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Geological Group-Formation


Geological Group-Formation a stratigraphic sequence that is presumed to divide in whole or in part into formations in the future. A formation is a body of rock identified by lithic characteristics and stratigraphic position; and is mappable at the earth's surface or traceable in the subsurface. The stratigraphically important Groups-Formations of Bengal-Assam region are arranged below from the oldest to the youngest:

Gondwana Group The oldest sedimentary unit in Bangladesh is the Gondwana Group of Permian age, resting unconformably on the Precambrian crystalline basement. It is characterised by a sequence of conglomerates, feldspathic sandstones, carbonaceous sandstones, grey and carbonaceous shales and coal seams. These sediments were deposited in the faulted basins of the Pre-Cambrian era and attained considerable thickness. The thick coal seams and carbonaceous shales of the Gondwanas do not exhibit any oil or gas. The Gondwana Group is divided into the Kuchma Formation and the Paharpur Formation.

Kuchma Formation an Early Permian mappable body of rock in the bengal basin identified by lithic characteristics and stratigraphic position. The name Kuchma Formation was proposed by T Ahmed and MA Zaher for the subsurface section at Kuchma in the district of bogra. Its synonym is barakar stage and Barakar Series. The type area from subsurface section at Kuchma (24°42'N; 89°16'E) in the district of Bogra from subsea depth of 2,364m to 1,858m. The presence of this formation has not been reported from any other drill holes in the Bogra-Rajshahi area. The formation is composed of sandstone and grit, with subordinate shale and is interbedded with coal seams. Five coal seams of 6m, 22m, 13m, 4m, and 6m are present in the type section. The formation is 495m thick. The Kuchma Formation lies unconformably over the basement complex and conformably under the Paharpur Formation in the shelf facies of Bangladesh. The rock unit is tentatively correlated with the Barakar Stage of the Lower Gondwana Group in India, which is of Early Permian age. However, this correlation is done without any biostratigraphic information. It is problematic to make correlation solely based on lithology. The results of coal petrographic analyses, which could at least roughly date the various coal seams, are also not available. Considering these facts Klaus-Ulrich Reimann placed this unit in the next higher stratigraphic division, ie the Damuda Series.

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Paharpur Formation carbonaceous unit, encountered in a drill hole at the Paharpur-Jamalganj coalfield of naogaon district in the northwestern part of Bangladesh, lies above the basement complex and unconformably lies below the basaltic Rajmahal Trap of the Mesozoic era. The rocks of the formation (512m) consist of grey, medium to very coarse (gritty) feldspathic sandstone with thin lens of dark grey shale and four coal beds of bituminous type. The fixed carbon of coal varies from 33% to 54% with an average of 47%, ash content varies from 10% to 60% with an average of 22.4%, volatile matter 30% to 40%. The stratigraphic succession has been established partly by petrological, palaeontological and palynological studies and partly by stratigraphic and lithologic correlation with known areas. It was deposited in a broad shallow freshwater basin, the extent of which has been reduced by subsequent faulting and erosion. The formation has been correlated to the Late Permian age and considered to be equivalent to the Raniganj Stage of the Lower Gondwana Group of west bengal, India.

Rajmahal Stage is named after the rajmahal hills (24°30'N; 87°30'E) of India. The unit is also mentioned as the Rajmahal Traps Stage. The sylhet traps, exposed in the Khasi Hills, is considered as the eastern extension of the Rajmahal Stage.

The Rajmahal Stage in the type area consists of 450m to 600m of basaltic lava interbedded with contemporaneous shale and clay beds, some of these being silicified and porcellanoid. Two of the flows are of pitchstone. Total thickness of the intercalated sedimentary beds is only 30m, each being 1.5m to 6.0m thick. The intertrappean sediments between the lower four of the five flows contain rich plant remains. The Trap rocks are mainly hornblende basalt, olivine basalt and andesite and these are mostly amygdaloidal, cavities being filled with calcite, chalcedony and analcite. The basalts are commonly dark coloured, porphyritic with an unusually fine-grained matrix. The top-most flow is the thickest one and is 76m, but others are thinner.

In Bangladesh, Rajmahal Trap rocks have been encountered in the subsurface in the Bogra, Jamalganj, shibganj and singra areas of the western part of Bangladesh, having thickness ranging from 68m to 546m. Singra (24°29'52N; 89°10'50E) has two trap beds with one intertrappean bed.

The Rajmahal Stage in the type area as well as in Bangladesh lies unconformably over the Paharpur Formation (Barakar Stage) and is also unconformably overlain by the Shibganj Trapwash (Mahadek Trapwash) in Bangladesh.

