Nakattlky6 Formation - トップページ - 横浜国立大学学術情報リポジトリ

TyPe locality ^: The present formation crops out along the river‑side of the Nakatu‑gawa ^near the gorge of the Nakatu‑ky6, Hanbara, Aikawa･mati.

Lithic character ^: It consists largely of various kinds of accidental ^pyro‑

clastic rocks intercalated with marine sediments.

Compared with the other formations, the present formation is marked by the exceedingly variable lithofacies. The lateral and vertical transitions of the accidental pyroclastic rock into the marine sediment, especially the conglomerate, ^are found in various horizons of this formation, highly indicating

that the regressive conditions ^were relatively prevailing during the period of the deposition of the ^present formation.

Again, the rocks making ^up the present formation are commonly calcareous in composition, ^some of which contain abundant marine fossils. The fact is another feature uncommon in the other formations developing in the present reglOn.

GeograPhical distribution : This formation distributes widely, occupying the eastern margin of the ^present region

(Fig. 1).

The general trend of this formation roughly coincides with that of the preceding, although ^a gentle fold having an axis of N‑S direction is found

92 K. MIKAA(I

in the southern extension.

The. present

Lformation is in fault contact with the Kobotoke series ^at ^a dislocation line named the T6noki‑Aikawa line _at the eastern limit of the distribution, 女hile its northwestern and southeastern extensions ^are separated from the Tanzawa group by the Aonohara‑Susugaya line.

T血e total thickness is estimated ^to be over 1,loom. in the ^type locality.

Division : From the lithic character, the present formati()n ^can be sub･

divided into fotlr members, ^as described below (Table

1),

and their straigraphic relations ^are always gradational.

NakatukyるVolcanic･breccia

TyPe locality ^: This member is typically developed on the Nakatu‑ky6 hiking path, ^west of the Isi‑goya, Hanbara, Aikawa･mati.

Liihic character ^: It consists largely of ^a thick layer of massive volcanic breccia and lapilli･tuffwith ^an occasional intercalation offine‑tuft and ^coarse‑

tuff. The volcanic breccia and the lapilli‑tuff contain abundant blocks or

lapillis of various kinds of augite‑andesites and ‑basalts in their ^common matrix of coar∂sイuff, and the transition between the ^two is gradual. The 血e‑tuffand the coarse‑tuffare mostly of andesitic ^to basaltic composition, occurring ^as ^a thin layer, several ^meters in thickness. Hydrotbermal actions

glVe a Strong effect ^on all of these rocks; ^most of the

ejecta

^are ^converted

into _green, uprplisb, ^or reddish rocks, while the matrix is commonly chloritized and/or epidotitized.

Distribution ^: This member occupying the basalpart of the ^present for‑

mation is distributed in a lenticular area

ranging

^from ^the ^vicinity ^{of Naka･}

kaido, Tukui･mati, to the upper ^course of the T･valley. Besides the ^type locality, it is separably observed in the western part of Mt. Nan‑zan, and also in the upper ^course of the U‑valley, the north of Mt. Bukka‑sam. The southern limit of the distribution is in the upper ^course of the T‑valley, where it _passes into an alternation of coarse‑tuffand

lapilli‑tuffbelonglng

^tO

the Nakatuky6 tuft member.

In the river‑bed of the Kusi‑kawa near Nakakaido, Tukui･mati, there is

a small ^exposure of volcanic breccia litbologically identical with the above volcanic breccia, probably representing the northern limit of this member･

The general trend of this member is NE‑SW, and its dip declines inter･

皿ediately ^to the northeast･ This member is intruded by a few dykes of andesite in the ^type locality. T･he thickness,is estimated ^to be 600m･ in the type.

