It has been proposed that the Kosi River continuously migrated >113 km westward across the surface of the megafan over the last two centuries. Examination of a number of old maps published between ...1760 and 1960 shows that during most of this period the Kosi River occupied a position slightly east of the megafan axis. The apparent channel movement shown in these maps is oscillating in nature and not unidirectional. Instead of encountering deposits left behind by a sweeping braided Kosi-like stream, a preliminary study of the uppermost 2–5 m succession in the north-central part of the megafan reveals overwhelming dominance of meandering stream deposits. Assuming the existing notion of Kosi River migration, the rate of deposition averaged over ∼100–200 years for the uppermost ∼5–10 m of megafan deposits, works out to be unusually high (>50 mm/y). All these observations question the soundness of the hypothesis of rapid westward migration of the Kosi River over the last two centuries. The existing facies model for the uppermost 8–10 m of the megafan deposits also appears untenable.
The three-dimensional geometry of the Kosi megafan is similar to those of typical alluvial fans, but with much gentler gradient (0.05°–0.01°) and with larger area (>10,000 km
2). Based mainly on the patterns of paleo and modern channels recognised in the satellite images, three major accretionary lobes can be identified on the Kosi megafan. Relative age of the lobes determined from the truncating relationship of the paleochannels indicates a random shift of the trunk channel forming these lobes. Similar multilobate form and evidences of random switching of the loci of lobe aggradation are also found to be common in the Tista and Taquari megafans.
The factors known to favour avulsion and the results of the recent simulation studies of alluvial deposits are inconsistent with the notion of unidirectional shift of the channels for more than 100 km across the entire megafan surface. This study suggests that the relocation of the Kosi River in the past was through random nodal avulsion rather than systematic unidirectional shift. The recent avulsion of the Kosi channel by a large distance to the east follows this expected pattern. Further study and age dating is required for a comprehensive understanding of the depositional dynamics of the megafan and the pattern of channel movement on it. Future flood predictions and disaster management plans should be based on such comprehensive understanding.
Grain segmentation of sandstone that is partitioning the grain from its surrounding matrix/cement in the thin section is the primary step for computer-aided mineral identification and sandstone ...classification. The photomicrograph of sandstone contain many mineral grains and their surrounding matrix/cement. The distinction between adjacent grains and the matrix is often ambiguous, making grain segmentation difficult. Various solutions exist in literature to handle these problems; however, they are not robust against sandstone petrography’s varied pattern. In this paper, we formulate grain segmentation as a pixel-wise two-class (i.e., grain and background) semantic segmentation task. We develop a deep learning-based end-to-end trainable framework named Deep Semantic Grain Segmentation network (
dsgsn
), a data-driven method, and provide a generic solution. As per the authors’ knowledge, this is the first work where the deep neural network is explored to solve the grain segmentation problem. Extensive experiments on the images highlight that our method obtains better segmentation accuracy than various segmentation architectures with more parameters.
Marine deposits in the Neogene Siwalik succession have been documented from the Tista valley, Bhutan and Kameng river sections of the Eastern Himalaya. This study was designed to investigate possible ...marine influence further east. During this investigation, it was revealed that the current classification and description of the stratigraphic units were inadequate to describe the variations present in this succession fully. In place of the existing three-fold classification, a four-fold lithostratigraphic classification of the Siwalik Group in the Arunachal Himalaya has been proposed by adding a new Siji Formation. This newly proposed formation is completely different from the underlying sand-dominated Subansiri or overlying conglomerate-dominated Kimin formations. Nearly 1000 m thick succession of the Siji Formation is characterised by of alternating tabular sheet like beds of grey mudstone, siltstone and sandstone with uncommon thin conglomerate beds. On the basis of the four-fold classification, the Siwalik rocks of the Arunachal Himalaya have been mapped over a strike length of ∼230 km, demonstrating the lateral continuity of the individual formations and the large-scale deformational features affecting the rocks. Three lithologs prepared show the distinctive lithological characters and overall paleocurrent pattern of these formations. The lithological, sedimentological and ichnological features of the Dafla and Siji formations provide evidence for marine influence in the study area. The proposed new lithostratigraphic subdivisions are expected to facilitate regional correlation and evidence of marine sedimentation which will throw new light on the basin analysis of the Siwalik Group in the Eastern Himalayan.