The intertrappean beds in the Rajmahal Hills and particularly in the chert beds near Nipania (24°36'N; 87°33'E) have yielded important plant fossils. The fossils indicate Jurassic age for the Rajmahal Stage in the type area. The intertrappean bed at Singra in Bangladesh has not yielded microfloral or faunal fossils and as such no confirmed age could be assigned to the Rajmahal of Bangladesh; however it could be considered as Jurassic/Early Cretaceous in age.

Bolpur Formation a formation in the subsurface of Bolpur in West Bengal, India (23°37'46N; 87°45'25E). It occurs from 1,032m subseas, to 1,193m subsea having a thickness of 160m. The formation lies over the Rajmahal Trap with an unconformable contact and is overlain conformably by the Ghatal Formation (Late Cretaceous) or by the Jalangi Formation (Late Cretaceous). It is composed of red clay, claystone and greenish sandstone (trapwash), and dominant white sandstone elsewhere. No foraminifera has been found in the formation but it contains rare examples of fossil fruits (gyrogonites) of Chara and brackish lagoonal ostracodas from its transitional zone with the overlying Jalangi Formation. It also contains primitive dicotyledonous pollen at places. The formation is mainly a freshwater deposit with the upper part at some places associated with the estuarine deposition. The spore-pollen of the formation indicate an Early Cretaceous to early part of the Late Cretaceous age.

In consideretion of the lithological composition, a stratigraphic position in the succession and the age of the Bolpur Formation, it could be conveniently correlated with the subsurface Cretaceous Shibganj Trapwash of the shelf area of northern Bangladesh and the Cretaceous Mahadek Formation of Assam, India.

Tura Formation a Palaeocene/Lower Eocene predominantly arenaceous sequence exposed in the Garo Hills. It was named by CS Fox after the town of Tura in Assam (25°31'N; 99°15'E). The formation lies unconformably over the Shibganj Formation and conformably under the Sylhet limestone. In Bangladesh the formation crops out at Takerghat in the sylhet district, which is adjacent to the Bangladesh-India border. The unit has also been reported to be present in the subsurface of Bogra and rajshahi districts. The lithology consists of sandstone with subordinate shale and marl with occasional carbonaceous shale. The sandstone is light grey, white, dirty white and light brown. It is fine to coarse-grained and even pebbly at places. Cross bedding is also found. The shale occurs as thin beds in the sandstone and is of grey to greyish brown in colour. The marl bed is greyish brown and fossiliferous. The carbonaceous beds locally developed in the coal seams.

The formation in the Garo Hills is about 152m thick. Thickness recorded from Singra-1X, Kuchma-X1 and Bogra-X1 wells, all located in the Bogra Graben, at depth of 360m, 270m and 169m respectively. One 237.80m thick Tura Sandstone has been found in DH-5 well located at Lalghat in Sylhet, and it was found to have a maximum thickness of 105m in EDH-3 well in northern Bangladesh. Standard Vacuum Oil used the well-established name Cherra Formation for the description of the sequence in the Kuchma and Bogra wells.

Cherra Formation a predominantly arenaceous sequence of the Tertiary age located particularly in the Khasi and Jaintia hills of the Northern Foreland Shelf. Similar deposits in the Garo Hills are known as the Tura Formation. Palynostratigraphical analyses have revealed that the Mikir Formation forms the time equivalent of the Cherra and Tura formations along the southern slope of the shillong plateau and of the Teok Formation in Upper Assam. In Bangladesh the Tura Formation is characterised by alternating white, pink and brown, fine to coarse grained, extensively crossbedded sandstones, light-grey, ash-grey shales, mudstones and streaks of carbonaceous matter.

The age assignment of the Cherra Formation as well as that of the Tura Formation, both forming the basal succession of the Tertiary in the marginal portion of the shelf was long debated. Apart from lithological similarities, the correlation of the microfloral assemblages shows a close correspondence between the two. Palynological data reveal that the Tura Formation is divisible into 4 cenozones, while 3 palynological cenozones were recognised within the Cherra Formation, although only minor variations in the composition of the microfloral assemblages were seen within the rock sequence. In many publications, both formations were regarded as Palaeocene sediments. However, a few authors mentioned that certain palynomorphs in the studied sequences provide some evidence which points towards a Lower Eocene age for the Cherra Formation. A similar point was also later accepted by other authors. Standard Vacuum Oil of the USA adopted the well-established name Cherra Formation for the description of the sequence in the Kuchma and Bogra wells. Later, this rock unit was designated Kuchma Sandstone.

