Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Partha Jit Roy ¹, Mrinal Kanti Roy ¹, Debasish Shome ², Md. Abdullah-Al-Mahbub ¹, Dhiman Kumar Roy ¹, Sudip Saha ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, BD
2 Department of Geological Sciences, Jadavpur University, Kolkata, IN

Abstract

The rock sequences of the Sylhet Limestone Formation, Dauki River section, Gwainghat, Sylhet were studied to identify their various petrological units, non-clastic minerals and fossil content. Limestones are of two varieties crystalline in lower horizon and fossiliferous in upper horizon. The various petrological units identified in the non-clastic portion of the Sylhet Limestone Formation include micrites, dismicrites, intramicrites, pelrmicrites, spantes, intrasparites, pelsparites, and oosparites. Calcite, dolomite and fossils with minor amount of clay, mica, chert, quartz, zircon, tourmaline, rutile, and opaque minerals constitute the limestones. Fossiliferous limestone consists of smaller and larger foraminiferal genera, such as Nummulites, Discocyclina Alveolina, Operculina, Assilina etc. with some Bryozoans and Brachiopod shells. The various petrological units mght have deposited in shallow marine environment with nonreducing as well as reducing condition by tidal, winnowing and non-agitating currents. Algal growth aided the various units in formation.

Keywords

Sylhet Limestone Formation, Dauki River, Gwainghat,Northeastern Sylhet, Bangladesh.

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Mrinal Kanti Roy ¹, Sudip Saha ¹, Syed Samsuddin Ahmed ¹, Quamrul Hasan Mazumder ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, BD

Abstract

The present study on tide, morphology, lithofacies and evolution of the Meghna estuary is confined to its Hizla-Goshairhat-Damudhya-Burirhat( HGDB) branch which represents the relict main channel of the Meghna-Padma river system. Tide in this HGDB branch is semi-Diurnal showing daily, monthly and yearly time versus water level asymmetry. This branch is at present microtidal in nature having tidal range in between 0 41-1 78 m, though mean water level varies from 0 87-5 18 m above average mean sea level. Geomorphologically the study area is divided into estuarine plain in south and deltaic plain in north with estuarine channel in between the 3 m high bank cliff section of the 26 km long estuarine channel at 32 sites recognizes seven litho-Facies. The litho-Facies-Finely laminated sand and silt (facies A), finely laminated silt and clay (fac~ies B ), laminated sand-Clay (sub-Facies C₁), alternating clay and organic matter (facies D) and rippled laminated sand (facies G) are tide generated. Crudely laminated to masslive clay (facics E) and clay with ripple laminated silt and sand (facies F) are interdistributary bay/Swamp and deltaic plain deposits respectively. Sub-Facies C₂(massive sand) and some ripples within facies B and G are storm/Flood and wave influenced deposits respectively. The morphological straight-Meander-straight divisions of the HGDB estuarine reach can be zoned as funnel-Meander-Head on the basis of channel morphology, sedimentary structure, facies and sand+silt clay ratio of sedimentary succession. Rhythmic occurrence of sand-Silt, slit-Clay and clay-Organic matter with sedimentary structures of parallel-and wavy-Lamination along with occasional lenticular-, flaser-, and bi-Directional cross-Lamination and plained off ripples of facies A, B, D and sub-Facies C₁ are tide generated. Some ripples in facies B and G and sub-Facies C₂ are wave and storm/Flood generated. These laterally change to massive to crudely laminated clay with minor amount of rippled sand and silt embedded with soil peds and ischolar_main traces of delta originated facies E and F in headward direction and vertically up the succession. This is a well-Recognized geologically predictable spatial variation when estuary is linked up with delta in landward direction and when delta progrades down the estuary with time. The relative influence of tide(facies A, B, D and sub-Facies C₁), wave(part of facies B and G and sub-Facies C₂) and river (deltaic facies E and F) is well reflected in this zonation. The facies A, B, C and D were deposited in an estuarine environmental set up that was established in the study area immediately after the mid-Holocene 7000-5000 years BP marine transgression. The Ganges delta was shaped and started progradation towards southeast direction probably after the late Holocene sea level fall at around 3000 years BP when the Bhagirathi, now in West Bengal, shifted toward east through Mathabhang, Bhairab-Pushar, Garai-Madhumati-Haringhata, Arial Khan to Padma, the new main branch of the Ganges in Bangladesh. The Padma might have met the old Meghna at around 1110±90 years BP and facies E and F deposited in a deltaic environment at a later time when the Ganges delta prograded to the present position. Facies G is a tidal bar deposit within the present estuarine channel. The Ganges delta is still prograding southeastward, but at a much reduced rate due to present trend of global warming initiating a new sea level rise.