Geological and Petrographical Studies on the Tanzawa Mountainland, Part I 93

zytlnrei掃ge Conglomerate and Sandstone, Itid6 MtLdstone and ^Sandstone TyPe ^locality ^: ^The ^former is well exposed in the vicinity of the Zyunrei‑

Pass,

Atugi‑si,

and the latter in the vicinity pf ltid6, the ^same city･

Lithic character ^: The Zyunreit6ge conglomerate and sandstone member consists largely of conglomerate ･ and t･uffaceous sandstone. The conglomerate is composed lorgely of welトrounded pebbles, such ^as quartzite, greywacke, clayslate and chert, all derived from the Palaeozoic system･ With these pebbles ^are also intermingled ^a few gravels of ^green tuft derived from.the

strata lying conformably under the present members･ The tuffaceous sand‑

stone is rather subordinate, being intercalated between the strata of ^con‑

glomerate ^as ^a thin layer.

Blackish green mudstone is the most predominant rock ^type of･ the ltid6 mudstone and sandstone, associated with yellowish coloured, tuffaceous sand‑

stone in various proportions and in different horizons.

Disiribution ^: The above ^two members occupying approximately the ^same

･borizon ^as the NakatukyeI VOIcanic‑breccia ‑member locally distribute in the southern extension of the present formation, and the stratlgraphic relation

between the ^two is always gradational, both horizontally and vertically.

Greenish black coloured, tuffaceous mudstone with slightfissility is develop‑

ed in the western tributaries of the Koai‑gawa, northeast and ^east of Mt.

Haku‑san, probably representing the northern extension of the ltid6 mudstone and sandstone.

The absence of ^proper ^exposures makes it impossible to know the detailed structure of these ^two members,‑the ^structure which, however, is considered̲

to show ^a synclinal fold baying an axis of N‑S trend.

In each ^type locality, the present two members ^are estimated ^to be 270

m. and Boom. in thickness, respectively.

Fossils: The fossils found in the present members ^are ^as follows :

Members I^Locality ^Nos.

∃

^Fossils ^Localities ^{I Mother} ^Rocks

Zyunreit6ge conglomerate and sandstone ltid6 mudstone and

conglomerate

Calcareous _algae, Bryo･

Boa, Minor foramini‑

feres, Molluscan and plant fragments

Calcareous algae, Plant fragments

Southeast of^the Muzinazaka ^‑ Pass, Atugi‑si

Southwest of lwasaka, Atugi‑

Tuffaceous sandstone

Tuffaceous conglomerate

Nakattlky6 Tuff

Type ^locality ^: ^The ^type ^locality ^{is in the} _river‑bed of Nakatu･ky6 ^near

94 K. MIEAhII

the lsi‑goya, Ⅲanbara, Aikawa‑mati. It is also well exposed along the river‑

bed of the

T･心alley

^(Fig.

1)

^Bowing northe'astward from the Hanbara‑goe, Susugaya, Kiyokawa‑mura.

Lithic character: In the ^type locality, the present member is composed mainly of various kinds of pyroclastic rocks intercalated with tuffaceous sand‑

stone. Most of the pyroclastics ^are commonly of accidental ^orlgln, COnSisting of gravels of sandstone, mudstone and quartzite in addition ^to somewhat variegated

ejecta

of andesite and basalt. They ^are associated with dacitic coarse‑tuffor lapilli‑tuffof the mixed ^type, grading into ^a true conglomerate, in different localities and horizons.

Tbe subangular breccias of hornblende‑quartz‑diorite and hornblende‑diorite, about 5cm. and 3cm. ^across, respectively, ^are occasionally found in the lapiui･tuff of the lower horizons. The former _must have been directly torn o庁from the quartz‑diorite of the Tanzawa massif, and the latter seems to be derivatives from some intrusive ^mass occurring in intimate relations ^to the above quartz‑diorite.

