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 & hazards- nor’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

November 1, 1880 to Nov 1930 Born in Berlin, Germany 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 The

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.)