On some sedimentary structures observed in the pliocene sediments in Kagawa prefecture, Shikoku, Southwest, Japan-香川大学学術情報リポジトリ

Vol.

12,

No.

1

(1960)

31

O N SOME SEDlMENTARY STRUCTURES OBSERVED

IN

THE

PLIOCENE SEDIMEKTS I N KAGAWA

PREFECTURE,

SWIKOKU,

SOUTHWEST JAPAN

Minoru

SAITO

(Laboratory of Engineering Geology)

(Received June

20,1960)

Introduction

Sedimentary structures developed during deposition of the sediments in which they are preserved or before consolidation of the deposits are common features of extensive distribution in marine formations Such structures which are due to many different kinds of movement such a s vertical, horizontal, vertical

and horizontal, and oblique, result in the production of varied types of structures Although common in marine sedimentary environments their occurrences in lacustrine deposits are not abundant in literature.

Of the lacustrine deposits or lake deposits of Japan, recently, J A K U ~ S U ( ' )

(1959)

~ e p o r t e d on several types of sedimentary structures in the Pleistocene Shiobara lake deposits The structures reported by J AKUISU a r e stated t o be mostly developed in varve deposits, but many are also found in places of the lake where such rhythmical sediments do not develop No other sedimentary structnres have been described from the lacustrine deposits of Japan although it is quite evident that many kinds occur (verbal communication from D K HATAI)

In the present report the writer describes some of the sedimentary structures observed in the lacustrine Pliocene depoits extensively distributed within Kagawa Prefecture.

Acknowledgements

Here the writer wishes t o express his thanks to Professor Kotora HATAI of the Institute of Geology and Paleontology, Tohoku University, for his kind guidance in the fleld instructions and in the laboratory. Deep appreciation is expressed to Professor Tadao MAEKAWA of the Faculty of Agriculture, Kagawa Uni- versity for his continued encouragement concerning the present work

Lacustrine Deposits of Southwest Japan

Distributed from Ehime Prefecture in the southwestern part of Shikoku north -northeastwards through Kagawa Prefecture,and also in a part of Nara Prefecture, the northeastern part of Mie Prefecture, and the northern part of Chita Peninsula in Aichi Prefecture, are lacustrine deposits called by the names of Age formation in Mie Prefecture, the lower Tokoname formation in Aichi Prefecture, Gunchu and Okamura formations in Ehime Prefecture, Mitoyo formation in Kagawa Prefecture, and the Shirakawaike and Sabo formations in Nara Prefecture, and by the well known Alsashi group in Osaka Prefecture

In the northern p a r t of the Chita Peninsula in Aichi Prefecture, the lacustrine Lower Tokoname for mation is unconformably overlain with the marine shell-bearing siltstone referred to the Upper Tokoname forma. tion The molluscan remains from the Upper Tokoname formation were recently dealt with by S HAYA- SAKA and T IWAI(Q

(1960)

In Mie Prefecture, the Age formation i s unconformably covered with the marine Pleistocene Konobe and Mitachi formations The molluscan remains from the marine Pleistocene

32

Tech.. Bull F a c . Agr Kagawa Univ.. deposits was studied by Y ARAKI c4) (1959) T h e Obie formation of Okayama Prefecture which unconfor- mably overlies the basement of Palaeozoic group is also Pleistocene in age In the Osaka and Kobe dis- tricts, the Akashi group is unconfor mably superposed with the Harima group which yields marine fossils, the sediments yielding them being known as the Maiko beds. The molluscan fauna of the Maiko beds has been studied by J MAKIYAMA('O) (1923).

The lacustrine deposits are everywhere unconformably superposed with marine deposits of Pleistocene age where such are developed The lacustrine deposits are characterized with the abundant occurrence of Metasequoza, which has wide distribution Associated with the plant fossil is the occurrence of several species of elephants T h e elephants have been studied by T SHIKAMA (I2) (1937) in detail and also by F T A K A I ( ~ ~ - - ' ~ ) (1938, 1939), while t h e stratigraphy was described by T . SHIKAMA(~-') (1936) , who also discussed on the geological age of the Akashi group and also of the next younger Harima g r o u p ( T SHIKAMA, 1996, 1952) (11 13)

