PLE豆STOCENE SEA LEVEL CHANGES AND TECTONIC MOVEMENTS
IN THE BOSO PENINSU正A， CENTRAL JAPAN
Takao KIKUCHI
INTRODUCTION
Glacial eustasy during the Quaternary Period played an important role in forming marine deposits and terraces along sea side districts． In unstable areas， these deposits and terraces were diSlocated from their original levels where they were generated to different altitudes Imd remained there． Consequently， a relative sea level curve during the period can be obtained from the values represented by the present elevations of some dislocated marine deposits and terraces． Therefbre， an absolute eustatic sea level curve should be drawn by subtracting tectonic factors from relative sea level changes．
It seems that a stable area is more favorable than an unstable area for estimating eustatic changes in sea leve1． On the contrary， the latter has an advantage over the former in construc・
tion and preservation of marine deposits and terraces． A constant rate of uplift at every point during the Quaternary is dften assumed by many workers by subtracting tectonic factors from sea level changes． That is to say， the relation≦hip between the elevations of marhle terraces and their ages is simply linear．
But in fact， it is not so s㎞ple． It has been well・known that the Quaternary tectonic movements in the Boso Peninsula， Central Japan， were rather complicated． In the central and northern areas of the peninsula， subsidence first occurred， then succeed血gly uplift血g took place． The subsi砒Lg area gradually upl血ed through progression of the sedimentary basi 1．
Such inversion in the tectonic movement is general in the formative process of the 唐?хqentary㎞bricate structure as indicated by Fujita（1951，1953，1958）， which Shows a general structure that the incli㎞ation of strata decreases gradua皿y toward the center of the sed㎞entary basi1． Furthermore， it goes without saying that the rate of uplift at the Boso area was never constant．
The purposes of the pre sent study are： frrstly， to clarify the Pleistocene tectonic movements of Boso area；secondly， to elucidate the、sedimentary environments of the mar血e Meistocene developed in this area which was not untouched by such tectonic movements；
and thirdly， to extract eustatic movements from the Boso succession and draw its curve．
It seems that the districts having such tectonic character茎stics as the Boso Peninsula are not peculiar in the world． Therefore， the author s method proposed in this paper may be not only applicable to the Boso area but also to other similar tectonically unstable areas．
一77一
GEOLOGIC SEQUENCE AND STRUCTURE OF THE MARINE PLEISTOCENE IN THE BOSO PENINSULA
The Kanto Plain which is the most extensive in the Japanese ISIands is situated geologically at about the junction of the North−East Japm Arc． This plain is ca皿ed the Kanto Tectonic Basin（Yabe and Aoki，1927）from the view point of geomorphology and geology．
Many subsurface drilling data obtained from throughout the plain＄how that the Pliocene and Pleistocene sedirnents lying underneath attain more than 3，000 m thick（Kawai，1961，
1965）．In central and northem areas of the Boso Peninsula which are located on the south・
east side of the Kanto Tectonic Basin， a thick sequence of lower耳1iocene to upper Pleistocene fossiliferous marine sediments overlyilg the Miocene Miura Group is exposed almost continuously．
130° 1350 140°
F鱈．1Map showing the location of the Boso Peninsula and Quaternary sedimentary basils in Japanese Islands．
145°E 450N
400
350
The sedimentary enVironments of血e marine Pliocene and Lower Pleistocene called the Kazusa Group are generally treated as bathyal one from the f6ssil fauna included（e．g．
Naruse，1959；Aoki，1968）． They are overlain by the SA Group along the Nagahama Unconfbrmity（Nirei et al．，1975）． The upper is divided into two formations， i． e． the Nagahama and Kasamori Formations in ascending order， and contains bathyal to neritic monuscan fbss丑assemblages． The SA Group are overlain unconformably by Mid（ile to Upper Pleistocene neritic sediments named the Shimosa Group． This discontinuity between
these two Groups was also correlated to the Naganuma Unconformity in the western district of Tokyo Bay by Mitsunashi（1968）． These remarkable discontinuities are considered to be caused by the Island Arc Tectonic Movements（F両ita，1970）， which become more intensive since Middle Pleistocene and continued to the present in Japan． The Shimosa Group consists mainly of marine fossiliferous sandstone and reaches over 320 m in thickness（Kikuchi，1976）．
Numerous papers on the stratigraphy of the Shimosa Group have been previouSly published（Makiyama，1930；Mitsuchi，1934，1935；Sakakura，1935；Kozima，1958，1959，
1962，1963，1966；Hattori and Komura，1959；Nakagawa，1960，1961；Naruse，1961：
Narita Research Group，1962；Kumai，1964；Aoki，1967；Ueda，1969，1970，1973，1974；
Kanto Quaternary Research Group，1969；Aoki and Baba，1971，1973；Aoki， Baba and Horiguchi，1971；Aoki et al．，1968 etc．）． However， there is scarcely any consensus of opinions on the stratigraphic division among these researchers． In addition， some papers are unreliable since their correlations of strata in the fields are erroneous as compared with field evidences． And， there is hardly any study that analytica皿y discussed thg eustatic and the tectonic movements of this area， which were genera皿y treated without distinction between them．