Kopili Formation P Evans introduced the name Kopili Stage for the upper unit of the Jaintia Series after the Kopili river in Assam, India. This formation is exposed in Bangladesh only at the bank of the dauki river in Sylhet district. The thickness of the formation in the type area ranges from 350m to 460m. In the Western Foreland Shelf area of Bangladesh it has been encountered as subcrops at a depth of 88m at gaibandha and 40m in Singra. The maximum thickness of this formation is reported at 700m in Upper Assam. The Formation lies over the Eocene Limestone and underlies the Oligocene Laisong Formation.

The Formation is composed of dark grey shale with calcareous sandstone. At the Dauki river the shale is dominant but in the Gaibandha area the shale-sand ratio is almost 1:1. The formation is fossiliferous. The important fossils are: Quinqueloculina bouena, Q.carinata, Q. seminulum, Q. zealandica,' Q. praelongirostrata, Pyrgo bulloides, Triloculina gibba, Lagena globosa, L. hexagons, L. hispidula, Discorbis globiformis, Chiloguembelina parallela, Nonion ovatum, Globigerina, Cibicides mimulus, Pullenia quinqueloba, Pararotalia audouini, P, armata, Nummulites pengaroensis, Pellatispora inflata, P. irregulaaris, P. glabra, Guembelitria oveyi, Discocyclina, Hantkenina, Globorotalia, and Cancris. The palynological data indicate the presence of zonocostities ramonae, Dicolpopollenites kalewensis, Ephedra etc. The Kopili Formation was deposited in a brackish to marine condition in a shallow marine to transitional environment. The water was hyposaline to saline and the temperature varied between 10°C and 30°C. The age of this formation is established as Late Eocene.

Burdwan Formation embraces sediments of Oligocene (37-24 million years before present) and Miocene (24-5 million years before present) ages and grades transitionally into the Miocene Pandua Formation as well as into the Oligocene Memari Formation. It is characterised by freshwater deposits representing a regressive event of the Early Oligocene age in the lower portion and by marine, or at least estaurine sediments indicative of transgressive conditions in the higher ranges of the succession. Predominantly an arenaceous sequence, composed of fine to coarse-grained sandstones with interbeds of shales and lignitic shales, the latter particularly in the lower portion of the formation, it reaches a thickness of 163m in the well Burdwan-1. This well is regarded as the type section. The lower and the upper boundary of this formation are transitional in nature.

Bokabil Formation, Labanakhya Chhara, Sitakunda

The formation grades into the conformably overlying Neogene Pandua Formation in some wells, for instance in Galsi-1, Bolpur-1, Debagram-1, Jalanhi-1, and the type section. Some other wells, such as Memari-1 and Ghatal-1, show the Burdwan Formation overlain by the Memari Formation, the second Oligocene stratigraphic unit. By the evidence of palynomorphs the unit is placed in the Bengal Palynological Zone IV that corresponds to Oligocene. However, the uppermost portion of this unit includes the basal parts of Miocene.

Barail Group an Oligocene lithostratigraphic unit comprising several formations. P Evans named the rock unit as the Barail Series after the Barail Range of Assam and this was later emended by B Biswas to the Barail Group. It represents the geosynclinal development of Oligocene. It is well developed in the Surma valley, north Cachar and the Khasi and Jaintia hills of Assam. The unit comprises arenaceous and in places shaly and carbonaceous rocks. It has three formations, from bottom to top: the Laisong Formation, Jenam Formation and Renji Formation. In Bangladesh the area between jaintiapur in the east and the Dauki Nala in the west belongs to the Renji Formation based on the lithology and the fossil content of the exposed rocks. Rocks belonging to the Jenam Formation were encountered in the well Atgram-iX at depths ranging from 4,740m to 4,980m without reaching the base of the formation. Also rocks lithologically comparable to the Renji Formation are penetrated in the well Atgram-iX between 4,015m and 4,740m. It lies conformably over the Jaintia Series but is separated by a sharp unconformity from the overlying Surma Series. The unit has economic importance for its workable coal seams and oil-producing horizon in Upper Assam.

Surma Group lies unconformably over the Barail Group and is underlain conformably by the Tipam Group. The unit is subdivided into the Bhuban Formation and the Boka Bil Formation. There is a local unconformity between the Boka Bil and Tipam formations in Sylhet in northeastern Bangladesh. At present the unit has been recognised as a rock unit in Bangladesh as well as in India.