Thus from the general view it seems evident that considerable diastrophism took place after the deposition of the lacustrine deposits and before the marine transgression which brought into the different areas a Pleistocene marine fauna, abundant in individuals but few in species

This marine transgression occurred after the extensive lacustrine sedimentary basin was broken t o admit the free inudation of marine waters through the present day Ise Bay between Mie a n d Aichi Prefecture, and through the now existing Kii Strait between Shikoku and Wakayama Prefectures

Remarks on the Sedimentary Structures

The sedimentary structures to be described were found in t h e lacuetrine deposits above mentioned and several of the ones observed in Kagawa Prefecture will be treated in t h i s article Since the lacustrine deposits frequently show poor sorting of the sediments, rapid variation in their lithological facies both laterally and vertically, it may be added that t h e sedimentary structures developed would be somewhat different from the ones formed under a marine environment where the sediments a r e more uniform, thicker, better sorted and the bottom configuration of the sedimentary basin different from that of a lacustr ine sedimentary envior nment Because rhythmical alternations of sediments are not well developed in present area, fine-cut structures as reported by J A ~ u r s u ( I ) (1959) from the Shiobara lake have not been observed For the sake of convenience the different types will be described separately and because of page limitation only several will be described in each article of the ones expected to be published.

Loadzng Structures

Among the different types of structures resulting from the loading of coarse sediments upon finer ones resulting in load casts, the one (Fig 1) observed near the resevoir of Taman.toge in Ayagami-mura, Ka- gawagun, Kagawa Prefecture seems to be an interesting type. Here one of the outcrops of sandy silt- stone overlain conformably wilh a rubble conglomerate, shows a n interesting load cast In the sandy silt is found an accumulation of pebble to cobble size, subangular to more or less rounded gravels of the Cretaceous Izumi sandstone and rarely also of the basement of granite The longer axis of the gravels are a t random, some horizontal to the bedding. others at oblique angles and some nearly vertical in posi- tion The central part of the submerged gravels comprises rather larger size gravels while the marginal parts are more dominantly of the smaller sized ones The pocket measures about one meter in width and nearly

60

centimeters in depth, thus being longer than deep. The gravelly accumulation in contact with the surrounding silt shows almost no distinct boundary, because of the sandy silt impregnating the gravelly deposit and the matrix of the accumulated gravels admixes with the sandy silt Howevever, i t is evident that the gravels were loaded rapidly upon the unconsolidated sandy silt a s indicated by the random orientation of the longer axis of the gravels, the impregnation of the sandy silts into the gravelly depos- it, of the matrix of the gravel deposit admixing with the sandy silt Also because the load cast devel-

oped at angle diverging from the vertical and since the overlying gravels which become smaller in upward seguence are inclined showing that they were deposited a t an angle,it is considered that the loading was from the right t o left of the photograph This direction is also considered to correspond to that of the source of the supply. In nearby places the gravels are found t o have been submerged into the underlying granular sandy silts and also to have formed brozd load casts (Fig

2)

This also shows that the substra- tum was plastic a t the time of gravel deposition

A similar case (Fig 3) is observed near Nishiebata in Manno-cho, Nakatado-gun, where gravels are found deeply loading the underlying sandy silts in more or less wedge.shape In this case the underlying sandy silts are injected upwards into the gravelly deposits Also in this case there can be observed no definite arrangement of the longer axis of the cobble to pebble size rounded to subrounded gravels derived from the Izumi Cretaceous sandstone Some of the gravels are captured into the underlying sandy silts, thus proving t h e unconsolidated nature of the sdiments and its plasticity. Also the contact reveals clearly the conformable nature of the contact, even though the boundary i s very undulating when seen from a distance. Impregnation of the underlying sandy silts upwards into the lower parts of the gravelly deposits seems to be evidence that the accumulation of the gravels upon the substratum was rather rapid and t h a t t h e substratum was unconsolidated because some of the gravels have sunk deeply into it

Thus t h e first described load cast i s analogous to that of the one just mentioned in the rapid accumula- tion of the gravels, impregnation of the lower sediments into the upper, the admixing of the sediments a t and near the points of contact of the two, and in the more or less rubbly aspect of the gravels, particularly in the former case