In this respect， the results of the field hlvestigations by the present author are suf伍ciently di脆rent from those previous studies（Kikuchi，1976）． Some characteristics of the Shimosa Group are outlined i l the pre sent articles as fol［ows．
The Shimosa Group is distributed on the northwest of the Boso Peninsula． These strata strike nearly NE−SW and dip gently northwestwards， and their inclinations decrease toward the northwe st from 8 per ct． to nearly horizontal． They consist mairdy of well−sorted sands with muds， gravels and many thin tuff beds． Four disconformable boundaries formed dudng the eroSional stages divide the Group into five formations，∫．θ． the JiZodo， Yabu，
Kiyokawa， Kamliwahashi， and Narita Formations in ascending order・The Iast has a characteristic widespread depositional surface named the Sh㎞osa Upper Terrace（Sugihara，
1970）．Along the fringe of the Sh㎞osa Upper Terrace， the Shirnosa Lower Terrace composed of the Ryugasaki Sand are distributed with a cliff of 5 m or less in height．
The Younger Kanto Loam（weathered volcanic ash layers）is about 3 m thick and covers throughout these deposits． Brief descriptions on each formation are as follows：
1／Jizodo・For〃zation r130〃3∫η枷欲ηε∬ノ
The lower part of the fbrmation at Jizodo， which is the type locality， consists of fine sands and alternation of sand and mud beds． A muddy bed containing molluscan fbssil fauna， Izumiyatsu Fossil Zone， which indicates an environment of cold current，
is interbedded in the horizon of 45 m above the base． The middle part of the f（）rmation yields many molluscan fossil shells（Jizodo Fossil Zone）indicating a cold water environment except an warm one at the middle horizon（Aoki et al．，1962）． The upper part of the formation consists of well・sorted fine sands with some thin pyroclastic beds． The uppermost horizon contains the characteristic trace fossils created by marine isopod Excilolana which is an indicator of intertidal environment
（Kikuchi，1972）． These traces observed in several horizons in the Shimosa Group have an important role in paleoecologic explanation f6r the sediments．
一79一
2／Yabu For〃zation r120〃z in thickness／
The Yabu Formation overlies disconformably on the Jizodo Formation and is distributed around Yabu， the type locality of the formation， and in its northern area．
The basal conglomerate of the formation is about 2 m in thickness and is overlain by a bluish grey mud bed that is 5 to lOm in thickness． Both beds make a set and extend to Kongochi，25km northeast from type locality・The mud bed is overlain by thick sands with some thin interbedding mud beds． Many molluscan shells are included at the lower part of the sand bed． This shell bed is called the Yabu Fossil Zone and characterized by the presence of cold water species．
The trace fossils of Excilolana are contained at the uppemlost and middle horizons of the formation． There are some outcrops showing the disconfbrmity between the Yabu and Kiyokawa in the east of Kisarazu．
3／KiyokOwa Formation（40〃1 in thickne∬ノ
The Kiyokawa Formation mainly consists of wel−sorted me（lium−grained sand with abasal conglomerate， and a tuffaceous mud bed is intercalated at the lower horizon of the formation． This mud bed， of about 2m thick， interbeds some pumiceous and scoriaceous thin layers． It shows a particular rock−facies that can be traced over 35 km f『om Kisarazu to the eastern end of the peninsula． Many outcrops yielding numerous molluscan fossil shells characteristic of relatively warm current fauna are（listributed in the eastern district of Kisarazu． The trace foss皿s of、Excio lana are found at the top of the formation．
4／Ka〃ziiwahashi F（）r〃lation r〃zore than 20〃z th ick／
The greater part of the upper member of the formation is distributed around the Lake Imba・numa． It consists mainly of well・sorted medium to fine sands， including a moUuscan shell bed called the Ka血iwahashi Fossil Zone． This molluscan shell fauna indicates a relatively cold water environment． A mud bed with peat，of l to 2 m thick，
is widespread in the middle horizon of the f6rmation． This bed was deposited clearly above sea level， because it contains some fresh water molluscs， i． e． Erodona amurensis and Corbicula iaponica， and some autochthonous fossilized roots of trees． In this way，
the transgression in the Kamiiwahashi stage may be divided into two phases from the fbss皿evidence．
The lower member consists of well・sorted medium sands and is observed to be underlain below the mud bed． Though the outcrop which shows the bottom of the member is seldom in the Boso Peninsula， the basal boundary is definitely observed in the eastern area of the Shimosa Upland．
5／Nari ta・For〃vation rmore than 50〃m thick／
The Narita Formation is widely distributed over the Shimosa Upland， north of the Boso Peninsula． It is chiefly composed of well−sorted medium sands． Numerous molluscan fossils are included in the sands from place to place and is called the Kioroshi Fossil Zone． Lithofacies of the formation is quite similar to that ofunderly−
