Bengal Shelf Research Articles

Barcoding, structural, and genetic variation of morphologically adopted polychaetes (Annelida: Paronidae, Lumberineridae, Spionidae) using mitochondrial COI gene sequences from the Bay of Bengal shelf including Oxygen Minimum Zone

The structural and genetic variation of morphologically adapted polychaetes from the Bay of Bengal continental shelf (North Indian Ocean) including Oxygen Minimum Zone (OMZ) are described. The density and biomass of polychaetes such as Aricidea curviseta (Paronidae), Lumberineris aberrans (Lumberineridae), Prionospio malmgreni, Paraprionospio pinnata, and Prinospio sexoculata (Spionidae) were significantly differed (P<0.05) in depths (Non-Oxygen Minimum Zone - NOMZ and OMZ), transects and cruises. The response and morphometric measures of branchiae structures were inconsistent between OMZ and NOMZ depths. The species Aricidea curviseta (OR450003 - OR450005), Paraprionospio pinnata (OR436939, OR436941, OR436952 and OR436953), and Prionospio malmgreni (OR436956, OR436957, OR436945, OR436946, OR436947) were barcoded for the first time. The Maximum likelihood tree analysis indicated low ranges of similarity indices among the NOMZ (0.255–0.446 %) and OMZ (0.257–0.443) depths. The total number of mutations and polymorphic sites were 395 and 268, with low haplotypes (12) and nucleotide diversity (among depths: 0.228–0.246, and regions: 0.826–0.100). The groups of haplotypes were also independent of the geographical locations. The AMOVA revealed that the variation among populations was low (regions: −5 %, and depths: −1.86 %) and within populations were high (regions:105.45 % and depths: 101.86 %, respectively). The nMDS indicated that the oxygen factor significantly separated the distribution of density and biomass of polychaetes within transects (density: biomass: F=3.57) and cruises (biomass: F=2.41, P<0.05). Generally, 70–80 % of the total variation was described in four pCCAs by salinity (PSU), pressure, Co, Cu, Fe, Mn, Ni, Pb, Zn, Hg (ppm), and clay (%) with NOMZ and OMZ depths, different cruises, transects and or the interaction between this three. OMZs might not influence the genetic variations. Several additional studies, such as physiological, metabolic, enzymatic, and biochemical, are also be essential for understanding the ecology and evolution of organisms in this habitat.

Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata.

Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt

This article illustrates a multi-technique frontier approach for the provenance study of silt-size sediments. The mineralogical composition of low-density and heavy-mineral fractions of four samples of fine to very coarse silt deposited on the Bengal shelf was analyzed separately for six different grain-size classes by combining grain counting under an optical microscope, Raman spectroscopy, and X-ray diffraction. The geochemical composition was determined on both bulk-sediment samples and on their &lt;5-μm classes. Such a “multiple-window” approach allowed capturing the full mineralogical information contained in each sample, as well as the size-dependent intra-sample variability of all compositional parameters. The comparison between grain-size distributions obtained by different methods highlighted a notable fallacy of laser granulometry, which markedly overestimated the size of the finest mode represented by fine silt and clay. As a test case, we chose to investigate sediments of the Bengal shelf, where detritus is fed from the Meghna estuary, formed by the joint Ganga and Brahmaputra Rivers and representing the largest single entry point of sediment in the world’s oceans. The studied samples show the typical fingerprint of orogenic detritus produced by focused erosion of collision orogens. Bengal shelf silt is characterized by a feldspatho-quartzose (F-Q) composition with a Q/F ratio decreasing from 3.0 to 1.7 with increasing grain size, plagioclase prevailing over K-feldspar, and rich transparent-heavy-mineral assemblages including mainly amphibole with epidote, and minor garnet and pyroxene. Such a detrital signature compares very closely with Brahmaputra suspended load, but mineralogical and geochemical parameters, including the anomalous decrease of the Q/F ratio with increasing grain size, consistently indicate more significant Ganga contribution for cohesive fine silt. The accurate quantitative characterization of different size fractions of Bengal shelf sediments represents an essential step to allow comparison of compositional signatures characterizing different segments of this huge source-to-sink system, from fluvial and deltaic sediments of the Himalayan foreland basin and Bengal shelf to the Bengal Fan.

