User:Bettymnz4/Geology of Bangladesh

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This is a geologic map of Bangladesh.
Detailed geologic map of Bangladesh
This general geologic map of Bangladesh shows some of the geologic features and sediment depths.
From the northwest to southeast: Indian shield, Rajmahal Traps, West Bengal, Calcutta–Mymensingh hinge zone, Himalaya Mountains, Himalayan foredeep, Sillong plateau (B), Assam, Sylhet Trough, Chittagong–Tripura fold belt, Myanmar (Burma), Arakan Yoma

Bangladesh occupies the delta plain of the Ganges (Padma) and the Brahmaputra (Jamuna) rivers.[1] The country’s geological history is covered by several kilometers of 2-million-year-old sediments.[1] Bangladesh has begun to extract natural gas from the tectonic belt in the eastern part of the country to generate electricity.[1] Coal was discovered in northwestern Bangladesh in the 1960s.[1]

Tectonic history[edit]

The more than 6,000 km (3,700 mi) journey of the Indian landmass (Indian Plate) before its collision with Asia (Eurasian Plate). India was once well south of the Equator, near the continent of Australia.
Note the west-to-east docking

Bangladesh's geology began 350 million years ago when the Pangean supercontinent broke apart.[2] The Bengal Basin began 127 million years ago when the Indian Plate rifted away from Antartica[3]:Ch.4.3 at 18 cm (7.1 in) per year for 20 million years.[4] This rapid velocity stopped 55 million years ago and was followed by a period in which little or no spreading took place west of the Ninety East Ridge[4] for 20 million years.

To the north[edit]

This diagram illustrates a continent-continent collision.
Continent-continent collision

One of the Pangean fragments – the Indian subcontinent – began drifting 127 million years ago and continued until it collided with southern Asia.[2] This is the Himalayan orogeny – a classic continent-continent collision mountain-building event. The abrupt slowdown in the rate of convergence between India and Asia 55 million years ago had been regarded as the beginning of the collision; even though most of the effects attributed to this major tectonic event did not occur until more than 20 million years later.[5]:2 Refined estimates of the relative positions of India and Asia indicate that they were not close enough to have collided 55 million years ago.[5]:2 Based on evidence from Tibet and a reassessment of data, it appears that the Indian–Asian continental collision began 34 million years ago.[5]:2 Another source believes the current collision began 36 million years ago.[4] The Himalayan collision progressed from west to east.[6]

To the east[edit]

This diagram illustrates an ocean-continent collision.
Oceanic-continent collision
This diagram shows two anticlines with its syncline and axis.

Three forces – oceanic-continent collision, subduction and compression – formed the mountains in Myanmar (Burma) to the east, and produced the Arakan Yoma anticline in Myanmar and eastern India.[2] The folding of the earth's crust in response to the eastward-directed subduction of the delta's oceanic crust beneath the western Indo-Burman Ranges[3] and the scraping off of sediments[2] are responsible for the growth of a series of elongate north-south anticline structures in eastern Bangladesh.[3]:Ch.4.3 These are the fold belts of the Chittagong Hill tracts and the eastern margin of the Sylhet Trough.[3]:Ch.4.3

Ninety East Ridge[edit]

Illustration of three types of strike-slip faults.
Three types of strike-slip faults
This shows the topography of the Indian Ocean, including the Ninety East Ridge.
Ninety East Ridge

The Ninety East Ridge is a mechanical border in the Indian Ocean separating two distinct areas.[7] West of the Ninety East Ridge there is a continent-continent collision; east of the ridge oceanic crust subducts along the Sumatra Trench (which is off the west coast of Sumatra).[7] The Ninety East Ridge – which crosses the equator[8] and is on the Indian plate – is a transform fault along which the Indian Plate moved north without disrupting surrounding crustal plates.[3]:Ch.4.3 It is a long linear fracture which lies parallel to and slightly east of 90° longitude; it begins about 31°S and extends to 9°N latitude.[9]:1 The northern portion separates the Bengal Fan sediments from its eastern Nicobar Fan lobe.[9]:1 The northern portion of the ridge – from 3°N to 10°S – is seismically active, where both vertical and left-lateral strike-slip motions occur, which is consistant with the Indian side meeting resistance due to its collision with Asia.[9]:1

Bengal Basin[edit]

This map shows the Ganges, Brahmapura and Meghna rivers, along with the mouths of the Ganges.
Ganges, Brahmapura and Meghna rivers, along with the mouths of the Ganges. Note that India wraps around Bangladesh.

