Sylhet Basin Research Articles

Risk assessment and zonation of flash flood in Sylhet basin, Northeast Bangladesh using GIS-MCDM tool

Risk assessment and zonation of flash flood in Sylhet basin, Northeast Bangladesh using GIS-MCDM tool

Antecedent Topography and Sediment Dispersal: The Influence of Geologically Instantaneous Events on Basin Fill Patterns

AbstractFavorable topographic gradients and channel bed aggradation are often cited as primers for river channel avulsion. Sylhet Basin in Bangladesh subsides relatively rapidly and presents a topographically favorable channel course for the Brahmaputra River. Holocene occupations of this region remained routed along the basin's western margin, depositing tens of meters of amalgamated channel sands but leaving the central basin under‐filled. The localized backwater effect from a seasonal lake that forms in Sylhet Basin, referred to as a “hydraulic barrier,” has been proposed as a mechanism for keeping the channel pinned to the western margin. Using a 1‐D channel profile model and a 2‐D depth‐averaged hydrodynamic model, we test the preferred flow path between two possible channel courses under a range of physical conditions. Results show that the hydraulic barrier interpretation is implausible unless water depths are increased beyond the physical dimensions of Sylhet Basin. Reducing topographic slope along the two pathways does not sufficiently enhance the local backwater to steer the channel to the basin center. Only the introduction of a scoured antecedent channel along the western margin induces a strong preference for bypass of the central basin. These results corroborate field evidence that Holocene outburst floods scoured a paleochannel that became the preferred course for the Brahmaputra within Sylhet Basin in the mid to late Holocene. We conclude that modification of basin topography by large, infrequent flood events can exert a persistent influence on channel steering that is comparable to previously documented effects of monsoon variability and tectonic deformation.

Soil properties and pollution indices in four designated wetlands in the Sylhet basin of Bangladesh

Soil properties and pollution indices in four designated wetlands in the Sylhet basin of Bangladesh

The historical avulsion of the Tista River, and its relationship to the Brahmaputra: Map and archive evidence from 1750 to 1835

Alluvial fans are constructed when river deposition raises local elevations until the river “avulses” to an alternative lower elevation alignment. Avulsions may be triggered by exogenous events, such as floods that locally overtop the river’s banks; they may then cause extreme flooding on a fan and have major impacts. Rivers draining the Himalayas into the Gangetic plain have behaved historically in this way; for example, the Kosi River’s historical westward migration (1731–1963), and its recent eastward shift (1980). In the major fan‐delta of the Bengal Basin, the Brahmaputra and its tributary, the Tista, have both undergone historical avulsions, with some suggestion that these may have been causally linked. Between the Rennell survey of Bengal (1765–77) and the partial re‐survey by Wilcox and Ommanney (1828–35), the Brahmaputra River substantially changed its course. In Rennell’s time it turned south‐east into the Sylhet Basin leaving the higher land of the Madhupur Tract on its right; by 1830, it flowed due south with the Tract on its left. One hypothesis for the timing and cause of this avulsion relates it to the impact of a major flood and avulsion in the River Tista in 1787. This paper examines the evidence of the Rennell maps, the East India Company archives of correspondence between District Collectors about flooding and avulsion in the Tista, and the diaries of Francis Hamilton Buchanan who travelled along the Tista in 1807–11. From these contemporary sources it is possible to re‐construct the changes in the River Tista in the late 18th century, and to conclude that its avulsion had little direct influence on that of the Brahmaputra, although it went through some radical rearrangements of its own.

