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Sunderban Delta Complex
 
 
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GEOTECTONIC BACKGROUND
 

The Sundarban Delta Complex, having geo-genetic link to the tectonic Bengal Basin, geographically extends over the eastern India and Bangladesh. It is characterized by prolific growth of rich and diversified mangrove vegetation and forms an integral down drift coastal part of the Bengal Delta Complex that overlies huge thickness of Tertiary marine sediments of the actively subsiding Bengal Basin.

The Bengal Basin, being one of the world’s widest, deepest and most tectonically active basins, extends over parts of eastern India, Bay of Bengal Sea and Bangladesh and represents a classical asymmetric pericratonic basin, which originated through different phases of the Tertiary Himalayan orogeny. Being bordered all around by tectonic fabric, the basin has a relatively stable shallow (1-8 km thick sediment) shelf part in the west and northwest facing the Indian shield and a tectonically active southern and eastern fore-deep part centered below the present Ganges – Brahmaputra river mouths. These two parts are separated by a hinge zone marked by high gravity and magnetic anomalies. The Bengal Basin got filled up through the Tertiary marine geosynclinal and shelf sedimentation (>16km thick) followed by gradual progradation of the Quaternary Ganga - Brahmaputra delta fronts towards the southern sea producing the Bengal Delta Complex, the mangrove vegetated Recent – subRecent part of which is popularly known as the Sundarban Delta Complex. Ever since the advent of mangrove ecosystems on the earth in the tropical – subtropical coastal land masses after the breakdown of Gondwanaland, the temporal as well as geographical distribution of mangrove plants are largely controlled by continental drift and Pleistocene glaciation-related environmental and physiographic changes. The present Sundarban mangroves have their widespread ancient counterparts buried under deltaic sediment cover further inland.

 
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Satellite imagery showing Holocene vs. modern extensions of Sundarban Mangroves in relation to submerged delta front & Bay of Bengal Sea
 
 
 
 
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Intertidal Bakkhali beach, a dwelling ground of thick population of red crabs
 
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Mud bank exposed along the eroding beach sector between Bakkhali and Frazergunj
 
 
 

The Sundarban Delta Complex and its surroundings have a dynamic physiographical and geomorphological evolutionary history. The region, being a part of the Bengal Basin, represents coalesced multi-generation deltas that have prograded in phases during the positive interglacial eustatic sea level changes occurred during the Plio-Pleistocene time towards the Bay of Bengal leaving behind distinctive multilevel delta surfaces, terraces, palaeochannels and palaeoshorelines and migrating the successive coastline towards southern sea. The Sundarbans represents the mangrove-vegetated Recent-Holocene down drift coastal part of the Bengal Delta Complex. The Ganges and Brahmaputra river systems mainly drain the area that includes estuaries, few hundreds of delta lobes and islands of latest generation with network of rivulets and tidal creeks or inlets. Towards north the deltas coalesce together to form relatively flat terrain with narrow, scanty and meandering tidal channels. Extensive thick blanket of floodplain deposits, low ridges of natural levees, abandoned channel courses, small pockets of swampy and marshy areas and channel bars characterize a part that constitutes a lower estuarine zone. In the southern part that constitutes a coastal zone, the delta lobes fan out and are separated from each other by wide drainage systems. Here the islands are elongated mostly in north-south direction. The conspicuous coastal landforms include back dunal mudflats-backswamps-saltmarshes and tidal creeks with mangrove vegetation, coast-parallel older stabilized dune ridges & younger mobile dunes and sand sheets, low gradient supratidal and intertidal beaches with mud bank, berms, runnels, tidal flats etc. and offshore bars from land to sea. Biomats are developed locally in patches in the moist and depressed areas. Due to rising sea level, the present coastline is retreating at alarming rate.

 
 
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Upper intertidal beach with evidence of retreat of recent mangrove line at Bakkhali
 
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Upper intertidal beach with evidence of retreat of recent mangrove line at Bakkhali
 
 
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Recent dune sand ridge migrating towards mangroves, Bakkhali
 
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A creek bank with thick mangrove vegetation, near Sajnekhali
 
 
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A meandering creek, east of Bakkhali
 
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A rippled tidal flat with just made decapod trackway, near Bakkhali
 
 
 
 

The sediments of the area show limited grain size variation (clay to coarse sand) and firmness (soft to firm). The average sedimentation rate in the Bay was calculated to be 6.7cm/100years. The terrain largely experiences diurnal tidal action (low to moderate). Effect of wave and wind action is most conspicuous on the breaker zones and beaches. Frequent storm action marks the beginning of monsoon here. The delta fronts exhibit wide range of primary sedimentary structures, for example, various types of cross-beddings, ripples (wave, current, interference, double crested etc.), rill marks, scour marks, wrinkle marks, aeolian current lineation, mud balls, desiccation cracks, hummocky cross beddings and so on. The tidal flats exhibit a glossary of ripples (lunate, linguoid, current, wave etc.) with crests truncated, ripple-drift laminations, rill marks, flaser beddings, rhythmic tidal laminations and herringbone cross beds. The aeolian dune sands show wedge shaped high angle cross beds and slump structures. The terrain exhibits bewildering array of organo-sedimentary structures (ichnofaunas) that have immense geological & geotechnical application potential. Erosion at Digha beach resort has been attributed to prolific crustacean burrowing activities. The polychaete worm tubes provide precise annual rate of erosion and deposition in the beaches. Formation of Uca mud volcanoes indicates beach instability.

 
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An eroded older dune ridge, Bakkhali A stabilized older dune ridge, Bakkhali
 
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Bakkhali beach with modern mangroves Dissected stabilized dune with aeolian cross beds, Bakkhali
 
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Aeolian rill marks, Frazergunj beach Rippled tidal flat, Bakkhali – Frazergunj beaches
 
 

The terrain experiences average maximum and minimum temperature of 40dC (May to June) and 22dC (December), moderately high annual rainfall (1480 to 2400 cm), a high salinity range (23‰ in the inner vs. 38‰ in the outer delta), moderate pH (7.6 - 8.5), and a mean maximum semidiurnal spring tidal range of 6.5 - 7.6m and mean minimum neap tidal range of 2.0 - 2.5m. The dissolved oxygen content in open seawater ranges from 2.4 - 4.7 ml/I. Predominant wind directions are from S to N (March to October) and are reversed during November to February. Pre-monsoon tropical cyclones, locally called “Kalbaishakhi” (SE to NW, 80 to 120km/hr and two to four times/month from March to May), reorganize the beach and dune sands. Non-storm shallow waves (0.5 to 7 m wavelength) approach the beach at 70 to 85 degree.

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Lunate, lingoid and double-crested ripples on the tidal flat, Bakkhali – Frazergunj beach sector

 
 
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