Introduction
•The Churia
zone consists of the Tertiary (Neogene to Quaternary) fluvial sediments that
form the Churia hill ranges, the southernmost hill range of Nepal.
•Bounded to
the north by the Main Boundary Thrust (MBT) and to the south by the Main
Frontal Thrust (MFT) also called the Himalayan Frontal Thrust (HFT).
•Well
established three-fold classification of Siwaliks in Potwar region of Pakistan
and Western Indian Himalaya was freely applied to the equivalent Churia Group
of Nepal from the beginning of the geological studies in Nepal.
early attempts of subdividing the Churia Group
were mainly based on the lithological characteristics,
•Neogene-Quaternary
rocks of Nepal are essentially poor in vertebrate fossils unlike the type
locality in the Potwar basin of Pakistan.
•Magnetic
polarity stratigraphy and biostratigraphic zonations of Siwalik Group of rocks
have been established in the type locality
•The Sub-Himalaya
of the Himalaya consists of the Neogene sediments, which are known as the
Siwalik Group.
•It extends
from the Potwar basin in northWest to the Brahmaputra River in east throughout
the Himalaya.
•The group
is distributed in the southern part of the kingdom and represented by foothills
and runs throughout whole of the kingdom from East to West and within the
Siwalik Range.
•The Siwalik
Group provides important records of change of the depositional environment such
as hydrology, sedimentology, regional tectonism, climate change, and eustatic
sea level change as well as the flora and fauna during the rapid upheaval of
the Himalaya.
•The name of
Siwalik was derived after the “Shivalik Hills” of Haridwar in India (Medlicott
1864) where first vertebrate fossils were found.
•The Siwalik
is known as the Churia Group in Nepal, takes its name from the Churia Mai
temple near Hetauda in Central Nepal (Chaudhary, 1982).
•There is
development of the valley called as the Dun valley (e. g., Surkhet valley in
Far West Nepal, Dang and Deukhuri valleys in West Nepal, Chitwan valley in
Central Nepal and Trijuga valley in East Nepal;
•About 4 to
6 km thick continental sediments are accumulated in the foreland basin situated
in the south of the Himalaya.
•These
sediments are supplied from the Himalaya to the north as a result of orogeny as
well as denudation of the Himalaya.
•The group
is comprised of coarsening upward succession as a whole, which reflects the
rising Himalaya.
•Within the
coarsening upward succession as a whole comprises of numerous fining-upward
successions. Generally, the beds are dipping northward, and are locally folded
and faulted.
•In Nepal
Himalaya. Two- to six-fold classification is acceptable based on the lithology.
•Geographically,
the group is divided into the southern and northern belts by the Central Churia
Thrust (CCT). These structures are equivalent to the Main Dun Thrust (MDT).
•A detailed
lithostratigrahy of areas have been described in the Karnali River area in Far
Western Nepal, Khutia Khola area, Mid Western Nepal, Surai Khola, Mid Western
Nepal, Tinau Khola West Central Nepal, Bakiya Khola, Central Nepal,
Chatara-Barahakshetra and Kankai River Easten Nepal.
•The Siwalik
Group in the Karnali River section is newly divided into four mappable
lithostratigraphic units, named the Chisapani, Baka, Kuine and Panikhola Gaun
Formations in ascending order.
•Chisapani
Formation (Lower Siwaliks)
•Along
Karnali River exposures of north of the Chisapani Bazaar.
•The
Chisapani Formation is represented by alternations of very fine- to medium-
grained, and greenish-grey to reddish-brown sandstones and variegated,
reddish-brown and, bioturbated mudstones and attains 1500 m thick
•Divided
into lower and middle and upper members based on thickness and proportion of
sandstone and mudstone.
Baka
Formation (Middle Siwaliks)
•The type
locality of this formation is around the Baka village.
•The total
thickness of the Baka Formation is about 2740 m.
•This
formation is composed of thick bedded, medium- to very coarse-grained
sandstones and pebbly sandstones and mudstone interbeds.
•sandstones
contain biotite and white minerals quartz and feldspar showing “salt and
pepper” characteristics.
•These sandstones
are interbedded with greenish-grey to grey coloured laminated mudstones.
•The Baka
Formation is also subdivided into the lower, middle and upper members.
•Kuine
Formation (Upper Siwaliks)
•The type
locality of this formation is defined as river side exposures near Kuine
village.
•The Kuine
Formation consists of thick bedded, imbricated, well-sorted, clast-supported,
cobble and pebble- sized conglomerates.