ing the Kamiiwahashi Formation． It was supposed in some previous works that the resemblance of lithofacies was the evidence of comformable relation between both formations．
Okubo， et a1．（MS）， however， recently pointed out that there is a disconformity
between them． They stated that an undulating boundary between both fbrmations is clearly observed in some outcrops， though their geologic structures are nearly concor・
dant． The sedimentary facies of the Narita Formation is controlled by configuration of relief， fbr instance， thick muddy sediments containing sporadic shells of Ostrea gigas are fbund only in a trough ofthe basal boundary．
The Narita Formation distributed in the Boso Peninsula directly overlies the Kiyokawa Formation， without intervention of the Kamiiwahashi Formation． Muddy valley・fill sediments including many molluscan shells which are distributed．around Kisarazu and Anegasaki had been previously correlate d to the Kamiiwahashi Formation（Aoki and Baba，1971；Kikuchi，1974）． Basing upon the presence of the characteristic pumice bed called the Manazuru Pumice， however， Machida et al．（1974）
and Arai et al．（1977）mentioned that the muddy deposits at Kisarazu above stated correspond to the transgressive deposits of the Shimosueyoshi stage in the Miura Peninsula which are equivalent to the Narita Formation in the Boso area． Some recent field investigations made by the author indicate that the correlation proposed by these authors may be correct．
The uppermost of the f6rmation generally consists of well・sorted medium sands except in the two areas occupied by lagoonal muddy sediments；namely， the southern area of Chiba and the northwest area of the Shimosa Upland． At the uppermost horizon of the sands， there are many trace fossils ofExeilolana that are distributed in the surrounding muddy sediments which contain fresh water diatom assemblages as reported by Horiguchi and Ohara（1972）． Both muddy areas correspond exactly to the place where the tectonic subsidence proceeded during the Quaternary Period
（Kaizuka and Naruse，1958；Kikuchi，1972）． It is supposed， therefore， that the lagoonal environment has retained until later period． The top of the Narita Formation is covere d with the Shimosueyoshi Loam（weathered volcanic ash layer）， which is one of the formations of the Kanto Lgam（volcanic ash group）．
6／Rソugasaki Sand／lfember ro．5 to 6〃z in th ickness／
One may observe the line of cliff of about 5m in height， borde血g the Shimosa Upper and the Lower Terraces． In the latter， the sediments consist mainly of sands with pebbles， forming a marine terrace which is 2 to 3 krn wide along the coast of Tokyo Bay． In addition， there is a river terrace formed simultaneously around the downstream basins of the River Tone． It consists of cross−1aminated medium sands with pebbles named the Ryugasaki Sand Member（Sug血ara，1970）・The boundary between terrace deposits and the Narita Formation is shown by an obvious discon・
formity in the main areas． The relationship between them， however， is conformable around the southern area of the Lake Kasumiga・ura and the southern area of Anegasaki． It suggests that some inlets have been open until the beginning of the Ryugasaki Sand at both areas（Kikuchi and Hatori，1969）．
The Shimosa Lower Terrace is correlated to the Obaradai Terrace in the Miura Peninsula， west of Tokyo Bay， by Machida（1973）， on the basis of his tephrochrono・
logic studies． He conclud6d from his Miura data that at the time of the so−called Obaradai Transgression the sea level rose relatively more than 20 m and made the marine Obaradai Terrace． Evidence in the Shimosa Upland， however， does not support
81＿
such a remarkable fluctuation of sea level during this period because the sedimentation from the Narita Formation to the Ryugasaki Sand Member was rather continuous．
SUPERPOSITION OF TECTONIC AND EUSTATIC MOVEMENTS
The marine Pliocene and Pleistocene sediments in the Boso section are gently inclined to the northwest． Though they now make a h皿1y district about 300 m in height in the central Boso， it is clarified that the district had once been the center of sedimentary basin during the Pliocene， and that the center were shifted to the north or to the northwest subsequently
（Fuj ita，1953；Koike，1957；Kawai，1961；Nanlse，1961）． The exposure of successive marine sed㎞ents on land means properly the turning of tectonic movements from subsidence to uplift・On the basis of stratigraphic evidences obtained丘om some Neogene and Quaternary sedirnentary tectonic basins in Japan， F旦lita（1951，1953，1958）mentioned that the migra・
tion of a sedimentary b asin and the turning from sub sidence t o uplift are general movements of the block exhibiting the sedirnentary㎞bricate structure ， and pointed out， moreover，
that a sedimentary tectonic basin generally shows more or less such a structure．
