Journal of Geosciences, Osaka City University

Vol. 40, Art. 8, p.145-152, March, 1997

The mode of occurrence of leaf-fossils in the Churia (Siwal ik)Group of West Central Nepal

KONOMATSU Masahiko

Department of Geosciences, Faculty of Science, Osaka City University

Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558, Japan

AbstractAbundant leaf-fossils are found in the Lower and Middle Churia (Siwalik) Group in West Central Ne­

pal. In this paper, author describes the occurrences of leaf-fossils in fluvial sediments in the Group.Leaf-fossils have been found in parallel laminated and small-ripple-laminated sandstones, and also in

mudstones. The occurrences of leaf-fossils can be divided on the basis of the existence of carbonaceous mat­ter including cuticles and on the density of leaf-fossils. The density means some kind of distribution patternof leaf-fossils (show Fig.4). The occurrence of the former divided as follow, Ca, Ca' and Cb types. The Catype is characterized with leaf-compression. The Ca'type is characterized with leaf-compression which hasbeen cuticle coated with calcareous membrane. The Cb type is consists of leaf- impression. By the anothermethord of classification used of fossils density. They are divided into four types. The Da type indicatesthat the density of leaf-fossils on the surface of bed occupies more over 80 percent. The Db type shows 20 to80 percent. The Dc type shows less than 20 percent. The Dd type indicates that almost consists of only linearleaves. The contents of leaf-fossils decrease upward as the Da to Dd types. This tendency may indicatespaleoenvironmental changes from the flood plain to the natural levees of a meandering river.

Key Words: leaf-fossil, mode of occurrence, Miocene, Churia (Siwalik) Group, Nepal.

1. Introduction

Plant-fossil assemblages have generally been used to

reconstruct paleovegetation. There are leaf, seed, wood

and pollen fossils in plant-fossil assemblages. The sedi­

mentary.process of each fossil differs according to the

sedimentary environment. It is necessary, therefore, to

study taphonomy of the plant-fossils in order to recon­

struct details of the paleovegetation. It is important to

describe the occurrence of the plant-fossiis and to dis­

cuss how fossil assemblages were deposited in the sedi­

mentary environment. This paper attempts to describe

the occurrence of leaf-fossil assemblages of the Churia

(Siwalik) Group in West Central Nepal for reconstruct­

ing the paleovegetation and visualizing the

paleoenvironment in which the plants grew.

The Siwalik Group of Neogene fluvial sediments had

been supplied from the lifted up Himalayas, and it lies

along the foothill regions of the range for a length of

more than 2000 km. Evidence for the effect of the lifted

up Himalayas is preserved in the sediments of this

group. The group is characterized by the including of

rich plant fossils (leaf fossils, wood and seeds) with or­

ganic matter. They are well preserved.

There have been several paleobotanical investigations of

the Churia sediments m Nepal, as reported by

Prasad (1990), Awasthi and Prasad (1990), Sarkar (1990)

and Awasthi et al. (1994). These investigations recon­

structed the paleoenvironments of the assemblages of

plant leaf fossils and pollen, however they did not dis­

cuss the sedimentary environment.

2. Geological Setting

The name of Churia Group was ordered a part of

the Siwalik Group, Nepal. The Churia Group has been

studied in detail by Tokuoka et al. (1986, 1988, 1990) at

the Arung Khola- Tinau Khola area, West Central Nepal

(Fig. 1).