Disiribution ^: The present member constitutes the bulk of the Nakatsu‑

ky6 formation, and widely distributes _occupylng the eastern margln Of the present reg10n. The essential ^exposures ^are ^as follows, from 'northwest to southeast ^:

(1)

^the river‑side of the D6si‑gawa from Sindo, Tukui‑mati to the northeast of Abiko in the ^same ^town,

(2)

^the river‑side of the E.usi‑kawa from Nakakaido, Tukui‑mati, to Sakurano in the ^same ^town,

(3)

^the river‑bed of the UIValley

(Fig. 1)

^from ^Kasihara, Aikawa‑mati, to the northeast of Mt.

Bukka‑sam,

(4)

^the river‑bed of the T‑valley

(Fig. 1)

^from ^the Hanbara‑goe, Susugaya, to Tasiro, Aikawa‑mati,

(5)

along､ the mountain path leading to Mt.

Ey6gatake from Rokku, Aikawa‑mati,

(6)

along the mountain path leading to Mt. Ky6gatake from Y6no,

Atugi‑si, (7)

^the river‑bed of the Yude‑gawa from Funazawa to Hasiba, Atugi‑si, and (8)the river‑bed of the small valley且owing eastward from Mt. Takatori･yama. Several representative lithofacies of this member ^are shown in Fig. 3

(Section

^Nos.

84‑94).

In the above localities, conglomerate is commonly found as a small lens in various horizons of this member, especially ^ln its upper ^ones, and it in‑

creases gradually northwestwards until it becomes thickest in the northwestern

area. It is constituted exclusively of well rounded, pebble‑ ^to cobble‑ sized

gravels originating from the Palaeozoic _system. Another kind of conglomerate is observed ‑in ^a small tributary of the D6si‑gawa directly east of Mt. Sankaku･

yama, Tukui‑mati, where itoccurs as a belt, about 1 km. in width, elongated from north ^to south

(Fig. 1).

^This rock, consisting mainly of derivatives from both the underlying ^strata and the Palaeozoic system, shows ^strong

GeologlCal _and Petrograpbical Studies ^on the Tanzawa Mountainland, Part I 95

a氏nity ^to that of the Otiai formation, and it ^seems, therefore, ^to retain the transitional nature of the said formation into the Sekir6zan conglomerate and sandstone.

Besides the ^type locality, the gravels of the ^quartz ‑diorite and its related rocks ^are found in the following places ^:

(1)

^the river‑bed of the D6si‑gawa, north of Maedo, Tukui‑mati,

(2)

^a small tributary of the Kusi‑kawa, 300m.

northwest of Mt･ Minami‑yama, Kiyokawa‑mura, and

(3)

^the river‑bed of the tJ‑valley

(Fig･

1) 400m. _west of Kasihara, Aikawa‑mati. The r.cks c.ntain̲

1ng these gravels ^are commonly lapilli‑tuft of accidental ^orlgln, approximately occupylng the ^same horizon.

The general trend of the ^present member roughly coincides with that of the Nakatuky6 formation･ The total thickness i? about 500m. _at the ^type locality.

Fossils : As already pointed ^out, most of the rocks making ^up the present member ^are very calcareous, yielding abundant marine fossils as listed below.

Chlamys kaneharai (Yok･), ^one _of these fossils, is usually found in thin layers of accidental lapilli‑tuft or coarse‑tuff, and these layers approximate ^to ^ten sheets in di鮎rent borizons, some of which ^can be traced･ Calcareous algae, in many

cases, occurs as a colony ^2cm. ^across, which ^seems ^to be in close connection

with the above molluscan fossil.

Locality Nos･

≠

^Fossils ^Localities ^I ^Mother ^Rocks

Chlamys kaneharai

(Yo且.)

Chlamys kaneharai (YoK.)

Calcareous algae Plant fragments Chlamys kaneharai

(Yo王(.)

Chlamys kaneharai

(Yo且.)

Nonion _sp.

Milioliae _sp.

Texturalia _sp., Cal‑

careous algae, Bryozoa,

Molluscan fragments Chlamys kaneharai (Yo且.)

