GEOLOGY OF BANGLADESH

Folded Belts:

The folded structures of Bangladesh may be classified into four belts:

1. The folded sediments of the eastern hilly regions comprising the north-eastern and eastern parts of Sylhet and Comilla districts and Chittagong and Hill Tract districts.

2. A folded region of discontinuous small, low hillocks and hill ranges occupy a very narrow strip on the Shillong Platform and the Basin along the northern frontiers of Sylhet and Mymensingh districts.

3. The subsurface dipping sediments on the Dinajpur Shield and its southern platform.

4. The subsurface folded sediment of the Basin

The Folded Sediments of the Eastern Hilly Regions:- A large number of anticlines and synclines, both symmetrical and asymmetrical.

- Most of the structures trend NNW-SSE, and almost all the folds of Chittagong and Hill Tract exhibit the same trend designated as the Chittagong Strike and may be applicable to the strikes of the folds of Comilla and Sylhet districts.

- In the extreme north-eastern part of Sylhet district, the Chittagong Strike first swings to the N-S then to NNE-SSW and NE-SW.

Folded Rocks of the Northern Frontiers of Sylhet and Mymensingh:

- Along the northern frontier of Sylhet and Mymensingh districts trend steadily E-W may be termed as the Sylhet Strike.

- Chhatak anticline and Goyen syncline parallel the Sylhet strike.

- Sylhet and Dupi Tila anticlinal axes more or less follow the Bogra strike.

- The rocks show high angles of dip ranging generally from 450 to 550S between Bangali Bazar and Bhangerghat and from 300 to 400S in the Bijaipur area.

The Subsurface Dipping Sediments of the Dinajpur Shield and its Southern Platform:

- It will hardly be possible to identify more than three periods of Precambrian granitic intrusions on the Dinajpur Shield and Platform.

- Maddhapara hard rock mine, if ever developed, will offer large exposures of Precambrian rocks.

- The bands of the gneiss trend ENE-WSW and show high angle of inclination ranging from 500 to 700.

- Several anticlines or dome like features and synclines or trough like irregularities may be present in the basin of the Paharpur coal field.

- The dip of the Gondwana sediments ranges from 50 to 100 and in places 150 but their strike remains to be ascertained.

- The top of the Dinajpur Shield exhibits a good degree.

- The strike of the Tertiary rocks trend NE-SW.

- The northern slope of the Dinajpur Platform that grades into the Himalayan trough or foredeep awaits exploration.

The Sub-surface Folded Sediments of the Basin:- Vertical thickness of sediments ranging from 2 to 10 km in the entire delta region.

- Six small anticlines, two south and one west of Khulna and one each at Patuakhali, Bhola, and Muladi.

- The sediments between Chhatak and Jamuna; Mymensingh and Sylhet; and southern boundary of the Dhaka range in thickness from 3 to more than 8 km. Kamta, Munshiganj, and Daudkandi anticlines fall in this area and their axes make small angles with the Chittagong strike.

- Beneath the Bay of Bengal, the Chittagong strike trend dominates but a small number of strikes appear to be erratic in their directions varying from ENE-WSW to almost NW-SE.

Faulting:

The faults may be classified as follows:

1. Precambrian

2. Late Eocene (Kopili)

3. Late Middle Miocene (Girujan clay)

4. Late Pliocene (Dupi Tila)

5. Post Pleistocene (Madhupur clay)

Precambrian Faulting:The faults generally trend NW-SE, NE-SW, NNE-SSW, E-W, ENE-WSW. Faulting in the Precambrian of the Basin has been much less intense. The faults run mostly in the NW-SE and NE-SW directions.

Late Eocene Faults:The narrow frontier strip on the Shillong Platform between Bagali Bazar and Bhangerghat exposes a series of faults that run across and oblique to the Sylhet Strike. Two faults extend parallel to the strike. The horizontal displacement of the geological formations due to the dip faults generally range from about 50 feet to several hundred feet and their heads mainly dip from 450 to 900.