A rich and diverse palynoassemblage recovered from the Churanthi River section (26°53' 59.3" N, 88°34' 17.2" E), Darjeeling foothills Eastern Himalaya, has yielded 87 species assigned to 69 genera. ...The palynoassemblage is rich in angiosperm taxa (45.63%) followed by gymnosperms (0.45%), pteridophytes (18.49%) and fungal remains (23.88%). Based on their nearest living relatives, a wet evergreen to semi-evergreen forest under a humid tropical to sub-tropical environment during the Mio-Pliocene age has been suggested. A lot of angiosperms such as Palaeosantalaceaepites, Araliaceoipollenites, Malvacearampollis, Zonocostites, Neocouperipollis, Dicolpopollis, Palmidites, Palmaepollenites, isolated salt glands of mangrove plant leaves (Heliospermopsis) and Mediaverrunites type of fungal spores, along with ichnofossils like Planolites, Palaeophycus, Skolithos, Rosselia, Ophiomorpha and Teichichnus associated with rippled mudstone-siltstone suggest an environment strongly influenced by brackish water. Primary sedimentary structures in the associated strata indicate strong wave agitation common in shallow marine setting. Some high elevation components (5.14%) such as Alnipollenites, cf. Corylus (Betulaceae), Juglanspollenites, Engelhardtioipollenites (Juglandaceae), Quercoides, Cupuliferoidaepollenites, Lithocarpus, Castanopsis (Fagaceae), Abietineaepollenites (Pinaceae) represent hinterland vegetation possibly transported to the prograding deltaic coastline by the rivers. Reworked palynotaxa (Striatopodocarpites sp., Striatites sp., Faunipollenites sp., Circumstriatites sp., Crescentipollenites sp., Cuneatisporites sp., Parasaccites sp., Scheuringipollenites sp., Rhizomaspora sp., Marsupipollenites sp., Lophotriletes sp.) of Permian age have also been recorded in the palynoassemblage (11.55%) indicating the abundance of Permian Gondwana strata in the source area.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•The transition from the Lower to the Middle Siwalik took place around 7.5 Ma.•The Lower Siwalik of this area is a shallow marine/deltaic deposit.•A large marine embayment existed at ...least till ~7 Ma in the eastern Himalaya.•The Middle Siwalik of this area is an axial braided fluvial deposit.•Sea level fall and Shillong Plateau uplift reduced the marine accommodation space.•Reduced accommodation and additional sediment load forced marine-fluvial transition
An understanding of the depositional environment and paleogeography of the Siwalik foreland basin are crucial in interpreting the basin configuration, sediment transport pathways and its evolutionary history. This study examines the sedimentology of the Siwalik succession of the Kameng River valley, Arunachal Himalaya, northeastern India. The facies characteristics of the fine-grained, well-sorted sediments of the Dafla Formation and its complex, polymodal paleocurrent pattern in this section, reveals deposition in a variety of open marine to deltaic environment. The overlying Subansiri Formation, characterized by coarse-grained, thick, multistoried sandstone, and showing more consistent SW-ward paleocurrent, indicate deposition from a large, axial braided river system. The proposed redefinition of the boundary between the Lower Siwalik Dafla and the Middle Siwalik Subansiri formations implies their transition at around 7.5 Ma, instead of 10.5 Ma, suggested earlier. The revised age of the transition is consistent with the age of arrival of the Transhimalayan sediments at 7 Ma and also denotes the time of marine to fluvial transition in this area. Presence of marine sediments in the Kameng section, with similar records further west, indicates the existence of an extensive seaway in the eastern Himalaya during the lower Siwalik time. The extant paleodrainage reconstructions have been recast on the basis of new data on the sedimentology and paleocurrent from this section. It is inferred that the changing sea level, uplifting Shillong Plateau and drainage evolution in the eastern Himalayan foreland during the middle Miocene time controlled the marine to fluvial transition in the basin.