Bangladesh extends from latitudes 20°43' to 26°36'N and longitudes 88°3' to 92°40'E; most of the country is covered by thick layers of sediments.[3]:Ch.4.1 The country occupies most of the Bengal Basin[3]:Ch.4.1 which includes one of the largest delta complexes in the world, covering an area of more than 200,000 km2 (77,000 sq mi) of sedimentary fill.[10]:1 The Bengal foreland basin contains up to 16 km (9.9 mi) of deltaic deposits, eroded from the eastern Himalayas and the Indo–Burman Ranges, and carried by major river systems similar to the present-day Ganges and Brahmaputra.[10]:1



The basin is bordered by the Indian plate to the west; the Himalaya to the north; the Shillong plateau, India, to the northeast; the Indo–Burman Ranges to the east;[6] and the Bay of Bengal to the south.[10]:4 Its sediments are more than 20 km (12 mi) thick in the Sylhet Trough and in the southern part of the Ganges-Brahmaputra Delta.[2] The basement increases in depth from west to east, from 4.3 km (2.7 mi) deep on the Indian plate to depths of 10 to 11 km (6.2 to 6.8 mi) on the extreme eastern parts.[13]

Sedimentation in the basin has been almost continuous for 65.5 million years.[3]:Ch.4.3 During the first phase the basin remained under marine transgression until the Indian plate collided with Asia; then a major marine regression occurred.[3]:Ch.4.3 Twenty-five million years ago another major uplift began in the Himalayas which created the Sivalik Hills; the impact of the collision between the Indian plate, and the Tibetan and Burmese plates resulted in a large influx of sediments both from the west and the east.[3]:Ch.4.1&4.3

A paleo-Brahmaputra river system flowed from the eastern Himalaya, drained through the upper Assam Valley, India, bringing orogenic detritus to the eastern Sylhet Trough; entered the Bengal Basin, and eventually drained to the Bengal Fan.[10]:14 Data from eastern and northern Bengal Basin 23- to 5-million-year-old sediments are consistent with erosion of the nearby orogenic belts.[10]:10 The first clearly orogenic detritus is in the lowermost 23-million-year-old strata.[10]:10

Geologic features[edit]

Hinge zone[edit]

The entire Bengal Basin is bisected by the northeast-trending 5.1 to 15.5 km (3.2 to 9.6 mi)[14] deep Calcutta–Mymensingh hinge zone which separates the basin into two tectonic provinces: the Indian platform shelf zone – with its thinner sedimentary layers – to the northwest of the hinge zone; and thicker basin fill over a deep-subsided basement to the southeast of the hinge zone.[10]:4 The hinge zone is 25 km (16 mi) wide; it is connected with the Dauki fault in the northeast by a series of east-west trending faults.[3]:Ch.4.2 The east-west Dauki fault is on the southern border of the Shillong plateau;[3]:Ch.4.2 it is the border between Bangladesh and India. Parallel to the hinge zone to its southeast is the Bengal foredeep.[3]:Ch.4.2 Sediments are more than 12 km (7.5 mi) thick within 200 km (120 mi) southeast of the hinge zone.[10]:4 East of this hinge lies a deeper part of the basin with a greater rate of subsidence, and the rocks change from nummulitic limestone on the stable shelf to thick layers of clay and shale in the deeper part of the basin.[14] Subsidence increased 23 to 5.3 million years ago in response to western encroachment of the Indo–Burman Ranges which is attributed to south-directed overthrusting of the Shillong plateau on the Dauki fault.[15]

Indian platform[edit]

The red area on this map is the Indian State of West Bengal.
The red area is the Indian State of West Bengal; note that India wraps around Bangladesh.
This is a table of the ages and names of the stratigraphic layers of the Bengal Basin.
Deposition ages and names of the various strata of the Bengal Basin

The Indian plate and Indian Shillong plateau are connected by the 100 km (62 mi) wide Rangpur platform; its basement rock is more than 2 km (1.2 mi) deep.[3]:Ch.4.2 The northern section is the Dinajpur slope – the northwestern-most land which juts into India – and the southern section is the Bogra shelf – which lies parallel to and northwest of the Calcutta-Mymensingh hinge zone.[3]:Ch.4.2 Sediments of the Rangpur platform uniformally increase in thickness from 1 to 8 km (0.62 to 4.97 mi) going from the northwest to the southeast.[14] The Bogra shelf tilts to the southeast and extends to the Calcutta-Mymensingh hinge zone.[3]:Ch.4.2 The sediments over the shelf are 5 to 7 km (3.1 to 4.3 mi) thick and consist of layers deposited before and during the northward drift of the Indian subcontinent, and sequences that were deposited in mid- to late-Tertiary time.[2] Sedimentation in this part of the Bengal Basin comes from the Indian plate on the west, the Himalayan Mountains on the north and the Arakan Yoma on the east.[2]

Sub-Himalayan foredeep, Shillong plateau and Assam Valley[edit]

The Sub-Himalayan foredeep is an east-west belt extending along the southern foot of the Himalayas, with part of it crossing the northwest corner of Bangladesh.[3]:Ch.4.2