Delineation of Subtle and Obscure Structures in West Bengal Shelf:A Remote Sensing and GIS-Based Parallel Approach

Delineation of Subtle and Obscure Structures in West Bengal Shelf:A Remote Sensing and GIS-Based Parallel Approach

Trophic diversity, size and biomass spectrum of Bay of Bengal nematodes: A study case on depth and latitudinal patterns

Depth and latitudinal patterns of nematode functional attributes were investigated from 35 stations of Bay of Bengal (BoB) continental shelf. We aim to address whether depth and latitudinal variations can modify nematode community structure and their functional attributes (trophic diversity, size and biomass spectra). Global trend of depth and latitudinal related variations have also been noticed from BoB shelf in terms of nematode abundance and species richness, albeit heterogeneity patterns were encountered in functional attributes. Index of trophic diversity values revealed higher trophic diversity across the BoB shelf and suggested variety of food resource availability. However, downstream analysis of trophic status showed depth and latitude specific patterns but not reflected in terms of size and biomass spectrum. The peaks at different positions clearly visualized heterogeneity in distribution patterns for both size and biomass spectrum and also there was evidence of availability of diversified food resources. Nematode biomass spectra (NBS) constructed for nematode communities showed shift in peak biomass values towards lower to moderate size classes particularly in shallower depth but did not get reflected in latitudes. However, Chennai and Parangipettai transects demonstrated shift in peak biomass values towards higher biomass classes explaining the representation of higher nematode abundance. Our findings concluded that depth and latitudes are physical variables; they may not directly affect nematode community structure and functional attributes but they might influence the other factors such as food availability, sediment deposition and settlement rate. Our observations suggest that the local factors (seasonal character) of phytodetrital food flux can be very important for shaping the nematode community structure and success of nematode functional heterogeneity patterns across the Bay of Bengal shelf.

Provenance of the Late Quaternary sediments in the Andaman Sea: Implications for monsoon variability and ocean circulation

We present a geochemical and Sr-Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo-Burman-Arakan (IBA) mountain ranges and the Andaman Islands, and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30–60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11–14, 20–23, 36, 45, 53, 57, and 62 ka) of which five are synchronous with cooling of the northern hemisphere, suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southward from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.

Mass failures associated with the passage of a large tropical cyclone over the Swatch of No Ground submarine canyon (Bay of Bengal)

Acoustic surveys collected six months before and after the passing of a major tropical cyclone over the Swatch of No Ground submarine canyon (Bay of Bengal) revealed the formation of widespread mass failures 30 km offshore of the Ganges-Brahmaputra river delta. Mass sediment flows several kilometers wide with minimal run-out distances ( 1 km wide × 5–60 m thick) developed in areas with preexisting subsurface sediment deformation and evidence for active submarine fluid discharge. In contrast, narrow, steep-walled (7°–10°) gullies present before the storm did not fail, suggesting that the gully walls may be shear hardened by the preferential funneling of gravity flows from the Bengal shelf. Combined, the widespread mass failures and gullies are part of a rapidly accreting (5–50 cm/yr), net-aggradational canyon system that supports multiple mechanisms for sediment transport from the active river mouth to the canyon.

Forced regression systems tracts on the Bengal Shelf

A vertical succession of five composite sequences has been identified within the upper 100 m of the outer Bengal Shelf by means of high-resolution multi-channel seismic data. Each sequence consists predominantly of up to 100 km long and some 10 m thick forced regression systems tracts. The internal reflection pattern of the regressive units show mainly prograding oblique clinoforms. Intervening transgressive systems tracts are represented by seismically transparent or chaotic layers. On the outer shelf three of the sequences cause shelf aggradation and retrogradation, and two of them cause mainly shelf progradation. Based on the hierarchy of systems tracts, their calibration by comparison with eustatic sea-level curves and reconstructed paleoshoreline positions the composite sequences are interpreted as eccentricity driven eustatic 4th order (Milankovitch) cycles with a periodicity of about 100 ky. Internal unconformities mark cycles of 5th or higher order. An average subsidence of the outer shelf is estimated to be less than 0.4 mm/year during the last 345 ky. The correlation between the shelf growth pattern and sea-level fluctuations is consistent with the enhanced deposition on the eastern Bengal submarine fan from 465 to 125 ky B.P., as was observed by other authors.

