幸艮 文
551．762／．763（521．13）
Ay曲箆w箆E舘m魏置磯o飽恥eO3童訟摩e聡量聡s翻題，職ya琶茸Pre建ect躍e，No麓肚ea舘」呼鍾
By Fuminori TAKlzAwA＊
Abstract
The stratigraphy of the Ayukaw＆Formation，Ojik翫Peninsula，northeast Honshu， togetherwith thepaleontologicalnotes on the ammonites therein，is describedinthispaper．The Ayukawa Formation may’range from upPermost Jurassic（？）to Lower cretaceous（Lower Neocomian）in age and moreovcr，is assigned to the Berriasian age at least fbr its middle part。
This fbrmation is much thicker than estimated befbre乱nd than the equiv乱1ents ofother食reas in the south Kitakami Mountains．
且。㎞重⑱魂鵬曲鵬
The ojika Peninsula at the southemmost part ofthe sanriku coast is one ofthe Jurassic and． Cretaceous且elds in the southem part ofthe Kitakami Mountains．Conceming the geology ofthis area a number ofreports have been published by various authors．Nevertheless，theJurassic－Creta－ ceous boundaryof this食rea and the exact age of the Ayukawa Formation have remained undeter－
mined． In ordertosolvetheseproblems，thewriterfbrseveralyears engagedhimselfin the負eldworkon the Jurassic and．1。ower Crctaceous sequences of the Ojika Peninsula．In the co皿se of this survey
1／1，000route maps were made along several selected routes．As a result of this investigation，the stratigraphical scheme oftheJurassic－Cretaceous sequence，especially ofthe Ayukawa Formation， dif琵血gfhndamentally fピom the previous authors，schemes is established and the age oftheAyuka－
wa Formation c翫n be determined on the basis ofseveral ammonoid species。Thus，this paper deals
primarily with the stratigraphy ofthe Ayukawa Formationシadding to paleontological description of
the ammonites． Ac監聡⑪w亘e認g艦e就s－The writer is much indebted to Assistant Profbs白or Tadashi SATo of the Geological I血stitute，University of Tokyo fbr identifゾing ammonoid species and Dr．Itaru HAYAMI ofthe Department ofGeology，：Kyushu University fbr identification ofpelecypod species．
The writer also is gratefUI to the fbllowing staffmembers ofthe Geological Survey ofJapan：to Dr。Keisaku TANAKA and Dr。Nobukazu KAMBE fbr much help and valuable advice in the course of theworkンto Dr。Masatora KAwAI，Dr。Sachio IGI andDr．Masato KATADAfbr helpfUI suggestions on geology and also to Mr。Yoshio MAsAI fbr photographing specimens ofammonites． Further thanks are due to Dr。Haruyuki TAKAYAsHI of the Ibaragi University and Pro色ssor YoshioONuKIoftheMiyagiUniversityofEducationwhoprovidedthewriterwithvaluableinfbrma－ tion．
2・ 0服岨置聡e⑪蛋Geo且ogy
TheOjikaPeninsulaisoccupiedbyfbldedMesozoicstratawiththegeneraltrendofNNE－SSW
＊Geology Department
1一（567）
＿」 地質調査所月報（第21巻第10号）
2
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Fig．1a Geologic Map ofthe southem part ofOjika Peninsula
2一（568） Ayukawa Formation ofthe ojika Peninsula，Miyagi Pre免cture，NortheastJap＆n（E TAKlzAwA）
OjlkαPeninsu［α
月 0噂γα ん〆雌αレγα β 3θα／θレθ／ －0一 ↓ ↓ ・ぺ！z％∠4’＝．’辱 ’駿x巳・． （κの） ノくr』甲彫ミ9彫綴 耀』し 『！『1『l o ×｝ぐ・・一’貞 懸』灘聯鷺会 欝，N ■㌧ II『『II ，，三纐灘、 簸 ！ 纒 糠 一1一
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Fig．1b Geologic Profile
圃 砂70Xθηθ命詫31狛一一一一一一一一一一一一一一γα倣ゴ0アノFoγ加磁0η 醜 皿1◎＿，／一一・ 圃 醜 一 團 鰭難劉・一一一 塵 圃 3α伽γα1〃αノηα5かα／θ 卿κ1ηoα7α月o珊αオ10η 圃 010τ1オθ，θαか加0αησρoγρわツガオθ 吝 ∠o・α”妙9ヂ〃o〃α50砂だ・5ε1／3
which are intruded by several masses ofplutonics（：Fig。1）．TheJurassic to：Lower cretaceous ojika