Calcareous algae Ditto

Minor foraminifers

A, small valley ^east of Mt. Eaku‑sam, Atugi‑si A small valley northeast of ^Mt. Haku̲sam

Vicinity of Odai, Atugi･si

Bottom of the V･valley, west of ^Mayumi, Ogino, Atugi‑si

A _mountain _path, northeast of Y6no, Ogino, Atugi‑si A mountain path, northeast of ^Mt. Ky6‑

gatake, Ogino, Atugi‑si A mountain path, north‑

west of ^Y6no

Accidental lapilli･tuff

Ditto

Accidental lapilti‑tuff, sandy tuff, and granule

sized conglomerate

Accidental coarse̲ and lapilli‑tu牙

Accidental lapilli‑tu牙, and tuffaceous sandstone Tuffaceous sandstone and shale

96 ^K. ^MIKAMI

Locality Nos. 1 ^Fossils

Chlamys kaneharai (Yo且.)

Calcareous _algae

Chla仇yS kaneharai

(Yo且.)

Calcareous algae Minor foraminifers Chlamys kaneharai (YbE.)

Calcareous _algae Calcareous _algae Bryozoa

Molluscan fr･agments Chlamy8 _SP.

Mesophyllum _sp.

Minor foraminifers Molluscan fragments Chlamys kaneharai (Yo且.)

Calcareous algae Pelecypoda _･fragments

Calcareous algae

Ditto

Minor foraminifers Molluscan fragments

Localities River‑bed of the T‑

valley, southwest of the Siokawa‑no‑taki, Tasiro,

Aikawa.mati Ditto

River‑bed _of the U‑val･

ICY, southwest of Kasi‑

bara, Aikawa･mati River.bed of ^the ^Na･

katu‑gawa, near lsi‑

goya, Aikawa‑mati River.bed df the Naka‑

tu･gawa, east df lsi‑

gOya

A mountain p白.th, south･

west of Niraone, Tukui･

mati

A small valley east of Mt. Sankaku‑yama, Tu‑

kui̲mati

Bottom of ^the P‑valley, southwest of Sek.i, Tu･

kui _̲mati

Vicinity of Sakurano, Tukui.mati

A mountain path, north‑

west of Mt. Sekir6‑lan, Sagamiko‑mati

Mother Rocks Accidental lapilli‑tuff

Accidental coarse･ and1 1apilli‑tu牙s, and dacitic lapilli･tuff

Accidental lapi11i･tuff

Accidental lapilli･tu庁 and ^sandy tuff Accidental ^coarse‑ and lapilli‑tuffs,and dacitic lapilli‑tuff

Accidental coarse‑ and lapilli･tuffs

Conglomerate

Accidentallapilli･tuff

Ditto

Shale and conglomerate

Sekir6zan Conglomerate and Sandstone

TyPe locality ^: This member is well developed along the river‑course of the D6si‑gawa southwest of the reservoir of Aoyama, Tukui‑mati,

althoughit

thickens beyond the northeastern limit of the geological ^map, Fig. 1.

Lithic character ^: The most predominant rock ^type of this member is coarse‑

to medium‑grained conglomerate, associated with dark green to black sand‑

stone and shale.

The conglomerate is lithologically identical with the conglomerate found:･

in the present formation ^as ^a small lens, ^as already mentioned. The

pebbles ^are exclusively derivations from the Palaeozoic system, and the matrix is of basic pすroclastic materials. The sandstone and shale ^are well‑bedded and intercalated in the conglomerate in various proportions and in different horizons.

Disiribuiion : This member extends from the southwest of Seki, Tukui‑

mati, ^to the vicinity of Mt. Sekir6‑lan, Sagamiko‑mati, and further beyond

Geological and Petrograpbical Studies on the Tanzawa Mountainland, Part I 97

tbe､ northern limit of the napped ^area. It also is in the western ^area of Hanbara, Aikawa‑mati, Shokk6‑no‑taki, Tasiro, in the ^same ^town. The localities is shown in the columnar seccions Nos.

intermittently distributed and in the vicinity of the lithofacies of the essential 84, 85 and 90 (Fig. 3).

