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53
INTRODUCTION
The north-south trending Thakkhola-Mustang Graben is
located in the Tibetan-Tethys Zone of central Nepal between
83°50”-84° east longitudes and 29°-28°50” north latitudes
bounded by South Tibetan Detachment Fault System (STDS)
(Burchfi el et al. 1992) to the south and Indus-Tsangpo Suture
Zone (ITSZ) to the north. This graben is a part of normal
faulting system affecting the whole Tibetan Plateau (Molnar
and Tapponnier 1978). The basement rock of the graben is
consisting of a thick and nearly continuous lower Paleozoic
to lower Tertiary marine sedimentary succession.
The basement rocks of Paleozoic to Mesozoic ages
unconformably overlain by Neogene to Quaternary age
sediments (Fort et al. 1982; Yoshida et al. 1984). These
Neogene deposits have been divided in to fi ve formations:
the Tetang Formation, the Thakkhola Formation, the
Sammargaon Formation, the Marpha Formation and the
Kaligandaki Formation. Older Miocene Tetang and Thakkhola
formations are disconformably overlained by upper Pliocene
to upper Pleistocene Sammargaon and Marpha formations,
respectively (Fig.1B). The Holocene Kaligandaki Formation
is in a cut-and-fi ll relation with these older formations. The two
older Thakkhola and Tetang formations lie unconformably on
a substratum of the high strain rocks of the deformed Tibetan-
Tethys sedimentary sequences and they are separated by a
low angle (~5°) unconformity (Fort et al. 1982; Adhikari and
Wagreich 2011 a,b).
In this paper, we report the palynological content of samples
from a succession of the Tetang and Thakkhola formations.
The data are discussed in the context of paleoenvironmental
evolution of the southern margin of Tibet.
METHODOLOGY
Twenty-six samples were collected from the black to grey
clay beds of the Tetang and Thakkhola formations. The
samples were prepared in the laboratory following the
methods described by Zetter (1989), and Ferguson at al.
(2007). In order to remove any contamination from the
recent pollen from the atmosphere the samples were cleaned
with scrapper, crushed them to powder and treated with
Hydrochloric acid (HCl) to remove any carbonate contain in
it. This was followed by the treatment with Hydrofl uoric acid
(HF) and boiled for half an hour to remove silicate minerals
from the samples. The samples were then forwarded to
chlorination and acetolysis a then washed with distilled
water and glacial acetic acid in each step and centrifuged
at the rate of 2000 rpm for 2-3 minutes to remove the fi ner
fraction of the sediments from the sample. Finally the
organic material was separated from the inorganic residue
using heavy liquid Zinc Chloride (ZnCl2). The residue thus
obtained was washed several times with distilled water and
mixed with glycerin for microscopic observation. In general,
the state of preservation of the polynomorphs was very poor.
RESULTS
More than 19 families and genera of pollen were identifi ed
from all of the samples (Table 1). A high percentage of
Neogene pollen assemblage from the Thakkhola-Mustang Graben,
central Nepal Himalaya
*Basanta Raj Adhikari1 and Khum Narayan Paudayal2
1Department of Civil Engineering, Institute of Engineering, Pulchowk Campus, Tribhuvan University, Pulchowk, Lalitpur, Nepal
2Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal
*(E-mail: basanta58@hotmail.com)
ABSTRACT
The Thakkhola-Mustang Graben lies north of the Dhaulagiri-Annapurna ranges and south of the Yarlung-Tsangpo Suture
Zone. The basement of Thakkhola-Mustang Graben is composed of Tibetan-Tethyan sedimentary rocks of Paleozoic and
Mesozoic ages, which are unconformably overlain by continental debris of Neogene to Quaternary age. Stratigraphically, the
Thakkhola-Mustang Graben sediments have been divided into fi ve formations namely the Tetang Formation, the Thakkhola
Formation, the Sammargaon Formation, the Marpha Formation and the Kaligandaki Formation. Detailed palynological studies
were carried out to understand the paleoclimate. Lacustrine layers in the Tetang and Thakkhola formations are enriched with
pollen. Pollen analysis shows that the sediments contain dominant alpine trees Abies, Pinus, Keteleeria, Picea Tsuga and
Quercus with some steppe elements such as Artemisia, Compositae, Chenopodiaceae, Plantago and Poaceae. The results
show that during this period, the southern part of Tibet was covered mainly by steppe vegetation, indicating dry climate.