Group overlies the Triassic Inai Group with an unconfbrm三ty．The Ojika Group is stratigraphically divisible into three fbrmations，the Tsukinoura，Oginohama and Ayukawa Formations in ascending order（Table1）．The upper half of the Ojika Group is distributed in the westem Kozumi syncli－
norium and．the eastem Niiyama synclinorium which are separ翫ted fヤom each other by the Ohara anticlinorium and both pitch to the south on the whole．The Niiyama synclinorium is delimited on the east by the Kinkazan tectonic line（IGI and others，1968）．
The Tsukinoura Formation as awhole shows asingle cycle ofsedimentation and．is stratigr批phi－ cally divisible into two members．The lower member，Tsukinoura Sandstone is more than120m thick，andis littoral sediments composed ofsandstone andsandyshale withbasal conglomerate．The upper member，Samuraihama Shale is underlain by the Tsukinoura Sandstone with a gradual changeofrock一飴cies，andisrepresentedexclusivelybyblack，bedded，siltyshale．ltis500to600m
thick．
The Oginohama Formation is probably confbrmable to the underlying Samuraihama Shale．It
3一（569） 地質調査所月報 （第21巻第10号）
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Oo〃7θん1 00硲θorゲθ7γ00畝5θ一ダ乃伽θ¢磁OS105α〃ゴ 5α〃おオo〃θ 300 ー5ガo〃θ
5哲7αオノfノθ（プ 07’ノご7〃7ノノ70オθ（〆 か／αo片5！うζ～ノθ 刃y畝α腋 んご碑αオα5方ノ 咋ノ功砺θ5α〃σ訂0ηθ 汽or〃7αピ！o“ 5方α／θ 620 伽θ＝00鷹θ一gr⑳θ¢砕ん0εノ05α堀3ご0〃θ βθ岸1α5／o“ 00偽θor〆θη！COα声5θ一9潤σノ〃θゴ，α7ん051C5副 侮05凶 ？ 50〃的オo〃θ 670 ・b／σ面みα／θ 一3オoηθ縦力・oρワ／0卿θ磁 ？万孟加万ノα“
貸㍉okも◎，ミi甲む 万0〃7α7！ 5σ〃43ピo〃θ α〃（プ5かα／θ 〃7ノ刀θ（メ！u〃7一力θolグθ（メ 5α〃おど0〃θ 370 αノ加7〃認〆0〃 σ〃グ5確／θ κ伽〃7θ万 κozu伽 一⑳ノαη 0のoカα〃α ～850 ゐ／αoん5ど辺醐一θσ5方α／θ 5hα／θ 万07ノηα6ノ’oη 〃αんノ〃o加ノηα σoα7簾2「。臨勝濫一卿θ切鵬ぎo”θ Oxヂ07グ！α〃 Q，、り偽制k葛ト θα〃グ5オ0〃θ 320 加oん8〃α／θ 縦5〃〃θzαた1 5α〃誌オoηθαηゴ3加ノθ加〃7θσ1uノη一舶4詫ゴ 5αηα3オo〃θ 360 α／亡θπηαノ0〃 σ〃グδ加〆θ 0α／／0グノαη 5α〃7U搬1hαノηα 500 汐／α欲肥〃一蕊7αオ艀θσ5かαノθ た畝1ηoび7α つ 5加／θ ～600 石o．陥αぎ〆oη βα酌o〃10〃 乃uんノ〃o端α ／20＋ δ／鰍3αゆ5カσ／θ，ooαr5θゴ7α！ηθσεαηゴ 5α〃dδピ0ηθ ー5オo〃θαηグ00砂／0〃7θrαオθ βαゾooノαη
1“α！ θ70〃ρ
※乃！0えηθ33θ8αrθヂ70切卯θ8θ・オ10〃θ
Table l Stratigraphic Classification of the Ojika Group is stratigraphicallydivisible intofbur members．The lower Kitsunezaki Memberischaracterized by Hne－to medium－grained sandstone and laminated shale in rhythmic altemation，exhibiting a flysch appearance．Thismemberisabout360minthickness．ThemiddleMakinohamaSandstoneMember is about320m thick，and covers the Kitsunezaki Memberconfbrmably，butwith a rapid ch翫nge of litho癒cies．Itiscomposedmainlyofconglomerateandsandstonewithsubordinateblacksandyshale・ Pebblesofthe conglomerates are dominatedbyvarious types ofgranites，homfdsic rocks ofslate and sandstone，and．chert。Plant fbssils are abundant at several horizons．The upper member，Kozumi ShaleoverliestheMakinohamaSandstonewithagradualchangein£acies．Itconsistsexclusivelyof black，beddedshale，containingsomeammonites（e．g．perisphinctids）．TheuppermostTomariMem－ ber is made up ofmedium－grained sandstone and laminated shale in Hysch－1ike altemation。This
member is thicker in the east than in the west． TheAyukawa Formationis confbrmablyunderlain bytheTomari Memberofthe Oginohama Formation andunconfbrmablyoverlainbytheYamadori：Formationwhichis representedbyefhsive rocks．ltisabout1，600mthickanddominatedbyarkosicsandstone．Furtherdetailswillbegivenin the next chapter．
