3.6 GEOLOGY – QUATERNARY DEPOSITS OF THIMPHU VALLEY
Geology and climate are the two prime determinants, of
all ecosystems. Understanding of geological formations and their history will
reveal the base of the environmental system and the interrelationship between
other natural determinants.
The geological formation of the Thimphu Dzongkhag region mainly consists of
highly metamorphosed gneisses, schist, and subordinate quartzite, calc silicate
rocks and marbles. These formations belong to the Precambrian age (600 million
years ago). A detailed study of the quaternary deposits of Thimphu Valley
carried out by the Division of Geology and Mines has classified the valley
into three. They are Alluvial Deposits, River Terrace Deposits and Colluviums
formation.
The Alluvial Fans are distributed extensively in this area. Most habitations and agriculture fields are seen on gentle slope surfaces of the Alluvial Fan Deposits, which have been formed at outlets of each tributary flowing into the river. These Alluvial Fan Deposits consist of ill-sorted clastics, mainly attributed to landslides or local floods. In the Thimphu area seepage of underground water is often observed at the boundary of each fan deposit.
River terraces have not been distributed widely because of narrowness and the steep gradient of the river.
The Alluvial Deposits in the Thimphu region can further be classified into two types of Alluvial Fan Deposit “A” and Alluvial Fan Deposit “B”. The Alluvial Fan Deposits type “A” is formed by several layers of debris due to flows, or floods, on a large scale from the upper stream tributaries. The slope formation of these fans ranges from flat terrains to moderate slope, i.e. from 0% - 20% (1 - 10 Degrees) making the land most suitable for settlements. The, contour lines on the surface of the Alluvial Fan are arranged concentrically. The soil consists of polymictic gravels and a fine to coarse sand matrix and is suitable for agriculture.
The Alluvial Fan Deposits type “B” is derived from repeated landslides. The soil matrix contains mud and clay. The slope formations of these fan deposits range from 10% -30%, i.e. 5 - 17 degrees thus, making the slopes unsuitable for agriculture and settlements. Some areas may be prone to landslides. The detailed distribution and land use suitability for each geological classification is explained in Table 3.3 of Quaternary Deposits of Thimphu Valley.
The River Terrace Deposits have been formed by sedimentation of the Wang Chhu. These Consist of well-sorted and rounded gravels, which are filled with a little matrix composed of medium to very coarse sand. The River Terraces are of two types. They are the higher River Terraces and Lower River Terraces.
The Lower River Terraces have been recently formed and are almost at the level of the existing riverbed and are seen up to two meters above the riverbed. They have black-brown soil, which is fertile and best for agriculture. The Lower River Terraces are rich in subsurface water, the so-called under flow, and its water table keeps fluctuating along the nearby river water level. These terraces are prone to floods.
Higher River Terraces also have the same characteristics of soil and fertility of the Lower River Terraces, but they were formed much earlier than the Lower River Terraces. But these terraces are distributed in very small areas within the city limit and are not potential holders of ground water.
Colluviums are deposits, which have been formed by a single landslide or slump, on a small scale along the Wang Chhu. The matrix consists of silty to clayish sand.
3.6.1 ISSUES
1. Threat to Lower River Terraces (which are the potential
ground water resources and also prone to floods) due to the increasing demand
for flat lands.
2. Lack of detailed sub-surface geological data and soil data for the valley
restricts the scope of exploring the possibilities of development on steep
slopes and other areas.
Damage caused to a patch
of Lower River Terrace due to the express way alignment. The road will get
washed away in case of floods in Wang Chhu. Further damage will be caused
to this River Terrace if the automobile workshops get shifted to this area.
MAP
NO. 3.7 GEOLOGY- QUATERNARY DEPOSITS OF THIMPHU VALLEY(Click to view the map)
TABLE 3.3 Quaternary Geology of Thimphu Valley
|
Sr. No. |
Classification of Quaternary |
Characteristics and Significance |
Locations |
Land Use Suitability |
| 1 |
|
1.
River Terraces are deposits that have
been formed by sedimentation of the Wang Chhu.
2.
The Lower River Terraces were recently
formed and are almost at the level of the existing riverbed and are
up to 2 meters above the riverbed.
3.
They have black - brown soil, which
is fertile and ideal for agriculture.
4.
|
The eastern banks of the river in the Babesa-Simtokha
stretch, the western bank of the river near the confluence of Ola Rong Chhu
and the river, the eastern and western banks of the river near the Chang
Jiji and Chang Bangdu areas, the
Changlimithang Grounds and Vegetable Market
area, the river banks near the Dzong and SARRC
building area, the existing cremation ground area and the banks of the
river within India House, Western banks of the river north of Taba,
and the river banks north of Pangri Zampa. |
1.
2.
Development in these areas should
be restricted. Construction and hard paving of the ground surface should
not be permitted.
3.
Best suitable land uses in these terraces
are agriculture (wetlands), gardens, plantations and nurseries.
4.
Play grounds/sports grounds (with
restrictions against hard paving) can be allowed at certain areas. |
| 2 |
Higher River Terraces |
1.
Higher River Terraces also have the
same characteristics of soil and fertility of Lower River Terraces,
but they were formed much earlier than the Lower River Terraces.
2.
They occur from 5 to 16 meters above
the existing riverbed, but these terraces are distributed in very small
areas within the city limits and are not potential holders of ground
water. |
Very small portion of the land near Babesa
marshlands, and the existing sewage treatment tank. |
Agriculture will be the best suitable
land use for these terraces due to their fertile soil. Since they are
distributed in very small areas within |
| 3 |
Alluvial Fan Deposit
Type “A” |
1.
The Quaternary system of
2.
The soil is fertile and suitable for
agriculture.
3.
The slope formation of these fans
range from flat terrains to moderate slopes i.e. from 0% - 20% (1 -
10 Degrees), thus making the land suitable for settlements. |
The whole of Motithang, City Core
area, Changangkha, Northern part of Yangchenphug, Kawang Jangsa, Hejo, Samteling, Taba, Dechencholing, Lungtenphu, some
parts of lower Simtokha and some parts of
the land North of Ngabe Rong
Chhu. |
1.
The slope gradient of type “A"
formations are gentle and are the most suitable for residential areas
and for providing infrastructure facilities.
2.
Both the Alluvial Fan Deposits “A”
and “B” are most suitable for agriculture in terms of soil characteristics. |
| 4 |
Alluvial Fan Deposit
Type “B” |
1.
These deposits are derived from repeated
landslides.
2.
These Alluvial Fans are similar to
Alluvial Fan Deposits type “A” in soil composition characteristics.
3.
The slope formations of these fans
range from 10 -30%, i.e. 5 - 17 degrees, thus making the slopes unsuitable
for agriculture and settlements. Some areas may be prone to landslides. |
Changzamtog, Chang Bangdu,
some parts of Simtokha near the Ola
Rong Chhu, the Dzong area and Zilukha, some parts
of Samteling and Jongshina. |
These Alluvial Fan Deposits are suitable for all land uses
other than Industries. |
| 5 |
Collovium |
These are deposits, which have been formed by a single landslide,
or slump on a small scale, along the Wang Chhu.
The matrix consists of silty to clayey sand. |
Few areas north of Taba and west
of RBG golf course. |
This formation is very rarely found within |
1. Threat to Lower River Terraces (which are the potential ground water resources
and also prone to floods) due to the increasing demand for flat lands.
2. Lack of detailed geological data, and soil data, for the entire valley,
restricts the scope of exploring the possibility of development on steep slopes
and other areas.
Major Data Source: Department of Geology and Mines, Ministry of Trade and
Industry