dips of 70°−90°Ｅ or ぺＶ.ぺA^hereas,

the joints witli NE-SW and NW-SE

strike belong to the third rank and

are more dominant than those in the

Izumi Group bordering the Futaba

thrust fault.

ORIGIN

OF

JOINT

SYSTEM

 Based on the joint systems in each formation described in the foregoing section, the writers try to consider the origin of the joint systems in the investigated area｡

 Joints in the basement rocks, and in the Futaba-, Shiramizu-, and the Yunagaya groups are common, and their predominant trends have faces with ぺＶＮぺA^-ESE to WSW-ENE (N60°−N60°べｖｊ and ＮＮＥ−SSべｖ to

ＮＮＷ−SSE(N30°E-N30°W). This

tendency is recognized in the Izumi Group and the Yamadahama Forma-tion｡

 The dominant joints with NNW-SSE to ＮＮＥ一ssw strikes in the Yunagaya and Izumi groups are closely related to the Futaba thrust fault,

which is the outstanding major fault in the surveyed area. Also, the joints

with NE-SW and NW-SE trends

are better developed in the Izumi Group and the Yamadahama Forma-tion in comparision with those in the

pre-Izumi Group.

'ln the Futaba and Shiramizu groups 由ｅ joints with WNW-ESE and

NNW-SSE to NNE-SSW strikes

are developed dominantly {Fig.ｱ). These two dominant joints were for-med by the structural control of the basement rocks, from the following reasons; 1) The tendencies of their strikes agree with those in the base-ment rocks. 2) The Futaba and Shiramizu groups show dips of 10°− 30°E, and it is considered that the dips eχpress nothing but the inclina-tion ｍ the east wing of up-warping of the basement by the uplift of the Abukuma Mountains, and not the influence of the Futaba thrust fault｡  In the Yunagaya Group, which was strongly influenced by the Futaba thrust fault, the joints extending from the Kunugidaira up to the Misawa Sandstone Member of the Taira Forma-tion are common the predominant trends ａｒｅべＶＮＭ/一ESE to wsw-ENE and NNW一SSE to NNE-SSW (Fig. 8). The predominant joints with ＮＮべＶ一SSE to ＮＮＥ−SSぺＶ trends

were probably formed by the same stress as the Futaba thrust fault, in which the ｍａχimum compressive pri-ncipal stress lies with horizontal angle

towards the east or west, because the joints are nearly parallel to the Futaba

thrust fault which has faces with N-S to NNW一SSE strike and dips of 35°− 70°W.べA^hereas, the other predomi-nant ones with ぺＶＮＶＶ一ESE to wsw一 ENE trend were developed by,the strac-tural control of the basement rocks,

and not by the same stress as the Futaba thrust fault, from the following

Ｎ心千ト

片九

半

KIDO-RIVER NOHAMA D  1  2  3  4  5 KM

12

Ｎノ下Ｔト

高知大学学術研究報告  第21巻．自然科学  第１号 KIDO-RIVER OHI   J ０  １  ２・ ３  ４  5 KM Fig.  the Ｍβ

9. Joint pattern in the Izumi Group and Yamadahama Formation

 reasons ； 1) The tendencies of their  strikes agree with those in the Futaba  and Shiramizu groups and the base- ment rocks. 2）lt is observed in the  Goyasu Formation, distributed west of  Shimo-Kita-Hasama, Hirono-cho, etc..  幽ａt the joints with WNW-ESE to  wsw一ENE strike were cut by thど  joints with NNW-SSE to NNE-SSW  strike.

  In the Izumi Group and the Yamada- hama Formation the most predominant ・ joints have faces with NNW-SSE

 to ＮＮＥ一SSべｖ and 凧

 wsw一ENE, and the former is better  developed along the Futaba thrust fault  than the･ latter which is distributed ｏ丘

the thrust fault (Fig. 9). The other  predominant ones have NE-SW and  ＮべA^-SE strikes which are well de- ve loped off the Futaba thrust fault.  All of the joints in the Izumi Group  and the Yamadahama Formation are  only tension joints. The judgements  ‘ofthe tension joints are based on that;  O The joints have ａ small crevice of  ａｂｏリtseveral millimeters in width,  and are filled with limonite. 2）Ｔｈｅ

plumose markings are observed on the  fracture surfaces.