Theintrusive rocks in the Ojika Peninsula are composed ofporphyrite，diorite and gabbro with a small amount ofhypersthene basalt．All ofthe various kinds ofintrusive rocks mentioned above may be genetically related，to one another，because they grade in petrologic attributes into one翫n－
other．Thus，theyale generically called‘‘the Ojika complex”（IGI and．others，1968）．These intrusive
bodies，roughly speaking，occur in concordance with the general trend ofthe Ojika Group．
3． S醗魏量gra欝恥y⑪葺伽e Ay胆聖箆・w箆F⊂贈m就且o聡
3。且 Ge踊e蜜箆且Rema茸翫s TheAyukawa：Formation is typicallyexposed along the coastofKiyosakisouthwest ofAyukawa
4一（570） Ayukawa Formation of the ojika Peninsula，Miyagi Prefecture，NortheastJapan（F。TAKIzAwA）
andthesouthemcoast ofAjishima Island．．In these areas this蝕mationshows ahomoclinalstructure withageneraltrendofNNE－SSW，dippingatan angleof70。to90。eastward，andis occasionally evenovertumed．Rowever，theAyukawaFormationhashitherto been erroneouslyregardedasbeing complicatedly fblded and exposed only in the axial part ofseveral synclines．Thus，the thickness of thisfbrmationwasestimatedat250m（FuKADA，19511MINATo，1953，table IVl FuJI，195610NuKI， 1956）or450m（TAKAHAsHI，1962）。lnactu翫Hact，theAyukawaFormationisfbundtobemuch thickerthanestimatedbefbre，beingnearly1ン600mthickaltogether， TheAyukawaFormation，fhrthermore，showsasynclinalstructureplungingsteeplyto thesouth near Niiyama north ofAyukawa，and．a synclinal structure in Tashiro Island．
It should be added here t｝1at an iso1飢ed outcrop ofvery coarse－grained，arkosic sandstone prob－ ably belonging to the Ayukawa Formation was fbund by Dr．KAMBE我nd the present writer near Kanayama east ofIshinomaki City。 The stratigraphic sequence ofthe Ayukawa Formation is summarized in Fig．2．This角rmation isstratigraphicallydivisibleintothreemembers，the：Kiyosaki，FutawatashiandDomekiMembersin ascending order．Ofthese three membersンboth the Futawatashi and the Domeki Members are not
〃o〃u30αηFo3ε〃ε 5オ7αオ＠αρ方10 0δノu加〃αア防！o 加θ ％〃オ Ooou7万8ηoθoヂ 砺〆65 月アθα θθ〃θ7α／ 乙励0／・gy 酒9θ 鋤・η齢 乃臨な 砺ノηoη1加5 η∂／θoρoグα X X 砂70xθηθαησθ5ノ加 〃θ励θ7 X X X Y×驚X× U所七 X ×× ×X×X 惚雛辱 X X K X X X X 加ザカ7θoo1α Ψ（？）一 Φ 一 ！卜 Wy－yyン瓦蝉ウ s §ミ∂、 総蘂（D へ・、起Φミoq㌧判5加揃θゴvθ7γoo備θ一グ7伽認5αη43オoηθ 300 oo〃ワ／o艀θ剛θ こミ壷→（§ Dm 一；崇一り．』一』・’ 」＝’＿、 5αηd3ぎ0〃θ5〃0聡近脚〃θηオ0γ・35一δθグゴ1η9
m） 溢 Q◎偽ooα器θ一卿ノηθゴ5伽4εきoηθα〃ゴooηダ／0卯θ7α乞θ 載 d湯ミ リ 句判 d ≧聴配（F端＝罫芸 5乃α左ン〔ズη（プ 5（〃フ（メ5オoηθ ノη オh1η 07〃7θ（ガ〃ノ77一∠うθ（メ4θσ α／オθγ F七3 i… 轟…一． 一m如ηか／αoκ訪α1β5伽σ5オoηθ；訪o岸！卿0705ε一ノα〃7！ 一ηαオノ0η αη0’ ÷をひ強 だ ε／〃塑 黛o■、恥娼ミkok弓≧唱受㍉＼『 F 卯θぬ卯一かθddθゴ勲／θ1ηオθ7／αノη！ηαオθσ縦か5αη肉オoηθ、 つ 儒θ70α／α加ワoooα3／oηα／肋！η5αη45伽θ＠θ751 Ft2 620・ θんOW！η9舵4々θ舵5／U加ρノη9S，50／θ一倣承1η95αησ 糾α〃θ〃αsp． ， 一 響 07055－／α加ノη讃！oη 補－鴇 F§↓ 兀加吻伽ρ！oθ7 一σ8sp． 〆θぴα如伽オ ’r根F βθ7老ノα5θ伽S 七） （貸も■、丈もo逗） Ft I θ伽d5オoηθαησ5hαノθ1η加θσル伽かθ碍θσα／オθ徽オ10η →窒ミ皇π 〜ミbりo、ミ← 貸唱■、り◎■ミkΦ偽 O 00α脊θ0ηθη！00α贋θ一卿！〃θσ5α〃醐0〃θ5力彌 Φ Ky7一一一一入 50παゴ！0 Q£起 貸曙ミ 募象ミ（ 07085一クθ瞳一 Ky6 壽一 碁 カノα欲訪α／θ07銘の5縦 醐⑳ だη材〃燃一ゆθグσθ¢〆θび00硲θ一9副〃θ¢α承03κ3αη耐0〃θ 碁 倣831〆θγθ穿00諮θ一9盟！ηθ¢α7ん05／05α〃σ5オ0ρθ Ky5 畔励oo〃ワ／0〃7θ縦オθ 3αησ5オ0〃θ3カ〇四⑤ α056一かθσσ！ηダ Ky4 畠巴’一最610 ηアσ55ノレノθ 00α7二6』θ一97α！ηθσ 5’α／7（プ5オ0ηθ か／αoん5αη砂5かα／θ θθ卿α5θ〃αsp 〆θぴ伽曲〃亡 ［ Ky3 解賜oゑ1麟，辮蒲翻朧η5欝オ鑑kO8麗σ／θ I l 5かOWZ塗2＿g7055一∠）θグα！ηワ （1ησ50／θ一ノ77α7穴ノη43 壷 ＿L 雲…………一 象碁 （レVθ舘） 驚ミ てっも Ky2 一 一 一 一 か／αo締hα／θ，5伽グ8乞oηθ／α御1ηαθ：oo備oη 一 一 一 一 ⑳ rθぴ吻惚ηオ き§題 Ky） 邨θ1〆θ・γ5オ耐1伽0・α〆5θ一卯1ηθσ6αηむ±・ηθ， ρ ＠θ5オ♪ Q q⊃も 』図＿ 嗣 叉 5か0励907055一わθσゴ1⑳θ γθび伽〃伽オ 壽§ミ ー