Northeast India is bound by the Himalayan arc to the north and the Burmese arc to the east.[16] The Shillong plateau and Assam Valley lie at the boundary zone of the two arcs.[16] Micro-earthquake surveys were conducted in this area from 1982 to 1986.[16][17] Equipment recorded 1,400 earthquakes in the magnitude range 2.0 to 4.0,[16][17] at depths of 10 to 35 km (6.2 to 21.7 mi).[17] Stress orientations are compatible with the known tectonics of the region: collision of the Indian plate with the Asian plate at the Himalaya and subduction of the Indian plate beneath the Burmese plate at the Arakan-Yoma Range.[17] Most of these earthquakes seem to have occurred within the Indian plate, currently being subducted eastward beneath the Indo-Burman Ranges.[18]

Sylhet Trough (Surma Basin)[edit]

The Sylhet Trough – a sub-basin of the Bengal Basin – is in northeastern Bangladesh.[3]:Ch.1 It is bound on the west by the Indian platform, on the north by the Indian Shillong plateau, on the east and southeast by the Chittagong-Tripura fold belt of the Indo-Burman Ranges, and to the south and southwest by the main part of the Bengal Basin.[3]:Ch.1 This east–west trending trough is 120 km (75 mi) long, 50 km (31 mi) wide[19]:20 and 13 to 17 km (8.1 to 10.6 mi) thick.[3]:Ch.1 The basin gradually deepens toward the center, and is actively subsiding.[3]:Ch.1 Sediments of the Sylhet Trough were deposited in a large, mud-rich delta system that drained eastern Himalayas.[15] The northeastern part of the basin was affected by tectonic loading, both from the northeast – eastern Himalayas – and east – Indo–Burman Ranges – causing nearby parts of the basin to subside further which accommodated the thick Surma Group sediments.[10]:7


Chittagong fold belts[edit]

The Chittagong fold belts are tight north-northwest–trending folds along the eastern edge of the Bengal foredeep; the complexity of the folds increases eastward to merge with the Indo–Burman Ranges.[10]:4

Bengal foredeep[edit]

The Bengal foredeep is a large elongated trough, occupying the area between the hinge zone and Arakan-Yoma-Naga folded system.[3]:Ch.4.2 This is a deeper part of the Bengal Basin where the basement is deeply subsided and the subsidence is directly related with the uplift of Himalaya-Burmese mountain chain.[3]:Ch.4.2 It is about 450 km (280 mi) wide in the south of Bangladesh and narrows toward the northeast.[3]:Ch.4.2 The basement is 12 to 15 km (7.5 to 9.3 mi) deep.[3]:Ch.4.2 The folded belts of the Indo–Burman ranges mark the eastern boundary of the Bengal foredeep.[3]:Ch.4.2 Anticlinal structures range from tightly folded, elongate, and faulted structures on the east to broad, open, relatively unfaulted structures on the west.[2]

Tripura–Chittagong fold belt[edit]

The Tripura–Chittagong fold belt is on the eastern side of the Bengal Basin and trends north, as part of the Indo–Burmese Mobile Belt.[3]:Ch.4.2 In Bangladesh this belt is the hills of the Chittagong Hill tracts, Chittagong and Sylhet.[3]:Ch.4.2 They are characterized by the presence of long narrow folds composed of thick sandy shales of the Neogene age, which are 4 to 8 km (2.5 to 5.0 mi) thick.[3]:Ch.4.2


Stratigraphic units[edit]

The basement of the Indian platform is over 360 million years old and has igneous and metamorphic rocks.[10]:5 The overlying bituminous Gondwana coal layer was formed 360 to 250 million years ago in fault-bound basins and is 1 km (0.62 mi) thick.[10]:5 The total reserve of 1,053 million tons of coal is distributed among seven seams and offers good prospects for coal bed methane development.[21]:10 The 3.3 km (2.1 mi) deep coal seams are beyond current underground mining methods in regard to the financial, technical and management capacity of Bangladesh.[21]:10 The Rajmahal Traps overlie the coal and were in place by 56 million years ago. They are 500 m (1,600 ft) thick flood basalts with trapwash deposits composed of red ferruginous mudrock, claystone, and green and red sandstone.[10]:5

This photograph shows a nummulitic limestone. A person's finger is used to show scale; the fossils range in size from an eighth of a fingernail to a fingernail.
Nummulitic limestone

The 184 m (604 ft) thick Jaintia Group overlying the Rajmahl Traps is divided from its base upward into the 104 m (341 ft) thick Cherra Formation, the 38 m (125 ft) thick Sylhet Limestone Formation and the 42 m (138 ft) thick Kopili Formation.[21]:6 The Cherra Formation, formed 56 to 49 million years ago, consists of poorly sorted sandstone and mudrock.[10]:5 The Sylhet Limestone Formation was deposited 49 to 41 million years ago and is a nummulitic limestone[10]:244 interbedded with minor sandstones.[10]:5 The Kopili Formation was deposited 41 to 34 million years ago and consists of dark-gray to black mudstone, interbedded with calcareous bands, sandstones and limestones.[10]:5

The Barail Formation overlies the Jaintia Group and was deposited 34 to 23 million years ago, is 0.4 km (0.25 mi) thick and is sandy shale. This formation is overlain by the 1 km (0.62 mi) thick Jamalganj Formation (Surma Group) which was deposited 23 to 5.5 million years ago; and consists of alternating sandstone, shale and siltstone.[21]:6&7 Dupi Tila Sandstone is the top sedimentary layer; it was deposited 5.5 to 0.2 million years ago, and is 1.3 km (0.81 mi) thick.

