IntroductionSlope failure processes are the common sites in the hilly terrain
of Himalayas. These are one major natural hazards which not only results in the
loss of life and property but also cau economic burden on the society. Hence,
there is a necessity for better methods of landslide h evaluation and its
zonation.
An area of about 100 sq. Ian. Around Dadahu bounded by latitude 30
33'5"N to 30 38~277 longitude 77 23'48'T to 77 3 0"E (Fig. 1) was undertaken as
a pilot area for the detailed stu landslide hazard zonation. nation.
Geological Setting
The investigated area, forming a part of the major Krol belt of
Himachal, has been subjected to repeated phases of tectonic movements which has
resulted in a very complex geology. Dadahu area, where the poly-phased
deformation has been reported (Sinha, 1992), shows an intricate but interesting
geology. The rocks of the investigated area are highly disturbed due to repeated
folding faulting and thrusting. The stratagraphy of the area is shown in the
following table:
| Pleistocene to Miocene |
Siwalik Fm. |
Upper Siwalik
Middle Siwalik
Lower Siwalik |
| Eocene |
Subathu Fm. |
|
| Precambrian |
Krol Fm. |
Krol C+D+E
Krol B
Krol A |
| Precambrian |
Blaini Fm |
|
| Precambrian |
Chandpur Fm. |
|
| Precambrian |
Basantpur Fm. |
|
The identification of different litho-units
(Fig.2) was carried out on the basis of aerial photographs (1:30,000 scale) and
Satellite Image of IRS LISS-1.
The Basantpur Formation, comprising conglomerate
(at base), shale, slate and limestone is lying tectonically over the Subathus
along the Krol Thrust and runs parallel to the Jalal river (WNW-ESE direction).
These rocks are forming the base of the SianDhar syncline. -The calcareous and
shaly horizon of this formation shows beautiful drag folds puckers.
The rocks of
the Chandpur Formation are :
- A massive limestone, white grey in colour
(microcrystalline) and occurs at the base ( to Bansa Member of Auden, 1937).
- Quartzitic sandstones occurring more prominently in
the southern flank of the main Nigali Dhar syncline, north of the Giri and
stratigraphically above the Bansa member.
- Black, slaty, finely laminated blackish shales and
phyllites interbedded with silt from the bulk of the deposit.
The Blaini
Formation undoubtedly marks a major unconformity in this part as it is lying
over the Basantpur and Chandpur
Formations leaving the Nagthats. The
lithostratigraphy of the Blaini Formation of the Krol belt is :
- boulder beds at the base,
- carbonaceous shale, slate, grey limestone, varved
argillite, minor grey and pale quartzite in the middle and overlain by
- flesh coloured and purple dolomitic limestone,
interbedded with red shale in the upper part.
The outcrops of Blaini
along the Jagar ka Khala are of particular interest because only here one can
find the famous Blaini boulders of considerablesize . They are found randomly
enclosed in a finely bedded silty matrix of slates. The siz from few centimeters
to over half a meter in length.
The Krol Formation forming the core of the
SianDhar syncline in the study area are divided into three namely Lower (Krol A)
comprising of intercalations of thin bedded limestones and calcareous shales,
Middle (Krol B) comprising of purple green calcareous slates/shales, and Upper
(Krol C+D+E) comprising of thick bedded massive limestones, sometimes showing
micro-crystalline nature.
The Subathu/Dagshai/Kasauli formation in the area is
sandwiched between the two major thrusts of Himalaya namely Main Boundary Thrust
and Krol Thrust. They are mainly dark brown to purplish splintery shales, thinly
bedded greenish white, fine grained sandstones and occasional beds of
carbonaceous shales faulted at places.