  The tension joints with NNW-SSE  to ＮＮＥ一SSぺｖ strike are developed  mainly in the area along the Futaba  thrust fault, and it is considered that  φｅ joints are closely related to the  thrust fault, because the strikes are  nearly parallel to the trends of the  thrust. It is logical that these joints  with their strikes were formed iinder  the stress fields in which the minimum  compressive principal stress (or the

of the Jointsin the Futaba Area of the Joban Coal-Field, Fukushima Pref. 15

with horizontal angle towards the east or west. Whereas, when observed in the stress fields of the Futaba thrust fault, the maximum compressive principal stress ("3) acts with low angle towards the east or west, the intermediate principal stress 02）lieｓ with 10ｗ angle towards the north or south, and the minimum compressive principal stress （ら）ａｃts nearly with vertical angle (Fig. 4). Accordingly, it should be considered that the tension joints with NNW一SSE to ＮＮＥ一ssw strike were formed under the stress fields in which the maximum tensil principal stress (or the minimum compressive principal stress) (''l) acts with low angle towards the east or west and the maximum compressive principal stress （（y3） was released after the formation of the Futaba thrust fault. Whereas, it is thought that the tension joints with WNW-ESE to ＷＳぺA^-ENE strike were formed after the formation of NNW-SSE to NNE-SSW joints, from the following reasons: 1）Ｔｈｅ faults with E-W strike parallel nearly to the joints with their strikes cut the Futaba thrust fault (Fig.ろ）. 2）Ｔｈｅ joints become dominant towards the area separated from the Futaba thrust. 3) The tension joints are considered to have been formed iinder the stress fields in which ･71，the maximum tensil principal stress lies with horizontal angle towards the north or south, and （71 at the formation of this joints is different from ''l at the formation of ＮＮＷ一SSE to

NNE-SSW joints. Finally, the writers infer that the tension joints with ＮＥニsw to ＮべＶ一SE trends were formed under the same stress fields as the joints withぺＶＮＩＶ一ESE ｔｏぺA^SW − ENE strike, because the distributions of the joints with their trends ａrｅ･defined

in the areas away from the Futaba thrust fault.

SUMMARY

  The results of the present study can be summarized as follows ；

  1. The oldest joints in the Futaba Area are the ones withべＶＮぺA^-ESE to ぺVSぺA^-ENE  trends in the Futaba-, the Shiramizu-, and the Yunagaya groups, and they were formed  by the structural control 0f the basement rocks.

  2. The next old joints are the ones with NNW-SSE to NNE-SSW strike. These occur  mainly in the Yunagaya Group, and they were probably formed under the same stress fields  as the formation of the Futaba thrust which acted prior to the sedimentation of the Izumi  Group.       .，   3. The joint at the third stage are the tension joints with NNW-SSE to NNE-SSW  trends in the Izumi Group, and they were formed under the stress fields in which the  maximum tensil principal stress Cor the minimum compressive principal stress) lies with  horizontal angle towards the east or west, after the reaction of the Futaba thrust fault and

after the deposition of the Izumi Group.

  4. The youngest joints in the investigated area are the tension joints with WNW-ESE  tｏぺA^SW 一 ENE strike in the Izumi Group and the Yamadahama Formation, and they are  considered to have been formed after the formation of NNW-SSE to NNE-SSべＶ joints in  the Izumi Group.

  5. Based on the results of the mean density values of the joints, it became clear that  the Futaba thrust fault had strongly affected the Yunagaya Group but had only littleinfluence  on the Izumi Group and the Yamadahama Formation.

   （ｉｎJapanese with English Abstract)

 5) Hirano, M. , Joint System of the Rokko Mountain即hge-Tectonics of Uplifted Massif in the    Kinki District, Japan (I). Ｏｓａｋａ ＣｉりＶｎｉｖ.， Ｊｏｕｒ. ＧｅｏｓｌＣｌ. ,V.12, Art. 3。p. 25-44 (1969)  6) Hoshino, K. , Fracture System and Natural Gas Occurrence in the Joban Coal-Field.  Ｒｅｐ･    Ｇｅｏｌ.Ｓｕｒｖ. Ｊａｐａｎ,ｎ０. 210,p. 1-36 (1965)・

 7) Hoshino, K. , Mechanical Properties of Some Japanese Sedimentary Rocks under Confining    Pressure (1).Ａｐｐｌｉｅｄ Ｇｅｏｌ., V. 8, no. 3, p. 151-164 (1967Kin Japanese ｗit!1 English Abstract)  7) Iwai, Jﾝ, Interformational Abnormal Deposition Observed in the Kameno-o Formation. T砿oku   Uniｖ.，Inｓt.Ｇｅｏｌ， Paleont. , Contr･， ｎ０. 42， p. 1-22(1953) (in Japanese with English Abstract)  8) Iwao, S.