•The total
thickness of the formation is about 1000 m.
•The gravels
consist mainly of quartzite and carbonate rocks with some metamorphic rocks.
•Each gravel
ranges from 1 to 10 cm in diameter. Sandstones and mudstones are interbedded as
lenses within the conglomerate beds.
Panikhola
Gaun Formation (Upper Siwaliks)
•The type
locality of this formation is defined as exposures near the Panikhola Gaun
Village.
•This
formation reaches about 500 m in total thickness.
•The
Panikhola Gaun Formation consists of matrix-supported pebbles, cobble to
boulder conglomerates and coarse- to very coarse-grained sandstone and mudstone
interbeds.
•Gravels are
mostly angular to sub-rounded and are composed of quartzite and occasionally
Siwalik Group sandstone.
•Two
lithostratigraphic units, the Jagati Formation and Kala Formations in ascending
order has been proposed (Adhikari and Sakai 2016), which are equivalent
to the Lower and Middle Siwaliks, respectively.
•The Upper
Siwalik is not exposed in the study area. Each lithounit is subdivided into;
the lower, middle and upper members.
•The type
locality of this formation is around Jagati Village. This unit is named after
Jagati Village
•The Jagati
Formation is represented by bioturbated, variegated, reddish-brown,
yellowish-brown, brown, yellow, yellowish-grey, grey to greenish-grey mudstones
and very fine- to coarse-grained, brown, reddish-grey and grey to greenish grey
sandstones.
•Thickness
of the Jagati Formation is attains about 2,110 m. In general, a sandstone unit
grades upward into a mudstone unit to form an upward-fining succession.
•The Jagati
Formation is further divided into the lower, middle and upper members based on
dominant unit and, color and grain size.
Kala Formation
•The
Kala Formation is well exposed from Gajari Khola in the south to Sayal Village
in the north. The thickness of the Kala Formation is about 2,050 m.
•This
lithounit is represented by thin- to very thick-bedded, very fine- to very
coarse-grained, light grey, grey to greenish-grey sandstones and pebbly
sandstones inter bedded with reddish-brown, brown, grey, greenish-grey to
dark-grey mudstones. Biotite, quartz and feldspar are abundantly found in
sandstones showing "salt and pepper" appearance.
•The rock succession of this formation also exhibits
fining-upward successions as in the Jagati.
Surai
Khola area, West Nepal Himalaya (after Dhital et al.1995)
•The
lithostratigraphy of the Surai Khola area has been established by Dhital et
al. (1995) and Corvinus and Nanda (1994).
•Dhital
et al. (1995) divided into the Bankas, Chor Khola, Surai Khola, Dobata and
Dhan Khola Formations, respectively in ascending order
•Corvinus and Nanda (1994) subdivided of the Chor Khola Formation into lower and upper members.
Tinau
Khola area, West Central Nepal Himalaya (after Tokuoka et al. 1990)
•Lithostratigraphy of the Arung-Tinau Khola area, West Central Nepal is the Arung Khola, Binai Khola, Chitwan, and Deorali Formations, in ascending order (Tokuoka et al. 1986, 1990)
Chatara-Barahakshetra
area, East Nepal Himalaya (after Ulak 2004)
•Department
of Mines and Geology, Nepal (DMG 2002), Schelling (1992) have
established the lithostratigraphy of the Siwalik Group in East Nepal Himalaya.
•On the
basis of lithology and grain size, the Siwalik Group in the
Chatara-Barahakshetra section has been slightly modified from the previously
established stratigraphy (Ulak 2004), and subdivided mainly into the
Lower Siwaliks (lower and upper members) and Middle Siwaliks (lower and upper
members). The typical Upper Siwaliks are absent.
•The
Siwaliks crop out along the Chatara–Barahakshetra road section and their total
thickness is about 3,050 m.
Stratigraphy
of the Siwalik Group (after DMG 1987)
•After
establishing of the Department of Mines and Geology in 1959, worked in
stratigraphy of Siwalik (Churia) Group.
•The senior
geologists have established the lithostratigraphy of the Siwalik Group same as
Auden (1935); Hagen (1969) Itihara et al. (1972) as the Lower Middle and Upper
Siwaliks in ascending order
•But they
have subdivided the Middle Siwalik into the lower Middle Siwalik (MS1) and
upper Middle Siwalik (MS2).
Geology of Nepal Himalaya, Prakash Das Ulak (2016).
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