The Churia Group is bounded by the Main Boundary

Thrust (MBT) on the north and the Frontal ChuriaThrust (FCT) on the south. The Churia Group, in the

present study area, consists of sandstone, mudstone and

conglomerate, and is about 6000 m thick. The Group is

divided, lithostratigraphically, into the Arung Khola

Formation, the Binai Khola Formation, the Chitwan

Formation and the Deorali Formation in ascending or­

der (Tokuoka et al., 1986). This is a reversed graded

146 KONOMATSU Masahiko

84' 00'27' 45'

o 1 2 3km~

---- Interred fault

83' 30'

0­:::Joa:(OJ

~ Deorall Formation

EJ Chitwan Formation

g BU]SZ~ B 8m Binal Khola Formation

~ § BI ~ Anticline

! : ::] Arung Khola FOrmatio:",,, Syncline

o ~ AI -- Fault

D Midland Supergroup

83' 25'

- 27' 30'

Fig, 1. Geological Map of the Churia Group, West Central Nepal and location of colummar sections (after Tokuoka et al.,1990),

sequence. The Arung Khola and Binai Khola Formations

are subdivided into three members each as AI, Am, Auand Bl, Bm, Bu respectively.

3, Lithology and Stratigraphy

Leaf-fossils are dominant in the AI, Am, Au and BI

Members. The lithology of these Members is described

below.

The Al Member consists of alternating beds of very

fine- to medium-grained, calcareous or arkosic sand­

stone and variegated mudstone, with the mudstone

more dominant than the sandstone. Thickness of the

member is more over than 500m (Tokuoka et al., 1990),

and leaf-fossils are found m both black mudstone and

variegated mudstone.The Am Member consists of alternating beds of

fine- to coarse-grained, calcareous or quartzose sand­

stone and variegated mudstone, with more sandstone

than in the Al Member. It has a thickness of 850-1200m (Tokuoka et al., 1990). Leaf-fossils are found in the

fine grained calcareous sandstone.The Au Member consists of fine- to coarse-grained

sandstone and gray to black mudstone. This membershows a sand rich alternation and has a thickness of

850-1600 m.

The BI Member consists mostly of thick-bedded, me­dium- to coarse- grained sandstone with mudstone

intercalations. The mudstones are variegated or dark­gray in color, and conglomerates are found in the upper

part of the sequence. This member is about 1000 m

thick.

4, Description of facies bearing leaf-fossil

The terminology of leaf-fossils used in this paper is

after Hickey (1973) and Sugden (1984).

The Churia Group has a variety of facies from mud

to conglomerate. Fig.2 shows typical columnar sections

with zones of leaf-fossils of the Arung Khola and the

Binai Khola Formations along the Tinau Khola route.The location of the sections are shown in Fig.I. Single

leaf-fossils have sometimes been found in other horizons

with wave ripples. Many leaf-fossils have been found in

parallel, and small ripple- laminated sandstones(Fig.3a, 3b). Sometimes, plant fragments have been

found in black mudstone. The leaf-fossils are preservedin thin, mud layers of 0.2 to 2 cm thickness intercalated

in sandstone (Fig.3c). The thickness of mud layer de­crease upward from the lowest part of the laminated

a. AI Member(Loc.No.93T-1)

b. AI Member(931-2)

c. Au Member(931-6) Fig. 3b

d. 8m Member(931-11)

...... J~ Ca, Db

:::I~ Ca,Da

.0

~t.m.c.

mud sand

<:> . CJ'

Homonoialancelata(Shrub)

Grasses

Oipterocarupussiwa/ikus(Tall tree)

Dominant taxi

......!Ca,Dd Q......Jf} Ca, Dc Q~¢ Ca, Db~ll Ca, Db~~¢ Ca, DaQ

iU>

Da : density of broad leaf-fossils over 80% of an areaDb : density of broad leaf-fossils from 20 to 80% of an areaDc : density of broad leaf-fossils less than 200/0 of an areaDd : only linear leaves without the relation of densty

= AV=

---+-T-r-it.m.c.

mud sand

.~.

:~.

. :. ',',

..... ". :··"0"'

'.7/." ~ .' :- :........".-. : .....

.. .' ....... .

50cmJ

Fig. 3d ~ I====----==d

Types of occurrence of leaf-fossils in Churia Group

Fig. 3c .3e ~

~fJ Ca, Dc

...... p Ca, Da

...... IJ Ca, Db

~t.m.c.

mud sand

.......'

