The Godavari delta in India is a major wave dominated delta of a tropical monsoon-fed river with one of the largest sediment deliveries in the world. While several earlier studies revealed the nature of landforms and progradation style of the delta plain during the Holocene, the present study attempts to reconstruct the depositional environment of the Godavari delta through the analysis of core sediment and accelerator mass spectrometry (AMS) 14C dating from three locations. The sediment core obtained from a 27.06 m deep borehole at Vilasavilli (VV) supported by 13 14C dates revealed the complete succession of the Holocene deposits unconformably overlying Pleistocene sediments. Textural analysis indicates the lower upward-fining and upper upward-coarsening units in the Holocene succession. Total organic carbon (TOC)/total nitrogen (TN) ratios of >20; high content of TOC around 1.5–2.5%; and black to very dark colored sediment throughout the muddy part of the Holocene succession indicated a predominance of terrigenous material. The VV core and two other cores (DRP and SDG) with 14 14C dates indicated the thickness of the Holocene sediments in the Godavari delta plain is in the order of ~20–50 m unconformably resting on a seaward sloping Pleistocene basement. Sediment facies and sediment accumulation of the three cores show the evolution of the Godavari delta. A transgressive phase is recognized as an upward-fining succession in VV core, 8.4–8.0 cal ky BP, followed by a low accumulation period, 8.0–6.3 cal ky BP including the Holocene maximum transgression in the Godavari delta. After 6.3 cal ky BP, areas of high accumulation rates have changed laterally between the central part (VV site) and southwestern part (DRP and SDG sites), may be controlled by the location of river-mouths.Further the study coupled with 11 more 14C dates compiled from earlier works indicated that the strandplain of the Godavari delta prograded seaward in three stages, and that the rate of progradation accelerated during the past ~3 ky, particularly in the last millennium. However, pronounced shoreline erosion led to a net negative growth of the delta during the recent decades due to sediment retention by upstream dams. Although this study provided insights into the sedimentation patterns and rate of seaward progradation of the Godavari delta, further studies on three-dimensional volume analysis of the deltaic sediments are necessary to estimate the overall rates of past sediment discharge and the Holocene growth of the Godavari delta.

季刊地理学, 1997

The geomorphological study and the radiocarbon datings of the sub-surface sediments in the Krishna delta, indicate that the strandline around 2,200 years ago was located at more than 40 km inland from the present river mouth, and the delta tront has prograded more than 5 km in the last 250 years. These facts differ somewhat from the previous view on the formation of the alluvial plain in the east coast of India that has been thought to be closely related to sea level oscillations since the Holocene maximum transgression.

Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges-Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At w9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front w100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (w4-8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3-C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at w7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO 2 .

Quaternary Science Reviews, 2009

Sedimentology, carbon isotope and sequence stratigraphic analysis of subsurface sediments from western part of Ganges-Brahmaputra (GB) delta plain shows that a Late Quaternary marine clay and fluvial channel-overbank sediments of MIS 5 and 3 highstands are traceable below the Holocene strata. During the Last Glacial Maximum (LGM) sea-level lowering of >100 m produced a regional unconformity (type 1), represented by palaeosols and incised valley. C4 vegetation expanded on exposed lowstand surface in an ambient dry glacial climate. At w9 ka transgression inundated the lowstand surface pushing the coastline and mangrove front w100 km inland. Simultaneous intensification of monsoon and very high sediment discharge (w4-8 times than modern) caused a rapid aggradation of both floodplain and estuarine valley fill deposits between 8 and 7 ka. The Hoogli River remaining along its present drainage possibly acted as the main conduit for transgression and sediment discharge that was subsequently abandoned. C3 vegetation dominated the delta plain during this time. From 7 ka onward progradation of delta plain started and continued till recent. This period experienced a mixed C3-C4 vegetation with localized mangroves in the mid-Holocene to dominant return of C4 vegetation in the late Holocene period. The study indicates that while the initiation of western part of GB delta occurred at least 1 ka earlier than the global mean delta formation age, the progradation started at w7 ka, at least 2 ka earlier than thought before. The terrestrial vegetation change was modulated by changes in depositional environment, specific ecological niches and climate rather than pCO 2 .