Stratigraphic units of Sylhet Trough[edit]

Sylhet Limestone[edit]

The Sylhet limestone was deposited 56 to 34 million years ago, it is 50 m (160 ft) thick and 6.6 kilometres (4.1 mi)* deep.

Kopili Formation[edit]

deposited 56 to 34 million years ago, 100 m (330 ft), can't tell composition from chart

The Kopili Formation ranges from 40 to 90 m (130 to 300 ft) thick across most of the Bengal Basin.[10]:5 In the Shillong Plateau region the Kopili Formation reaches a thickness up to 600 m (2,000 ft).[10]:5 The upper part of the Kopili Formation grades into brownish siltstone and off-white sandstone with local carbonaceous streaks.[10]:5

Barail Formation[edit]

The Barail Formation was deposited during a major marine regression that exposed most of the Indian platform of the Bengal Basin.[10]:5 The Barail Formation comprises a variably thick sequence of medium- to coarse-grained sand and sandstone intercalated with siltstone, mudstone and intraformational conglomerate.[10]:5 Sand-rich units are range in color from yellow, yellow–brown, red–brown to red; and they are thickly bedded and locally cross-laminated.[10]:5 Interbedded siltstone is gray and compact; mudstone is gray, locally laminated and rippled; and conglomerate is reddish brown.[10]:5 Barail rocks exposed along the northern edge of the Sylhet Trough at the foothills of the Shillong Plateau and near the Dauki fault range from 800 to 1,600 m (0.50 to 0.99 mi)thick.[10]:5 This formation was deposited about 33 to 23 million years ago and is a pink, medium- to coarse-grained sandstone.[10]:244 The Barail Formation strata came from the eastern Himalayas.[15]

Surma Group[edit]

The Surma Group contains the lower, middle and upper Bhuban Formation strata; and the Boka Bil Formation.[3]:Ch.1 It is divided into these formations because of differences in their lithologies.[3]:4.4

The Surma Group is a thick sequence of clastic sediments consisting of alternating layers of sandstone, shale and siltstone that filled the Bengal Basin.[3]:6 In the subsurface, the unit is represented by thick sand-shale sequences.[3]:6 The Surma Group unconformably overlies the Barail Group and is overlain by the sandstone dominating the Tipam Group.[3]:6

Sediments of the Surma Group consisting of the Bhuban and the Boka Bil formations, make up thick and widespread accumulations of mudstone and quartzolithic sandstone derived from the neighboring orogenic belts.[10]:3

The Surma Group consists of alternating shales, sandstones, siltstones and sandy shales with occasional thin conglomerates, indicative of repetitive deposition from pro–delta, delta front with intermittent, wholly marine facies.[3]:Ch.1 The group is divided into the Bhuban and the Bokabil formations, based on differences in their gross lithologies.[3]:Ch.1

Mineralogical and geochemical data from the Surma Basin suggest that the detrital input was especially intense during the Neogene probably because of tectonic forces linked to the active upliftment of the Himalayas.[3]:Ch.9

These trends suggest that deltaic deposits of the Surma Group filled the Sylhet Trough from the east.[10]:1 There appears to have been a major drainage system similar to the modern Brahmaputra River 23 to 5 million years ago, which carried orogenic sediments eroded from theHHimalayas and Indo–Burman Ranges to the eastern Bengal Delta.[10]:1 Clay minerals of the Surma Basin come from sediments derived from the Himalayas.[3]:Ch.9 Data from the Surma Basin suggest that the detrital input was intense during the Neogene probably because of tectonic uplift linked to the upheaval of the Himalayas.[3]:Ch.10 The sediments consist of granites which are a major constituent of Himalayan rocks.[3]:Ch.10

Bhuban Foramtion[edit]

The lower level of the Bhuban Formation was deposited 22 to 18 million years ago, the middle layer was deposited 20 to 10 million years ago and the upper layer was laid down 18 to 14 million years ago.[10]:244

Potential source rocks in the exposed part of the section include the marine shales and carbonaceous shales of the Bhuban and Boka Bil formations.[2] The shale-dominated units may be as much as 500 m (1,600 ft) thick.[2]

