Kaligandaki
section is geologically located at the Higher Himalaya also called the Higher
Himalayan Crystalline in Western Nepal. Kaligandaki River cuts the Higher
Himalayan Crystalline making the deepest gorge in the world. As the Kaligandaki
gorge lies between two major mountains peaks i.e. Dhaulagiri on the west and
Annapurna on the east respectively. Both peaks being more than 8000m, Kaligandaki
gorge is expected to have a depth of 6000m. The upper part of Kaligandaki gorge
is also called Thak Khola. This area has been studied by many geologists.
Bordet et al. (1972) subdivided the Higher Himalayan Crystalline first into
these four units:
1)
Kyanite
–Sillimanite gneisses
2)
Pyroxenite
gneisses
3)
Finely
banded gneisses
4)
Augen
gneisses
Later Le
Fort (1975) modified this classification and divided the Higher Himalayan
Crystalline into three units as:
1)
Formation I
2)
Formation
II
3)
Formation
III
Here the classified unit of both Bordet (1972) and Le Fort (1975) are
equivalent with each others;
1)
Formation I
= Kyanite-Sillimanite gneisses
2)
Formation
II = Pyroxenite gneiss+ banded gneiss
3)
Formation
III = Augen gneisses
1)
Formation I:
It is composed of
mylonitic quartzite alternating with gneisses which is mostly micaceous. It is
followed by the small passage of rare marble along with lenses of quartzite
towards north. And it is further followed by kyanite zone of thickness about
1400m and sillimanite mostly occurs at the top of this succession.
2)
Formation II:
It lies above the kyanite zone
of Formation I and mostly comprises calcareous gneisses along with impure
marbles (200m) thick. Metasomatism (mineralogical replacement of the rocks) is
observed near the contact of gneiss and marble. Above the calcareous gneiss
there settles a succession of banded gneiss (1600m) thick along with small
appearance of calcareous gneiss.
3)
Formation III:
The main lithology of
this unit is augen gneiss. The pegmatite in augen gneiss sometimes comprises
small ruby crystal of diameter 1-2mm. Further north there is alternating layer
of banded gneiss within the zone of calcareous gneiss up to the limestone bed
of Tibetan Tethys Himalaya.
|
Zone |
Litho
unit After Le
Fort (1975) |
Main lithology |
Thickness (m) |
age |
Litho
unit Bordet
et.al (1972) |
|
Higher Himalaya |
Formation
III |
Augen
gneiss |
3000 |
P R |
Augen
gneisses |
|
Formation
II |
Banded
gneiss, Quartzite, marble |
1700 |
E C A M B |
Pyroxenitic
Gneiss/
banded gneiss |
|
|
Formation
I |
Gneiss schist |
5000 |
R I A N |
Kyanite-sillimanite
gneiss |
Table:
lithostratigraphy of Kaligandaki section (Thak Khol)
Source:
Geology of Nepal Himalaya
Nicer
ReplyDelete