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Introduction
:
The
Dibrugarh District is located in the north eastern part of the state of
Assam. Lakhimpur District lies to its west and north-west , Tinsukia
district is to its east and south east, and Sibsagar district to its
south. The northern boundary is delineated by the Brahamputra river.
Physiographically, this
district represents the tapering eastern end of the Brahamputra valley.
The Lohit, Noa Dihing, Buri-Dihing, Namchik, Tirap, Dirak, Disang and
Namsang are the main rivers of this district. The drainage pattern of
these rivers is of antecedent type and subsequently fortified. With loamy alluvial soil and annual rainfall between 250cm and 290cm,
the district forms a vast fertile land supporting luxuriant paddy fields,
tea gardens and thick forest with several species of valuable trees.
Animals like elephant, tiger, panther, buffalo, bear, wild pig, wild cat,
deer monkey and variety of minor games and birds are found in the
district.
Geology
A fairly thick
group of sedimentary rocks occurs in this district ranging in age between Eocene and Pleistocene and are exposed
mainly along the foot hills bordering the southern boundary of this
district. The eastern part of the district and the valley of the
Brahamputra river are covered by thick alluvial deposits belonging to
sub-Recent and Recent periods.
Tertiary rocks :
The Tertiary group of rocks of this area are classified into
the following Groups and Formations and are described from the oldest to
youngest.
Disang Group : (60 million
years old)
The oldest
formation of the Tertiary group is a thick series of dark grey splintery
shales with thin partings of sandstones and sandy sghales.
Barail Group : (40
million years old)
It is sub-divided
into three formations viz- ( 1 ) Naogaon ( 2 ) Boragolai and
( 3 ) Tikak
Parbat.
(1)
Naogaon Formations comprises sandstones with partings of shales and
occasional carbonaceous shales.
(2)
Baragolai Formations comprises several alternating facies of
sandstones, clay and carbonaceous shales with many thin unworkable seams
of coal.
(3)
Tikak Parbat Formations comprises sandstones alternating with sandy
shales, shales, clays and carbonaceous shales with several thick seams of
coal at the base.
Tipam
Group : (25 million years old)
It
is sub-divided into two formations viz – (1) Tipam sandstones Formation
and the (2) Girujan Formation.
(1)
Tipam sandstones Formation is comprised of sandstones with subordinate beds of clay, sandy clays and occasional shales, the basal
beds of this stage are often conglomeratic.
(2)
Girujan Formation is represented by a thick group of mottled clays
and subordinate beds of sandstone.
Dihing
Group : (10 million years old) This group consist of thick alternating
pebble beds with clays and sandstone.
Alluvial
Deposits :
The alluvium covers a major part of the
district and is of two different types viz, Older Alluvium and Newer
Alluvium.
Geological
Structure :
The Brahmaputra valley in this district, is of a nature of a “ramp
valley” developed during the simultaneous upheavals of the Himalayas on
the north and northeast and the patkai ranges on the south and southeast.
The thick sedimentary rocks of the Tertiary period have been buckled
and over thrust due to the tectonic forces directed towards south
from the Himalayas and to the northwest from the Shan-Burma Plateau region
consequent on the upheavals . Eventually the Tertiary rocks along the
Patkai ranges were structurally disposed along several folded anticlines,
often cut off by several parallel imbricating thrusts viz. Naga
thrust,Haflong –Disang thrust and Margherita thrust. Naga thrust,passes
along the northern edge of the Jaipur-Tipam –Digboi range of hills
bordering the alluvial plains of Brahamaputra on the north. The next
important one is the Haflong Disang thrust which runs E.N.E.,along the
northern base of the Namsang –Barduria hills,south of Jaipur and
continue further eastward along the southern boundary of the Makum coal-field.In
between the above two, there is another thrust.known as the Margherita
thrust running along the northern boundary of the Makum coalfield. The
thrust finally merges with Disang thrust near Dirak towards the west.
MINERALS
:
Coal and petroleum are the chief minerals for economic and industrial development of this district.
Next is the natural gas associated with the petroleum from the Naharkatia
area, which is now gaining importance in various industrial uses.
Besides,clays for brick making and pottery and gravels for road metal ling
and other useful purposes, are abundantly found within the district.
Coal
:
Large deposits of coal exist in two different fields, viz.,Makum and Jaipur.
The Makum coalfield is the well developed and
important one occurring near Ledo-Margherita and having a length of30 km
and a width of 5 km including Baragolai, Ledo, Tipang and Namdang mines
along the southern boundary
of the Dibrugarh district. At least, five workable seams of coal
successively 18 m,2.30m, 6,0m, 1.50m are well developed. The workable
indicated reserve is of the order of 235 million tones to a depth of 300m.
The Jaipur coalfield covers a tract 15 km long and roughly 1 km wide at the base of the Jaipur-Tipam hills along
the course of the Disang river which separated Dibrugarh and
Sibsagar District.
Six coal seams have
been recorded in this field out of which the lowermost seam is 11.89m
thick,and in seam No.3 the thickness varies from 2,70m to 4.25 m .The
others very in thickness between 0.30 m and 2.0 m.Richest development of the seams are in the vicinity of the
Disang river. Inferred workable reserves
of the coal up to a depth of 100m have been estimated at about 100 m have
been estimated at about 10 million tones.
Petroleum:
Seepages of crude petroleum with bubble discharges of natural gas are seen
along the base of the Jaipur-Tipam –Digboi range of hills as well
as along with valley of Buri-Dehing river near Margherita . The important
ones amongst them are in the Barapatra and Naharjan streams in
Jaipur and Digboi ;Makumpather,Hilikapani and Namangpani and around
Margherita and Namchik towards east of the Makum coal field.
The discovery of a big oil seepage in Digboi in Digboi jungle during the
construction of the Dibrugarh –Ledo railway line in 1882, eventually led
to the discovery of Digboi oilfield.
Crude
oil at Digboi occurs in 24 different oil sand horizons within a stratigraphic
thickness of 1,065 m of Tipam sandstone. The Digboi crude is
of mixed paraffin and asphalt base with fair proportion of cyclic hydrocarbobns.
The
Naharkatia oilfield covers Naharkatia-Hungrijan area in the Dehing valley Oil occurs in this field in 5 main
producing oil sand horizons within a thick stratigraphic unit of the Barails struck at a depth between 3000 m and 3,926 m.
Natural Gas
:
Large reserves pf Natural gas are found in
association with oil in the Naharkatia field. For utilization of the gas ,
a thermal power station and a
fertilizer factory have already been set up in Namrup.
Clays
:
Several thin bands if fire clay containing a little amount of impurities
are found in association with coal seams in the Makum and Jaipur
coalfields. They might be suitable for manufacture of firebricks after
proper benificiation and blending.
Suitable clays for brick manufacture are extensively found in the areas covered by Dihing group of rocks
as well as by older alluvium clays suitable for pottery are available
throughout the district. A blackish variety of clay found both in the
Dihing and the older alluvium is suitable for use in oil drilling and is
being used by Oil India Limited and Oil and Natural Gas Commission.
Ground
Water Potential :
Ground water level is progressively deep towards the southern margin of
the district and has a gentle slope towards the river Brahamputra on
the northern margin. In the flood plain, water table occurs generally at
shallow depth, mostly less than 4 meters.
The flood plain areas are completely saturated by the monsoon rainfall
leading to rise of water table to near surface. During peak monsoon
flooding, water logging and marshy conditions prevail in low lying areas.
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