)and Matsuii H. , Explanatory Text of the

Geological Map of Japan, Seal 1 : 50,000,   Taira and Kawamａｅ.  Ｇｅｏｌ｡Siiｒｖ，Ｊａｐａｎ,Tokyo (1961) (in Japanese with English Abstract)  9) Kitamura, N. , Preliminary Report on Systematic Jointing in the Taishu Group developed in   Mitsushima-cho, Tsushima Islands, Japａｎ. Toiiofeｕ Ｕｎｉｖ.， Ｓｃｉ.Ｒｅ＊･，２ａ

  Vol., no. 5, p. 303-319 (1962)

10) Mitsui,･S. , Studies on the Mechanism of Deformation of Sedimentary Rocks in the Iwaki Area   of _{the Joban Coal Field, Fukushima} Ｐｒｅtｅｃtｕｔｅ. Toliofeｗ. Ｕ･niｖ.， Ｓｅt.Ｒｅｉ）･，２, ｢. Ser. (Geol. )>   V. 42, ｎ０. 3， p. 199-272 (1971)

11) Murai, I. , The Fracture System Deve】oped in the Vicinity of Katsuura, Boso Peninsuχａ.  Ｅａｒthｑ.   Ｒｅｓ. 7z7μ.，￡ ｢/. , V. 43, no. 4， p. 829-869 (1965) (in Japanese with English Abstract) 12) Murai, I. , Fracture System developed on the Island, Awa-shima, near the Epicenter of the Niigata   Earthquake in 1964. ￡αΓthq. Iしs. Inst., B ｢/., V. 43, no. 3, p. 611-624 (1965) (in Japanese   with English Abstract)

13) Murai, I. , Tectonic Analysis of the District between Murakami and Tsuruoka. ＧｅｏＬ  Ｓｕｒり.   Ｊａｐａｎ., Ｓ＊ｅｃ. Ｒｅ＊。no. 3， p. 15-25 (1966) (in Japanese with English Abstract)

14) Murai, I. , Fracture Analysis of the Seismic Area where the Matsushiro Earthquake Swarm has   been occurring. Ｅａｒth･ｑ. Ｒｅｓ. Inst., ＢｕＵ.,ヽV. 45√no. 2, p. 505-536 (1967) (in Japanese with   English Abstract)

15) Ninagawa, S., Seismic Prospecting in Tomioka District, Joban Coal-Field.Ｇｅｏｌ..Ｓｙ・. Japan･   Ｂｕll･， Ｖ. 5， ｎｏ.･ 9， p. 483-492 (1954) (in Japanese with l?:nglish Abstract)

16) Sugai, N. , and Matsui, H., Geological Maps of the Ｃｏａ! FieMs of Japan (0, Geological Map   and Explanatory Text of the Joban Coal Field, Scale l1 :.50,000. p. 1-143,Ｇｅｏｌ. Ｓｕｒてノ，Ｊａｐａｎ，   Tokyo (1957) (in Japanese with English Abstract)

17) Yabe, H. , and Aoki, R., Mesozoic and Cenozoic History of the Abukuma Plateau and its   Foreland along the Pacific Coast. Ｊａｐ. Ｊｏｕｒ. ＧｅｏｌﾚGeogr. , V. 3, nos. 3-4, p. 23-27 (1924) 18) Yamaguchi, T. , Tectonic Study of Rock Fractures. Ｇ・ ･.Ｓ・c. Ja戸αzz･ Jour. , V. 71, no. 837,   p. 257-275 (1965)

19) Yanagisawa, I. , Geology and Paleontology of the Takakura-Yaguki Area, Yotsukura･cho.   Fukushima Prefectuｒｅ.  Tohofeｔt Univ. ,Ｓｃｉ. Ｒｅｐ･，２ｎｄ Ｓｅｒ.（ＧＥ。/. ), V. 39, no. 1. p. 63-112   (1967)

 −●Ｗ哺

E二]]二]I

YA : YAMADAHAMA FORMATION

｢T￣T￣｢]＼ sK:SEKINOUE FORMATION

[で￣う￣乙子Ｉ NA:

NAKAYAMA FORMATION

rt￣lJTy]I MI:MISAWA SANDSTONE MEMBER

 一一一

E三万三五二I HO:

HONYA MUDSTONE MEMBER

EΞΞΞ] KM:KAMENO-0 FORMATION

nrai]

Mz: MIZUNOYA FORMATION

匹回ごI

GOI:GOYASU

FORMAT

IOh､j

F三三司KU:

KUNUGIDAIRA

FORMATION

 −−

｢三￣三Ｔﾖ!SI : SHIRASAKA FORMATION

Ｆ耳:呂:呂Trr

AS: ASAGAI FORMATION

｢うてylTyllｗ:IWAKI FORMATION

Ｅこ二召ＩＦＵ:ＦりITABA FORMATION

｢ﾆFて:i:でRI   GRANITIC

・ROCKS

  Ａ        ･-  o一 一250   ｍ Ｂ

Fig. 3. Geological map of the Futaba Area

Ｎ Ｏ   一一  １   ●‘  ２      ３      ４      5 KM 250 ｍ 250 ｍ    m    0    25 0 −

くよずノ

DO-RIVER

YAMADAHAMA

Ａ

  B’