δ／αo没 5／～α／θ ⑬α伽ゴα尻 £ノηθ一47α！ηθゴ5αη45オoηθ ρ加卿η近5 〃7θ07U〃7一かθ07（メθ（メ （Zノオθ万ηαぎ1017 00脇θ0ηθぴ00α鷹一ダr面θσ5α〃d5オ0ηθ c卿／o加θ7αオθ
長 ノηo〃び30αη！b55！Z3 萎ρ／α万昂・5θ1／θ
Fig。2 Generalized stratigraphic succession ofthe Ayukawa Formation
5一（571） 地質調査所月報（第21巻第10号） exposedinTashiro Island．The：Kiyosaki Memberwith severalplant beds and the Domeki Member are charactcrized by the pred．ominance of very coarse－grained，arkosic sandstonel the midd．1c Futawat翫shi Member between consists mainly ofblack，ma血e shale．
3・2 亘⊆量y⑫s談遇K量S麗鵡遇s慮o龍e騰雄〔㊨朧恥e暫
This member is typically exposed along the coast ofKiyosaki southwest ofAyukawa．It consists
of coarse－grained，arkosic sandstonc and subordinate black shale in vαy thick－bedded alternation， beingintercalatedwithsomeconglomerates．TheKiyosakiMemberisabout670mthickinthetype area。Thesandstone is composedofangular grains ofquartz andfeldspar andf｝agmcnts offdsicvo1－ canic rocks with a small amount ofmatrix．：Pale－greenish dacitic to rhyolitic tuffoccurs at several
horizons．
Thepresent member is very variable in litho飴cies fピom base to toP，thus being divisible into
seven units（Fig．2）．Some units are rich in sandstone，but others are dominated by shale．A depo－
sitional break as represented by a diastem or partial disconfbrmity is observed between unit Ky4
乱nd unit Ky5．The ch＆racteristics ofeach stratigraphical unit are as fbllows． UnitKy1．一This unitisInadeup ofcoarse－grained，arkosicsandstone accompaniedbysiltstone and acid tuff．At Atsuizaki there occurs black bituminous shale bea血g coaly fピagments．Thc sand－ stoneofthisunitisthicklybeddedintheeastempartofthestudied area，whilemassiveinthewestem
Table2 List ofthe molluscan fossils丘om the Ayukawa Formation
Spe，i∈，～一』璽ly一．＿ 2 5 3 1 4 7 『一 Ammonoid
R 銑％禰αη濡67α3sp．c£T．歪50斥㍑56n5雰（KoBAYAsHI et FuKADA）
R κぎあαη8JJαsp。
C β677勉58戯z sp． Pelecypoda
S Pα7認あJo40ηκ856ημ御8n認5HAYAMI
R £yπ（ツoJon6η昭（P）sp．
S O7α窺η観哲040πsp。
C ハη6露h6召n。sp。
R 7握η05卿碗lJαsp．cf T。κ06妙45h露（TAMuRA）
R Soη忽ψ86∫6πsp．cf．8．κ4η麗窩αηθη5乞5KIMURA
瓢翻6Jl襯4め㈱α8（HAYAMI） A
S Mα濡6〃μηz sp。 S ∠霊ηψh歪40漉8sp。c£“Exogッ7α”Kz脚z6η豆5KIMURA V．A R （7碗05舵oηsp、c£0ψ70ゐo厩46nηz（SowERBY）
ハ勿ψho76伽（P70nηψho78伽）o漉π如傭KoBAYAsHI and TAMuRA C c ハ4。 （P． ）sp． V．A M． （∬漉磁ぎα）676襯傭αKoBAYAsHI and TAMuRA R
∠4吻吻h匂06η5露TAMURA S
∠L sp、c£ψ痂6η語5SToLlczKA R
G オ． sp。
R． Oや∫5（T7忽oηoゆ茗5）sp。c£0．渉07彪05麗θη認5KIMuRA
R PJ8‘∫o砂α（？）sp。
Gastropoda R S S Brachiopoda S
Locality（See Fig，1） V．A：very abundant A：abundant G：common S：sporadic R：rare l。Niiyama． 2。The Quarry near K批nayama，north ofAyukawa 3．Minamizaw批，about I km nQrth ofAyukaw＆ 4．Nakashoji，Ajishima Isl批nd。 5．YokQne，southem coast ofAjishima Island。 7．Mitsuishi，Tashiro Islana．
6一（572） Ayukawa Formation ofthe ojika Peninsula，Miyagi Prefecture，NortheastJapan（F・TAKIzAwA） part（Tashiro Island）．Bivalves arc vcry abundant in sand．stone in Tashiro Island．
Unit Ky2．一The unit consists of shale interlaminated．with sandstone，carrying occasional interbeds of fine－grained sandstone．The shale is dark gray in color and rich in plant fbssils in the eastem part ofthe area，whereas in the westem part ofthe area（Tashiro Island）it is black in color and intercalated with some marine shell beds．Unit Ky2was fbrmerly regarded asbeingequivalent to unitKy6whichis similarly represented by shale．In reality，the shale of unit Ky2is rich in sandstone lamina，whereas that of unit Ky6is monotonous without including sandy materia1． Unit Ky3．一This unit consists of a thick－bedded altemationofverycoarse－grainedsandstone and bl＆ck shale，each of them is generally several meters thick．Some sandstone beds begin with