The glacial eustasy recorded in marine terraces or sediments are modified by subsequent tector直c movements． Provided that a sed㎞entary region was tectonically immovable and deposits were supphed sufficiently correspond to rising of sea leve1， the actual sea level in the past may be estimated from geologic evidences preserved in such a region．Japan has no district， however， where Quatemary tectonic movements are not recognized in the least．
On the premice that the rate of vertical diSlocation was constant during the Quatemary Period， the past sea level can be caluculated from the height of marine terraces or raised coral reefs whose ages were already estimated． By the easy assumption， many previous workers draw the curves of sea level changes． The method， however， is not available fbr the Boso sequence， because the rate of uplift of this area was not constant． Another method must be prepared to est廿nate the past elevation of sea leve1丘om the geologic data of the Boso sequence． Recently， Stearns（1976）po㎞ted out lucidly that the rate of tectonic uplift in the Barbados lsland was not uniform． The writer agrees with that opinion．
It is usually said that the rate of sea level change is much larger than the rate of tectonic movement even in the Japanese Island area， which is one of the most active archipelagoes in th・w・・1d（Y・・nk・w・・nd K血uk・，1956；K・b・y・・hi，1962；Sugimu・a，1968）． F，。m thi，
P・血t・f・i・w・it w皿b・ab1・t・b・・t・t・d th・t・h・・t・y・1・・f・ed㎞・nt・ti・n i・cau・ed by glacial・u・t・ti・・ea 1・v・1・i・e and th・t止e ch・ng・丘・m・equ・n…f・・ub・id血9、，ea t。
that of an uplifting area in the succession， including some relative short cycles of sedtrnenta．
tion， is caused by tectonic movements．
Fig．2 shows three types of geologic and geomorphologic hypothetical succession constructed by the composition of tectonic movement is constant rate and cyclic glacial eustasy（Kikuchi，1974）． In the figure， it is drawn that erosion and deposition come about
㎞mediately in response to sea level fluctuations． Therefbre， the difference between the rates of sea level rise and deposition， the rate of erosion， and the length of thne of sea level stagnation must be considered in actual cases． Moreover， the eustasy concerning giacial advances and retreats are only represented in this figure． Thus， the eustatic changes
（a）
（b）
（c）
1 ＿＿＿．1
2
2− 『 一 一 一 一 一 一 一 一 一 ＿ r
3− 一 一 一 ｝ 一 一 一 r
、、A 、
@、@、
@ 、@ 、
@ 、、
闇一一一
、、
@、A 、
@ 、黶̲奮＼
@、、 、 A、A 、
、、A 、
Q 、、＼ 1 、 、
@ 、
A 、
、、A 、 、、
A 、
ゴζ・・：Qill∴：、， 3 2 1
『・二：1・：1・：
￨r−一一 ＿＿一一一一r−一 一一 一
3 2 1
・1瀕≧タL・＿メ・は・二ら・≒ 占
ノ@Y
養≒＃、
！
ノ ノ
@ ，
@ ， ！@
輔麟 @ @ 〆 〆
＿一
←TIM↑
Fig 2 Three types of geologic and geomorphologic successions form−
ed by various movements of the crust・ （a）： uplifting area， （b）：
stable area，（c）：subsidi㎎area，
1： tectonic curves， 2： sea level curves． （after Kikuchi，1974）
caused by other factors must be contained in the tectonic curves if their cycle is sufficiently long． The following aspects on sedimentation corresponding to each tectonic type may be stated．
1／Uplifting area
It may be said that existence of a set of marine terraces suggests tectonic uplift． A rise in sea level builds up marine deposits and a terrace which is genera皿y composed of relatively thin and coarse・grained deposits， or is lacking in any deposits by denuda−
tion． The areas developing marine terraces in the Japanese Islands may be classified in this type．
2／Stable area
The valleys formed on land by the processes of subaerial erosion and the shallow marine deposits filling submerged valleys are caused by eustatic sea level fluctuations in a stable area． The lower half of the sediments shows muddy facies indicating frequently a submerged environment， and on the other hand， the upper sandy facies suggests an shallow littoral environment． Several regressions and transgressions must make a complicated succession in a geologic profile． Though there is scarcely any stable area in the Japanese Islands， it seems that tectonic movement at the margin ofa tectonic sedimentary basin is relatively moderate， and， therefore， a similar stratigraphic succession to that at a stable area may be formed． The geologic profile of the Yokohama district（Fig．3）shows such a structure of a stable area， because the district is situated at the east end of the sedimentary tectonic basin named the Sagami
83
場
ヨヨ
（UIJ」．i・t B・。b・g・u・a E
触NaganumG F
4km
100m
ShirrpsLN？yoshi F
Moio㎞F
『瀞 認
Fig．3Distribution map of the Sagami Group in the Yokohama district and geologic profile． （compiled from Kanto Quaternary Research Group（1974）and unpublished data）
Sedimentary Basin（Naruse，1960）and seems to be relatively stable．
3／Subsiding area
Each transgressive formation in a subsiding area accumulates one after another． The sediments are well preserved in comparison with that of other types of tectonic movement． On the shallow sea−bottom which is not exposed above the sea even at the regressive stage， the following transgressive deposits must conformable overlie the former sedim合nts． On the contrary， the following sediments accumulate discon・
formably when the previous sediments have been exposed above the sea and have su ffe red marine or subaerial erosions． These sediments will not be exposed above the sea if subsidence is continuous． In the Japanese Islands， the areas classified in this type are represented by the central area of the Kanto Plain， Nobi Plain， Osaka Plain， Ntigata Plain， Ishikari Plain， and other sedimentary tectonic basins（Fig．1）．