Sedimentary Geology, 2000

Borehole data from the Ganges-Brahmaputra delta system unveil an intriguing Late Quaternary history controlled by immense sediment discharge, tectonics, and eustasy. Sea level first intersected a major portion of the lowstand surface ϳ10,000-11,000 cal yr BP, marking the onset of sediment trapping and delta growth. Despite rapid sea-level rise (Ͼ1 cm/yr), sediment load was sufficient to maintain relative shoreline stability during this time. By ϳ7000 cal yr BP, accommodation was quickly infilled with slowed sea-level rise, leading to upstream channel migration and widespread dispersal of sands. This forced coastal progradation along the western delta front where the Ganges was situated, and much of the river's fine-grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf. Concurrently, Brahmaputra sediments were sequestered to an inland tectonic basin, thus starving the adjacent shoreline and leading to transgression along the eastern delta. By ϳ5000 cal yr BP, the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river-mouth estuary. Present delta configuration appears to have developed by ϳ3000 cal yr BP. Evolution of the tectonically active Ganges-Brahmaputra (G-B) system reveals important similarities and differences with other deltas. Overall facies succession follows that of basic models, progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain. However, the timing, thickness, and controls of these deposits differ. Immense sediment discharge from the Ganges and Brahmaputra rivers allowed initial delta growth 2000-3000 yr earlier than the global average, despite debouching onto a canyon-incised, high-energy margin. Subsequently, a thick (ϳ50 m) transgressive systems tract was formed during the early Holocene phase of delta aggradation, contrasting with extensive shoreline retreat documented along most margins. Highstand progradation of coastal and shelf sequences ensued by the middle Holocene, but subsiding inland basins also favored accumulation of a thick (to 40 m) highstand sequence in the lower floodplains. Unlike many other delta systems, subsidence in the subaerial G-B delta is not dominated by compaction, but rather by plate-driven tectonic processes that generate rates up to 4 mm/year. Overall, the huge sediment load, tectonic subsidence, major seismic events, and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta, shelf, and deep-sea Bengal Fan throughout the Late Quaternary. ᭧

2014

Three main rivers-the Ganges, Brahmaputra, and Meghna-coalesce in the Bengal basin to form the world's largest delta system, which serves as fi lter and gateway between the Himalayan collision and vast Bengal fan repository. New insights into the Holocene construction of the Ganges-Brahmaputra-Meghna delta, with a focus on river sedimentation, channel migration, and avulsion history, are presented here using the Sr geochemistry of bulk sediments as a provenance tracer. The sediment load of each river transmits a distinct Sr signature owing to differences in source rocks from the Himalaya, Tibet, and local regions, allowing for effective tracking of river channels and stratigraphic development within the delta. In the early Holocene, vigorous delta aggradation occurred under rapid sea-level rise and high river discharge and supported the construction of sand-dominated stratigraphy by laterally mobile, braided-stream channels. However, the vertically (i.e., temporally) uniform, but geographically distinct, Sr signatures from these deposits indicate that the Ganges, Brahmaputra, and Meghna fl uvial systems remained isolated from one another and apparently constrained within their lowstand valleys. By the mid-Holocene, though, delta stratigraphy records spatially and temporally nonuniform Sr signatures that hallmark the onset of avulsions and unconstrained channel migration, like those that characterize the modern Ganges and Brahmaputra fl uvial systems. Such mobil-ity developed in the mid-Holocene despite declining discharge and sea-level rise, suggesting that earlier channel behavior had been strongly infl uenced by antecedent topography of the lowstand valleys. It is only after the delta had aggraded above the valley margins that the fl uvial systems were able to avulse freely, resulting in numerous channel reorganizations from mid-Holocene to present. These channel-system behaviors and their role in delta evolution remain coarsely defi ned based only on this initial application of Sr-based provenance tools, but the approach is promising and suggests that a more complete understanding can be achieved with continued study.