Bulletin of Nepal Geological Society, 2012, Vol. 29, pp. 53-58
54
grassland taxa and a low percentage of temperate forest
taxa characterize the pollen assemblages. One sample from
Tetang village of the Tetang Formation and one sample from
the Tange Village of the Thakkhola Formation contain more
pollen compared to other samples.
Fig.1: Location map of the Thakkhola-Mustang Graben
(Adhikari and Wagreich 2011a). A) Regional geological
map of the Nepal Himalaya B) Geological map of the
Thakkhola-Mustang Graben showing the graben fi ll
units. The location B is shown in Fig. A
Seven samples out of seventeen samples collected from
the Tetang Formation at Tetang village contain pollen. All
samples were carbonaceous clay with plant fossils in some
samples. Sediments display a higher percentage of Pinus,
Quercus and Keteleeria with Abies and Tsuga and low
concentration of Betula and Juglans (Plate I, II and III).
The upper horizon of the Tetang Formation is dominantly
lacustrine sediments, which are rich in pollen.
Similarly, only fi ve samples out of nine contain pollens in
the Thakkhola Formation. Sample collected from the Chaile
section and Tange sections are rich of pollen and spores.
Sample taken from the Chaile section is characterized by
a presence of Angiosperms and Gymnosperms pollen.
Pinus, Quercus, Tsuga, Fagus, Juglans, Betula, Tilia, Salix,
Acer, Fraxinus and Plantago are the dominant pollen grains.
Pteridophyte spores are found in the sample taken from Tange
section. Lycopodiaceae, Polypodiaceae and Pteridaceae
families dominate these spores. Most dominant pollen grains
are Pinus, Quercus, Plantago, Poaceae, Compositae and
Artimisia in the Tange section (Plate I, II & III).
Table no. 1: Neogene pollen assemblage of the Thakkhola-
Mustang Graben
Phylum Family Genus Fig.
No.
Pteridophytes LYCOPODIACEAE Lycopodiaceae 1
POLYPODIACEAE Polypodium 2
PTERIDACEAE Pteris 3
Family indetermined 4
Gymnosperms PINACEAE Abies 5
Keteleeria 6
Picea 7
Pinus 8
Tsuga 9
Angiosperms FAGACEAE Quercus 10
Fagus 11
Fagaceae ? 12
JUGLANDACEAE Juglans . 13
BETULACEAE Alnus 14
Betula 15
TILIACEAE Tilia 16
SALICACEAE Salix 17
ACERACEAE Acer 18
OLEACEAE Fraxinus 19
Ligustrum 20
CARYOPHYLLACEAE Caryophyllaceae
gen. indet. 21
ROSACEAE Rosaceae gen. indet. 22
VITACEAE
Parthenocissus 23
Vitaceae 24
CHENOPODIACEAE Chenopodiaceae gen.
indet. 25
PLANTAGINACEAE Plantago 26
POACEAE Poaceae gen. indet. 27
COMPOSITAE
Tubifl orae
Compositae gen.
indet.
28
ASTERACEAE Artemisia 29
Undetermined 30
Basanta Raj Adhikari and Khum N. Paudayal
55
DISCUSSIONS
The pollen assemblage include arboreal taxa (such as Pinus,
Betula, Keteleeria, Juglans, and Quercus), shrub taxa
(such as Oleaceae) and herbaceous taxa, which are mainly
Chenopodiaceae, Artemisia, Poaceae and Rosaceae. The
palynofl oras of the Thakkhola and Tetang formations may
reveal that paleotemperature is the key factor in controlling
long-term trend and fl uctuations in Neogene vegetation
in Thakkhola-Mustang Graben. Pinus is usually over-
represented because of its high pollen production and long
distance dispersion (Denton and Karlen 1973; Wang and
Wang 1983) and the percentage of Pinus below 10% are
insignifi cant (Faegri and Iverson 1989). Therefore, they are
generally regarded as being exotic. Presence of Keteleeria in
Tetang Formation may indicate the warm climate but it can
be reworked from the older sediments. Yoshida et al. (1984)
suggested the dry climate during the deposition of Tetang
Formation based on the high percentage of Ephedra spores.