Sediment transport in the shelf canyon “Swatch of No Ground” (Bay of Bengal)

The Swatch of No Ground is a shelf canyon that deeply incises the Bengal shelf near the Ganges–Brahmaputra river mouth, cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and Bengal fan. Sedimentation rates in the Swatch of No Ground are highest near the canyon head at ∼50 cm a−1, decreasing to ∼15 cm a−1 in 600 m water depth. The canyon deposits consist of intercalated fine (silt–clay) and coarse (silt–sand) grained deposits. In seismic profiles, small-scale sedimentary structures and parallel-bedded layers reveal that sediment in the canyon is mainly deposited from suspension. During fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon, forming fine-grained silt–clay layers. During storm conditions, sediment is resuspended on the inner shelf and subaqueous delta east of the canyon. Storm-generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt–sand layers. Channels and gullies in the Swatch of No Ground indicate active gravity-driven currents with an erosional character in the upper canyon and non-depositional character below ∼450 m, suggesting that persistent or ephemeral currents presently export sediment to the Bengal fan. Numerous slumps and slides observed in the canyon show small-scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin. These mudflows move downslope and halt where the canyon gradient decreases. One widespread acoustically transparent layer with an age of approximately 140–160 years BP can be traced throughout much of the canyon and is probably the result of large-scale sediment remobilization triggered by a catastrophic event like an earthquake. This may represent decennial-to-centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling.

Acoustic facies and depositional processes in the upper submarine canyon Swatch of No Ground (Bay of Bengal)

The Swatch of No Ground (SoNG) is a canyon that incises the Bengal shelf deeply over more than 100 km length landward of the shelf edge and ∼100 km west of the Ganges–Brahmaputra–Meghna river mouth. It acts as conduit for the fluvially discharged sediment that bypasses the shelf and reaches the deep sea fan. What transport, deposition and mass-wasting processes contribute to the sediment export, and what sediment structures and main pathways are associated with these processes is studied by closely spaced bathymetric HYDROSWEEP swath sounder and PARASOUND sediment echosounder profiles over the upper SoNG. An analogue side-scan sonar profile completes the acoustic data set. From the PARASOUND sediment echosounder data three main acoustic facies types related to undisturbed normal deposition, mass wasting and erosive transport are defined and mapped with respect to the HYDROSWEEP bathymetry of the SoNG. Generally, nearly all facies types show an unusually high sediment echosounder signal penetration of ∼80–100 m, indicating that unconsolidated, high-porosity sediments occur on the canyon floor, flanks and banks. Additionally, the acoustic facies distribution map yields that about half of the canyon floor is covered by undisturbed, well-stratified fine-grained, muddy and coarse grained, graded beds. They are deposited from a suspension plume transported to the canyon either during fine weather conditions by westward oriented tidal currents or by storm events that mobilize sediments on the inner shelf. Curved or inclined faults or growth faults are found at all steep flanks, indicating that active mass-wasting processes occur along all channels and gullies on both canyon flanks. Such mass-wasting processes are probably initiated by severe storms, earthquakes, high sediment load or excess pore pressure and lead to slides, slumps or mudflows. They stop in regions with low inclination angle and form acoustically transparent deposits of various extents on the channel and gully floors and on the southwestern canyon thalweg. Current marks identified on the canyon floor by high backscatter amplitudes in the side-scan sonar images indicate that low-density turbidity currents episodically flow down the canyon. They are generated in an area of very high sedimentation rates close to the canyon head by high-energy conditions like severe storms or earthquakes. Additionally, a small gravity-driven flow along the main channel(s) obviously continuously erode the steeper northeastern branches and inhibit sediment deposition in the more gentle southwestern channel below ∼450–500 m water depth. Together with larger-scale mudflows and low-density turbidity currents this gravity driven flow contributes to an active growth of the channel levee system in the deep-sea fan.

Sedimentology of the Bengal shelf, Bangladesh: comparison of late Miocene sediments, Sitakund anticline, with the modern, tidally dominated shelf