Estimation of Carbon Stock in the Sylhet Basin Soils of Bangladesh

Wetland basin soils are the major store houses of organic carbon where there is a scope to use this carbon in mitigating the climate change. A study was conducted in these basin soils at 100 cm depth regarding their carbon stock. The study showed that total soil organic carbon (SOC) stock in the Sylhet basin soils of Bangladesh is 0.094 Pg where the SOC stock was 0.044 Pg in medium low land sites and it was about 0.050 Pg in lowland sites. There was no previous study on SOC stock in the Sylhet basin soils of Bangladesh. These may act as benchmark SOC stock datasets for the future agricultural planning. The soil organic carbon stock is higher in the lowland than medium lowland sites. The contents of SOC are low is compared to its threshold levels. Moreover, it is apprehended that basin soils may lose their carbon due to the decrease of inundation level by climate change, and other eco-environmental changes. So, it is very much urgent to take steps in preserving the organic carbon of lowland basin soils.
 Asiat. Soc. Bangladesh, Sci. 46(1): 49-60, June 2020

Assessment of soil carbon storage and nutrient contents in some wetlands soils of the northeastern region of Bangladesh

An investigation was carried out in four designated wetlands to assess soil organic carbon (SOC) storage and evaluate soil nutrients of the northeastern Sylhet basin of Bangladesh. SOC storage was the highest in the Nikli wetland (4.1 Tg), followed by Hakaluki (4.0 Tg), Hail (2.8 Tg) and Balai wetland soils (2.6 Tg) at 100 cm depths. It is found that the total soil C storage across the medium low land (MLL) and low land (LL) sites covering the four wetlands of the Sylhet basin is about 13.5Tg. C storage across the MLL and LL sites at 100 cm depths was estimated about 5.1Tg and 8.4Tg respectively. It is found that SOC storage was higher in the low land sites in contrast to medium low land sites. The soil property varies depending on land types, soil depths and spatial distributions. Among the investigated wetland soils, Hakaluki wetland stored higher amount of SOC in the deeper soil layers whereas an inverse relationship between soil depth and SOC storage was noted for rest of the wetlands. It is apprehended that SOC storage thus gradually lessening in greater magnitude due to climate change and other anthropogenic reasons. An integrated management approach should be developed to restore the SOC sink.
 Dhaka Univ. J. Biol. Sci. 30(1): 115-124, 2021 (January)

A Review on Hydrocarbon Prospectivity in Chittagong Hill Tracts and Adjacent Area

Bangladesh is a country with limited sources of energy and since 1990 natural gas has been its main source of energy. Most of the exploration approaches had been conducted onshore especially in the central and eastern part of Bangladesh, particularly in northeastern Sylhet basin. Among the hydrocarbon provinces, the East Delta Hill Tract province is an under explored petroleum province in Bangladesh. An exploratory well drilled in Sitakund anticline was found dry but no reasonable cause was perceived why that well went dry. Although many works had been carried out in Chittagong Hill Tracts but none of them was cumulative and descriptive. In this study, the overall hydrocarbon prospect of the Chittagong Hill Tracts was analyzed by mapping of potential zones on the basis of the evaluation of regional structure and construction of lithocolumn of the prospective zones. The five elements of the petroleum system discussed thoroughly to find overall petroleum prospect of the study area. Source rocks of Chittagong Hill Tracts are mainly Bhuban shale, reservoir rock is sandstone from Bhuban-Bokabil formation, the way of migration path is both through longitudinal and cross fault. The data of source rock and seal is collected from previous researches. Multiple types of traps have been found there. Conventional anticlinal traps which are highly disturbed due to tectonic instability & the core part are shale diapirism. Most of the anticlines are plunging and the nose or plunge area might be prospective to HC for being comparatively less faulted. Broad synclinal areas between tight narrow anticlines are another prospective area for HC. These similar types of synclines are also found in Tripura, India which is a highly prospective area for petroleum and their small anticlinal hums within syncline are also prospective. Some stratigraphic traps have also been found in Tripura from where production has been started already. Both Chittagong Hill Tracts and Tripura Fold Belts are parts of great Arakan Fold Belts, so similar type of structures might be prospective here. Apart from these, Bangladesh is a deltaic country. So stratigraphic trapment like channel sand, pinch-outs is possible. Considering all the elements of petroleum prospectivity of the area and factors discussed above, it is quite clear that Chittagong hill tracts might be the next target for HC exploration program.