Betula, Quercus and Juglans are very sensitive to humidity
and their pollen contents increase with rainfall (Sun et al.
1996). Therefore, presence of these taxa in Thakkhola-
Mustang Graben indicates temperate forest increased
signifi cantly, implying a relatively humid climate. Presence
of high percentage of Plantago, Poaceae, Artimisia and
Chenopodiaceae in Thakkhola Formation indicate the
arid climate. They show that mostly steppe vegetation
was dominant during the deposition time of Thakkhola
Formation. This steppe vegetation might caused by the
Himalayan barrier. The uplift of the Tibetan Plateau could
exert a profound effect upon atmospheric circulation and
environmental changes of Asia. It would form a water vapor
barrier, so that the water vapor carried by the south-west
monsoon could not reach the Tibetan Plateau (Ruddiman
and Kurzbach 1989), leading to the decrease of rainfall
and gradual vegetation change to arid grasslands in the
Thakkhola-Mustang Graben.
CONCLUSIONS
Graben sediments are composed of braided fl uvial deposits
with lacustrine deposits in different level of the succession.
Lacustrine layers in the Tetang and Thakkhola formations
are enriched with pollen. Pollen analysis shows that the
sediments contain dominant alpine trees Abies, Pinus,
Keteleeria, Picea Tsuga and Quercus with some steppe
elements such as Artemisia, Compositae, Chenopodiaceae,
Plantago and Poaceae. The results show that during this
period, the southern part of Tibet was covered mainly by
steppe vegetation, indicating dry climate. It is presumed that
the paleoclimate during the sediment deposition time of the
Thakkhola-Mustang Graben was signifi cantly warmer than
the present-day climate.
ACKNOWLEDGEMENTS
This paper is part of the Ph. D. thesis by the fi rst author at
the University of Vienna, Austria. This study was supported
by the Austrian Academic Exchange Service (OEAD). We
would like to acknowledge Michael Wagreich, Department
of Geodynamics and Sedimentology, University of Vienna
for fruitful discussion. We are very greatful to K. K. Acharya
and Y. N. Timsina for fi eldwork assistance. Our deep
gratitude goes to Ilse Draxler, Department of Palynology,
Geological Survey of Austria, Reinhard Zetter, Department
of Paleontology, University of Vienna for their help in pollen
analysis and identifi cation.
REFERENCES
Adhikari, B. R. and Wagreich, M., 2011a, Provenance evolution of
collapse graben fi ll in the Himalaya-The Miocene to Quaternary
Thakkhola-Mustang Graben (Nepal). Sedimentrary Geology,
v. 223, pp. 1-14.
Adhikari, B. R. and Wagreich, M. 2011b, Facies analysis and basin
architecture of the Thakkhola-Mustang Graben (Neogen-
Quaternary), central Nepal Himalaya. Austrian Journal of
Earth Science 104/1, 66-80.
Burchfi el, B. C., Chen, Z., Hodges, K. V., Liu, Y., Royden, L. H.,
Deng, C. and Xu, J., 1992, The south Tibetan Detachment
System Himalayan orogen. Extension contemporaneous
with and parallel to shortening in a collisional mountain belt,
Special paper Geological Society of America, v. 269, p. 41.
Denton, G. H. and Karlen, W., 1973, Holocene climatic variations-
their pattern and possible cause, Quaternary Research, v. 3,
pp. 155-205.
Fægri, K. and Iversen, J., 1989, Textbook of pollen analysis, John
Wiley & Sons, 328 p.
Ferguson D. K., Zetter R. and Paudayal, K. N., 2007. The need for
the SEM in Palaeopalynology, Comptes Rendus Palevol, v. 6
(6-7), pp. 423-430.