The t w o kinds of load casts described above differ from the cases generally found i n marine sedi- ments in showing rubble aspect, ill-sorting and generally less rounded gravels. In marine sediments when such types occur generally they are associated with rubble deposits and or turbidity currents, which may be the most important method t o account their rapid supply of sediments

A n o t h e ~ particular type is found in an alternation of sandy silt and ar kose sands exposed near Mita in Manno-cho,Nakatado-gun Here (Figs

4 ,

5,

6)

the silty layers show wavy structures due t o sagging, wave loads, pocket development and because of loading of arkose sandy sediment upon the sandy silts, partial stretching of the layers apparently resulted In some places of the same outcrop there appear wash- out like structur es, which are really due to rapid loading and not to scouring The cause of the overturne- d load waves may be due to that rapid accumulation was associated with current action, by which the soft sandy silts were compelled to bend in the direction of the current. Although the outcrop shows an alternation of layers of different kinds of sediments, the arkose sands more or less scrambled in aspect in good agreement with the directions of the loading, detaching of the silty layers by stretching in some places, pinching and swelling of those layers, all of which point to that considerable current action must have accompanied the development of the load casts

Under quiet sedimentary environment it would be almost impossible t o develop such structures as shown in the photographs Admixing of the sandy silt and arkose sands particularly in the layers of the latter sediments, the more or less scrambled aspect of those sediments and the above mentioned features all point to the same view Also upholding such a view is that a t places there is found rounded lumps of sandy silt with the marginal parts built of a mixture of the sandy srlt and arkose sand, and even some of the arkose sand is captured in the lump which measures about

20

centimters in short and about

35

centimeters in longer diameters, thus appearing more or less flattened parallel with the general dip of the layers

Tech. Bull Fac Agr Kagawa Univ.

Significance of the Loading Structures

The loading structures or load casts described in the above lines seem to have certain significance o n the development of the lacustrine deposits Judging from the type of load casts described by J. AKU'IS(~)

(1959) from the lacustrine deposits of the paleo Shiobara lake and those from the marine formations of t h e Chichibu Basin, Saitama Prefecture by J A R A I ( ~ ) (1960), it may be considered that the present types described in this article resemble more closely those of t h e latter author. This is because the ones treated by AKUTSU seem to have been formed under tranquil conditions while those of ARAI were formed under conditions of current action, turbidity currents and more or less quiet conditions, and compared with the present ones those made under conditions of current action show morc or less similarity with the present ones The similarity of the lacustrine and marine load casts just mentioned is in the rather rapid supply of sediments resulting in the formation of rather large size, admixing of sediments a t the marginal parts,random orientation of the filling gravells, the accumulation of the coarser ones in the middle parts and of finer materials in the outer parts, and the more or less relationship with the direction of supply of the materials However, the lacustrine ones differ from those of marine environment even though having mutual characters in the degree of sorting, roundness of the included gravels in general, thickness of the sediments, presumed duration of the current action associated therewith

Crowded S o f t Rock Pebbles

Near the entrance to Ebata in Manno.cho, there i s an outcrop showing sandy silt in the lower and cross-laminated gravelly sands in the upper part The upper part shows small load casts against the fine sands of the lower (Fig 7) but none (Fig

8)

in another part of the outcrop The lower part

6f

fine cross-laminated sands (Fig 7) and of non-cross-laminated sands (Fig

8)

, the latter grading laterally into the former The gravelly sands in the upper part have inte~calated in them soft rock pebbles derived from the underlying silty sands. These pebbles have their longer axis more or less parallel with the angle of cross-lamination The silty sands underlying the gravelly sands have developed in their upper part a crowded accumulation of angular to subrounded fragments of the sand rock, and this grades downwards into massive silty sands This outcrop needs special consideration because different interpretations may be possible, and these are mentioned below

Although the stratigraphic relationship between the lower silty sands and covering gravelly sands is conformable. according to interpretation the relation may be possibly an unconformity This may be explained by that the closely crowded angular to subr ounded fragments which downwards grade into massive silty sands a r e with a matrix of the same material, and this suggests the residual nature oi the phenomenon The ment~oned closely crowded fragments are more rounded in their upper parts par ticularly near to and at the contacts with the overlying gravelly sands and less so downwards. Laterally this aspect is lost because of grading into cross~laminated silty sands above which there are well developed load casts showing a very highly irregular surface