The lower Bhuban Formation consists of a 50 m (160 ft) thick layer of light-grey to light-yellow siltstone sandwiched between a 75 m (246 ft) layer of fine-grained sandstone and a 75 m (246 ft) layer of bluish-grey mudstone.[10]:244 The middle Bhuban Formation consists of four layers of sandy mudstone, separated by layers of blue to yellowish-grey silty mudstone.[10]:244 The upper 715 m (2,346 ft) thick Bhuban Formation consists of several layers of light-grey to light-yellow bedded siltstone, fine-grained sandstone and bluish-grey sandy mud.[10]:6&244

Bhuban Formation sediments accumulated in a large, elongate trough.[10]:1 The grain size and sand thickness both decrease away from the trough; the trough meanders westward from the northeastern Bengal Basin and curves southward toward the Bay of Bengal and the Bengal Fan.[10]:11 This sediment lobe can be traced down to southern Bangladesh.[10]:11 A second lobe distributed sediment southward through the Chittagong Hills of southeastern Bangladesh, where sand contents and thicknesses are high relative to the surrounding regions.[10]:11

The other location in which the Bhuban Formation has a comparably high sand content is in the hinge zone more than 200 km (120 mi) west.[10]:7. In the hinge zone the sand percentage in the Bhuban Formation (25%) is over twice that of the Boka Bil Formation (11%).[10]:8 Shale thickness decreases from the Bhuban (1,105 m (3,625 ft)) to the Boka Bil Formation (698 m (2,290 ft)) in the southeastern area.[10]:8

Boka Bil Formation[edit]

The Boka Bil Formation consists of both a top and bottom layer of 100 m (330 ft) thick layer of bedded and rippled mudstone,[10]:244 and was deposited 23 to 12 million years ago. The second-lowest layer is a 100 m (330 ft) thick layer of calcareous concretions, with the next layer a 250 m (820 ft) thick layer of siltstone and fine- to medium-grained sandstone.[10]:244

The middle part of the Boka Bil forms natural gas reservoirs in the Bengal Basin.[10]:6 The top of the Boka Bil is popularly known as the Upper Marine Shale, marking the last marine transgression in the Surma Group.[10]:6

Strata of the Boka Bil Formation show a similar geographic trend in deposition of coarsest and thickest sediment; the major depocenter had shifted northward relative to that of the Bhuban Formation by 30 km (19 mi).[10]:1 The lowest sand-shale ratio for the Boka Bil Formation is in the extreme northeastern corner of Bangladesh.[10]:8 Sand content in depocenters of the Boka Bil Formation is generally higher than that in the Bhuban Formation.[10]:11

Tipam Group[edit]

Subsidence rates in the Sylhet Trough increased three to eight times when the Tipam Sandstone and Dupi Tila Formation were deposited.[15] The Surma Group is unconformably overlain by the 12.5- to 5-million year-old Tipam Group[10]:6 which consists of Tipam Sandstone and Girujan Clay.[10]:244 Tipam Sandstone consists of yellowish-brown to orange, coarse-grained, cross-bedded, sand and sandstone.[10]:6 Tipam Sandstone ranges from .076 to 2.565 km (0.047 to 1.594 mi) thick, it is thickest in the Sylhet Trough.[10]:6 Girujan Clay is brown, blue, purple and gray mottled clay, ranging from .68 to 1,077 m (2.2 to 3,533.5 ft) thick, also being thickest in the Sylhet Trough.[10]:7

Sands of the Tipam Group and the Dupi Tila Sandstone contain abundant argillitic and low- to medium-grade metamorphic lithic fragments and feldspar grains, suggesting continued orogenic unroofing.[6] These younger sands are rich in potassium feldspar relative to plagioclase phospherous-rich Bhuban and Boka Bil sandstones, suggesting a granitic source, probably the Miocene leucogranites of the High Himalayan crystalline terrane.[6]

The Tipam Sandstone is {[convert|960|m|ft|abbr=on}} thick and the Girujan Clay is 600 m (2,000 ft) thick.[10]:244 The Tipam Sandstone has two 30 m (98 ft) thick layers of gravel within the sandstone layers.[10]:244 It is yellow-brown to orange, medium- to coarse-grained, massive and cross-bedded.[10]:244 The sandstone has pebbles and coal fragments.[10]:244

Dupi Tila Sandstone[edit]

The 5- to 0.2-million-year-old Dupi Tila Sandstone unconformably overlies the Tipam Group; in the Sylhet Trough it comprises a sandy lower unit and an argillaceous upper unit.[10]:7 The lower unit is composed of medium- to coarse-grained, cross-bedded sandstone with pebbles of sedimentary and crystalline rocks.[10]:7 The upper unit – which has not been identified elsewhere in Bangladesh – consists of fine- to medium-grained siltstone with intercalations of mottled clay horizons.[10]:7 Its thickness ranges from 92 m (302 ft) in northwest Bangladesh to 2,393 m (1.487 mi) thick in the Sylhet Trough.[10]:7

The lithofacies maps also suggest that the Shillong Plateau, an uplifted massif of Precambrian basement rocks to the north of the Bengal Basin, was not a source for Surma Group deposits.[10]:10 This finding is consistent with the results of petrologic studies of sediments from the Sylhet Trough, which suggest that the plateau was uplifted during Pliocene time.[10]:10