Characterisation of latest Miocene facies of the Bay of Bengal reveals a range of tidal and fluvially dominated sediments that represent sedimentation on a broad, shallow shelf that was strongly influenced by tidal and seasonal cyclicity. These tidally influenced sediments are excellently exposed within growth anticlines of the westward-migrating Chittagong–Tripura Fold Belt (CTFB), and permit detailed study of the late Miocene Bengal shelf for comparison with the modern subaqueous deltaic sediments. At the outcrop scale, facies associations record a subtidal shelf, migrating intertidal/subtidal bedforms, tidal and incised channels. The subtidal shelf comprises extensive sand sheets interbedded with thick clay horizons, while the migrating intertidal/subtidal bedforms exhibit a decrease in bedform size from large dunes to small ripple-scale cross-stratification that reflect a decrease in current velocity and sand/silt grade sediment supply as the bedforms migrated. Rootlet horizons within these intertidal/subtidal bedforms may represent a lower delta plain environment. The tidal channel facies consists of metre-scale sandstone beds that are rich in locally derived clay clasts, with up to 0.5 m of incision into a rhizolith horizon below. Incised channel facies records a coarse, pebbly sandstone with clasts derived from a reworked clastic source, with metre-scale scour at the base of this facies association into laminated and rhizolith clays. A 534-m one- and two-dimensional section within the Sitakund anticline, north of Chittagong, is characterised by 20–100-m thick, tidally dominated, progradational parasequences. Mudstones with isolated sand lenses and abundant trace fossils of the Cruziana ichnofacies, coarsen upwards to fine–medium-grained sandstones deposited in shallower, high-energy environments. These sands were deposited under both uni-directional and bi-directional currents resulting in dune-scale cross-stratification, and are overlain by 2–5-cm thick clay drapes, considered to be of a thickness greater than that typical of deposition from a single slackwater episode. The main controls on sedimentation are the seasonally influenced river and sediment discharge onto the shelf from the Himalayan source region, combined with the influence of daily, spring–neap tides, channel and bedform migration and minimal wave action over the shallow shelf. The modern Bay of Bengal shelf provides an analogue for interpreting processes recorded within the Miocene succession, allowing construction of a depositional model for the late Miocene shelf of the Brahmaputra/Ganges delta system, with sedimentation dominated by tidal and seasonal cyclicity.

Absolute Dating of Recent Sediments in the Cyclone-influenced Shelf Area Off Bangladesh: Comparison of Gamma Spectrometric (137Cs, 210Pb, 228Ra), Radiocarbon, and 32Si Ages

A geochronological survey of the Bengal shelf area involved results from more than 20 sediment cores dated using gamma spectrometry and the nuclides 137Cs, 228Ra, 226Ra, and 210Pb. In some cores, which contained older sediments, 32Si and 14C were determined to examine the possibility to extrapolate the obtained chronologies to century and millennial scale. Geochronological work in this region is faced with problems of cyclone-induced sediment reworking, grain-size effects on fallout nuclides, scarcity of carbonates, unknown 14C reservoir effect and sedimentation rates that are too high to obtain sediment cores long enough to establish a chronology. Despite these problems, comparison between the results of the different dating methods provided the most reliable sediment balance to date for the submarine delta of the Ganges-Brahmaputra river system and indicated that on a time scale of several centuries at least 35% of the annual sediment load is deposited.

Oolitic beach barriers of the last Glacial sea-level lowstand at the outer Bengal shelf

During RV Sonne cruise 93 the Bengal shelf was surveyed using sediment echosounding, side-scan sonar and sediment coring techniques. A sequence of shallow ridges was discovered, each between 100 m and 260 m wide and extending for kilometres parallel to the 120–130-m isobaths. The ridges mark the top of a barely covered regressive sediment sequence and consist of oolitic limestone of Late Pleistocene age. They are interpreted as contouring fossil shorelines of sea-level lowstands of the Last Glacial Maximum. Their lithology indicates that fine-grained sediment supplied by the Ganges/Brahmaputra river system did not `pollute' the nearshore environment during Glacial times; this argues for sediment bypassing of the Bengal shelf and a direct supply to the Bengal deep-sea fan.

Late Quaternary Seismic Stratigraphy of the Eastern Bengal Shelf

An ultra-high-resolution seismic study of the eastern Bengal Shelf with the parametric narrow-beam echosounder Parasound allows the interpretation of late Quaternary depositional patterns in terms of seismic stratigraphy. Accommodation space was still present on the outer shelf during the last lowstand, where a prograding delta developed in the western survey area. Oolitic beach ridges were later formed on top of this lowstand delta. Farther east, large parts of the shelf were exposed to subaerial erosion and a river system extended seaward across the area. A subaqueous highstand delta prograded southwards following the maximum transgression about 7,000 years ago. Its foreset beds exhibit acoustic voids very likely generated by sediment liquefaction, possibly caused by episodic energetic events such as major cyclones and/or earthquakes. Bottomset sediments extend seaward close to the shelf break in the west, whereas no Holocene sediments cover the outer shelf in the east.