Equisetum (Equisetaceae) has long been a focus of attention for botanists and palaeontologists because, given its extensive and well-documented fossil record, it is considered the oldest extant ...vascular plant and a key element in understanding vascular plant evolution. However, to date, no authentic fossil evidence of Equisetum has been found from the Indian Cenozoic. Here, we describe a new fossil species, namely, E. siwalikum sp. nov., recovered from the middle Siwalik (Late Miocene) sediments of Himachal Pradesh, western Himalaya. We identified fossil specimens based on morphological and epidermal characters. In addition, X-Ray diffraction (XRD) analysis was used to determine the mineral composition of compressed stems of Equisetum. The close affinity of our recovered Siwalik fossils to Equisetum is supported by the presence of both macromorphological and epidermal characters. Because Equisetum generally grows in wet conditions around water reservoirs, our findings indicate that the fossil locality was humid and surrounded by swamp and lowland regions during deposition. Ample fossil evidence indicates that this sphenopsid once existed in the western Himalaya during the Siwalik period. However, at present Equisetum is confined to a particular area of our fossil locality, probably a consequence of severe environmental changes coupled with competition from opportunistic angiosperms. Our discovery of Equisetum fossils in appreciable numbers from the Siwalik sediments of the Himachal Himalayas is unique and constitutes the first reliable recognition of Equisetum from the Indian Cenozoic.
•Evidence of the oldest extant vascular plant (horsetails) from the Indian Cenozoic.•First-ever fossil record of Equisetum from Siwalik.•Description of a new fossil species of Equisetum based on morphological and epidermal characters.•Review the Cenozoic fossil history of Equisetum in detail.•Discussion of palaeoecological implications of this sphenopsid.
The present work analyses the effect of three spatial filters, modelling the psychophysics data on brightness perception, when applied to a difficult computer vision problem. This pertains to the ...automatic grain-matrix segmentation of cross-polar images of sedimentary rock thin sections, gathered through petrographic microscopy. These three filters are assumed to be modelling three modes of vision, viz. vision-at-a-glance (VG, through Magnocellular or M channel in primate visual pathway), vision-in-strong-contrast mode (VSC, through Parvocellular or P channel of visual pathway), and vision-with-scrutiny (VS, through Magno-Parvochannels combined referred as the PM filter). The aforementioned biologically inspired filters related to brightness–contrast modelling are applied on the cross-polar microscopic images. Then binary segmentation is performed on the original, as well as the filtered, images using a k-means clustering algorithm which effectively segment the input images according to the homogeneity present. The binary segmented output when compared with our manually built grain-matrix segmented ground truth using suitable indices, forecasts noticeable difference in terms of the applied filter on them and the grain size distribution in that image, manifested sometimes by the resolution scale of capturing the image. Mostly for images with higher resolution of microscope lens while capturing, or for images displaying larger grain size, the PM filtered image has shown better grain-matrix segmentation, though the effect of filtering is not very pronounced for such images. On the other hand, for lower resolution of microscope lens for capturing the image or for smaller grain size featured images, the P filtered image has consistently shown more accurate grain-matrix aggregation result as also distinct improvement over those not filtered. The novelty of the approach lies in the application of a psychophysics model that attempts to mimic the manual segmentation performed by the trained eye of the expert in various visual modes. The proposed method achieves a reasonably higher accuracy in grain segmentation when compared with relevant existing methods especially while dealing with more difficult images containing many small-sized grains.