Origin and Accumulation Mechanism of Gas Condensate in Kailashtila Gas Field, Sylhet Basin, Bangladesh

The Kailashtila gas field (KGF) is situated in the northeastern part of Sylhet basin, Bangladesh. This paperpresents chemical characteristics of extractable natural gas in drilled well KTL-2, in order to examine their potentialsource and maturity of organic matter, and hydrocarbon accumulation mechanism in the basin. The gas condensate inthe KTL-2 composed primarily of methane (85.81 wt.%), ethane (6.68 wt.%), propane (2.13 wt.%), and traces of higherhydrocarbons (i-butane, 0.69 wt.%; n-butane, 0.73 wt.%; i-pentane, 0.50 wt.%; n-pentane, 0.44 wt.%; hexane, 1.27wt.%; heptane, 0.99 wt.%; octane, 0.24 wt.%). Nitrogen and CO2 contents in the gas condensate are low (0.46 wt.%and 0.05 wt.%, respectively). Average dry coefficient (C1/C1–5) value in the gas condensate is 0.93 (0.91–0.95), whichreflects relatively mature hydrocarbon migrating from nearby deeply buried source rocks. The δ13C1 (–39 to –40‰) andC1/C(2+3) (19.77) variation diagram show that gas condensate in the KGF is mainly controlled by type III kerogen, andthe organic matter was thermally mature in nature. However, the relationships between stable isotope value of methane(δ13C1), ethane (δ13C2) and propane (δ13C3) indicate mainly thermogenic origin of the studied gas condensate, andminor input from mixed thermogenic and bacteriogenic processes.

Origin and Accumulation Mechanism of Gas Condensate in Kailashtila Gas Field, Sylhet Basin, Bangladesh

The Kailashtila gas field (KGF) is situated in the northeastern part of Sylhet basin, Bangladesh. This paperpresents chemical characteristics of extractable natural gas in drilled well KTL-2, in order to examine their potentialsource and maturity of organic matter, and hydrocarbon accumulation mechanism in the basin. The gas condensate inthe KTL-2 composed primarily of methane (85.81 wt.%), ethane (6.68 wt.%), propane (2.13 wt.%), and traces of higherhydrocarbons (i-butane, 0.69 wt.%; n-butane, 0.73 wt.%; i-pentane, 0.50 wt.%; n-pentane, 0.44 wt.%; hexane, 1.27wt.%; heptane, 0.99 wt.%; octane, 0.24 wt.%). Nitrogen and CO2 contents in the gas condensate are low (0.46 wt.%and 0.05 wt.%, respectively). Average dry coefficient (C1/C1–5) value in the gas condensate is 0.93 (0.91–0.95), whichreflects relatively mature hydrocarbon migrating from nearby deeply buried source rocks. The δ13C1 (–39 to –40‰) andC1/C(2+3) (19.77) variation diagram show that gas condensate in the KGF is mainly controlled by type III kerogen, andthe organic matter was thermally mature in nature. However, the relationships between stable isotope value of methane(δ13C1), ethane (δ13C2) and propane (δ13C3) indicate mainly thermogenic origin of the studied gas condensate, andminor input from mixed thermogenic and bacteriogenic processes.

An Empirical Investigation on Hydro-Morphological Process of Surma River: Substantiation from North-East Zone of Bangladesh