conglomerate．Sole markings and graded bedding are commonly developed in sandstones。 Unit Ky4．一The unit is represented by black sandy shale ofmarine deposition and contains pelecypod fbssils very abundantly．Some ammonites（e．g。．867吻5611α）also批re fbund in this unit。The unit is much thicker in Ajishima Island than in the northem part ofthe peninsula。 Unit Ky5．一This unit is made up mainly ofvery coarse－grainedsandstone withf琵quentinter－ calation ofconglomerates．Thesediments ofthis unit as a whole are coarsergrainedthanthose ofthe
other units．Grad．ed bed．ding is almost imperceptible，but cross－bedding is common。 Unit Ky6rThisunit consists largelyofblackshale accompaniedbysiltstone containingplant fbssils．
Unit Ky7．一This unit begins with a thin layer ofconglomer乱tewhich，in tum，isfbllowed by a 40msequenceofcoarse－orverycoarse－grainedarkosicsandstone． 3．3羅翻置εS肱且e Memb舩 This member is typic＆11y cxposed．near Futaw翫t＆shiンAjishim＆Island．。The main part of the memberconsists chieflyofmud．dyflysch－likesedimentswhich arerepresented．byblack，1aminatedto medium－bedded shale丘equently interlaminated and interbeddβd withsandstone。Thethicknessof the member in the type area measures about620mンalthough its exact value can not be obtained
owing to丘equently steeply dipping overthmsts， Some ammonoids such asκ冨伽n611α，7’h％7飛αn繍6郷and B67吻5611αンand pelecypods are fbund in the shale．It should be noticed that the Futawatashi Member is essentia11y different in sedimentary 飴ciesfピom the othermembersofthe OjikaGroup．Thatis to say，themember is characterizedby the common occurrence ofdisturbed beds and penecontemporaneous deformational structures such as slump overfbld．ba11－and－pillow structure．Thin interbeds of丘ne－grained s翫ndstone display a variety
ofcurrent sole markings，10ad structures，convolute lamination，cross－lamination and current ripple
marks． 3。4Do㎜e腫S鋤翻⑪聡e Mem恥舘 This member is typically exposed near Domeki at the eastem extremity ofAjishima Island。．It consists mostly of very coarse－gr抗ined，arkosic sandstone and is lithologically very similar to the Kiyosaki Sandstone．Black shale is intercalated．only in the lower part of the member．At several horizons there occurs conglomerate，pebbles mainly rhyorite，granite and chert． The thickness of themember increases to the south on thewhole，measu血gmore than330m at Domeki．The stratigraphic relation between this member and the underlying strata，in spite ofthe rapidchangeoflitho飴cies，isprobablyconfbmable、Ontheotherhand，theDomekiMemberisun－ confbrmable to the overlying Yamadori Formation．
7一（573〉
＿．動 地質調査所月報（第21巻第10号）
乱 Geo亙09且c Age⑰亙藤e Ay覗翫題w鼠E儲鵬魏置⑰聡
The Ayukawa Formation has hitherto been refヒrred to the earliest Cretaceous ortheKochian series oftheJapanese time－scale on the basis ofinsu伍cient paleontological evidence．However，thc
ammonites fピom the Ayukawa Formation are very similar to those f士om the Berriasian Isokusa and Nagasaki Formations（SATo，1959）ofthe Karakuwa district north ofthe Ojika Peninsula．This fact makes it possible to determine the age ofthe Ayukawa Formation． Among the idenri且ed ammonoid species，乃躍解α痂66郷c銅50ん％56漉5and魏1加611αsp．surely indicate a：Lower Cretaceous Berriasian age．Accordingly，the lowerpart ofthe Futawatashi Mem－
