SIWALIKS OF ARUN- TAMAR REGION

1. SIWALIKS OF ARUN- TAMAR REGION
Prepared by:-
Samyog Khanal (13/077)
M.Sc. Engineering Geology
(1St Semester)
Central Department of Geology
Tribhuvan University
Email:- Samyogkhanal2017@gmail.com
Submitted to:-
Institute of Science and Technology
Tribhuvan University
Central Department of Geology
Engineering Geology
Kirtipur, Kathmandu, Nepal

2. 1. Introduction
• The region is compressed into tight Anticline and
open Synclines, interrupted by many imbricated
Faults.
• A larger part of the belt is represented by the Middle
and Upper Siwalik, where the subordinate lower
Siwalik Strata are Intermittently distributed.
• The widest piggyback basin of the region is
developed in the Kankai Khola. Its structural position
is fairly similar to that of the Siwalik belt in the
Budar–Jogbudha area of the Mahakali–Seti region.

3. Fig 1:- Simplified geological map of the Siwalik exposed in the Arun- Tamar region.
Source Based on Pradhan et al (2006).

4. 2. Stratigraphy
• Stratigraphy of this section is divided into three
Formations:
a. Lower Siwalik
b. Middle Siwalik
c. Upper Siwalik

5. a) Lower Siwalik
• Lower Siwalik of this section was well exposed in the
Barahakshetra-Dharan area, Sordu Khola and in inner belt of the
Kankai Water Shed.
• Bharahakshetra-Dharan area:- Fine grained, grey green to blue
green sandstone interbedded with red purple, yellow green or
brown mudstone. Mudstone rich in plant remains.
• Sardu Khola:- Thick to very thick bedded, medium to fine
grained, light grey green to dark grey sandstone interbedded with
green gray red purple variegated Mudstone with coal seam
(5cm).
• Inner belt of Kankai Basin:- Fine grained , moderately
indurated grey sandstone, interbedded with purple and green
shales. A thick more than 3m yellow brown residual soil is
generally found on the top of the sandstone.

6. b) Middle Siwalik
• Middle Siwalik of this section was well exposed in the
Barahakshetra-Dharan, Chonjo Khola, Kankai Basin and in
outer belt of Domukha.
• Barahakshetra-Dharan:- Coarse to very coarse grained,
cross stratified, biotite rich, Pepper and salt sandstone,
containing few conglomerate lenses and thin beds. The
sandstone gradually pass upwards to grey mudstone containing
plant remains.
• Chonjo Khola:- Light gray, cross-stratified, very thick
bedded, coarse to very coarse grained and spherically pebbly,
wit coal or peat beds, pepper and salt sandstone interbedded
with grey to dark grey mudstone.
• Multisteriod Sandstone is interbedded of 10-30m thickness.

7. • The pebble size generally range from 2 to 10 cm thickness
with clast of quartzite, banded gneiss and schist.
• The sandstone exhibit climbing ripple lamination and
mudstone is parallel laminated.
• Kankai Basin:- Fine to medium grained arkosic, pebbly
Sandstone with rare grey to dark grey clays and occasionally
silt sandstones and conglomerates.
• Massive sandstone interbedded with shales with thin bands of
Pseudo Conglomerate and Massive plant and animal fossils are
present in clays and shales.
• Domukha (Outer Beds):- Thick-bedded, yellowish grey
sandstone, siltstone and greenish grey mudstone with petrified
wood fragments (diameter about 20cm)

8. c) Upper Siwalik
• Upper siwalik of this section was well exposed in the Kankai
watershed, Biring Khola and Barphalyang.
• Kankai Watershed:- Coarse boulder conglomerate with
irregular beds and lenses of sandstone and thin intercalation of
yellow, brown and grey sandy clays.
• Bring Khola and Barphalyang:- Very thick, gently north –
tilted, alluvial deposits are observed. Where the Upper Siwalik
gravel beds gradually passes into the late Pleistocene-
Holocene alluvial deposits.

9. Fig 2:- The Siwalik constituting the Kankai watershed. Source Based on field
survey in 2009 by A M S Pradhan and A Regmi, and in 2011 by M R Dhital.

10. Fig 3:- The siwalik Constituting Chatara- Barahakshetra area after Adhikari et
al 2018.

11. Fig 4:- Generalized Lithostratigraphy of Chatara Barahakshetra area.

12. Table 1:- Stratigraphic correlation of Chatara Barahakshetra area with other section
of Siwalik.

13. Photograph 1:- South dipping Middle Siwalik Exposed on the Left Bank of
the Ratuwa khola Ilam. Source by M R Dhital.

14. 3. Discussion
a) Structures
Sub Himalayan Fold and Thrust belt of the Arun Tamar can be
divided four Sectors
i) Western Segment Between the Sapt Koshi River and
Dharan
• Represented by the gently folded Lower and Middle Siwalik.
• The Lower Siwalik was repeated twice owing to an
imbricated fault passing through the Kokha Khola.
• Towards north, the folds gradually disappear, giving way to a
homoclinal running along the left bank of the Kokha Khola.

15. ii) Dharan and Madhumalla Segment
• Made up of various imbricated fault, lateral ramps, tear fault
and eroded frontal anticlines.
• The tear faults disrupt the folds and the faults indicating their
late stage of development.
iii) Madhumalla- Kundunabari Segment
• To the north, the basin is cut out by the Mai Khola Thrust.
• Upper Siwalik was developed on its Hanging wall.
• The Kankai anticlines is well exposed in the Chonjo khola,
where gently dipping Middle Siwalik strata make a beautiful
east-plunging arch.
iv) Kundunabari and Mechi River:-
• Characterized by the proliferation of blind imbricated thrusts.
The Sharp anticline fold of the Bring khola is developed above
such fault.

16. Fig 5:- The Ratuwa–Kankai frontal anticline exposed in the Thulo Chonjo
River, Ilam. Source M R Dhital.

17. b) Geomorphology
i) Low trace alluvial fans, badlands and a thin soil covers are
the characteristics of the Sub Himalayan range of this region.
ii) There is a strong Geological control on the physiography of
the hills.
iii) A majority of the rivers originating from this range are
ephemeral.

18. 4. Conclusions
• The greater part of its South limb has been removed by recent
erosion and covered by alluvial deposits.
• The Siwaliks of this area are represented by grey sandstone,
blue nodular marls and Clays with coal seam and
conglomerate.
• The rocks are very similar to the Siwaliks exposed in the
southern belt of the Kankai river.
• The Variation in tectonic style with the Siwalik belt for a
distance of less than 100km, points to a rather shallow nature
of deformation and a transfer zone.

19. 5.Refrences
• Heim A, Gansser A (1939) Central Himalaya: Geological Observations of
the Swiss Expedition 1936. Denkschriften der Schweizerischen
Naturforschenden Gesellschaft, Band LXXIII, Abh. 1, 245pp (with maps,
sections, and plates).
• Drona Adhikari, Keshav Shrestha, Purushottam Adhikari, Khum N.
Paudayal, and Lalu Paudel(2018) Geological study of Chatara
Barahakshetra section, Sunsari-Udayapur District, eastern Nepal, Bulletin
of Department of Geology, Tribhuvan University, Kathmandu, Nepal, vol.
20-21, 2018, pp. 49-58.
• M R Dhital (2015) Geology of the Nepal Himalaya Perspective of the
Classic collidal Orogen:, Springer Publication.

20. Thank you
Any Questions?