From the above description of the outcrop it may be probable that the peculiarly arranged gravel-like deposits represents residual materials, that is to say t h e original exposed land surface andthat the gravel.like materials are weathering products later covered with lake waters and ther efore partially eroded by i t If so. then i t follows that the weatheted zone represents and unconformity whose upper part i s blended with the covering sediments However, this may be difficult to accept because later- ally the gravel like silty sediments grade in one direction into fine sandstone which is cross-laminated and in the other into similar sediments. Also load casts are developed above the fine sands which are cross -laminated and also above t h e sediments in the other direction, thus suggesting t h e conformable contact Incipient scouring and backfilling is also noticed i n the parts above the corss-laminated fine sands Thus from the evidence of rather strong currents i t may also be inferred that the structures

Vol

12,

No.

1

(1960)

35

present are not those produced during an erosion interval. Therefore, i t is considered tbat the contact is a conformable one.

Another explanation to the outcrop may be that the gravel-like or blocky or lumpy aspect shown by the silty sands, fragments of which are incorporated into the overlying gravelly sands was due t o current action possibly associated with the instability of the sedimentary basin The lumpy fragments may be nothing but the ripped u p and partially water rounded portions of the more or less firm lake bottom deposits This may be further accepted from that the conformably superposed sediments are those result- ing from current action.

That currents were sometimes rather strong during deposition of the lacustrine deposits can also be inferred from other phenomena observed i n many outcrops within Kagawa Prefecture. Among the many outcrops, one is here described merely to show that current action persisted at the time of deposition of the sediments making u p the lacustrine formation. This evidence is in the form of transported, rather large, irregular -shaped blocks of silty material from the original source to he deposited in ar kose sand foreign t o the original environments This outcrop i s a cliff situated near the Suga resevoir in Manno. cho

Included in the arkose sands situated there (Fig.9) is a nearly one and a half meters long and about

30

centimeters thick silty block, fractured irregularly in several parts, and the whole of more or less elongate form with irregular margins In the fractured parts there may be observed the partial injections of arkose sands into the silty block and a t the marginal parts there can be seen arkose sand grains shallowly impregnating the silty material. Also beside the large block there are also severaI smaller ones and more rounded in aspect, situated nearby The arkose sands do not show any significant structures beca-use of being homogeneous and massive, aside for the included silty lumps and blocks a s well a s sometimes of flakes or shavings Upwards the arkose sands become gravelly and incorporate angular fragments of silty sediments thought to have been derived from the same source as that of the large blocks and lumpy fragments embedded in the ax kose sands.

The injections of arkose sands into the silty block, the many fractures developed, the impregnation of arkose sand grains shallowly i n t o the marginal parts of the silty block, the elongated but irregular outline of it, the associated small silty lumps are all characteristic features of the transported silty block The source of this block is thought t o be the silty sediments underlying the arkose sands a n d situated not far distant from the present exposure The silty fragments incorporated into the overlying gravelly deposits are thought also t o have been derived from the same source as that of the described block

The features a s presented show that either current action or some other kind of agencies took place in the transportation of the silty block a s well a s of the smaller fragments. If the transportation was due t o currents then i t may be inferred that such aqueous agencies were active at times during deposition of the sediments preserving the structuses If not due to water currents, then some process as turbidity curreut, subaqueous mudflow, subaqueous land sliding or rock-fall may have been responsible for the transporta- tion of the erratic block of silty material Whether the instability of the sedimentary basin associated with the rising of the hinterland had relation with the phenomenon is a t present reserved for another opportunity

R e f e r e n c e s

(1) AKUTSU, J,, : Types of Sedimentary Structu (2) ARAI,

J..

: Some Load Cast Structres in the res Observed i n the Pleistocene Shiobasa Lake De- Akahira Fosmasion (Oligocene), Chichibu Basin, posits, Tochigi Prefecture, B u l l . Ut.sunomi,ya Saiama Prefecture, ,Japan, Bull. C f ~ i c h i b u Mu.s..

36

Tech. Bull. Fac. Agr Kagawa Univ.

(3)

ARAI, J. : The T e r t i a r y System of the Chi- chibu Basin, Saitama Prefecture, Central Japan P a r t 1-Sedimentology, Jap. Soc Prom. Scz

,

Tokyo, 1-122, 30 PIS (1960)

.