Keywords

Tide, Morphology, Facies, Zonation And Evolution, Meghna Estuary, Bangladesh.

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Mrinal Kanti Roy ¹, Basudeb Chandra Karmakar ¹, Sudip Saha ¹, Sibendu Chaudhuri ²

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi - 6205, BD
2 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN

Abstract

The Mio-Pliocene Dupitila Formation is exposed in its type area in a sub-latitudinal hill range known as Dupitila Hill Range, located at the foothills of the Shillong plateau within Jaintiapur Police Station, Sylhet, Bangladesh. These sediments are exposed at the southern margin of the homoclina1 fold that is separated from the Shillong plateau by the Dauki fault. Lilhostratigraphically the Dupitila sediments can be conveniently divided into a sand dominant lower Dupitila member and clay dominant upper Dupitila member. The sandstones are poorly sorted and strongly fine skewed and show a dominant unimodal distribution with mostly angular to subangular grains. Low ZTR (Zircon, Tourmaline, and Rutile) index indicates a nearby source. The lower Dupitila sediments are represented by lithofacies Gm, Gt, Gp, St, Sp, Sr, Sh, Ss and F1 which form an association resembling fine gravel mid fan, whereas F1 and Frn with facies P and Ss dominating the upper clay rich member suggesting alluvial plain deposits associated with mid fan (Rust, 1984a). This facies association is also similar to medial to distal facies association of alluvial fan of Walker (1 984) and McGowen and Groat (1971). The stratigraphic cross section of this formation has a convex up profile that resembles alluvial fan profile of Blair and McPherson (1994). The radial length of this alluvial fan is assumed to be 12 km and the plunges of channel axes are relatively high (10°-30°) towards south. Palaeoflow expansion angle distinctly varies from 120° to 248° making a variance angle of 128" which may be upto 180" at the maximum. Cross profile relief of lower Dupitila fan is 292 metres at its central part and that of upper Dupitila is 240 metres. Radial slope value ranges from 16° to 20°. An alluvial fan environment during the Dupitila sedimentation (Late Miocene to Middle Pliocene) at the southern piedmont slope of the Shillong plateau is suggested.

Keywords

Depositional environment, Dupitila Formation, Sylhet, Bangladesh.

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Mrinal Kanti Roy ¹, Syed Samsuddin Ahmed ¹, Tapas Kumar Bhattacharjee ¹, Sultan Mahmud ¹, Md. Moniruzzaman ², Md. Masidul Haque ¹, Sudip Saha ¹, Md. Ismail Molla ³, Pulan Chandra Roy ¹

Affiliations
1 Department of Geology and Mining, University of Rajshahi, Rajshahi - 6205, BD
2 Atomic Energy Commission, Savar, Dhaka, BD
3 Gas Seepage Control and Drilling Project, Bangladesh Gas Field Co. Ltd., Bakhrabad, B.Baria, BD

Abstract

The Dupi Tila Formation is composed of yellow to light brown medium to very fine moderately hard to loose sandstone, siltstone, silty clay, mudstone and shale with some conglomerates with clasts of petrified wood. The lithofacies of matrix supported conglomerate, trough cross bedded conglomerate, massive sandstone, trough cross bedded sandstone, planar cross bedded sandstone, ripple cross laminated sandstone-siltstone, flaser laminated sandstone-shale, lenticular laminated sandstone-siltstone-shale, parallel laminated sandstone-siltstone, wavy laminated shale, parallel laminated blue shale, and mudstone are delineated within this formation. Based on the grain size, sedimentary structures, water depth and genesis of individual facies, facies are grouped into three types of facies associations like (i) coarse-grained conglomerate facies association in relation to tractive current deposits of alluvial fan set up at the base of litho-succession (FAC), (ii) medium to fine-grained sandstone-siltstone-mudstone facies association or facies association in relation to strong tide (FAT) characterizing the middle part of litho-succession, (iii) very fine-grained sandstone-siltstone-mudstone facies association in relation to less frequent weak tide or heterolithic facies association (FAHL) characterizing upper part of litho-succession and shallow marine facies association (FASM) composing the uppermost litho-succession. Presence of gluconite indicates that the depositional environment was shallow to deep marine. The dominant paleoflow direction during the deposition of Dupi Tila Formation was toward southeast to southwestern direction. The rivers were of braided type at the piedmont alluvial depositional set up at the lower part, which later changed to estuarine-tidal flat type environmental set up in the middle part to upper part and paleo-environment was shallow marine in the uppermost part.

Keywords

Paleoenvironment, Dupi Tila Formation, Lalmai Hills, Comilla, Bangladesh.

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