Subaqueous delta of the Ganges-Brahmaputra river system

The Ganges-Brahmaputra is among the world's three largest river systems in terms of sediment load, but, until now, no high-resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers' mouths. The other two (Amazon, Huanghe) discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy. High-resolution seismic reflection profiles of the Bengal shelf reveal similar stratigraphy: topset beds dip gently (0.036 °) and diverge offshore; more steeply dipping foreset beds (0.190 °) converge farther seaward; and relatively thin, gently dipping bottomset beds (0.022 °) extend across the outer shelf, overlying an erosional surface presumed to be of Late Pleistocene age. Sediment accumulation rates are highest in the foreset region (≥ 5 cm/year) and reduced in the bottomset region (< 0.3 cm/year), corroborating the relative thickening and thinning of strata observed in seismic profiles. Taken together, these data indicate a subaqueous delta is actively prograding across the Bengal shelf. Volume estimates for the Holocene subaqueous delta reveal that about one third of the total load of the Ganges-Brahmaputra has accumulated on the shelf. The remainder is likely partitioned between the river floodplain/delta plain and off-shelf transport via the submarine canyon, Swatch of No Ground. The canyon incises the shelf in the area of highest sedimentation rates (foreset), and growth faults and slumping of modern sediments near the head of the canyon support the idea that significant off-shelf transport of sediments to the Bengal Fan is occurring.

Sedimentary structures on the Bengal shelf: a multi-scale approach to sedimentary fabric interpretation

Sedimentary structures on the Bengal shelf are examined using X-radiographic, thin-section, and scanning-electron-microscopic (SEM) techniques, which reveal contrasting depositional processes for the inner and middle shelf as well as for either side of a major submarine canyon indenting the western shelf margin. The inner shelf is characterized by tidally produced interlaminated silts and muds that reveal a neap-spring cyclicity. The cyclicity is more pronounced for the winter season, when high-discharge/monsoon conditions do not obscure the tidal signature. On the middle shelf, alternating beds of silts and muds probably represent high-discharge and low-discharge conditions, respectively. Silty beds on the western margin of the submarine canyon are thinner than those found on the eastern margin. The western margin receives predominantly fine-grained Ganges sediments, in contrast to the eastern area which receives both Ganges and coarser Brahmaputra sediments; these observations indicate that the submarine canyon presently intercepts westward along-shelf sediment transport. Muddy beds and laminae that appear internally homogeneous in the X-radiographs and exhibit faint bedding in thin sections often are composed of resistant (protruding) and less-resistant (recessed) microlaminae as observed with the SEM. On the inner shelf, resistant microlaminae are composed of fine-grained polymineralic, equant particles. The less-resistant microlaminae are composed of large, platy illite grains rimmed by smaller platy or lath-like illite particles. Resistant and less-resistant microlaminae within middle-shelf muddy sediments have the same textural characteristics observed on the inner shelf; however, both types essentially are monomineralic. On the middle shelf the smallest particles within the less-resistant microlaminae exhibit an open flocculated structure that may result from higher salinity, and/or decreased bottom shear stress associated with tidal currents. Biological structures predominantly occur on the western inner and middle shelf, particularly in intervals of high smectite concentrations. This relationship may exist because of a preferential organic affinity for expandable minerals, providing a favorable substrate for benthic organisms.

Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan

Research Article| December 01, 1989 Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan Steven A. Kuehl; Steven A. Kuehl 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Tina M. Hariu; Tina M. Hariu 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Willard S. Moore Willard S. Moore 1Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Geology (1989) 17 (12): 1132–1135. https://doi.org/10.1130/0091-7613(1989)017<1132:SSOTGB>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Steven A. Kuehl, Tina M. Hariu, Willard S. Moore; Shelf sedimentation off the Ganges-Brahmaputra river system: Evidence for sediment bypassing to the Bengal fan. Geology 1989;; 17 (12): 1132–1135. doi: https://doi.org/10.1130/0091-7613(1989)017<1132:SSOTGB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The nature of shelf sedimentation seaward of the world's largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples. Sediments from the Ganges-Brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m, forming a clinoform-like deposit similar to subaqueous deltas found off other major river systems. The highest sediment accumulation rates on the shelf occur near the head of the Swatch of No Ground, a major submarine canyon that indents the shelf west of the present river mouths. This observation, together with textural data, suggests that river sediments are transported seaward and westward and that the Swatch of No Ground is currently a major conduit for the transport of sediments from the Bengal shelf. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cretaceous—Eocene sedimentation, tectonism and biofacies in the bengal basin, India