Barail Formation[edit]

subsurface, 4.3 km below the surface, 34 to 23 mya, 1.6 km thick, sandstone with a thin layer (.1km) of sandy shale on top

Surma Group[edit]

The Surma Group consists of the Bhuban Formation and overlying Boka Bil Formation.[10]:5 These formations are more than 4 km (2.5 mi) thick in the eastern fold belts and the deeper part of basin; the equivalent unit in the Indian platform – the Jamalganj Formation – ranges from 0.150 to 1.3 km (0.093 to 0.808 mi) thick.[10]:5

Bhuban Formation[edit]

23 mya, mudrock, sandstone, .6 km thick

Bhuban sequences as prodelta and delta-front deposits of a large mud-rich delta system similar to the modern Ganges-–Bengal delta.[10]:6

Boka Bil Formation[edit]

Boka Bil sandstone, mudrock and sandy shale were deposited in subaerial to brackish environments[10]:6 18 to 11 million years ago. At 1.3 km (0.81 mi) thick, the Bhuban Formation is less than half that of the Boka Bil Formation.[10]:8

Tipam Group[edit]

.5 km thick, 11 to 5.5 mya, sandstone, sandy shale

Dupi Tila Formation[edit]

The Dupi Tila Formation was formed 5.5 to 0.2 million years ago and lies in a 0.4 km (0.25 mi) thick band .2 to .55 km (0.12 to 0.34 mi) below the surface.[21]:7 It consists mostly of loosely consolidated medium- to coarse-grained sandstone with minor amounts ofshale–clay.[21]:6 That is overlain by recent alluvium of sand, silt and clay.[21]:6

Seismic stratigraphy reveals that there were major sandy tidal mega-channels (8 to 10 km (5.0 to 6.2 mi) wide and 100 m (330 ft) deep) in the southwestern part of the Bengal basin between 10.5 and 5 million years ago.[10]:10 These data suggest that this area was an active part of the Ganges–Brahmaputra delta, in which a distributary channel shifted through the Hazipur area.[10]:10

The Indo–Burman Ranges contributed sediment to the Chittagong area, but the major sediment source for the Bengal Basin was the early uplifts of the eastern Himalayas.[10]:14 When significant uplifts began in the western Himalayas, the region to the east was a shallow sea or only shallow uplifts.[10]:14 Initial uplifts in the eastern Himalayas and the Indo–Burman Ranges in the Oligocene, and more intense uplift in the Miocene, funnelled orogenic detritus along a major drainage (paleo-Brahmaputra?) through the remnant ocean basin.[10]:14 Depocenters in the Bengal Basin migrated from east to west and from north to south, toward the Bay of Bengal, as underthrusting of India beneath southeast Asia along the present-day Java Trench and its northern extension continued.[10]:14

The upliftment of the Arakan–Yoma–Chin geoanticline and basin-wide movement took place in Early Oligocene.[3]:Ch.4.1 The sea regressed from the Shillong Plateau area and fluviomarine Barail sediments were deposited along the southern rim of the Shillong Plateau; at the same time the area extending from the Surma Basin to the Chittagong Hill Tracts subsided and was filled with fine-grained marine Barail shales and siltstones.[3]:Ch.4.1 The thickness of the Barail Group generally decreases toward the shelf.[3]:Ch.4.1 The deposition of the Barail Group in the foredeep basin and the mobile belt varies from 800 to 1,000 m (2,600 to 3,300 ft) thick whereas on the shelf it is only 163 m (535 ft) thick and is represented by the Bogra Formation.[3]:Ch.4.1

Hydrocarbon resources[edit]

The basin is rich in hydrocarbons, with estimated reserves of 400,000,000,000 m3 (5.2×1011 cu yd) of natural gas and 40 million barrels of condensate.[10]:4

Buried deeply in the subsurface, source rocks have been generating hydrocarbons since the formation of the delta.[2] The eastern Chittagong-Tripura fold belt contains large anticlinal folds that are traps for hydrocarbons in the Surma Basin and throughout much of eastern Bangladesh.[2] These geologic structures have been forming for 5 million years and contain the largest gas fields in Bangladesh.[2]

Reservoir rocks in the Surma Basin are sandstones of the Boka Bil and Bhuban formations that were deposited in fluvial, deltaic and estuarine environments.[2] Reservoir sands range from thick channel fill and marine-bar deposits to sandstones thinly interlaminated with shale and siltstone that were deposited in tidally influenced environments.[2] At the top of the Surma Group, a widespread unit – Upper Marine Shale – serves as a regional hydrocarbon seal in the Surma Basin and Bangladesh.[2] These sedimentary strata have been folded into several large-scale anticlines that are unfaulted, or in the western and central parts of the basin are moderately faulted .[2] Folding and faulting is more intense toward the east within the Chittagong-Tripura fold belt and becomes progressively less intense to the west, where the folds are more broad and gentle.[2]