Objective of the study is to know the inherent morphological process of the river system in the Haor areas of sylhet basin. The specific objective is to “validate the existing conceptual Model of the CEGIS”. This research for the validation of the CEGIS Model. Primary data was collected through routine measurements of discharge, sediment concentration, measurements of cross-sections, sediment concentration measurements. Secondary data was collected on water level, discharge, velocity and cross-section from the BWDB. Satellite images have been collected from USGS. A thorough review of the manuals of different numerical models were carried out. After evaluation of the models HEC-RAS 5.0.3 has been selected. Validation of the CEGIS conceptual Model was tried using both conventional way of data analysis as well as from model output. Both the models have been fine-tuned and simulated to predict the future scenarios with 20% increase of discharge as well as 20% decrease of discharge at the upstream. Findings of the study confirms the acceptability of Hypothesis 1, Hypothesis 2 could not be (conclusively) validated. Concluded that, the bankfull water levels at the downstream decrease, changes in channel dimension, the change of both the area and the top width shows a scattered pattern and change of average depth shows a decreasing trend towards downstream direction. (i) Conventional analysis, Hypotheses 3 may be considered validated (ii) From Model output, it may be stated that the Hypothesis 3 may be considered as not validated. Hypotheses 4 and 5 relate to the hypothetical ‘Regime Condition’ of the river. Its clear that the Surma River is not in ‘Regime Condition’. So the hypothesis could not be validated through the model output. But ‘Regime Condition’ is a theoretical condition of a river, the validity of these two hypotheses (4 and 5) can be accepted on Theoretical explanation basis.

Sedimentology and geochemistry of Middle Eocene-Lower Oligocene sandstones from the western Salin Sub-Basin, the Central Myanmar Basin: Implications for provenance, source area weathering, paleo-oxidation and paleo-tectonic setting

The Salin Sub-Basin is a well-known Cenozoic hydrocarbon-rich basin in central Myanmar, and contains a succession of siliciclastic sedimentary rocks. The analytical results of petrographic and whole-rock geochemical data indicate that the sandstone sequences in the Salin Sub-Basin were derived mainly from at least three different provenances: the Western Myanmar Arc, the Myanmar terrane basement, and the westerly Indo-Myanmar (Burman) Ranges (IMR), where sediments were subjected to initial-intermediate weathering conditions and rapid erosions, and were finally transported to the studied area. Based on the comparison of the geochemical variations between the Salin Sub-Basin in Myanmar and Sylhet Basin in Bangladesh, the provenance terranes of these two sedimentary basins were found to be significantly different from each other. This is most likely due to the rising of IMR and the IMR probably acts as the barrier between the Salin Sub-Basin in the East and Sylhet Basin in the west, and prevents influences of Myanmar sediment sources on the Sylhet Basin. As a result of major and trace-elementtectonic setting discrimination diagrams and geochemical parameters, sedimentation in the studied area was likely to occur at the Continental Island Arc (CIA) setting under a well- oxygenated condition. It was most probably formed under the site of a fore-arc setting associated with the India-Asia convergence and collisional processes.

Coupling Mass Extraction and Downstream Fining With Fluvial Facies Changes Across the Sylhet Basin of the Ganges‐Brahmaputra‐Meghna Delta

Quantitative interrogation of grain sizes in sedimentary systems has the potential to improve predictions of stratigraphic architecture, facies distributions, and downstream reservoir characteristics. To quantify these relationships, downstream fining data are coupled with rates of mass extraction, with input grain‐size distribution, accommodation, and sediment input from multiple transport pathways providing primary controls on resulting sediment dispersal patterns. We spatially apportioned mass distribution along three sediment delivery pathways with distinct accommodation characteristics within the Ganges‐Brahmaputra‐Meghna Delta to calculate chi (χ), the total fraction of supplied sediment flux lost to deposition at any given point. Low rates of downstream fining and sand‐rich channel facies characterize a bypass‐dominant pathway along the western margin of Sylhet basin, whereas two splay deposits that prograde into the underfilled basin interior exhibit higher rates of fining and preservation of mud‐rich facies. Both splay deposits show a shift from sand‐dominated to mixed sand and mud facies and increased mud preservation (above 30%) at a χ value of ∼0.8. No comparable increase in mud preservation occurs along the bypass‐dominated pathway, suggesting that this course operated in an inherently different extraction mode due to limited mid‐Holocene accommodation. A similarity solution model effectively reproduces most of the spatial patterns of mass extraction observed in Sylhet basin, except in one location receiving lateral sediment input from a distributary channel. These field and modeling results indicate that grain‐size data and sediment volume measurements can be used to not only reconstruct paleodynamics of transport networks and resulting stratigraphy but also lead to predictive insights on subsurface heterogeneity, and thus improved reservoir and aquifer characterization.