The Bengal Basin, located in the eastern part of the Indian subcontinent, is one of the thickest sedimentary basins in the world. The basin started downwarping in the Early Cretaceous and the sedimentation has since been continuous. From an initial transgressive marine phase, lasting almost up to end of the Eocene, the basin has evolved through a subsequent regressive marine and a later continental deltaic phase. In the absence of detailed information available from the deeper basinal parts, an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the Bengal Basin pertaining to the initial phase of its evolution is attempted. This is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in West Bengal, Meghalaya, Mikir Hills and Assam, India. The subsidence of the Bengal Basin was the result of differential adjustments of the crystalline basement along certain well-established trends and was initiated soon after the Gondwana tectonism, sedimentation and volcanicity. The earlier sediments were not typically marine but were more paralic to estuarine in nature. The entire Bengal Basin, except the area now comprising Assam, came under the influence of the first regional marine transgression during the Late Campanian—Maestrichtian. The coarse clastic sediments were deposited under shallow coastal marine conditions. A more widespread marine transgression took place during the Early—Middle Eocene which covered almost the entire shelf area including Assam. During this transgression, the sediments belonging to the orthoquartzite—carbonate association were formed in the shelf. The sea started receding in the Late Eocene during which the entire Bengal shelf came under the influence of heavy unloading of fine clastics. The top of this fine clastic shaly sequence is an important sedimentary datum in the Bengal shelf which marks the end of the first phase in the evolutionary history of the basin.

Geological and Geophysical Studies in Western Part of Bengal Basin, India

Just beyond the western boundary of West Bengal, the great Indian shield disappears below a blanket of alluvium. The exposed part of the shield bordering the Bengal basin is marked by a row of intracratonic Gondwana basins, a series of thrust zones in Singhbhum, and extensive exposure of basic volcanics in the Rajmahal Hills. Intensive geophysical surveys and deep drilling in the alluvium-covered plains of West Bengal have revealed a thick section of Cretaceous and Tertiary sediments lying on a basement of basalt lava flows, presumably of the same age as the Rajmahal Group volcanics. An extension of the easternmost Gondwana basin farther east, below the Bengal alluvium, also is suggested. A series of buried basement ridges, marking the western margin of the Bengal basin, resumably kept the Gondwana continental basins isolated from the main Bengal basin through most of Tertiary time. Locally, during the late Tertiary, the sea transgressed over these basement ridges and onlapped parts of the Indian shield. Flanking the eastern margin of the buried ridges is a row of basin-margin en echelon faults and scarps, possibly the shallower expressions of some deep-seated movements in the basement. East of this marginal fault zone lies the stable shelf of West Bengal with a homoclinal dip toward the southeast. Seven seismic reflectors mapped in the Mesozoic and Tertiary sediments of the shelf indicate uniform increase of thickness (3,000 to 27,000 feet, approximately) of these sediments toward the southeast. Except for a few normal faults, the area is practically undisturbed structurally. An extensive unconformity between the Miocene and Pliocene has been recognized. Locally, weak evidence of another depositional break, at the top of the Oligocene, is present. Around Calcutta, the Eocene key horizon (Sylhet Limestone) shows a conspicuous basinward flexure (the hinge zone) at a depth of about 15,500 feet. East of this hinge, which traverses the whole Bengal basin, lies the deeper part of the basin with a greater rate of subsidence and a different lithofacies. Seismic interpretation suggests a sharp lithofacies change at this zone, from the Eocene nummulitic limestone of the stable shelf to a thick sequence of clay and shale in the deeper part of the basin. In the younger Tertiary sediments is a similar change of facies from the arenaceous sediments of the stable shelf to the dominantly argillaceous sediments downdip. Marine transgression on the West Bengal shelf occurred during the Late Cretaceous (locally), late Eocene (extensively), and Miocene (in the eastern parts only). Except for these periods of marine transgression, sedimentation took place under fresh-water, estuarine, or deltaic conditions. A summary of the tectonic and depositional history of the whole region, from the eastern margin of the Indian shield to the folded belt in Assam, is given in conclusion. This integrates the work done in West Bengal by the Indo-Stanvac Petroleum Project with that done in Assam by the Burmah Oil Company and its affiliates.