Late Middle Miocene Faults:The displacements of the sediments of most of the folds, being simultaneously limited to the formations of the Surma and Tipam groups. These faults have given rise to many stream and gullies that flow west and east and also from north to south. The dip faults trend mostly oblique to the strike and arre concentrated in the districts of Sylhet, Hill Tract and Chittagong. The strike and the thrust faults have mostly affected the folds of the Chittagong and Hill Tract districts.

Late Pliocene Faults:The faulting during Late Pliocene took place on a much smaller scale compared to the Late Middle Miocene faulting. The faults trend mostly oblique to the strike, cause displacements of small magnitude, and remain limited to the Dupi Tila formation.

Post Pleistocene Faults:The Madhupur area shows nine faults and the Barind Tract exhibits ten. Most of the faults trend NW-SE and form low scraps. The Madhupur clay formation happens to be the least studied geological formation in Bangladesh. The Dupi Tila formation occupies the tips of nearly all the anticlines of the Sylhet, Comilla and Noakhali districts. But in the Chittagong and Hill Tract districts the sediments of the Surma Group form the crest of all the anticlines baring a few which expose the Tipam group.

Horst and Graben:Shillong Plateau including the Mikir hills represent a horst bounded the faults on all sides and formed during Mio-Pliocene orogeny. The Dinajpur Shield constitute the corresponding graben down thrown as a result of multiple faults along the Jamuna and the Ganges.

Mineral Deposits of Bangladesh:

Precambrian:

Hard Rock: In the Maddhapara area, the Precambrian igneous and metamorphic rocks lie within 400 to 500 feet of the surface in a sizeable area.

Iron Ore Mineral: The iron ore mineral composed mostly of siderite occurs as veins both in the coarse grained, light coloured banded gneiss and in the fine grained gray to dark fray banded gneiss. The colour fo siderite varies from creamy, pale yellowish to brownish red.

Precambrian:

Pegmatite: Pegmatite is disposed in veins, sill, and dykes in the Precambrian rocks of the Maddhapara area. It exhibits fleshy colour and is readily identifiable in core samples. It is hard and massive but has developed minute fractures which are generally filled by magnetite, siderite, hematite, and silica.

Kaolin: Kaolin directly overlies the Precambrian rocks and underlies a thick bed of glass sand. It shows milk-white colour and is mixed up with quartz grains. The unwashed kaolin contains 69% SiO2, 24% Al2O3 and 0.11% Fe2O3.

Glass Sand: Glass sand overlies the kaolin. The glass sand mostly consists of colourless and smoky quartz grains. The unprocessed and unwashed silica sand contains 97.04% to 97.31% SiO2 and 0.19% to 0.64% of Fe2O3. The thickness of glass sand ranges from 20feet to 45 feet averaging 31.8 feet.

Paleocene to Early Eocene:

Tura Sandstone: Two workable coal seams occur at subsurface depths ranging from 150 feet to 319 feet between Lalghat and Takerghat, a distance of about 10,000 feet. The coal is black, shiny, hard, and massive. Its streak is dark brown. It has very mild odour of sulphur.

Middle Eocene:

Sylhet Limestone: Limestone outcrops as small hillocks in Bagali Bazar, Lamakata, Lalghat, Takerghat, and Bhangerghat area

Upper Eocene:

Kopili Shale: In Jaipurhat thick calcareous and non-calcareous shale overlies the limestone. The calcareous shale may provide much better and cheaper substitute for the fertile Pleistocene and Recent alluvial clay in the neighbourhood of Jaipurhat.

Early Miocene:

Surma Group: Some of the numerous sandstone interbeds of the Surma group hold commercial accumulation of gas. The total reserves of gas are estimated at 11 trillion cubic feet.

Late Miocene:

Dupi Tila: White clay interbeds with the sediments of the Dupi Tila formation of the Bijaipur area in the northern frontier region of Mymensingh district.

Pliocene:

Lalghat-Lakhmachhara Area: The white sandstone in the Dihing formation of this area contains 99% SiO2, R2O3 0.66%, Fe2O3 0.01%, MgO 0.10% and traces of CaO and insolubles. The minimum depth of the white sandstone from the surface is 78 feet and the maximum is 223 feet. The sandstone is semi-consolidated and is saturated with water.