Journal of Coastal Research, 2012

Lithoand chronostratigraphic analyses of radiocarbon-dated cores are utilized herein to distinguish Holocene deltaic and underlying transgressive units and late Pleistocene alluvial deposits in the western Ganges-Brahmaputra delta. Regional distribution of these facies indicates that neotectonic displacement, including differential land subsidence, of delta plain sectors is one of the major controls of late Quaternary depositional patterns in this depocenter. The spatial and temporal configuration of Holocene deltaic sediment thickness, mud and sand layers, peats interstratified in Holocene sequences, and modern mangrove forests that form the Sundarbans are attributed to NE-SW, and to a lesser extent NW-SE, neotectonic trends. Holocene sedimentary and stratigraphic configurations closely parallel geological structures, some of them deep seated, that affected this region during most of the Tertiary and have continued to the present. Extensive mangrove forests developed along the NE-SW zone of thickened Holocene deltaic deposits. Their present configuration is related to natural factors, such as eastward tilting of the delta, rapid sediment accumulation (to 0.7 cm/yr), marked land subsidence (to 0.5 cm/yr), and increasing anthropogenic influences, including large-scale land reclamation and decreased river flow influx. The diverse and extensive mangrove tracts of the delta have significant environmental and economic implications for the rapidly growing population, including serving as a buffer zone that helps to reduce the impact of landward-driven tides, storms and cyclones. Interpretation of Holocene facies in the subsurface by means of radiocarbon-dated cores provides a mean to more precisely define the interaction between contemporary Holocene depositional patterns and neotectonics. This information, in turn, can be utilized to develop realistic measures needed to minimize further degradation of this biologically unique ecosystem.

Annual Review of Marine Science, 2015

We present a review of the processes, morphology, and stratigraphy of the Ganges-Brahmaputra-Meghna delta (GBMD), including insights gained from detailed elevation data. The review shows that the GBMD is best characterized as a composite system, with different regions having morphologic and stratigraphic attributes of an upland fluvial fan delta; a lowland, backwater-reach delta; a downdrift tidal delta plain; and an offshore subaqueous-delta clinoform. These distinct areas of upland and lowland fluvial reaches and tidal dominance vary in time and space, and we distinguish late-Holocene phases of delta construction, maintenance, and decline similar to delta-lobe cycling in other systems. The overall stability of theGBMD landform, relative to many deltas, reflects the efficient, widespread dispersal of sediment by the large monsoon discharge and high-energy tides that affect this region. However, we do identify portions of the delta that are in decline and losing elevation relative to sea level owing to insufficient sediment delivery. These areas, some of which are well inland of the coast, represent those most at risk to the continued effect of sea-level rise.

Sediment cores from the Ganges -Brahmaputra delta in Bangladesh were examined for sedimentological character, clay mineralogy, elemental trends (C, N, S), and 14 C geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late Holocene. A widespread facies succession from Muddy Sand to Interbedded Mud records progradation of shoal -island complexes and the transition from subtidal to intertidal conditions. Mangrove-vegetated islands and peninsulas represent the final phase of progradation; a Mottled Mud that is deposited by penetration of turbid coastal water into the mangroves during high water events. Organic matter preservation is generally low ( < 1% TOC) in most of these well-drained deposits that are characterized by a permeable, silt-dominated granulometry. Clay mineralogy in the cores records the relative influence of smectite and kaolinite-rich Ganges sediments and illite and chlorite-rich Brahmaputra material. The lower delta plain west of the modern river mouths was deposited as a Ganges-dominated delta in three phases since 5000 cal years BP, with Brahmaputra influence confined to the Meghna estuary area and to the supratidal section of western delta deposits. Evolution of the lower delta plain in the late Holocene was influenced by regional subsidence patterns in the tectonically active Bengal Basin, which controlled distributary channel avulsion and migration, and the creation of accommodation space. D