Identifying multiple deep aquifers in the Bengal Basin: Implications for resource management

AbstractIn the Bengal Basin of Bangladesh and West Bengal (India), where arsenic (As) and salinity adversely affect groundwater in shallow aquifers (<150 m deep), consumers increasingly turn to “the deep aquifer” as a source of low‐As and low‐salinity water. We show that “the deep aquifer,” which has traditionally been regarded as a single entity, can be divided into at least eight deep aquifer units each with distinctive chemical, isotopic, hydraulic, and piezometric characteristics. Except in the rapidly subsiding Sylhet Basin, the deep aquifers are largely As‐safe (<50 μg/L and mostly <10 μg/L) but differ in terms of iron (Fe), manganese (Mn), salinity, stable isotopic composition, groundwater age, and in their piezometric response to pumping. We use isotopic data to identify recharge from five periods, of which two are quantitatively dominant: (a) from the Last Glacial Maximum, isotopically heavy water (δ18O >−3.5‰) with low As, Cl, and Fe concentrations occupies hydraulically trapped positions in the South‐Central, Southeast, Southwest, and Madhupur aquifer units and (b) terminal Pleistocene to Early Holocene recharge of isotopically lighter water (δ18O <−3.5‰) that has high Fe and slightly elevated Cl concentrations and was recharged along palaeochannels that facilitated deep circulation up to the mid‐Holocene. Piezometric responses to development change in style towards the coast. In the north, water levels at all depths tend to be coincident and transition through a seasonally divergent pattern to become parallel in the south with a near constant downward gradient. The delineation of the deep aquifer units provides a practical framework for the assessment and management of groundwater resources for water supply.

Holocene Brahmaputra River path selection and variable sediment bypass as indicators of fluctuating hydrologic and climate conditions in Sylhet Basin, Bangladesh

AbstractThe Holocene stratigraphy of Sylhet basin, a tectonically influenced sub‐basin within the Ganges‐Brahmaputra‐Meghna delta (GMBD), provides evidence for autogenic and allogenic controls on fluvial system behaviour. Using Holocene lithology and stratigraphic architecture from a dense borehole network, patterns of bypass‐dominated and extraction‐enhanced modes of sediment transport and deposition have been reconstructed. During a ~3‐kyr mid‐Holocene occupation of Sylhet basin by the Brahmaputra River, water and sediment were initially (~7.5–6.0 ka) routed along the basin's western margin, where limited downstream facies changes reflect minimal mass extraction and bypass‐dominated transport to the basin outlet. Sediment‐dispersal patterns became increasingly depositional ~6.0–5.5 ka with the activation of a large (~2250 km2) splay that prograded towards the basin centre while maintaining continued bypass along the western pathway. Beginning ~5.0 ka, a second splay system constructed an even larger (~3800 km2) lobe into the most distal portions of the basin along the Shillong foredeep. This evolution from a bypass‐dominated system to one of enhanced mass extraction is well reflected in (i) the rapid downstream fining of deposited sand and (ii) a change in facies from amalgamated channel deposits to mixed sands and muds within discrete depositional lobes. The persistence of sediment bypass suggests that seasonal flooding of the basin by local runoff exerts a hydrologic barrier to overbank flow and is thus a principal control on river path selection. This control is evidenced by (i) repeated, long‐term preference for occupying a course along the basin margin rather than a steeper path to the basin centre and (ii) the persistence of an under‐filled, topographically low basin despite sediment load sufficient to fill the basin within a few hundred years. The progradation of two 10–20‐m thick, sandy mega‐splays into the basin interior reflects an alternative mode of sediment dispersal that appears to have operated only in the mid‐Holocene (~6.0–4.0 ka) during a regional weakening of the summer monsoon. The reduced water budget at that time would have lowered seasonal water levels in the basin, temporarily lessening the hydrologic barrier effect and facilitating splay development into the basin interior. Overall, these results place basin hydrology as a first‐order control on fluvial system behaviour, strongly modifying the perceived dominance of tectonic subsidence. Such coupling of subsidence, fluvial dynamics and local hydrology have been explored through tank experiments and modelling; this field study demonstrates that complex, emergent behaviours can also scale to the world's largest fluvial system.