Bhuban Formation A Miocene body of rock identified by lithic characteristics and stratigraphic position, and mappable at the earth's surface or traceable in the subsurface. P Evans designated the lower unit of his Surma Group as the Bhuban Stage after the Bhuban Range (24°40'N; 93°02'E) of Assam, India. B Biswas has emended this sequence to formation rank. Outcropping rocks of the Bhuban Formation occur throughout the Frontal Folded Belt and particularly in the cores of anticlinal trends in the eastern part of the chittagong hill tracts. Further outcrops are known from three northern margins of the surma basin between the Atgram area in the east and Jaintiapur in the west, where the outcrops parallel the E-W trend of the Dauki Fault. The entire Bhuban Formation has been penetrated in Atgram-1X and probably also in Rashidpur-2, while drilling for hydrocarbons. The lower and upper members are made up predominantly of sandstone, while the middle member usually shows a higher percentage of shale. The Bhuban in the Chittagong Hill Tracts has a thickness of about 3,500m. The unit is conformable with the overlying Boka Bil Formation and unconformable with the underlying Barail Group.

The oldest rocks of the Lower Bhuban Member of the Frontal Folded Belt are exposed in the cores of the easternmost anticlines near the Indian border. Tests of foraminiferas and hystrichospherids encountered in the shaly sequences of this member indicate deposition in a marine to brackish environment. Rocks of the Middle bhuban Member are also confined to the cores of the anticlines in the eastern part of the Chittagong Hill Tracts. This member indicates a deltaic to near shore depositional environment. The Upper Bhuban Member crops out in most of the anticlines throughout the Chittagong Hill Tracts, in the cores of the Harargaj Structure and Patharia Structure in the southeastern Surma Basin and along the northern margin of the Surma Basin between Atgram-1X and Jaintiapur. Sandstone dominates and commonly shows crossbedding, flaser bedding ripple marks and channel deposits. The Upper Bhuban Member is dominated by gymnospermous pollen. This indicates that the orogenies in the region were already so highly elevated that the climatic conditions were favourable for the growth of gymnospermous plants in these zones. The Bhuban Stage is devoid of identifiable faunal fossils, though at Kanchanpur in the Surma valley a collection of molluscan fossils, believed to be of the Aquitanian age (Early Miocene), was obtained. However, palynomorphs (spores and pollen) are available. The unit is a natural gas bearing horizon in Bangladesh.

Boka Bil Formation the upper unit of the Surma Series. P Evans named it the boka bil stage after a locality (24°26'N; 92°32'E) in the Hailakandi valley, northern Cachar, Assam, India. The lithology plays an important role in identifying the Boka Bil unit and as such a Boka Bil Stage is referred to as the Boka Bil Formation, a rock unit, in Bangladesh as well as in India. In Assam the Boka Bil Formation is represented by a 900m to 1,500m thick succession of sandy shale alternating with siltstone and ferruginous sandstone.

The formation is exposed in the hill ranges of greater Sylhet, Chittagong district and the Chittagong Hill Tracts. It is mainly composed of alternating bluish-grey to yellowish-grey, well-bedded siltstone and shale with subordinate light yellowish-brown to brown, fine to medium-grained, bedded to massive, hard sandstone, locally calcareous and conglomeratic. Generally shale and siltstone dominate the lower and the upper parts while in the middle part the sandstone percentage is much higher. It occupies a transitional zone between the underlying dominantly sandy Bhuban Formation and overlying Tipam Sandstone Formation. The formation contains heavy minerals and is characterised by the presence of zoisite, epidote, staurolite, kyanite and hornblende.

From the Boka Bil Formation, more than 100 fossil species have so far been identified from two localities of the Garo Hills. Most of them belong to Pelecypoda and Gastropoda. The following species of Foraminifera have also been recorded: Chiloguembelina globigera; Globigerina bulloides;G, falconensis-G, cf bradyi and G.quinqueloba. In the Sitakunda hills the Boka Bil Shale contains Ostrea digitalina, O. gryphoides and numerous plates of Bolanus, fragments of Arca, Pecten, Trochus, Oliva and Corals. The formation may have been deposited under shallow marine to deltaic and estuarine environments. The Boka Bil Formation contains natural gas in economic quantities.

Tipam Group a Pliocene lithostratigraphic unit in the Bengal Basin comprising several formations. FR Mallet used the name Tipam Series after the Tipam Hills (27°16'N; 95°30'E) of Assam in India. Its synonym is Tipam Group. Its type area is the Dihing river of Assam (27°15'N; 95°25'E). It is exposed in Upper Assam and the Surma valley and extends to the Arakan coast. However, the Tipam Group of the Bengal Basin is not related in any way to the Tipam Group proper of the Dihing Basin. The group is subdivided into two formations: the Tipam Sandstone Formation and the Girujan Clay Formation. The Tipam Sandstone Formation is a coarse-grained sandstone sequence overlying the Boka Bil Formation. The contact between these two units is unconformable in the marginal portions of the basin as well as in the shelf areas in the west. In the deeper sections of the basin the contact is conformable. The upper contact to the Girujan Clay Formation is also conformable. In the case of absence of the Girujan Clay Formation, the Tipam Sandstone Formation is unconformably overlain by the Dupi Tila Formation.