Quaternary International, 2017

Sedimentary record of a 40 m deep borehole drilled in a paleodelta region of southwest coast of India has been studied for sedimentary proxies -texture, clay minerals, geotechnical parameters, and pollens. The sedimentary record, spanning from 12 ka to Recent in age, exhibits two different sedimentary environments of deposition. Sediments record high values of moisture content, organic carbon and plasticity index. Sediment texture and geotechnical properties indicate a distinct change in depositional environment from marine to fluvial during the sea level fall i.e., after ~7 ka. It also suggests that major rise of sea level from ~11 to ~7 ka and regression from ~7 ka to ~5 ka contributed to the changes in the environment of deposition. The downcore increase of illite and decrease of kaolinite at 12 m depth (~6 ka), and an upward increase of smectite and kaolinite, and decrease of illite concentration support the major fall in sea level in the region that accounted for the change of depositional environment. The pollen records reveal the abundant occurrence of semi-evergreen type of mangroves during early-to mid-Holocene. Thus, the multi proxy record provides an evidence of change in the environment of deposition from marine to fluvial, which might have been influenced by neotectonics, sea level variations, and monsoonal intensity in the region.

AI

We investigated the Holocene evolution of the Godavari delta along the east coast of India, by analyses of land- forms and 11 borehole cores collected from the delta plain during 2010–2013 with 113 14C dates. The Godavari delta covering an area of 5820 km2 is one of the world's largest wave-dominated deltas fringed by a ~ 30 km wide beach-ridge plain. Our analysis of sediment facies and their ages indicated that the Holocene marine sequence thickens seaward from ~10 m to more than 50 m and that the local relative sea level rapidly rose from 43.5 m to 7.5 m below the present sea level during 10.9–8.0 cal ky BP followed by a slow rise up to its current level around 5 cal ky BP. Spatio-temporal variations in sediment accumulation rates and palaeo-shorelines for the last 6 ky revealed successive changes of the main depocentre (delta lobe). We recognised six stages of delta evo- lution with distinct cyclic shifts of delta lobes controlled by avulsions. A linear beach ridge located 12–14 km in- land from the present shoreline, which separates the first three lobes from the last three, represents the effect of coastal erosion of past cuspate delta lobes, as does the current linear shoreline of the central part of the delta. Our study documents continuous net progradation during the Holocene on millennial to centennial time scales. How- ever, a considerable decrease of sediment discharge due to dam construction and water diversion since the 1970s has exacerbated coastal erosion, resulting in shrinking of the Godavari delta plain and tipping it into a persistent destruction phase. © 2015 Elsevier B.V. All rights reserved.

Marine Geology, 2020

The Holocene evolution of the Krishna Delta was inferred using landform characteristics and 11 sediment cores with 59 Accelerator Mass Spectrometry 14 C dates. The landform assemblages in a 5880 km 2 area of the Krishna Delta indicate an upper (landward) river-built fluvial plain and a lower (seaward) marine-built beach-ridge plain. Holocene sediment unconformably overlying a Pleistocene basement consists of 11 sedimentary facies, mainly shallowing-upward and deltaic succession. Sea level indicators such as the basal mangrove peat layers above the Pleistocene basement and the foreshore sediments indicate that the sea level rose from −9 m to −3 m during 8.3-6.3 cal ky BP, and stabilised at the present level around 5 cal ky BP. The 3539 km 2 area of the Krishna Delta beach-ridge plain developed in five major stages during the last 6 cal ky, with distinct lateral and seaward migration of delta lobes (depocentre shifts). Delta progradation accelerated in the last 500 years, forming an outbuilding lobate delta. However, predominant erosion along the sediment-starved coast during the past five decades, due to impoundment of riverine inputs at the burgeoning upstream dams, has pushed the Krishna Delta into a persistent destruction phase. many rivers such as the Subarnarekha, Brahmani, Baitarani, Mahanadi, Godavari, Krishna, Penner and Cauvery building their deltas (Fig. 1a). However, the deltas of these rivers are some of the least studied, leaving an information gap in the Holocene environmental changes and sealevel history of this part of the world. Previous studies based on interpretation of aerial photographs and satellite imagery have revealed 30-35 km wide beach-ridge plains in the Mahanadi-Brahmani-Baitarani composite Delta (