In the east and the southeast, the present member is bounded by the T6noki‑Aikawa line, while, in the northeast, it is in fault _contact ‑with an

alternation of andesitic coarse‑tuffand lapilli‑tuffbelonging ^to the Nakatuky6 tuff. This fault is observed in ^a small valley immediately _west of the reservoir, where it strikes N40oW, and dips nearly vertically.

This member attains ^to 500m. ^or ^more in thickness in the ^type locality｡

Fossils ^: A lfew gastropoda‑fragments ^are enclosed in conglomerate of

a dale in Aoyama, Tukui‑mati (Loc. 77).

Geologic Age and Correlation

The fossils collected by the writer from the present region ^are shown in Tables 2 and ^3.

It is evident from these fossils that both the Tanzawa and the Aikawa groups ^are largely of marine doposit.

Following the writer's discovery of LePidocyclina

(NephrolePidina)

ⁱⁿ

the Otiai formation of the Susugaya subgroup (K. MIKAMI,

1955a),

^H. ^SHIBATA (1957) _reported ^a ^new ^occurrence _of the ^same fossil in a dacitic tuffexposed in the river‑side of the Hayado‑gawa, the geological horizon of which definitely corresponds ^to the northern extension of the Fud6ziri dacite‑tuff member of the

6sawa

formation of the ^same subgroup. Accordingly, the Susugaya subgroup is the lower Miocene

(Fl)

ⁱⁿ ^age, and is correlated with the Onuma group of the Misaka massif in that it _occupleS the upper part of the Tanzawa _group, associates with dacitic, pyroclastic rocks, and also contains LePidocyclina _and ^Chlamys

kaneha;ai (YoK.).

As already stated (K. MIEAMI/

1955a),

^the LePidocyclina limestones ^re‑

ported by S. HANZAWA

(1931)

^from ^the ^present mountainland ^seem ^to belong to the lower horizon, probably the Hondani‑gawa formation, of the Oyama subgroup･ Thus the presence of three LePidocyclina‑bearing strata is Con‑

firmed in different stratigraphic horizons of the Tanzawa group, and the difference between the upper and the lower limits of these horizons exceeds 6000 m. in thickness. As already discussed by several authors

(S.

^HANZIAWA,

1943, N. IEBTBE, 1948, and J.MAKIYAMA,

1947),

^if LePidocyclina (NePhrolepidina) is slgni丘cant ^as ^an index fossil of the lower Miocene age, ^we may wonder bow the materials composing the Oyama and Susugaya subgroups have been

98 K. MIEAMI

Table 2. List of ^the fauna from the eastern region ^of ^the ^Tanzawa mountainland

Fossils Locality Nos.

1 2 3 4 5 6 7 8 9 10 ll 12 13 14 15 16 17

18 19

20 21 22 23 24 25 26 27 28 29

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

Pelecypoda Sole耽ya tOkunagai YoEOYAA(A ‥... ‥

Glyeym?ris osozawe%sis KANNO ^･ ‑ ‑ ‑ ‑ ･

Limopsw naたanurai ^KANNO ^‥ ^‥ ^‥ ^‥ ^‥...

Mytilu8 Sp‥.‥..…‥‥‥….‥...‥‥‥

Chlamy8 kaneharai YoEOYAMA ^‥ ^‥ ^‥ ^‥ ^‥ Ch. aff. kaneharai (YoI(OYAMA) ^‥ ^‥ ^‥ ^‥ ^‥

Ch. nipponensis KtlRODA ‥...‥‥‥‥‥..

Ch.sp. ...,...,

Lima (Aee8ia)^sp. (n._sp.) ^{..‑‑‑‑‑}

08trea _Sp. .‑‑‥..‥.‥‥.‥‥.‑‑‑‥.