It consists of at least 10 major anticlinal structures that bring strata as old as Miocene to the surface on their crests.[2] The Tertiary-age stratigraphic units mapped in the tightly folded anticlinal structures in this area are the same as those recognized in the subsurface of the Surma Basin.[2] As a result of the intense folding and faulting, the source rocks and reservoir rocks of the Surma Basin are exposed at the surface in this area where the anticlines are breached by erosion.[2] The strata exposed in the anticlinal structures are about 3 km (1.9 mi) thick.[2]

Migration is generally vertical along fractures and lateral through porous strata.[2] Hydrocarbon generation began a few tens of millions of years ago and has continued during the formation of the major anticlinal traps, which began only about 5 million years ago.[2]

Kopili Formation[edit]

These compositions are similar to sands derived from the Indian craton, suggesting that they underwent intense chemical weathering, likely in a source with low relief and considerable transport.[6] Himalayan tectonism during this time was probably significantly farther from the Bengal Basin than at present.[6] This fossiliferous mudstone was deposited 40 to 35 million years ago and is 50 m (160 ft) thick.[10]:244

Surma Group[edit]

Dupi Tila Sandstone[edit]

The Dupi Tila and Dihing formations are thick sequences of 2.5 km (1.6 mi) of fluvial and deltaic facies. [3]:Ch.4.1 These sandstones were deposited during the Pliocene to Pleistocene time and are 1,550 m (5,090 ft) thick.[10]:244 They are medium- to coarse-grained variously colored sandstone.[10]:244

Accumulation rates increase in low-lying distal basins, where several meters of annual precipitation and associated runoff rework sediment discharge annually.[11]

The Surma Basin is partly a fault-bounded trough.[3]:4.4.2 The basin covers an area of roughly 10,000 km2 (3,900 sq mi) and is bounded in the north by the Shillong Plateau.[3]:4.4.2 In the west, the Surma Basin gradually ascends toward the hinge zone, while passing into the Bengal foredeep.[3]:4.4.2

The Surma Basin is a sub-basin of the Bengal Basin, which was formed about 127 million years ago when the Indian Plate rifted away from Antarctica.[3]:4.4.2 The onshore part of the Bengal Basin is the site of the world's largest delta (about 60 000 km2) formed by rivers (Ganges, Brahmaputra/Jamuna, Padma, Meghna) that drain a large portion of the Himalayas.[3]:4.4 This delta feeds the world's largest submarine fan (Bengal Fan), which extends more than 3,000 km (1,900 mi) south into the Bay of Bengal.[3]:4.4 The Bengal Basin is being encroached upon by the Indo–Burman Ranges, a 230 km (140 mi) wide, active orogenic belt associated with eastward subduction of the Indian plate below Myanmar (Burma).[3]:4.4 As the collision between the Indian and the Eurasian plates continued – 20 million years ago – there were further major phases of uplift in the Himalayas.[3]:4.4

The Surba Basin has topographical elevations of 5 to 20 m (16 to 66 ft), numerous lakes and swamps, and is actively subsiding.[3]:4.4 On the basis of seismic data, the Surma Basin comprises a17 km (11 mi) thick sedimentary column the Sylhet Limestone to recent clastics.[3]:4.4

Surma Basin was structurally evolved by the contemporaneous interference of two major tectonic movements, i.e. the emerging of the Shillong Massif in the north and the west prograding mobile Indo-Burman Fold Belt.[3]:4.4

The northern and eastern parts of the basin are far more complicated than the southern and western portions.[3]:4.4 The relief and complexicity increases towards the east.[3]:4.4 The anticlines are commonly faulted and many produce gas.[3]:4.4 Structural relief between paired anticlinal crests and adjacent synclinal troughs may be as much as 7 km (4.3 mi), and the synclines have acted as major late Neogene and Quaternary depocenters.[3]:4.4 The folds decrease in amplitude westward, and are not present west of about 91°, where the Sylhet Trough merges with the main part of the Bengal Basin.[3]:4.4 The Surma Group (Early Miocene - Quaternary) is a diachronous unit consisting of a succession of alternating shales, sandstone, siltstones and sandy shales with occasional thin conglomerates, indicating repetitive deposition from pro-delta, deltafront and paralic facies with intermittent, wholly marine facies.[3]:4.4

Two major lithofacies were identified in the Surma Group: the less-common sandstone lithofacies A which consist of massive, thinly inter-bedded and inter-laminated, fine to medium-grained sandstone; and more the abundant facies B consisting of claystone, mudstone and shale.[3]:6.1 Facies B consists of laminated bluish, bluish gray and gray to black shale from gray to yellowish-gray siltstone to very fine grained sandstone.[3]:6.1

Ganges–Brahmaputra–Meghna rivers[edit]