2005

The peninsular part of the Indian subcontinent is traversed by a number of rivers most of which flow from west to east and in the process build large deltas at their mouths along the east coast of India bordering the Bay of Bengal. These river systems are essentially monsoon driven and as such the sediments embedded in their deltas are considered as excellent repositories of paleo monsoon records. Of the many deltas that dot the 2,300 km long east coast of India, the Mahanadi, Godavari, Krishna and Cauvery are the major ones. While very little is known about the nature of offshore parts of these deltas, there have been a few studies, however, on the Holocene evolution and shoreline dynamics of their subaerial parts.

Frontiers of Earth Science in China, 2010

The present study is an attempt to estimate the rates of subsidence in the Holocene sediments of the Godavari delta along the east coast of India. Two boreholes dug at Panangipalli village in the delta revealed Early Historic culture material such as potsherds including Coarse Red ware, Red-Slipped ware, Coarse Grey ware, and Black and Red ware, between 3.5 m and 9.0 m below the surface level. This suggests that the location was a human settlement of Andhra Satavahana period which flourished in the region between 3rd century BC and 3rd century AD. The fossil shells of Anadara sp. recovered from the borehole further downward at 11.5 m below the surface are considered to represent the intertidal swampy/lagoonal environment. The age of the shells was determined through AMS 14C dating as 6400 cal a BP. The presence of about 2.5m thick intertidal swampy/lagoonal material between 9.0 m and 11.5 m depth, suggests post-depositional subsidence at an average rate of less than 1.0 mm·a−1. However, a higher rate of subsidence of about 2.0 mm to 4.0 mm·a−1 is estimated for the upper 9.0 m thick floodplain sediment unit which embeds the Early Historic culture remains. The increased rates of subsidence during the last two millennia when compared to the earlier period in this part of the Godavari delta could be due to anthropogenic activity of deforestation and agriculture leading to accelerated soil erosion in the catchment and increased sedimentation in the delta.

SEPM Special Publication No. 83, 2005

The Godavari delta, on the east coast of India, is prograding into a microtidal and low to moderate wave environment, fed by a highly variable water discharge in a monsoon-driven hydrographic regime. Extensive strandplains that prograded 30 to 35 km across the continental shelf during the Middle to Late Holocene characterize the overall arcuate seaward bulge of the delta. At present, the delta has two lobes: the Gautami in the northeastern part and the Vasishta in the southwestern part, which have been active during the past one thousand years. Although the bigger size of the Gautami lobe with extensive mangrove swamps apparently indicates a larger riverine influence, both lobes, in fact, exhibit strong wave-influenced morphologies. As revealed by a series of historical maps, pronounced growth of spits characterizes the northern Gautami lobe. The 21-km-long Kakinada spit, which is prominent among all such sand bodies, had even deflected the Gautami distributary course in the initial stages of its growth under the influence of a strong northeastward longshore drift. Analysis of multi-date satellite images shows that spits are growing at the updrift sides of the mouths of the two terminal branches of Gautami, whereas erosion is dominant on their downdrift sides. On the downdrift side of the Vasishta mouth, development of mouth bars is followed by their emergence as barrier islands that migrate landward, and subsequent infilling of the backbarrier bays by riverine discharge, typical for wave-influenced asymmetric deltas. The nature of coastal landforms and sedimentation processes indicate prevalence of a northeastward drift at the Gautami lobe and a southwestward drift at the Vasishta lobe.

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