Venerioardia _sp. ...

Luoino肌a Shinokii EIRAYAMA ‥ ‥ ‥...

Lu. otukai HATAI and NISIYAMA ^‥‥...

Conchoeele dis3'uncta GABB....,...,....,.

C.sp‥‥.‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥.

Nemocardium _sp. ...

A8tarte _Sp. ‑.‑‑‑..‑‑...‑‑‑‑.

.40, 54, 64,

‥‥‥.31 48, 49, 50 48, 49, 50 48. 49. 50 55, 60, 61, 62, 63, 65, 67, 68, 69, 72 .‖‥‥.48, ^49, ⁵⁰

‥‥‥‥‥.46

‥‥‥.30, ⁷¹

‥‥48, 49, 50 47, 48, 49, 50 .‥.48, 49, 50

‥‥‥‥‥.31

‥‥‥‥‥.46 48, 49, 50

‥‥‥.31

‥‥‥.46

‥‥‥.46 Scaphopoda

Dentalium yokoyamai MAKIYAn(A‥,‥.‥‥…‥.･･･‥‑･‑‑‑‑‑‑･‥‑.31 D.sp...‑...30,48,49,50

Gastropoda Diodora?sp... ... ...･

Tegula naru8ei SHIBATA.... ...

Umboniu仇Sp. (n. sp.).‥‥‥‑‑.‑.‑.

Turbo sp. ...

Astraea (Pachypoma) omorii SHIBATA ‥..

Ceratostoma _sp. ‥‥‥‥.‥.‥‥.‥.‥‑.

Siphonalia obliquinodus KANNO (MS). ^‥ ^‥

S.sp. ...,...

Kelletia nodulosa ^KANNO (MS)‥^‥.. ^‥.. ^‥

48, 49 48, 49 48, 49 48, 49 48, 49 48, 49 48. 49

50 50 50 50 50 50 50 40 48. 49. 50 Nassariu8SP...‑...34

Antho2:Oa FlabeLlunsp‑‑‑‑‑‑･････‑‑･･‑‑I‑‑‑‑･‑‑･･‑.:･･‑‑44

Foraminifera Cyola仇ina _sp.

.‥‑‑‑‑‑‑‑..‑.‑..‑‑‑‑‑‑‑･･･‑‑‑‑‑･.57 Pyrgoverpertilio‥.‥.……‥‥.‥･.･‥‑‑‑‑‑･･‑‑‑‑‑‑･･‑.32 Pyrgo sp.

...･.･･･...I.････････..･･....･.30, 35 0rbulina sp‥‥..‥‥.‥...‥‑...‑.‑‑‑‑..‑‑.‑‑‑....‑‑‑.‑‑.17

Robulus _sp.

‑‑...‑‑‑.‑.‑...‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑.30

Nonion sp...‑...52, 63 Miliolide sp...63

Texturaliasp･

･:･･･････････‑‑‑‑･･･‑‑･･･････････‑･････････‑‑‑･63 Lepidoeyolina n%pponica HANZAWA

‥.‥‥‥‥.･.･‥.･‥.‥･‥‥.A.･.･.･‥.52 Amphi8tegina radiata FICHTBL _and Moll‑‑‑‑‑‥‑.･.･･･････‑.･.･.‥52 Cyeloolypeu8 annulatus MARTIN..‥‥‑･･‑‑･･.･･･‥.･‥‥･･‑‥‥.･‥‥.52 Gypsma.globulu8 REtJSS

･･‑･‑‑‑･･････‑.I‑‑････････‑･･･‑‑･･･‑･52

Globigerma sp‑‥‑..‑‑‑‑.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ll, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 24, 25, 55, 57 Pelecypoda‑fragments

