Geological overview of Southeast Himalaya

1. Himalayan Geology
Southeast Himalaya
Prepared By :
Sudhan Kumar Subedi
Engineering Geology First Semester
©Central Department of Geology
Tribhuvan University

2. Himalayan Geology
Southeast Himalaya
- Southeast Himalaya consist of the Eastern part of Nepal, Darjeeling, Sikkim and Bhutan
- 3 windows : Arun and Tamar Window of Nepal and Rangit Window of Sikkim (Gosh, 1956)
- Consist of interrupted Siwaliks, narrow belt of Lesser Himalaya and Higher Himalyan Crystallines
Source : Google Earth Pro v. 7.3.2.5776 (2019)

3. June 12, 2019
Himalayan Geology
Southeast Himalaya

4. Himalayan Geology
Southeast Himalaya
Siwaliks
- Interrupted, missing in some places in Bhutan
- Gap is between Chel and Jaldhoka River in southwest Bhutan
Godwin-Austen (1865, p.106, 1868, p.122) ; Medlicott (1865. p.436), Mallet (1874, p.48),Heim (1938),
Heim and Gansser (1939, p. 6), and Gansser (1983, p. 16).

5. Himalayan Geology
Southeast Himalaya
Siwaliks
- Interrupted, missing in some places in Bhutan
- Gap is between Chel and Jaldhoka River in southwest Bhutan
Godwin-Austen (1865, p.106, 1868, p.122) ; Medlicott (1865. p.436), Mallet (1874, p.48),Heim (1938),
Heim and Gansser (1939, p. 6), and Gansser (1983, p. 16).
- Also about 50 km gap in Hattisar embayment between Sankosh and Manas River, where there is presence of
south tilted alluvial terrace (Nakata, 1972).

6. Himalayan Geology
Southeast Himalaya
Lesser Himalayan Sequence
Darjeeling Area
Dalings ( Schist with quartzites)
Garnetiferous mica schist
(Gansser, 1939, pp.16-17)
- Presence of Gondawana outcrop as a narrow thrust strip over Siwalik of Sikkim and Darjeeling and in Rangit Window of
Sikkim
- The narrow belt of Lesser Himalayan Sequence of Darjeeling, Sikkim and Bhutan is represented by following successions :
Age Geological Units Lithology
Gondawana Unit
Permian Damudas SubGroup arenite, shale, coal beds, slate
Permian Setikhola Formation sandstone, shale, graywacke
Late
Carboniferous -
Early Permian
Diuri Formation conglomerate, quartzite, phyllite, slate
Paleo-Mesoproterozoic ? Baxa Formation
dolomite, limestone, clac. Shales, slate,
quartzite and schist; containing
stromatolites and oncolite fossils
Paleoproterozoic ? Dalings and Shumar Group
garnet schist, quartzite, carbonate, calc-
silicate schist, mylonitized gneiss
Table : Lithostratigraphy of Lesser Himalayan Sequence of Southeast Himalaya, after Gansser (1983) and Bhargava (1995)

7. Himalayan Geology
Southeast Himalaya
Central Nepal
Southeast Himalaya
Bhutan
Dumri Formation Permian
Damudas Sub-Group
Bhainskati Formation
Taltung Formation
Setikhola Formation
Amile Formation
Sisne Formation L. Carb- E.Permian Diuri Formation
Robang Formation Unconformity
Baxa Group
Malekhu Limestone
Benighat Slate
Dhading Dolomite
Paleoproterozoic –
Mesoproterozoic ?
Nourpul Formation
Dandagaon Phyllite
Fagfog Quartzite Paleoproterozoic ?
Dalings and Shumar Groups
Kuncha Formation
Permian
(Sakai et al, 1999;
Stocklin & Bhattarai, 1977)
(Gansser, 1983 ;
Bhargava, 1995)
Correlation in lithostratigraphy

8. Himalayan Geology
Southeast Himalaya
Higher Himalayan Crystalline
- Darjeeling gneiss in Darjeeling (Mallet, 1874); containing garnetiferous psammitic gneiss and injected
garnetiferous muscovite-biotite gneiss as well as sillimanite-biotite gneiss (Gansser, 1939. pp. 18-19)
- Presence of Lime Silicate intrusions, in lenticular form with bent and crumpled ends and confined in lower
horizon of Darjeeling gneiss;
- Also presence of muscovite- tourmaline rich Pegmatite( and also alpite) Dikes in garnetiferous mica schist in criss-
cross pattern and also in form of pegmatitic sills structure

9. Himalayan Geology
Southeast Himalaya
Inverted Metamorphism
Sillimanite
Kyanite -Sillimanite
Garnet
Biotite
Chlorite
Upper
Low Lower level
Upper level
Metamorphic
grade
Structurally
MCT
Upper Structural Level
P-T increases progressively
Auden (1935); Ray (1947)
Neogi et. al, (1998)
- Swapp and Hollister (1991) explains the inverted metaphorphism in Bhutan as Polymetamorphic process
Inverted Barrovian Succession

10. Himalayan Geology
Southeast Himalaya

11. Himalayan Geology
Southeast Himalaya
Proposition of Huge Recumbent Folds
- Superposition of metamorphosed sequence over sedimentary suites
- Couldn’t explained by Medlicott (1864) and Mallet (1874)
- First time described by a Hungarian Prof. L von. Lôczy in 1907, as a enormous recumbent fold or Nappe
- But again, couldn’t found such by Auden (1935, p.162) and Gansser (1964)
- Investigation of Frank et al. (1977) in Northwest Himalaya and Searle and Rex (1989) in Zanskar Himalaya revealed
it as a large recumbent fold and nappe

12. Himalayan Geology
Southeast Himalaya
-Frontal Anticline is partly eroded found in outer Dafla hills of east Himalya by Godwin-Austen (1875)

13. Himalayan Geology
Southeast Himalaya
Any Question ??

14. Himalayan Geology
Southeast Himalaya