Map of Bangladesh showing its many rivers.
Bangladesh and its many rivers
This map shows that the Indus, Ganges and Brahmaputra rivers all originate in approximately the same place in the Himalaya Mountains. The Brahmaputra River flows east-southeast for a long distance before turning south and then west-southwest to join with the Ganges River in Bangladesh.
The Indus, Ganges and Brahmaputra rivers all originate in approximately the same place in the Himalaya Mountains
The Ganges River Delta is the largest inter-tidal delta in the world.
Ganges River Delta

Changes in the courses of the Ganges and Brahmaputra rivers through Bengal during the last few hundred years can be attributed to faulting and resultant tilting of fault blocks.[20] These changes have caused the Ganges to abandon numerous western distributaries to join the Brahmaputra-Meghna system to the southeast.[20] 30,000 km2 (12,000 sq mi) of former Ganges deltaic plain in southwest Bengal has been abandoned.[20]

Heavy-metal accumulation[edit]

The increased heavy metal concentration in the 63 m (207 ft) fraction of surface sediments shows similarity among major segments of the G-B-M system in the basin.[12] The differences in heavy metal concentation in the lower G-B-M system with that of its upper and middle counterpart is related to the contrast between Himalayan rivers and the other major South Asian rivers.[12] Heavy metals in the lower G-B-M system have an affinity toward the clay fraction of the sediments.[12] The correlation matix of heavy metals in the lower Brahmaputra and Meghna suggests the importance of iron-manganese oxyhydroxides in their accumulations.[12] Iron, Ti and Mn are higher in the Meghna main channel, Zn is higher in the Meghna tributaries, and Cr is higher in both the Brahmaputra and Meghna compared to the value for standard shale.[12] lower Ganges system shows relatively higher concentration in the nondetrital fraction of heavy metals, probably due to the presence of petroleum refinery, industrial and mining effluents, and agricultural runoff in the drainage basin.[12] The Bengal Basin represents a relatively unperturbed alluvial basin with regard to heavy metal pollution.[12]

Bengal Fan[edit]

Main article Bay of Bengal

Ideal sketch of an alluvial fan with text in Italian.
Alluvial fan
This map shows where the Sunda arc is.
Sunda arc

The Bengal Fan is the characteristic feature of the Bay of Bengal[22]:507 and is the largest submarine fan in the world, with or without its eastern Nicobar Fan lobe.[23] It is 3,000 km (1,900 mi) long, 1,000 km (620 mi) wide and has a maximum thickness of 16.5 km (10.3 mi);[24] with an area of 2,800,000 to 3,000,000 km2 (1,100,000 to 1,200,000 sq mi).[23] The western boundary is the continental slope of India and Sri Lanka; the eastern boundary north of 10°N is the Sunda subduction zone – where the Australian plate is subducting under the Sunda plate[25] – and the eastern boundary south of 10°N is the Ninety East Ridge.[23] There is an estimated 1 billion tons of fluvial sediment discharged annually.[11]

The Bengal Fan formed as a result of the Indian plate colliding with the Eurasian plate, the orogeny of the Himalayas and the uprising of the Tibetan Plateau.[24] It is supplied by the Ganges and Brahmaputra rivers, with smaller contributions of sediment from several other large rivers in Bangladesh and India.[24] The overall surface of the fan is smooth and has gradients varying from about 6 m/km (32 ft/mi) on the upper fan to less than 1 m/km (5 ft/mi) on the lower fan.[23]

The largest submarine canyon on the Bangladesh shelf – the Swatch-of-No-Ground Canyon – is the most conspicuous feature in the northern Bay of Bengal.[22]:507 The canyon is 2,500 km (1,600 mi) long;[23] it is a 300 m (980 ft) deep and 18 km (11 mi) wide depression trending north-northeast.[22]:507 The canyon is not associated with faults or folds; it was formed by major river flows and underwater currents in the northern Bay of Bengal.[22]:507 The canyon formed 125,000 years ago and has been a conduit for sediment dispersal to the largest fan in the world.[22]:507 The head of the canyon lies in 38 m (125 ft) deep water on the continental shelf of Bangladesh.[22]:507 It continues seaward as a linear trough for 160 km (100 mi), with an average slope-gradient of 8.2 m/km.[22]:507

During the initial phase of sediment accumulation – prior to the collision of the Indian plate with the Asian plate – the Bay of Bengal's sediments came from the eastern continental margin of India.[22]:507 Since the continent-continent collision, sediments have come from the Himalayas via the Ganges and Brahmaputra rivers – which annually transport billions of tons of sediments into the Bay of Bengal1 –[22]:507 through the Swatch-of-No-Ground Canyon and into an extensive, meandering net of fan valleys.[26]

[27] The deepest sediment accumulation rates on the shelf occur near the head of the Swatch-of-No-Ground Canyon.[27] The sediments decrease in thickness toward the south – at latitude 7°40′S – where they abut exposed basement topography.[22]:507


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