ber，the middle part ofthc Ayukawa Form乱tion can be safも1y corrclatedwith the Berriasian Isokusa and Nagasaki Formations．Furthermore，the relatively upper part ofthe Ayukawa Formation may be refbrred to the Valanginian．
In the Karakuwa area the stratigraphic relation between the Isokusa－Nagasaki：Formation and the Upper Jurassic Kogoshio Formation is unknown because of poor exposure．Judging fヒom the writer’spreliminaryfieldsurvey，however，itmaywell besaidthatthe Isokusa andNagas＆ki Forma－ tions are re狙erred to the upper part ofthe Kogoshio Formation。In actua1£actシthe lower part ofthe AyukawaFomation（i．e．theKiyosakiMember）andthe：KogoshioFomationareextremelysimilar to eachotherinlitho飴cies，modeofcyclicsedimentation，thickness andkindofpebbles。Therefbre，it is reasonable that the Ayukawa Formation as a whole is correlated to the Kogoshio Formation in－
clusive ofthe Isokusa－Nagasaki Formation．
The Ayukawa Formation is covered with the Yamadori Formation which is correlated・to the KanaegauraFormationin Oshima IslandoftheKarakuwa district．Itis establishedthatthe Oshima Formationoverlyingthe KanaegauraFormation is Hauterivio－Barremianin age。Fromthescfゑcts it is concluded that the age oftheAyukawa Formation ranges fヒom upPermostJurassic Tithonian（P） to：Lower Neocomian（Valanginian～）．
8一（574） Ayukaw＆Formation of the Ojika Peninsula，Miyagi Pre免cture，Northeast Japan（F・TAK－zAwA）
A昼囎e聡謹x
On Some Berriasian Ammonites f士om the Ayukawa Formation，Ojika Peninsula
By
Tadashi SATo＊＆Fuminori TAKIzAwA
One ofthe writers（TAKlzAwA）discovered a number ofammonites fヒomtheAyukawaForma－ tion，the upPer part oftheJurasso一αetaceous ojika Group，ofthe ojika Peninsula，and then sATo identified them．As a result，among the ammonites there are discriminated three species which are
quite similar to the ammonites reported by SATo fピom the Berriasian Isokusa and．Nagas乱ki Forma－ tions ofOshima Island in the Karakuwa district．Therefbre these characteristic ammonites indicate 》 a Berri抗sian age at leastfbr the middle part ofthe Ayukawa Formation and．then supPorts the a£ore－ mentioned stratigraphical scheme ofthe same fbrmation established by thejuniomuthor． Ofthe ammonites fピom theAyukawa FormationB67吻56Jlαoccurs in abundance atYokonc and Bando－no－hama ofAjishima Island． Brief paleontological remarks are given on fbur identi丘ed ammonoid species including one species fヒom thc underlying Kozumi Member ofthe Oginoh訊ma Formation．
Family Berriasellidae SpATHフ1922 Sup飴mily Berriasellinae SpATH，1922 Genus．B6漉α5詔αUHLIG l905 ，
β877宛58JJαsp。PL1シFig．1．
Material－Scveral specimens，all fヒagmenta1． Description－A large adult or rather senile specimen，though highly corroded，presents Ber－ riasellid characters such as irregular，somewhat Hexuous，generally bifhrcated ribbing。The ribbing becomes obsolete at thesenilestage．’Noventral aspectisknown。But aspecimenfヒomYokone，well－ preserved outer mold ofthe ventral portion，exhibits clear and smooth siphonal band，bounded by
the swelled terminals ofthe ribs．There is no guarantee，however，that all the specimens belong to
one and the same species。