Implementation of Carbon Dioxide Gas Injection Method for Gas Recovery at Rashidpur Gas Field, Bangladesh

Natural gas plays an important role for the economic development of Bangladesh. It is the primary options to satisfy the environmentally clean energy, whereas coal is a dirty energy source and oil creates an unhealthy environment. Bangladesh is the seventh-largest producer of natural gas in Asia. Gas supplies meet 56% of domestic energy demand. The proven natural gas reserve in Bangladesh is only 19.73 Tcf. The Rashidpur Gas Field (RGF) is located in the Sylhet Basin, Northeast Bangladesh. It is 35 km long and 7 km anticlinal structure and asymmetric in nature with steeper eastern flank (22˚ to 25˚) and gentler western flank (8˚ to 12˚). There are two gas zones in depth between 1380m to 2787m below surface. Sandstone reservoirs of Miocene-Pliocene age and are considered to have been originated shallow marine depositional environment. The reservoir porosity-permeability values are very good, with estimated gas initially in place (GIIP) of the RGF was 2.242 Tcf with 58% recovery, thus recording an initial gas reserve is 1.309 Tcf. Five gas producing wells (RP-1, 3, 4, 6 and 7) in the RGF are producing 50 MMscf gas per day. Due to the demand of natural gas with decreasing production rate, this enhanced natural gas plays a vital role in the national economy of the country. This research depicts the development of the daily production of the RGF from 50 MMscfd to 99 MMscfd using software from the existing production wells. Thus the natural gas in the RGF would be enhanced/recovered using carbon dioxide (CO2) gas injection by Enhanced Gas Recovery (EGR) method from the RGF reservoir. Applying this method would play a vital role to increase the daily production rate of the RGF.

Reproductive Biology of Great Snakehead Channa marulius from Sylhet Basin in the North East Bangladesh

Reproductive Biology of Great Snakehead Channa marulius from Sylhet Basin in the North East Bangladesh

Petrography and whole-rock geochemistry of the Tertiary Sylhet succession, northeastern Bengal Basin, Bangladesh: Provenance and source area weathering

Detrital modes and the geochemical compositions of sandstones and mudrocks from the Paleocene to Plio-Pleistocene Jaintia, Barail, Surma, Tipam and Dupitila Groups of the Sylhet basin in Bangladesh have been examined to identify their provenance and source area weathering conditions. Barail and Surma Group sandstones are quartz-rich, feldspar-poor, and are rich in sedimentary and metamorphic lithic fragments, indicating a recycled orogen source. Geochemically the succession is divisible into three parts. Paleocene–Late Eocene Jaintia Group mudrocks form separate trends, and have very high Chemical Index of Alteration (CIA) values, suggesting derivation from a stable cratonic source. The second part, the Late Eocene to Miocene Barail and Surma Groups, forms single trends on variation diagrams. Maximum CIA ratios decrease from the Barail to the Surma Group, suggesting rapid influx of detritus from a common felsic source undergoing active uplift, compatible with weathering and erosion proxies from the Bengal and Indus fans. Average bulk geochemical and REE characteristics of these two groups are identical, as are A–CN–K trends. These features and comparison with potential Himalayan sources support derivation of both the Barail and Surma Groups from that orogen, and hence supply of Himalayan detritus to the Sylhet basin as early as the Late Eocene, rather than from the Neogene. The third part, the non-marine Tipam (Late Miocene–Pliocene) and Dupitila (Pliocene–Pleistocene) Groups, forms slightly differing geochemical trends and has higher maximum CIA values. REE fractionation between sandstones and mudrocks is more marked, but REE patterns and parameters remain unchanged, suggesting that the overall source remained unchanged. Higher CIA and greater REE fractionation in the Tipam and Dupitila Groups may be a product of their fluvial deposition and more intense weathering on the floodplain during intensification of the South Asian monsoon from 4 Ma onwards.