,", ... '

:::~i"_:~:e.~

::~.~

Ca : leaf-fossil with carbonized matterCb : leaf-fossil without carbonized matter

50cmJ

......~ Cb, Db

-t-r-nf.m.c.

mud sand

..... ~ .

.: .... :.:..:. " :

..." .'.

~~~I Cb,Dc

0 broad leaves

[Z] linear leaves

~ fruits

CD rootlets

0 burrows

~ mottles

D sandstone...

• black mud

n reddish mud

5J ripple cross laminae

~ parallel laminae

~~ .' .. ' ~ '.. trough cross bedidng

50cmJ

Fig. 2. Columanar sections with typical facies bearing plant-fossils of the Churia Group in the Tinau Khola route.

148 KONOMATSU Masahiko

The mode of occurrence of leaf-tossils in the Churia Group, Napal 149

sandstone toward upper part. The density of leaf-fossils

on the surface of bed occupies is higher in the lowest

part of the laminated sandstone above black mud (Figs.

2c, 3c and 3e) and the density gradually decrease up­

ward (Fig 2c). These fossils are mainly composed with

broad leaves. Most of the leaf-fossils with linear form

contain assemblages of only one type (Fig.3d) above

the broad-leaved fossils without linear form assem­

blages (Figs.2a, 2b and 2c). The linear form leaf-fossils

include monocotyledons with parallel dormouse vena­

tion. Lanceolate leaf-fossils such as the monocotyledon

Bambusa, have also been found with other broad leaf

assemblages in parallel to small, ripple-laminated sand­

stone. Complete leaves of the monocotyledon are not

found in the assemblages.

Carbonaceous matter with cuticles of leaf-fossils

have been preserved in light gray, parallel to ripple­

laminated sandstone, but they are very rare in black

mud. The matter tend to stick to the lower surface of

the bedding plain (Fig.3e). This facies has been ob­

served to be dominant in the Au and Bl Members. The

occurrence of leaf-compression with coated membranes

of carbonaceous matter are rarely observed (Fig.3f). On

the other hand, the carbonaceous matter with cuticles

do not exist in variegated sediments such as laminated

sandstone and mud (Figs. 2a, 3g). In short, only leaf­

impressions are observed. This facies is dominant in the

Al and Am Members, but is rare in the Au Member of

the Tinau Khola route.

Assemblage changes have been described in the lami­

nated sandstone of the Au Member (Fig. 2c). Leaf and

seed fossils of Dipterocarpus siwalicus which indicate

that tall trees of a tropical rain forest have occurred in

the lowest part of the laminated sandstone. Linear

leaves of Homonoia lanceolata indicate that shrubs of

river bank occurred in the middle part of the sandstone.

Moreover, linear leaves of grasses have been found

above the Homonioa zone.

5. Discussion

5.1. Classification of occurrence based on leaf-fossils

The occurrence of leaf-fossils is classified according

to the presence of leaf-compressions with cuticles and

the density of leaf-fossils. The occurrence can be divided

into five types, namely, Ca and Cb types, based on the

presence of cuticles and Da, Db, Dc and Dd types, based

on the density of leaf-fossils on the surtace of bedding

plain (FigAa) .

A. Existence of leaf-compression with organic matterCa (Ca') type: Leaf-compression

The cuticles of leaf-fossils with black color are car­

bonized. The carbonized matter occurs parallel to

small, dark to light gray ripple-lamination sediments in

the Arung Khola and Binai Khola Formations. The fos­

sils are particularly rich in the Au and Bl Members.

The carbonized matter with cuticles have a tendency to

stick to the lower surface of the sediment and to the

upper epidermis (abaxial surface) of the leaf-fossils

(Fig.3e). Variations can be easily recognized as the

primary and secondary veins of a leaf show clear, nar­

row nses in the adaxial surface. There are two types

of cuticle surface. One types has a lustrous surface, and

the other has a dull surface. Lustrous leaf-fossils have

conaceous textures, as In Dipterocarpus, and

Calophyllum. Rich fossils have been found in the Au