Remarks－Despite the impossibility of specific determination，the general aspect of the shell omamentationsuggestssome ofthemoreorless advancedforms ofB8γ吻58伽．Suchfbrms cancoexist with the7伽7伽編66榔as in the case ofthe Nagasaki and Isokusa Formations ofOshima Island。 Occurrence－Black sandy shale at Yokone ofAjishima Island。Probably Berriasian。
Sub魚mily Neocomitinae SpATH，1924
Genusτh％襯磁競6郷CossMANN 1901 ，
7ゐ％7窺αη痂667α5sp．c£T。250ん郷6η5乞5（KoBAYAsHI an（i FuKADA）Pl．1，Fig．2．
σoηψα76－1947。P67碑）h歪ηo診65（1）2560功h乞n6孟85）おoん％58n5おKoBAYAsHI and FuKADA，
＊ Geological Institute，University of Tokyo
9一（575） 地質調査所月報（第21巻第10号）
p。55，p1．XIII，Hgs．2－4，
1958．乃嬬襯編06郷乞50ん塀η5乞5，SATo，p．592艶p1．XXVIII，丘gs．9－12． Material－A single immature specimen．GSJ5513＊． Description－Whorl rather rapidly growing，with somewhat convexsideandslightly且attened ventral region，Ribbing fine and．dense，somewhat sinuous and prorsiradiate，but irregularly bifUr－ cated at various heights ofthe side，and sometimes even bidichotomous，terminated by small swell－ ings＆t the border of the smooth siphona，I band．Fa，int constriction present．
Remarks－Since the specimen is small and immature，the comparison with the described species of Z勧7η2観痂067α5is di伍cult．The attribution to the genus is，however，justified，by the charac－ teristicribbingwhichde且nitelydif艶rsf士omthatofOloo5妙hα郷fbrmsknownfヒom the corresponding horizons ofOshima Island． Occurrence－Black bedded shale at a quarry near Kanayama north ofAyukawa．Berriasian．
Genus流1襯61」αUHLIG，1905
π乞1ぎ観θJlαsp．PL1，Fig．3
Compare－1958．Kilianella sp．SAToンp．595，pl．XXXVIII，fig．8． Materia1－A single specimen．GSJ5514． Description－Widely spaced，stout ribs are characteristic．Strong and probably prorsiradiate constriction present，accompanie（i by triplicate rib behind．Ribs are mostly simple，but there are
some bi負1rcated ones at the ventral shoulder．
：Remarks－The specimen at hand．ンthough f士agmental and de£ormed，closely resembles the describedκ茗伽η6Jlαfyom Oshima Island（SATo，1958，P1．XXVIII，丘9．8）on thegeneralaspect ofthe
shell．
Occurrence－Derivedfセom arolledblockatN翫kashqli ofAjishima Island．ProbablyBerriasian。
Family Perisphin6tidae STEINMANN，1890 Sub飴mily Perisphinctinae STEINMANN，1890
Perisphinctinid gen．et sp．indet．PL1，Fig。4。
Materia1－A single large specimen．
1）escription－The specimen is poorly preserved except small part ofthe body chamber，and is
di伍cult to be determined even generically．The rounded ventral region，regular，biplicate and un－ interruptedribbingandpτesenceof£aintconstrictionsaccompaniedbypeculiarribs are allindicative ofsome Perisphinctid genera instead ofsome similar Berriasellids。 Occurrence－Black shale of probable：Kozumi Shale Member along the westem co乱st of Ajishima Island（Nagatoro－hama）．Probably uppermostJurassic。
Re筍ere鵡ces
FuJI，N．（1956）：On the fbssil plants fヒom the Cretaceous Ayukawa Formation，Ojika Peninsula， Miyagi Pre飴cture，Japan．σh乞御κα9ぬ，no．30，P．32－35（inJapanese with English
＊ The preHx GSJ for numbers ofcatalogue is an abbreviation for the Geological Survey ofJapan．
lO一（576） Ayukawa Formation ofthe Ojika Peninsula，Miyagi Prefecture，NortheastJapan（F。TAKlzAwA）
abstract）。
FuKADA，A．（1950）：On the occurrence ofP67細h乞鰯65（s。s．）fyom Ojika Peninsula in the southem
Kitakamimount乱inland。Jo％7．Fα6．36乞．，∬oんん碗o Z吻．U励。，S67．1V，vo1。7，no．3，p．211
－216．
HANzAwA，S．（1956）：R6g乞oπα」（穿8010gクヴ読吻η，7bhoん％D乞5励診，Asakura Book（】o．，Tokyo（inJapa－