In the case that sediments formed in a subsiding basin are observed above the sea level at the present， it means tectonic movement of the area turns over from subsiding to uplift
in the past． The sedimentary imbricate structure in the Boso Peninsula was surely fbrmed by such a reverse on tectonic movement， because the sedimentary structures of the Jizodo， Yabu， and Kiyokawa Forrhations indicate a succession of subsidence and those of the Kamiiwahashi and Narita Formations show a structure of stable and uplifting areas．
According to Kozima（1972）， the sands of Kamiiwahashi Formation in the neighborhood of the Lake Imba・numa， Shimosa district， were supplied from the southern area and referred to the uplift of the Boso Peninsula at that time． The presumption of the tectonic movement of the Boso area by the author is not inconsistent with Kozima s conclusion．
TECTONIC MOVEMENT DURING THE PLEISTOCENE
IN THE SOUTHERN KANTO DISTRICT
Many geophysical and geomorphical studies on the tectonic movements in the southern Kanto district have been carried out． Recent studies on the tectonic movement associated with the Great Kanto Earthquake of 1923 cleared that the remarkable uplift in the north and east coastal areas of Sagami Bay and the south area of the Boso Peninsula was caused by thrusting along the Sagami Trough（Ando，1971；Kanamori and Ando，1973）． Sugimura and Naruse（1954，1955）pointed out that the highest Holocene marine terraces called the Numa Surface in the southem Kanto had been displaced by the tectonic uplifts associated with repeated earthquakes rated as great as the 1923 earthquake． Isopleth maps expre ssing vertical displacements of the Numa Surface as determined by several workers are， however， a little different from the pattern of seismic uplift in 1923 in曲owing high values along the east coast of the peninsula（Naruse，1968；Machida，1973）． Matsuda et al．（1974）and Yonekura
（1975）delineated contour maps of the Numa Surface based on their data newly obtained，
respectively， and explained that the vertical displacement of the surfaces is a re sUlt of composite crustal defbrmation s㎞丑ar in mode with those of the l923 earthquake， and 血eGenroku Earthquake in 1703． In association with the Genroku Earthquake which occurred along the Sagami Trough off the southern end of the Boso Peninsula， the south and east districts of the Boso Peninsula were remarkably uplifted as compared with that of 血e1923 earthquake． The uplifting district was regarded as corresponding to the region of co・earthquake deformation facing the Sagami Trough （Kaizuka，1974；Yonekura，1975）．
Kaizuka（1974）designated the swelling recognized in submarine topography off the east coast of the Boso Peninsula the Boso−Kashima Uplifting Zone and also suggested the existence of the region of co・earthquake deformation facing the Japan Trench ． From these geodetic and geomorphologic studies， they assumed the mechanism and cause of tectonic uplifts in southern Kanto， but they did not interpret those of sub sidence of 止eKanto Tectonic Basin．
Besides，丘om geologic studies of this area， the following interpretation were given．
The Miocene and lower Phocene series in the southern part of the Boso Peninsula show 証ight zonal stnLctures with some fblds and faUlts extending east and west， and fbmユthe so−called the Hayama・Mineoka Uplifting Zone， which was active during the Middle Miocene to Pliocene． The stnlcture of Pliocene and Pleistocene sed㎞ents overlying the Miocene series with the Kurotaki Unconfbrmity in the central part of the Boso Peninsula extellds
一85＿
northeast and southwest㎞the strike， and tilts toward the center of the Kanto Tectonic Basin as mentioned above． It was explained by Mitsuna血（1973）and Kak㎞i（1974）that 1血edifference in structure between both regions was caused by a new type of tectonic movement which began ill the Late Pliocene or Early Pleistocene。 Mitsunashi（1973），
moreover， proposed that the Miocene to Pleistocene formations were deposited in the basin which was fbrmed by a collapse subsidence in the northem part of the Boso Peninsula，
after which， an upheaving area was f6rmed in the basin in the Middle Pleistocene and new collapse basins apPeared in the northeast area of the peninsula． The apPearance of the upheaving area in a subsiding area seems to correspond to the turning of tectonic movements from sub Sidence to uplift mentioned in the present paper．
The conception of coUapse subsidence in the Pleistocene sedimentary basin is proposed by Fujita（1970）， and is considered to have relation with the Island Arc Tectonic Movement．
Although mechanism of the subsidence in the Kanto district is not sufficiently clear，
the tectonism of this district is undoubtedly composed of tectonic uplift and subsidence
（Namse，1968；Murai and Kaneko，1973；Kaizuka，1974；Kikuchi，1974， Research Group of Neogene Tectollics of the Kanto District，1977）．
CALCULATED PROCESS OF TECTONIC MOVEMENTS IN THE SEDIMENTARY IMBRICATE STRUCTURE