Member. Dull fossils with chartaceous texture have

been found in the Arung Khola and the Binai Khola

Formations. Rich fossils have also been found in the Bl

Member.

The occurrence of leaf-compressions with coated

membranes on the cuticle are sometime observed (Fig.

3f), and this is named the Ca' type. This sedimentary

structure has parallel laminae of fine sandstone, and

hence, the cuticle is well preserved. The leaf-fossils with

chartaceous texture sometimes tear off when mem­

branes are removed by hydrochloric solution. This type

can also be found in the Am Member.Cb type: Leaf-impression

These fossils are characterized by impressions In

variegated mudstone to fine sandstone (Fig.3g). The

Fig. 3. a: Parallel laminated sandstone (arrow) bearing mainly leaf-fossils in the B1 Member along Tinau Khola. b: Parallellaminated sandstone (arrow) bearing mainly leaf-fossils in the Au Member along Tinau Khola (Fig .2c). c: A close-upview of parallel laminated sandstone and black mud (lower arrow in Fig. 3b indicates position); arrows are horizonsbearing leaf-fossils; pen scale: about 13 em. d: A occurrence of grasses of linear type in the Au Member along TinauKhola (Fig.2c). e: An occurrence of leaf-fossils in the Au Member. Almost all carbonized leaf-fossils (black color)tend to stick on the lower surfaces of sediments in the Au Member. f: A occurrence of leaf-compression with coated cal­careous membrane on the carbonaceous matter (black color) in the Am Member along Arnng Khola. g: Leaf­imperssion without carbonaceous matter in variegated sandstone of the Au Member along Tinau Khola.-

150

a

KONOMATSU Masahiko

area : over 20X20 emExistence of

Density of leaf-fossilscarboneceous matter

Types Ca (Ca') Cb Da Db Dc Dd

broad leaf with linear leaf linear leaf

Conditions 0 X > 80% 20 ~80% <20% over 5 leaves

grayishreddish,

horizontalP~PPle or wavy laminaeRemarks variegated·mud

mud bioturbation

b Da Db Dc Dd

Fig. 4. a: Classification of the existence of compression and the density of leaf-fossils. b: Typical sketches of the classification,based on the density of leaf-fossils.

sedimentary struCtures are small ripple and parallellaminations which indicate similar structure to type Ca.Most of the variegated mudstones are reddish and yel­lowish, in which the upper parts are reddish and thelower parts are yellowish. The broad leaf-fossils havebeen found mostly in reddish mud. Most of the mono­cotyledons occurs as grasses in variegated, yellowishmudstone in the Arung Khola Formation. The Cb typeis dominant in the Al and Am Members. This type can

be found mainly in the Tinau Khola route, rather thanin the Arung Khola and Binai Khola routes.

B. Density of Leaf-fossils on the surface of beddingplain

There are four types of density of leaf fossils,namely, Da ,Db, Dc and Dd types (Fig. 4a, 4b). TheDa to Dc types are based on the density of broadleaves, while the Dd type is based on the density of lin­

ear leaf, with grasses as monocotyledons (Fig. 3d).Da type: Density of leaf-fossils on the surface of bedoccupies more over 80 percent.

The midribs of the leaf-fossils have a random orien­tation in the piles. The sedimentary structure consistsof almost parallel laminations, composed of reddish

mud to light-gray fine-grained sand (Fig.Z). The mudlayers in laminated sands bearing leaf-fossils of broadleaves are thick, having a maximum thickness of 2 cm.These occurrences can again be divided into two types.In the first type, the leaf fossils have a tendency to lieflat along parallel lamina, most of the fossils being ob­served on the abaxial surface. In the second type, thecurved leaf-fossils tend to pile up in parallel laminawith transformation structures. These leaf-fossils are

more prominent than any other type in many taxa.Rich fossils were found in these assemblages 1ll theArung Khola Formation.

Db type: Density of leaf-fossils on the surface of bedoccupies 20 to 80 percent.