‥..‥..‥...‖‥‥..‥‥.‥‥.‥‥‥..‥‥...35, 56, 73

Gastropoda‑fragments‥..‥..‥.‥‥‥...‥‥‥‥‥‥..‥..,.‥.‥.‥56, 77

Molluscan fragments..‥.∫.‥.‥‥...29, 31, 32, 33, 34, 36, 37, 39, 40, 41, 44, 45, 47, 51, 52, 55, 58, 63, 70, 71, 76

Bryozoa..‥..‥‥ノ‥.‥...‥.‥‥‥....‥‥‥..‥‥‥‥55, 58, 63, 64, 70, 71

Fish _scale

‥.‥‥.‥‥.‥.‥‥‥‥‥‥‥‥‥‥‥‥‥.‥.‥‥.‥‥‥‥‥‥.30 Coral fragrrlentS

‥‥.‥‥‥‥...‥.‥‥‥.34, 39, 40, 45, 47, 48, 49, 50, 52

Spines of Ecbinoidea .…‥.‥‥‥‥‥.‥‥‥‥‥‥‥‥.‥.‥‥32, 33, 34, 35

Minor Foraminifera

‥‥.‥‥‥‥.1, 3,､7, 8, 13, 16, 20, 23, 26, 27, 28, 29, 31, 32, 33, 34, 42, 43, 55, 58, 66, 68, 71, 76

Geological and Petrographical Studies

,on t壬1eTanzawa Mountainland, Part I 99

Table 3. List _of the flora from the eastern region Of the Tanzawa mountainland

Foss.ils

1 2

Mesophyllu仇SP.

Locality Nos.

Algae

‥‥.‥‥‥‥‥‥‥‥‥.‥‥‥‥‥.‥‥‥‥‥‥‥‥.‥‥‥71 Lithotha仇niu飢Sp.

..‑‑‑‑‑..‑‑.‑..‑‑.‑‑‑‑....‑‑..‑48, 49, 50

3 Calcareous algae‥‥‥40, ^51, 52, 56, 58, 59, 61, 63, 64, 65, 67, 68, 69, 70, 72, 74, 75 4 Plant fragments

‥‥‥‥‥.‥‥.‥‥‥‥‥30, ^35, 38, 51, 52, 53, 56, 58, 59, 61

accumulated ^so thickly during that age. There is ^an exceptional ^reason, however, that these materials ^are exclusively products of ^gross volcanisms having taken place in a geosynclinal environment and ^are petrographically

very similar ^to each other. The age of the Oyama subgroup, therefore is also thought ^to be the lower Miocene

(Fl).

In addition, ^a quite similar example has been found by K. WATANABE, J.

ARAI, S. KANNO, and others

(1956, ‑57)

in the Tertiary system of the Chichibu basin. According to their results, LePidocyclina

(NePhrolePidina)

^occurs ⁱⁿ

the upper‑middle horizon of the Akahira _group besides the Cbichibumachi 宮rOup, and the difference between the ^two attains ^to about 4,000m. in thick‑

ness. This fact seems very favorable for the writer's consideration of the

above problem.

Judging from the stratigrapbic position and litbological characters, the Aikawa group ^can be correlated with the Nishikatsura group. K. WATANABE

(1954)

pointed ^out that OPerculina comPlanata

japonica

^HANZAWA in the Furuya formation of the lower _part of the ^same ^group is characteristic of his Matu･

yamian ^stage. These facts seem to indicate that the Aikawa _group IS Of the middle Miocene _age.

The age of the T6gatake subgroup is uncertain because of the absence of

palaeontological

^evidence. ^But ^the ^facts ^that ^this ^subgroup ^of ^more

thickness than 3,500m. lies under the Susugaya‑Oyama subgroup of which total thickness exceeds 6,000 mリand that the stratigraphic relations between the ^two is _very similar ^to those of the Chichibumachi and the Akahira 貰roups in the Chichibu basin, will probably lead to

Lthe corlClusion that at least a part of the T6gatake subgroup is of the Oligo‑Mioαne age

(E).