In order to clear『the process of tectonic movements， the succession of sedimentary
㎞bricate structure was simplified a mode1（Fig．4）（Kikuchi，1976）． The border of subsid・
Ott．i；
δ譲・
■ ，
．
12
皇2，
， 層
1
ノ
R1 ii 2 i
Fig．4Schematic model displaying the formative process of the Sedimentary lmbricate Structure ．1−3：deposits，
！一皇：depositional surfaces，①一③：apParent axes of tilting． （after Kikuchi，1976）
ing and upliftilg areas is regarded as an axis of tilting or rotation of the block， and the spread of uplifting area as migration of the axis ． That is to say this axis is assumed one and not actual．
We may suppose that the rate of vertical di訊ocation at a particular point in a simple tilting block is proportional to the distance from the axis of tilting， i． e．
dh
＝Cx， （1）
dt
盤ti旛識k膿 麟盤呈1・融r畷離賭望1認膿
movement continua皿y proceeds in a sedimentary imbricate stnlcture and， thus， the assumed axis of titing migrates at a constant rate， the rate of vertical dislocation of the terrace（or stratum）must be shown by another equation． The distance from the
≠?奄刀hof t丑ting x is given by
X＝Vt＋XO， （2）
when V is the migrating rate of the ax 鰍刀h， t is the age of the terrace（or stratum）， and xo is the origillal distance from the poillt to the axis at that time． Then， substituting equation（2）into equation（1），
dh ＝c（Vt＋Xo）＝CVt＋CXo．
dt Substituting
1） ＝ C17 and Vo ＝ CXo ，
−Zi｝dh ＝ Dt＋v・・ （3）
Dmeans the coefficient of vertical diSlocation at the concerned po㎞t having a distance xfrom the axis ， and vo is the rate of vertical diSlocation in the past， when the terrace
（or stratum）has been formed（t＝O）． The height of the terrace（or stratum）h is， therefore，
h＝∫dhニ∫の ＋・・）dt＝÷D 2＋・・ ・h・， （4）
where h o is the height of the terrace（or stratum）when it was formed（t＝0）．
Equation（4）indicated that the height of the terrace（or stratum）at the particular point 血ablock exhibi廿ng a sedimentary㎞bricate structure is expressed in a quadratic
fUnction of time if the axis migrated at constant rate，and that the rate of vertical diSloca・
tion at the point unifbrmly accelerated． In other words， the locus of the height of the terrace（or stratum）at each point shows a respective parabola with the passage of time as Shown Fig．5（a）． It may be stated that the rate of subsidence gradually decreases with 廿1e apProach of the axis at a point that has been subsiding originaUy， and then sub sidence turns into uplift at the transit of the axis ， and thereafter the rate of uplift gradually
＿87一
increases． A conceptua1 sequence taking account of the effect of an assumed sea level change is shown in Fig．5（b）．
While，equation（4）is simplified when V＝ O as follows；
h＝Vot＋ho． （5）
Equation（5）means that the relationSliip between h and t is linear． In other words， the height of the terrace（or stratum）in a sirnply tilting area where the axis is relatively fixed increases in proportion to proceeding of time， and is the fotmula that assumes a constant rate of uplift which many researchers have accepted in several active areas in the world．
It may be said that equation（4）is more general formula showing the relationShip between h・ight・nd・g・・f・terrace（・r st・atum）， f・・it in・1ud・・equ・ti・n（5）whi・h can b・apPli・d only to a specific area where the aXis of tilting has never moved．
（Q）
（b）
T3
うろ
ん
ん
2
t
v．t← h。
懸：蜜：1；玉：二
畿・・y ：謹・一一一
；ill…1…琴il…ll……雛
、 、
艦艦鎚顎：‡｛灘1 な＼＿！錫 @ も、、 ．、、 、
転 ，・ 噛
7、、 、
、 ，亀、、一 、
t
Fig．5（a）Change of height of terrace with time drawn by the model for tectonic movements．
（b）Sequence of terraces and deposits by superposition of sea level fluctuations ・nd tect・ni・m・v・m・nt・・ ・−t5・ag・・f d・p・・iti・n・1・u・face， Tl−TS・d・p・・i−
tional surface formed at stage of high sea level stand．
EUSTASY AND TECTONIC MOVEMENTS ESTIMATED FROM SEDIMENTARY STRUCTURE OF THE SHIMOSA GROUP
Naruse（1971）est㎞ated the rates of tectonic movements during the last 16，000x103 yea「s from the thickness of Neogene and Quaternary deposits in the Boso area． His study is unique in taking the consolidation process of deposits into account， but he shows the reverse
of tectonic movements in only the Toyooka Group（Pliocene）by a co功ectual diagram and hardly refered to glacial eustasy in the Narita Fomlation． The tectonic movements during the Quaternary Period surely turned in reverse direction not only at the time of the Toyooka Group but also the Kazusa and Shimosa Group， as mentioned above． In this chapter， the author s method fbr estimating amount of tectonic movements will be applied to the sedimentary stnlcture of the Shimosa Group， and the mode of tectonic movements and sea level curves will be proposed．
1），yo， and 乃o in equation （4） take constant values and 1） talくes a variabl e amount especiany at e ach pohlt，but their quantities are yet unknown． A correlative formula between height and age ofthe terrace at a particular point w皿be established by substituting some
㎞own numbers into the equation．
1） ：age of the terrace（or stratum）
We have no radiometric data of Pliocene and Pleistocene deposits in the Boso area．