The Db type is the occurrence of leaf-fossils assmall leaves in piles, with the midribs of the small leaffossils oriented a particular direction. Sedimentarystructures are parallel to small ripple laminations andare composed of reddish mud to light gray medium­grained sand. The mud layers bearing broad leaf-fossilsin laminated sands have a thickness of 0.3 to 1 em.This type is plentiful in the Arung Khola and BinaiKhola formations.

The mode of occurrence of leaf-tossils in the Churia Group, Napal 151

Dc type: Density of leaf-fossils on the surface of bed

occupies less than 20 percent.

The leaf-fossils of this type may have similar direc­

tions of the midribs. Sedimentary structures have paral­

lel to ripple laminations and are composed of mud to

medium-grained, light gray and mottled sand (Fig. 2a).

This type is plentiful in the Arung Khola and Binai

Khola Formations.

Dd type: The fossil-leaves are only linear leaves.

In this type the density of the leaves shows varia­

tion. Sedimentary structures have parallel lamination to

small ripple lamination and are composed of mottled

mud (Fig. 2c, 2d, 3d). These leaves are being to grasses

and shrubs. This type in plentiful in the Au Member

and the Binai Khola Formation.

5.2. The changes of occurrence of leaf-fossil assemblages

in one sequence.In general, the typical change of occurrence of leaf­

fossils in the Churia Group (Fig.2c) is as follows.

Abundant fossil-bearing, parallel laminated

sandstones occur above black massive muds. Seeds andleaf fossils of the Da type occur in lowermost part of

the sandstone. The taxa of the seeds are similar to the

taxa of the leaf-fossils. For example, Dipterocarpus

seeds and Dipterocarpus leaves occur in the same beds

in the Au Member.

Leaf-fossils of the Db or Dc type occur in the mid­dle and upper parts of small ripple laminated

sandstones. As a result, the density of leaf-fossils de­

creases upward from parallel to small ripple laminated

sandstone. Linear leaves as grasses of the Dd type andleaves of Homonoia lanceolata occur in the upper parts

of small ripple-laminated sandstones (Fig.2). Compara­

ble extant species of Homonoia lanceolata is H. riparia

(Konomatsu and Awasthi, in press). This H. riparia is

shrubs which distribute in river bed including river bank

(Brandis, 1921). Therefore, this sedimentary environ­

ment may indicate natural levee and point bar.

As a result, leaf-fossil assemblages change in each

bed in one locality, and careful sampling of fossils is

important in order to describe the character of sedimen­

tary facies. The occurrence of leaf-fossils with changing

sedimentary environments is now examined.

6. Conclusion

1. Leaf-fossils have been found in very fine to medium

grain sandstone which have been characterized with

parallel to small ripple laminae and mudstone. They aregray, red and yellow in these facies. The cuticles of

leaf-fossils are mainly present III gray laminated sand­

stone.

2. The made of occurrence of leaf-fossils can be divided

into tree types on the basis of. the existence of carbona­

ceous matter including cuticles. The Ca type ischarac­

terized with leaf-compression. The Ca'type is character­

ized with leaf-compression which has been cuticle coated

with calcareous membrane. The Cb type is consists of

leaf- impression.

3. The made of occurrence of the occupational density

of leaf-fossils on the surface of bed can be divided into

four types. The Da type indicates that the density of

leaf-fossils on the surface of bed occupies more over 80

percent. The Db type shows 20 to 80 percent. The Dc

type shows less than 20 percent. The Dd type indicates

that almost consists of only linear leaves.

Acknowledgments

I am grateful to Dr. Satoshi Tanaka of Kyoto Uni­

versity of Education for many suggestions regarding

sedimentary facies. I thank all members of NHG project

teams for their encouragement and discussions.

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