The Tipam Sandstone Formation typically consists of grey-brown to pale-grey, coarse-grained, crossbedded, massive sandstone. Intercalations of grey shale, conglomerate horizons, pebbles, wood fragments and petrified trunks also occur. Outcropping Tipam Sandstone deposits occur throughout the Frontal Fold Belt from the southernmost portion of the Chittagong Hill Tracts, through the western section of the Indian state of Mizoram and across Tripura to the northeastern section of the Surma Basin. The formation occurs regularly on the flanks of the anticlinal trends in the Frontal Folded Belt area, forming steep cliffs. The microfloral assemblage found in this formation is featured by the dominance of gymnospermous pollen and monoporate pollen grains belonging to the Gramineae. The poor preservation of the palynomorphs can be linked with repeated deposition, erosion and redeposition and subaerial conditions prevailing during deposition.

The name Girujan Clay Stage was given by P Evans after a small stream at Digboi (27°23'N; 95°38'E) in Assam. The formation develops conformably and gradationally from the underlying Tipam Sandstone. The arenaceous deposits of the Dupi Tila Formation unconformably overlie it. It is composed mainly of mottled clay with subordinate mottled sandy clay, sandstone with subordinate ferruginous sandstone. Fossil wood and lignite are also present in the unit. Its deposits are preserved mainly in the large valleys coinciding with major synclinal trends of the fold belt. The thickness of the formation is 915m to 1,829m in Assam, 168m in the sitakunda, 107m near Cox’s Bazar, 366m in the Harargaj area, 762m in the Hari river and 1,077m in the Lubha khal area. The Girujan Clay Formation represents lacustrine floodplain and overbank deposits. The sedimentation took place under subaerial conditions.

Dupi Tila Formation a Pliocene-Pleistocene mappable body of rock in the Bengal Basin. P Evans named it after the Dupi Tila hills in the Hari (Shari) river (25°06'N; 92°08'E) valley of Sylhet district. It is exposed in Chittagong division as well as along the southern margin of the Jaintia hills of Assam and Bangladesh. In northern Bangladesh the presence of the unit has been reported from a number of drill holes. The unit unconformably overlies the Tipam Group and is in turn overlain either by the Dihing Formation or by Recent alluvial deposits. At Atgram, the unit overlies the Boka Bil Formation. In Sylhet, the formation consists of two distinct units. The lower part is composed chiefly of massive sandstone interbedded with subordinate claystone. The sandstone is yellow to yellowish brown, medium to coarse, cross-bedded and in places pebbly. The upper part consists of claystone and siltstone with subordinate sandstone.

In Chittagong, the Dupi Tila Formation consists of yellowish-brown to brown, fine to medium grained pebbly and cross-bedded sandstone with subordinate claystone and siltstone. In the subsurface of northern Bangladesh the formation consists of pebble beds, coarse to fine-grained sandstone and shale of grey colour. Well preserved leaf impressions in Dupi Tila sandstones are also reported from kaptai lake near rangamati. Folding and faulting were common during deposition of this unit and extremely thick sequences were deposited in the rapidly subsiding areas. The thickness along the Hari river in Sylhet is about 1,722m, in the Goyain and Kushiara troughs 4,419m and 3,669m, and in the subsurface of northern Bangladesh ranges from 20m to 276m. In the Chittagong-Cox's Bazar area the unit contains pockets of white clay, though white clay beds are found in the same formation in mymensingh district.

Dihing Formation a Pleistocene mappable body of rock. It has scattered occurrence in the Bengal Basin and is merely of local importance. FR Mallet gave this name after the Dihing river (17°16'N; 90°24'E), near Jaipur in Assam. The formation has a patchy distribution within Bangladesh. The thickness of 24m as observed in the ramu area in Cox's Bazar district does not match the thickness in Assam, where it ranges from 305m in the type section to 1,525m in the Mana Bum environs. The formation consists of yellow and grey, medium-grained, occasionally pebbly sandstone and clayey sandstone with interbeds of mottled clay. The rocks are in most part poorly consolidated. The unit lies unconformably between Dupi Tila and alluvium. At places the unit is reported to contain white clay at the base. [Sifatul Quader Chowdhury, Mujibur Rahman Khan and Md Nehal Uddin]