However， some data on age of tephras based on the fission track method in the Oiso and the Yokohama districts， west of Tokyo Bay， are recently accumulated． In this way， data of some terraces covered by these tephras have been known． Machida and Suzuki（1971）dated the Shimosueyoshi Terrace in the Yokohama district fbrmed about 120x103 years B．P．， which is correlated to the depositional surface of the Narita in the Boso． Machidaθ al．（1974）mentioned that the age of the characteristic pumice bed（called GoP ）among the Tama Tephra in the west of the Kanto Plain is 275xlO3 years B．P．， and they regarded the pumice bed as intercalating the Yabu Formation in the Boso． As a result， these estimated dates in the Boso succession may be apPlied to equation（4）．
2）乃：present height of the terrace（or stratum）at the point concerned
This term means the present height of the terrace（or stratum）， whose age is known，
at a particular point． This point adoped will be mentioned later．
3）yo：rate of vertical dislocation at the point in the past when the terrace（or stratum）has been formed（ 瓢0）
Evaluating this term is impossible excepting the expedient method mentioned later．
4）ho：past height of the terrace（or stratum）when it has been formed（ ＝0）
substituted for the elevation of sea level at that time
The problem on the elevation of sea level during the interglacial epoch has been discussed by many authors． Drawing a sea level curve during the Quaternary Period by the present method is impossible in so far as the problem is unsolved． Estimating the past elevation of sea level at about 120x103 years ago， however， is possible to some extent because of many papers on the subject that have been recently published．
Butzer and Cuerda（1962）stated that the Tyrrhenian II correlated to Eemian was generally found at about 5 to 10 m high above sea level on stable coast in the Mediterranean Sea． On the basis of the dating of foss且coral reefs from Florida that seems to be stable， Osmondθ aL（1965）mentioned that the sea level at about
130xlO3 years ago was approximately l O m higher than today． Veeh（1966）indicated that the Pleistocene fbssil corals dated about 120x103±20xlO3 years ago occurred between 2 and g m above sea level at many locations in the Pacific and lndian Oceans・
一89一
Landθ α乙（1967）stated that sea leve1110xlO3 years ago rose at least l l m and perhaps as high as 20 m at Bermuda lsland where no evidence fbr Pleistocene tectonism has been discerned． Broecker etα乙（1968）assumed that the sea level at Barbados Terrace lIl stood about 6 m though which is one of the tectonically uplifted island． Guilcher（1969）described that Upper Normanian beach estimated betWeen 200x103 and 120x103 years B．P． stands between 12 and 18 m above the present sea． He also emphasized that the beach probably remained approximately at its original height in most places． Oka（1970）estimated contemporary sea level to be about 12 m on the basis of summarizing Some previous reports that had discussed Late Pleistocene sea levels． Hopkins（1973）represented in his geochronologic study in the Bering Sea region that sea level during Pelukian I correlated by him to Barbados III（125x103 years old）stood no higher than lOm．
These data on the elevation of sea level are discrepant， although they seem to be contemporaneous with each other and were obtained from locations regarded to be situated in stable regions． The author adopts 10m±as the elevation of sea level at about 120x103 years ago．
We have no satisfactory data on height of sea levels at other interglacial epoch．
Although it is no more than mere presumption， the following discussion is based on an estimation that sea level reached 5 m±high above the present sea level at each older interglacial sea level rising．
5）1）：acoefficient of vertical dislocation at the concerned point
According to equation（3）， the numeric of the term 1）must be obtained from the original distance of the concerned point to the axis of tilting xo， the rate of migration of axis V， and the past rate of vertical dislocation at the pointレo，or the
・g・・fterrace ・nd th・p・e・ent・at・・f…tical di・1・cati・n・t th・p・int fk： H・w・ver・
we cannot obtain the numeric of term xo， V， and vo． Moreover， the period of geodetic survey in recent several tens of years is too short to extrapolate the present rates to more than 100x103 years past． Only the expedient method mentioned later，
therefore， is possible to obtain an approximation．
It is almost irnpossible to formulate the curve of tectonic movement on the basis of equation（4）on account of many indefinite factors as itemed above． If we have more data on the ages and the original heights of the deposits， we could express and solve simultaneous equations with the same number of equations as yet． The writer， accordingly， adopted a temporary expedient method that the uppermost horizon of the Yabu Fo㎜ation was deposited at the same height of the present sea level 260x103 years ago．
This speculative assumption is based on the following three evidences， namely，（1）
