Download 3.1 Geological Background cont..

3. The Natural Environmental Setting
3.1 Geological and tectonic aspects (Origin & evolution of
the Bengal Basin and formation of the great delta)
3.2 Relief & Physiography (elevation of various regions of
BD from the sea level, three major classification of
landforms)
3.3 River Systems & Wetlands (Number of rivers, origin &
networks, contribution of rivers, major wetlands, wetlands of
especial importance, Ramsar site/convention, ECA)
3.4 Climate & Climatic Hazard (climate type & hazardsnor’westers, tornadoes, cyclones etc.)
3.5 Soils, agroecological zones
3.1 Geological Background cont..
3.1 Geological Background cont..
3.1 Geological Background
-Bangladesh is a part of the BENGAL BASIN & one of the
largest geosynclines on earth
“Geographically, the Bengal Basin is a surface
physiographic unit, comprising the entire lowland
bounded by the Chhota Nagpur Plateau and
Rajmahal Hills in the west, the Himalayan foothills
in the north, the Meghalaya Plateau in the
northeast and the Manipur-Tripura hills in the
east”…..Banglapedia (ASB)
3.1 Geological Background
3.1 Geological Background
Geosyncline is a term occasionally used for a
subsiding linear channel that was caused by the
accumulation of sedimentary rock strata deposited
in a basin and subsequently compressed, deformed,
and uplifted into a mountain range (internet).
2.1 Geological Background
-A River basin is the portion of
land drained by a river and its
tributaries. It encompasses all of
the land surface dissected and
drained by many streams and
creek that flow downhill into one
another, and eventually into one
river. The final destination is an
estuary or an ocean.
3.1 Geological Background
-The river catchment, or drainage basin, is all the
land from the mountain to the seashore, drained by
a single river and its tributaries.
Abraham Ortelius (Abraham Ortels) (April 14, 1527 – June 28,
1598) was a Flemish cartographer and geographer. He is believed
to be the first person to imagine that the continents were joined
together before drifting to their present positions.
3.1 Geological Background cont..
-Continental Drift Theory elucidates the drifting of
continents from one single landmass (Pangaea) in the
Triassic Period between 250-200 million years ago.
-The rest of earth surface was covered by ocean called
Panthalassa.
-Alfred Wegener in 1912 explains the drifting of the single
landmass
-Later continental drifting was proved in 1960s after the
Plate Tectonic Theory was established
-Pangaea broke up into two continents, the northern part
called LAURASIA & the southern part known as
GONDWANALAND
Alfred Wegener
November 1, 1880 to Nov
1930
Born in Berlin, Germany
Died in Carinetania,
Greenland
3.1 Geological Background cont..
-LAURASIA broke up to form
North America, Greenland, Europe
& Asia
-The southern part split to form the
continents of South America, Africa,
Australia & Antarctica
-The shallow sea of Tethys lay
between these two landmasses
-Gondwanaland broke up from the
northern part of Pangaea around 180
m years ago & the continental plates
started drifting
3.1 Geological Background cont..
-Later by around 150 m yrs ago
Gondwanaland separated into distinct
landmasses (Africa, South America, India
& Madagascar, Australia & Antarctica)
3.1 Geological Background cont..
-Further drifting and separation continued through geologic
years
-In the Jurassic period between 195 & 135 m yrs ago, the
Indian portion of Gondwanaland split off & moved
northward in the direction of Laurasia (Rasheed, 2008).
-India collided with the Eurasian Plate of Laurasia in the
Eocene epoch around 50 m yrs ago (Park, 1997).
-The collision resulted in the uplift of the Himalayan
mountain range & the Tibetan plateau.
-During the Oligocene epoch (between 38 & 26 m yrs ago),
part of the northeastern Indian plate fractured & subsided
below sea level.
3.1 Geological Background
-this is gradually filled up to form the eastern part of the
Bengal Basin.
-the Himalayas after being uplifted had been continuously
reduced by erosion.
-Practically the Himalayan uplift has emerged as one of the
world’s largest sediment dispersal complex to feed the Bengal
Basin & the delta.
-The Ganges & the Brahmaputra from the Himalayan range
& the Meghna from the northeastern Indo-Burman range
drain large catchment areas & transport huge supply of
sediments toward the Bengal Basin
3.1 Geological Background
-the transported loads are from the metamorphic, magmatic &
sedimentary rocks.
-The Bengal Basin is a sedimentary basin having sediment
thickness in excess of 18 km in some parts.
-The Bengal delta occupies a large part of the Bengal Basin
extending across southwestern BD & adjoining parts of West
Bengal
-The delta started to evolve in the Quaternary epoch (about 2
m yrs ago)
-Tectonically the delta is situated in the subduction related
active margin of the northeastern Indian Plate
3.1 Geological Background
-the delta building in the Holocene epoch (since 10,000
yrs ago) has experienced the strong influence of
tectonism.
-Therefore, some parts within BD have been uplifted in
the recent times & some parts are believed to be still
sinking
-The local structure, rate of sediment input & different
phases of the rise of the Himalayas played a key role in
the subsidence scenario of the Basin.
3.2 Relief & Physiography
• Bangladesh is crisscrossed by a larger number of rivers
maintaining an intricate network with their numerous
tributaries & distributaries.
• The elevation in the major part of BD is low since the most
of the country’s land is originated through fluvial processes.
• Nearly 50% of the country’s elevation is less than 10 m
from sea level.
• The southeastern hilly region exceeds an elevation of 300
meters.
• Around 20% of the land consists of low-lying tidal plain
having an average elevation of 3 m above the sea level.
BANGLADESH:
Physiographic Unit
Ref. (Rasheed,
2008)
Physiographic Classification
• Based on the age & formation of landmasses, the
physiography of BD can be divided into three categories
a) Hills (Tertiary period) b) terraces (Pleistocene age) & c)
flood plains (relatively recent) .
Hills: developed in Tertiary period (35-15 m yrs ago)
occupying 12% of the land area
-located mainly in the southeastern regions of Ctg & CHTs
and northern part of great Sylhet District.
-topography is largely determined by geologic structure
(folded, faulted & uplifted) & rock type (sandstone, siltstone
& shale of mid-Tertiary age).
Physiographic Classification
•Terraces
-major terraces are the Barind (located in Rajshahi, Dinajpur
& Bogra districts with an area of around 8000 sq km) &
Madhupur Tracts are located in the northwestern & central
parts of BD respectively.
-Lalmai (with an area of 33 sq km), west of Comilla Town is
also a smaller terrace.
-These terraces cover 8% of the land which are also
recognized as OLD ALLUVIUM (flood plain deposits of the
earlier Ganges & Brahmaputra rivers).
-the Madhupur Tract extends from north of Dhaka in the
south to Jamalpur & Mymensingh to the north with an area of
around 4000 sq km.
Physiographic Classification
•Flood plains
-are also known as alluvial plains through fluvio-deltaic
processes.
-flood plains are created due to the deposition (alluvium) of
the GBM rivers covering 80% of the land in Bangladesh.
-there are as many as 9 types of flood plains in BD, of which
the Ganges plains cover a major part.
Based on the age of formation of delta, several types of delta
formation exists in BD
-Moribund delta, immature & mature delta, active delta
(active Padma flood plain, Mehendiganj islands, Meghna
estuary islands & chars, Meghna estuarine floodplains etc.)