Influence of organic matter type on the distribution of tri-aromatic hydrocarbons in Tertiary mudstones in the Sylhet Basin, Bangladesh

The distribution and behavior of tri-aromatic hydrocarbons (especially alkylphenanthrenes) have been investigated in twenty-four mudstones from the Tertiary succession in the Sylhet Basin of Bangladesh. Phenanthrene (P) and methylphenanthrenes (MP) were abundant in most samples, whereas anthracene and methylanthracene (MA) were detected only in some samples. Abundances of the 1,7-dimethylphenanthrene (DMP) isomer are signifi cant relative to other DMP isomers. Overall correlation between the methylphenanthrene index 3 (MPI 3) and Tmax (426 – 449 °C) is poor in this basin. Abundances of MA, pimanthrene (1,7-DMP) and 1-MP are relatively high in the lower part of the succession. Higher plant organic matter is abundant in the lower part of the succession (middle Eocene to early Miocene), whereas planktonic organic matter is relatively abundant in the middle and upper parts (middle Miocene to Pleistocene). These alkyl-isomers thus originated from terrigenous sources. High phenanthrene/alkylphenanthrene ratios in the mudstones can be related to oxic environmental conditions. Based on detailed aromatic distribution patterns and their isomer ratios, we conclude that the compositions of P, MP and DMP isomers in the Sylhet Basin were probably controlled by organic matter sources, rather than by thermal maturity in the early stage of the oil window. * Department of Geoscience, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan a Corresponding author. Tel: +81-852-32-6453; Fax: +81-852-32-6469. e-mail: sampei@riko.shimane-u.ac.jp (Y. Sampei).

Characterization of organic matter and depositional environment of Tertiary mudstones from the Sylhet Basin, Bangladesh

The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene–late Eocene), Barail (late Eocene–early Miocene), Surma (middle–late Miocene), Tipam (late Miocene–Pliocene) and Dupitila Groups (Pliocene–Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya. The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C 30 hopane, n-C 29/ n-C 19 alkane, Tm/Ts [17α(H)-22,29,30-trisnorhopane/18α(H)-22,29,30-trisnorhopane] and sterane C 29/(C 27 + C 28 + C 29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. T max values of ca. 450 °C, vitrinite reflectance ( R o) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area.

Mineralogy of the Ganges and Brahmaputra Rivers: implications for river switching and Late Quaternary climate change

Late Quaternary sediments of the Bengal basin contain a history of river switching and climate change as revealed from sand- and clay-size mineralogy of boreholes and modern riverbed grabs. Epidote to garnet ratios (E/G) in sand fraction sediments are diagnostic of source, with high (>1) E/G indicating Brahmaputra provenance and low (<1) E/G indicating Ganges provenance. In the clay fraction of surficial sediments smectite is diagnostic, with high values (∼39%) in the Ganges and low values (∼3%) in the Brahmaputra. In contrast, the Brahmaputra contains more kaolinite (29% vs. 18%), more illite (63% vs. 41%), and more chlorite (3% vs. 1%) than the Ganges. Analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the Holocene. Extended periods of mixed river inputs appear to be isolated to the Early Holocene, suggesting rapidly migrating braided channels during sea level lowstand. Tectonically driven accommodation in the Sylhet Basin may have contributed to the favored easterly course of the Brahmaputra during much of the Holocene. Relative abundances of illite and chlorite (IC) vs. smectite and kaolinite (SK) record varying degrees of physical and chemical weathering, respectively. High IC values in early post-glacial deposits suggest a dominance of physical weathering at that time. However, a general increase in SK concentrations throughout the Holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions. Notably, a peak in SK concentrations that corresponds to an Early Holocene warm period (∼10–7 ka) suggests that weathering patterns in the catchment respond quickly to climatic shifts.