the uppermost part of the formation may be thought to have been deposited in an intertidal sea environment of the past， because it included trace fo ssils ofExcilolana，（2）the thickness of the Yabu Formation（about 120 m）nearly correspond to the quantity of interglacial sea level fluctuation， which suggests that the formation was deposited at a time ofhigh sea level during an i㎞terglacial epoch， and（3）the age of pyroclastic key bed GoP interbedded in the middle horizon of the fbrmation is dated about 275x103 years B．P． Thus， the following process can be expected at a particular station where the uppermost horizon of the Yabu
Formation lies on the level of the present sea， namely， a stratum began and continued to subside from the original level simultaneouSly with deposition， then turned to uplift after 130x103 years when half of its age passed， and thereafter， conthlued to uplift to the present when it returned to its original elevation． The process of tectonic movements is illustrated㎞
Fig．6．
ん
hr
v＝Vr
⊥tr
V＝O
．．・?B
レ！＝ Vo＝ −Vr
t
Fig．6Two different tectonic curves． Curve I shows tectonic curve of older terrace formed t？ year ago and curve II younger one formed（tr−tp）years ago．
●
Two parabolas in the figu↑e express changes of elevation with t㎞e a食er constmction Of two terraces or strata at one point whose ages and original elevations are known respectively・Both terraces or strata hl the present case correspond to the upPemlost horiZons of the Yabu and the Narita Formations． The parabola 1 shows the forrner whose orig血al elevation is co㎞cident with the present sea level． The height h， on the parabola II indicates the present elevation of terrace or stratum II at that point， and hp indicates its orig血al elevation（when ＝ ρ）． The height h o on the extention li㎞e of parabola II曲ows the imaginary height of the stratum hl the past when stratum I had been deposited， and is equivalent to hr． The following explanation is fbrmulated．
As the rate of vertical diSlocation vo at the particular point when stratum I had been fo rmed（ ＝ o＝0）is equal to the negative quantity of the present rate of vertical diSloca・
tion yr（when ＝tr），
yo＝−Vr・
Equation（3）， thus， can be expressed as fbllows，
dh ＝1）t
dt Vr・ （6）
Further， the time of reverse from subsidence to uplift in tectonic movements corres・
ponds with the just time that the pretended aXis of tilting had passed． The rate of
一91一
・・丘i・al・di・1・cati・n筈・th・・ef・・e・bec・m・・ze・・at去．t・wh・叫i・at・㎜丘・m止・d・p・・iti・n
of stratum I to the present． Equation（6）， therefore，is
警＝D・去 ・一・，＝・，
so that， the coefficient of vertical diSlocation D is reduced to
1）＝2．h ．
tr
（7）
As indicating in Fig．6， ho is equivalent to hr based on the above mentioned assumption．
From equations（4）and（7）， accordingly，we obtain
h・＝ ?f・2
v，tp＋hr， （8）
where tp is the time interval from the older horizon（e．g． the top of the Yabu Formation）
to the younger one（e．g． the top of the Narita Formation）， and hp is the original height of the younger． Adopting a certain standard succession at the particular point where the uppermost horizon of the Yabu is at the present sea level， we must be able to estimate the value of Z）from equation（7）and then formulate equation（8）．
By the way， we can adopt Fukashiro in the geologic profile in Fig．7， as the standard poilt because the uppermost horiZon of the Yabu Formation is at present sea level．
The maXimum thickness of the Shimosa Group was prese rved in this profile，which indicates typical sedimentary facies as compared with other profiles in the Boso Peninsula（Kikuchi，
1976）．This profile， therefbre， is very suitable fbr the examination of the method．
The data in Tab．1were obtained from the Fukashiro standard section． Thus， from equation
（8）the rate ofvertical dislocation at this point is computed as follows，
・・＝÷1（告…lli＝・・774x1σ3・ （9）
The est㎞ated presellt rate of uplift at Fukashiro， about O．8 m per 1，000 years， may be a reasonable value compared with Sugimura s，0．9 m per 1，000 years， which was estirnated at Kururi situated about 15km south of Fukashiro（Sugimura，1967）． The value of D is caluculated from this amount． It is 5．954x 10の9． Using these values，hp can be expressed as；
hρ ＝ 2．977x10幽9 tρ2 0．774x10−3 tp ＋ 60． （10）
This fbrmula represents tectonic movemellts of the sedimentary surface of the Narita Formation at Fukashiro，which is graphed in Fig．8．
On the basis of the expedient method considered above， we can draw the tectonic curve amd the sea level curve dur血lg the sed㎞entation of the Sh㎞osa Group in this area．
The standard columnar section is obtained from the geologic proMe in Fig．7 by the following expedient co！1stmcti6n． The strata observed in the field were extended to some underground data bored in the northem recla㎞ed land along Tokyo Bay． From the underground profile determined by the expediellt method， a columnar section at Fukashiro
（O卜9
b
oゆ o
濃
隔 し
、 、、 、 、、
，@．辱『
@●
@■■．諱D．．・
@「．．H．r■，
1，P「
．さ｝：ミミ』欝蔓こごこへ
薫
㌧・： ，，．，9
f．∴
「甲
・∴1・寧♂
D．∴・馬呉 ・，．・∴・．∴，
、°
Eミ
2ら．，， ．
．■
X ．
・・焉F韓☆
璽C、 も
、．二
幽¶
● 幽噺 P 「，，・
L
」℃
o
．・
X
＼．・・ρ
X ． 8
N
，一
＼・ 一
・C
9 C轡1
邑．
｡層
「 7
「■
・，， 唱7
、．・… 「、 ．
．「
A・・
．圏
．．．惜 r r● 9
@．●・ ● ．．
@層 居言
，．」
@卜
o》
Aボ ，
．L ，・ °■
．ら ．，
9「怩V，，
脚．
喧．・ 鳳 ・ﾁ・｝二
● 霊｛， 凸
．・灘
，「■．，
『 ．、 ・
@ F．
E・●， 」
@ 呈 o ぢ
コへ
，．雫
レ・
，．，F
：陰潤
、：ゼ：：
● P
．， 」 一 6
■一L
・， o
Z
∈Xゆ
@
W看ξ此呈翌。ξ 台c田 蔓∈
O
翻
㎜ 囲 口
一93一