Geologic Structure

ln the present chapter the detailed structur,e of this

region

^is ^to ^be

･described丘rst ^; and ^next will be summarized the general trend of the whole Tanzawa mountainland based on the available evidence.

100

K. MIKAM:Ⅰ

S (a) N

(a) (b) (c) (d) (e) (f)

S̲こ (ち) N

･1

ミE

S (a)

!t)

LW!

岳

(I))

Fig. 5. Orientation diagrams for faults.

Poles of 395 faults found in the T6gatake and Oyama _subgroups. Contours

at 13%, 10%, 7%, 4% and 2%, per l% ^area.

Poles of 130 faults found in the Susugaya subgroup. Contours _at 6%, 4%,

3%, 2% and l%, per l% ^area.

Poles of 26 faults found in the Aikawa _group. Contours _at 4%, 3%, 2%, 1%,

per l% ^area.

Poles of 110 faults found in the metamorpbic ^area. Contours _at 4%, 3%, 2%

and l%, per l% ^area.

Poles of 150 faults found in the Fudakake fault･block area. Contours _at 7%,

5%, 4%, 3%, 2% _and l%, per l% area; maximum concentration 8%.

Poles of ⁸⁹ faults found in the western part of the Hadano basin. Contours

at 4%, 3%, 2% and l%, per l% ^area.

Geological and Petrographical Studies on the Tanzawa Mountainland, Part l 101

A. Structtlre Of the ^Present Region

Tbe Tanzawa and Aikawa groups in the present region ^as investigated by the writer ^are of comparatively simple monoclinal ^structure with ^a gentle

≡

anticline with ^an axis pitching eastwards and increasing the deviation westwards, ^as shown in the geological ^map

(Fig. 1).

^This structure is best interpreted as the southeastern representative of the dome‑shaped _structure

･surrounding the.quartz‑diorite ^mass.

As already stated, ^a ^great ^many faults are actually observed throughout the whole region. To bring out the degree and ^pattern of preferred orientation of these faults, the trend of 900 representative faults ^was determined by measuring the strike and the dip of each fault, which ^was then plotted by regional ^groups ^upon ^an equaトarea

projection

^by ^means ^of ^a ^Scbmidt ^net

(Fig. 5).

^From ^these

projections

it is clear that the faults occurring in the present reglOn ^are COmmOnly characterized by the predominance of the NE‑

SW trend, although their dips _vary in places.

The faults, ^moreover, ^are most abundant in the distribution area of the T6gatake and Oyama subgroups, and ^most of them ^occur in the southwestern

area of this region where the above ^two subgroups ^are strongly deviated by these faults, _suggesting that they have been formed in close connection with both the crustal ^movement during the ^severe volcanisms represented by the Tanzawa _group and the intrusion of the quartz‑diorite, ^as mentioned later.

The amount of the displacement causred by these faults _appears ^to be

insigni丘cant, but some marked exceptions ^are described below:

Tbnoki‑Aikau)a line : A prominent ^reverse fault which ^separates the Aikawa _group from the Kobotoke series is found along the eastern margln of the present reglOn. This fault represents the southeastern extension of the T6noki‑Aikawa line, ^now _under consideration, which ^was first designated

as the ^" T6noki‑Funazawa Line by H. FuJI班OTO

(1941),

and is probably ^one

of the most important tectonic lines that bound the major geologic units in

･tbe southern Fossa Magma:

It can be traced from Nenzaka, Sagamiko‑mati outside the map, south‑

eastwards ^to Tasiro, Aikawa‑mati, and its essential ^exposures have been reported in the previous ^paper

(R.

^SEINOKI and K. MIEAMI,

1954),

As already outlined, the Aika.wa group is the middle Miocene in _age. In the east‑side of the distribution ^area of the Kobotoke series, ^on the other hand, the Pliocene Nakatsu series lies unconformably ^on the said ^group without ^any

trace of disturbance. From these facts, the present fault ^seems to have

もeen formed during the late Miocene, probably at the end when the crustal

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