Fort, M., Freytet, P. and Colchen, M., 1982, Structural and
sedimentological evolution of the Thakkhola Mustang Graben
(Nepal Himalaya). Z. für Geomorph. Suppl. Bd., v. 42, pp.
75-98.
Molnar, P., and Tapponnier, P., 1978, Active tectonics of Tibet,
Journal of Geophysics Research, v. 83, pp. 5361-5375.
Ruddiman, W. F., and Kutzbach, J. E., 1989, Forcing of late
Cenozoic northern hemisphere climate by plateau uplift in
southern Asia and The American west. Journal of Geophysical
Research, v. 94, pp. 18409-18427.
Sun, X. J., Wang, F. Y. and Song, C.Q., 1996, Pollen-climate
response surfaces of selected taxa from Northern China,
Science in China, Series D 39 (5), pp. 486-493.
Wang, K. F. and Wang, X. Z., 1983, Palynology Conspectus.
Beijing University Press, Beijing.
Yoshida, M., Igarashi, Y., Arita, K., Hayashi, D., and Sharma, T.,
1984, Magnetostratigraphy and pollen analytic studies of the
Takmar series, Nepal Himalayas, Journal of Nepal Geological
Society, v. 4, pp. 101-120 (Special Issue).
Zetter, R., 1989, Methodik und bedeutung einer routinemäßig
kombinierten lichtmikros-kopischen und raster
elektronenmikroskopischen untersuchung fossiler
mikrofl oren. Cour. Forsch.-Inst. Senckenberg, Frankfurt am
Main, v. 109, pp. 41-50.
Neogene pollen assemblage from the Thakkhola-Mustang Graben, central Nepal Himalaya
56
EXPLANATION OF PLATE
1. Lycopodiaceae, LM X 200, 2. Polypodiaceae, LM X 200, 3. Pteris sp., LM X 200 4. Fam. indet., LM X 200, 5 . Abies sp.
LM X 200, 6. Keteleeria sp. LM X 200, 7. Picea sp., Equatorial view, LM X 200, 8a. Pinus sp., Equatorial view, LM X 600,
8b. Pinus sp., Polar view, LM X 600, 9. Tsuga sp., Polar view, LM X 600, 10a. Quercus sp., Equatorial view, LM X 600, 10b.
Quercus sp., Polar view, LM X 600, 11a. Fagus sp., Equatorial view, LM X 600, 11b. Fagus sp., Polar view, LM X 600, 12.
Fagaceae (?), Equatorial view, LM X 600
Basanta Raj Adhikari and Khum N. Paudayal
57
EXPLANATION OF PLATE
13. Juglans sp., LM X 600, 14. Alnus.sp., Polar view, LM X 600 , 15 . Betula sp., Polar view, LM X 600, 16. Tilia sp., Polar
view, LM X 600 , 17. Salix sp., Equatorial view, LM X 600, 18. Acer sp., Polar view, LM X 600, 19a. Oleaceae (Fraxinus),
Equatorial view, LM X 600, 19b. Oleaceae, Polar view, LM X 600, 20a. Oleaceae (Ligustrum), Equatorial view, LM X 600,
20b. Oleaceae, Polar view, LM X 600, 21. Caryophyllaceae, LM X 600, 22a. Rosaceae, Equatorial view, LM X 600, 22b.
Rosaceae, Polar view, LM X 600, 23a. Parthenocissus sp., Equatorial view, LM X 600, 23b. Parthenocissus sp., Polar view,
LM X 600, 24a. Vitaceae (?), Equatorial view, LM X 600, 24b. Vitaceae (?), Polar view, LM X 600
Neogene pollen assemblage from the Thakkhola-Mustang Graben, central Nepal Himalaya
58
EXPLANATION OF PLATE
25 . Chenopodiaceae, LM X 600, 26. Plantago sp., LM X 600, 27. Poaceae, LM X 600, 28. Compositae, Polar view, LM X
600, 29. Artemisia sp., LM X 600, 30. Unidentifi ed, Polar view, LM X 600
Basanta Raj Adhikari and Khum N. Paudayal