nese）．
且AYAMI，1。（1960）：Pelecypods ofthe UpperJurassic and Lowermost（】retaceous Shishiori Group in NortheastJapan．」ψJoπ7．0801。06097．，voL31，no。1，p．85－98．
HAYAMI，1．（1961）：on theJurassic pelecypod fauna inJapan。Jo雄，F46。s6乞．，u銘甑7－bゆo，566。π，voL
8，pt．2，p．247－343。
EAYAMI夕1．（1961）：successions ofthe KitakamiJurassic（Jurassic stratigraphy ofsouth Kitakami，
Japan I）．Jlψ．Jioz67．（穿601．06097．，vo1．32，no．2，P．157－177。
HAYAMI，1．（1961）：Sediments and correlation ofKitakamiJurassic（Jurassic stratigr乱phy ofSouth
Kitakami，Japan II）。1う鼠，p．179－190．
HAYAMI，1．（1961）＝Geological Ristory record．ed in the KitakamiJurassic（Jurassic stratigraphy of
south Kitakami，Japan II）．乃24。，P．191－204。 IGIンS．，KATADA．M．，TAKlzAwA，F．＆ABE，T．（1968）：Thegabbroic rocks fヒom Kasagai islet and
the ojika Peninsula，Miyagi Prefbcture，Northeast Japan（abstract）．Jo躍．（ヌ601．so6。」α一
ραn，vo1．75，no。2，P。80（inJapanese）． INAI，Y．＆TAKAHAsHI，N．（1940）10nthe geologyof thc southem Kitakami Mountainland．σo肱
1n5渉．0601．Pα160n孟．，7－bhoん％∫ηψ．ulzづ∂．，no．34，P．1－40（in Japanese）。
：KAMBE，N．＆．sHIMAzu，M．（1961）：ExplanatorytextoftheGeol．MapofJapan，K．esennumasheet， scale1：50，000．Geo1。surv．Japan．（inJap批nese with English abstract）。 MATsuMoTo，T．（1967）：」灘繍P6吻4翻α伽60郷P6吻4，勉研5♂07翻σ60Jogク，voL2，Asakura Book co．，Tokyo（inJapanesc）． MINATo，M．（1953）：σhぎ509α加（s翻α⑫ψ妙）．Iwanami Book co．，Tokyo（inJapanese）． MINATo，M．，GoRAIラM．＆HuNAHAsHI，M．（1965）：7乃8σ60卿61）6∂吻窺6漉φh6」卿n65615」姻5．
Tsuk弓i－shokan Co。，Tokyo。 ONuKI，Y．（1956）：Geology ofthe Kitakami Mountalns．Explanatory text ofthe Geologic Mapof
Iwate Prefbcture（1：100ン000）Iwate Pre£ecture（inJap批nese）。
SAToンT．（1958）：Pr6sence d．u Berriasien d．ans la st撒tigraphie d．e plateau d，e Kitakami（Japon
septentriona1）。β麗ll．So6．σ601。F7αn66，SJ7。6，Tom．8，p、585－599．
TAKAHAsHI，H。（1962）：Mesozoic Form翫tions ofOjika Peninsula。B観。Fα6．温765Soε。，伽γα彪U物．，
ハベごz6．s62．，no．13，P．89－99．（in Japanese）．
YAMAsHITA，N．（1957）：The Mesozoic II．060562，36726M5506。060乙Co〃αゐ。」卿η，no。11ラp、1－116（i耳
Japanese）．
11一（577） 地質調査所月報（第21巻第10号）
宮城県牡鹿半島の鮎川層について
滝 沢 文 教
要 旨
牡鹿層群（中部ジュラ系～下部白亜系）の最上部層である鮎川層の層序と構造について新知見を述べた。本層につ いては，従来等斜摺曲構造を示し，層厚250mあるいは450mとされていた。筆者の調査では，本層は主分布域におい て基本的に同斜構造を呈し，厚さ約1，600mに達することがわかった。本稿では新しい層序区分を示し，対比に関連 してアンモナくトの記載を付記した。鮎川層は岩相的に下位から次の3部層に区分される。 （下部） 清崎砂岩：浅海～陸成（P）の粗粒アルコーズ砂岩を主とする。厚さ670m （中部） 長渡頁岩：泥質フリッシュ型の海成黒色頁岩，厚さ620m （上部） ドウメキ砂岩：粗粒アルコーズ砂岩厚さ約300m 本層の粗粒砂岩は酸性火山岩の礫および岩石片を多量に含有することで特徴づけられ，田代島の砂岩が層相から清崎 砂岩と同層準であることは確実である。アンモナイトおよび海棲二枚介は下部層～中部層下半より産出し，それらは 下部白亜紀Berriasianを示している。しかも唐桑地域の磯草・長崎層産のものに酷似する。
12一（578） βz〆乙（ヌ601。Sπ7∂．」ψαπ，1晒oJ。21 PJ伽44
2
3
灘噴 4 Explanation of PLATE I （Pl伽44）
Fig。1． B677∫α5611αsp．
A gypsum c＆st fピom extemal mold，GSJ5512丘om Ioc。5，Yokone，Ajishima Island，Unit Ky．4，Kiyosaki Sandstone Member，Ayukawa Formation（Coll。TAKlzAwA）．Lateral view×0．7． Fig．2。 乃π禰砒痂67α5sp。c£Tj50ん獅漉5（KoBAYAsHI and FuKADA） G町5513，倉om loc．2，near KanayamanorthofAyukawa，UnitF3，FutawatashiShaleMember，Ayukawa Formation（ColL TAKIzAwA）。Lateral view×2．8．
Fig・3。 1百1宛η6JJαsp。
GSJ5514，from loc。4，Nakashoji，Ajishima Island，probably derived fピom Unit F3，Futawatashi Shale Member，Ayukawa Formation（Coll．TAKIzAwA）．Lateral view×1．1．
Fig。4。 P67乞ψh乞n6あn歪4gen。et sp．indet． GSJ5515，肋mlloc・6，Nagatoro－hama，westem coast of邸shima，Kozumi Shale Member，Oginohama Formation underlain by the Ayukawa Formation（ColL TAKIzAwA）。Later＆l view×O。7。