(4)

ARAKI, Y. ; A Pleistocene Marine Fauna from near the Cities of Tsu and Yokkaichi, Mie Prefecture, Southwest Japan, T r a n s . Proc Pale-

ant.

Soc. Tapan, N . S , (331, 19-22,

1

PI (1959) (5) FUNAYAMA, Y. : Sedimentary Structure from the Northwesrn Part of the Yamagata Basin, Ya- magata Prefecture, Japan, Saito Ho-on Kaz Mus

Res BzdN , (25) 3-9,l PI (1956)

(6)

HAYASAKA, S

,

IWAI, T : Remarks on a Pleistocene Marine Fauna from Chita Peninsula, A ichi Prefecture, Japan, Jour Geol Soc. J a p a n ,

66

(776), 3538-360,

1

f i g , 1 table (1960)

(7) HAYASHI, T

,

IWAI, T : A Sedimentary Structure in the Kumura Formation (Miocene) ,

Chita Peninsula, Aichi Prefecture, J a g lour G e -

o l . Geogr

, 30,

195-200 (1959)

(8)

HAIAI K , et a1

,

A Preliminary Note on Some Sedimentary Structures Observed in the Cen- ozoic Rocks of Boso Peninsula, Chiba Prefecture, Japan, Scz. Reg , Tokyo Kyozku Dazgaku, Sec

3

.

4

(33), 185-2208 (1956)

(9)

HAIAI, K., FUNAYAMA, Y : Load Cast in

the Miocene Hirashio Formation, Western Margin of the Yamagata Basin, Yamagata P r e f e c t u ~ e , Ja- pan, Sazto Ho-on Kai Mus., Res. Bull

,

(261,

11-

15,

1

PI. (1957).

(10) MAKIYAMA, J. : Some Pliocene Molluslts from Maiko near Kobe, Tap Jour. Geol. Geogr

.

,

2

(a),

19-26,

1

PI. (1923).

(11) SNIKAMA, T. : On the Akash iGroup, Jour

Geol Soc. l a p a n ,

43

( 5 1 4 , 565-589 (1936)

(12) SHIKAMA, T . : Parastegodon infrequens s p . nov from the Akasi District, Jap. J o u r , Geol

Geogr.,

14

(2-3), 127-131,l

PI

(1937).

(13) SHIKAMA, T : The Japanese Quaternary,

its Outline ane Historical Review, Bull. Yokohama

N a t . Univ

,

2

(I),

32-53 (1952)

(14) TAKAI, F . : O n Some Cenozoic Mammals from Japan (Preliminary Report), Jour Geol Soc

l a p a n .

45

(541), 715-763 (1938)

(151 TAKAI, F : On Some Cenozoic Mammals from Japan ( P a r t l ) , Jour Geol. Soc J a p a n ,

46

(552). 481-489 (1939)

U6) TAKAI,

F

: A Summary of the Mammalian Faunae of Eastern Asia and the Interrelationships of Continents since the Mesozoic, l a p Jour. Geol

Geogr ,

2 2 ,

169-205 (1952)

Fxplanation to plate

Fig. 1 - A n interesting type of load cast exposed near the resevoir of Taman.toge in Ayagami-mura, Kagawa- gun. Kagawa Prefecture

Fig 2-Broad load c a s t ; the gravels are found to have been submerged into the underlying granular sandy silts in nearby places (Fig 1)

Fig. 3 -Load cast near Nisiebata in Manno-cho,Nakatado-gun, Kagawa Prefecture.

Figs 4, 5,

6 -

The silty layer shows wavy structure due to sagging, wave loads, pocket development and because of loading of arkose sands. Outcrop of a road-side cliff on road leading from Fukurami to Mita , Manno-cho, Nakatado-gun,Kagawa Prefecture

Figs 7, 8-The gravelly sands in the upper part have intercalated in them soft rock pebbles derived from the underlying silty sands. Cliff near the entrance of Ebata, Manno-cho, Nakatado-gun, Kagawa Prefecture.

F i g 9-Transported large irregular shaped blocks of silty material embedded in arkose sands Cliff near the Suga resevoir in Manno-cho, Nakatado-gun, Kagawa Prefecture

Vol. 12, No.

1

(1960)

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