In order to investigate the effect of intervention structures like dams and barrage on the sediment grain size distribution, we examined a ~75 km stretch of the Tista River in the foothill alluvial ...plain of the eastern Himalayas. However, studies on the impact of dams on streamflow and sediment in the Himalayan rivers are constrained due to inadequate historical gauge data. Therefore, we employed a simple method to track the initial formation of some of the existing river landforms that formed before the dam/barrage operations from yearly satellite images (1987–2017) for in situ sediment sample collection. Hence, the collected sediment grain sizes analysis is used here as a proxy to comprehend the pre‐, syn‐ and post‐dam river sediment‐transport pattern. We have grouped 23 landforms into three temporal clusters: (i) those formed before 1997, earlier than the dams/barrage interventions (TOL); (ii) formed between 1998 and 2010 during the upstream dam construction (TML) and (iii) formed after 2010 (TNL) when upstream dams and the barrage were fully operational. A total of 116 sediment samples from these landforms indicate the overall grain size reduction for TML and TNL sediment due to reservoir sequestration of coarser grains. In comparison, the coarsest TOL sediment with downstream fining trend represents the pristine river condition. The coarsening trend with a poor correlation coefficient of TML is primarily assigned to partial trapping of the coarse bedload and massive sediment generation by the upstream under‐construction dams, resulting in the downstream erosion of the riverbed and transport of the coarser bedload further down. Similarly, post‐dam sediment trapping and controlled flow were responsible for the weaker trend with a distinctly finer size of TNL. The historical satellite images and data on streamflow and suspended sediment reveal that contemporary river behaviour is consistent with the earlier interpretation.
The historical satellite image time‐series analysis used to assign the age of the preserved landforms across the sandy braided river. Sediment samples were collected from these spatiotemporally clustered landform deposits which preserved till date during the field investigations. The collected samples were analysed further using grain size parameters (D84, D50 and D16), which is used as a proxy to understand the changing fluvial process in the downstream river stretch due to upstream dams and barrage.
This paper reports a study of the Tista megafan in the foothills of Darjeeling Himalaya. Spread over parts of India and Bangladesh, the megafan is bounded by the Mahananda River to its west and the ...Tista River to the east. The Atrai and Karatoya Rivers flow through its axial part. The megafan covers an area of ~
18,000
km
2. Near its apex the surface slope is ~
0.19°, that declines to 0.01° near the toe. The east–west transverse profile of the megafan is broadly upward convex, with gently sloping (~
0.01°) flanks. Maximum width and the length of the megafan are about 145
km and 166
km respectively. The highest point near the megafan apex is ~
150
m above the Brahmaputra alluvial plain. The Tista River flanking the megafan has an average annual discharge of 609
m
3/s with highest average monthly discharge exceeding 2000
m
3/s during monsoon. Most of the other channels currently traversing the megafan are plains-fed, and compared to Tista and Mahananda Rivers these channels have lesser discharge, higher sinuosity, and decreased widths. A radiating network of abandoned channel belts can be identified in satellite images of the megafan. Each of these major paleochannels is associated with numerous crevasse channels in the distal part of the megafan, forming an intricate network of radial drainage on the megafan surface.
Three distinct depositional lobes can be recognised on the Tista megafan. Each of the lobes is identified by a set of ancient and modern radial drainage systems. The lobe boundaries are marked by discordance in drainage network of adjacent lobes. The relative ages of the lobes, as tentatively determined from the drainage discordances, indicate that the megafan first built up the eastern lobe (lobe 1), then shifted to the west to form lobe 2, and finally switched again eastward giving rise to the smallest lobe located close to the mountain front (lobe 3). Although broadly upward convex in cross profiles, subtle reflections of multiple lobes are apparent in some of the cross profiles. Study of the old maps published between 1794 and 1945 reveals that some of the present-day plains-fed rivers, like the Tangon, the Atrai and the Karatoya, were directly connected to the Himalayan catchment basins prior to the late eighteenth century. Archaeological excavations along the banks of these rivers appear to support this paleodrainage configuration. Eight facies and five facies associations were recognised in the uppermost megafan sediments. Facies associations suggest deposition from high-energy sandy streams in the proximal area and deposition from mixed load, sinuous streams and flanking marsh or lake in the distal part of the lobes. Paleocurrent data are consistent with a southwestward spreading paleochannel pattern recognised within the studied part of the megafan.
Multiple lobes typify megafans ocurring in different parts of the globe. The existence of the mutiple accretionary lobes in Tista megafan, its radial drainage pattern, its concave-upward longitudinal and convex-upward transverse profile shape make it morphologically comparable with other high-gradient alluvial fans observed in nature and those produced in the laboratory. These morphological features denote the similarities between the small, high-gradient alluvial fans and large, low-gradient megafans, and that in turn may be indicative of the commoness in certain controlling factors between these two depositional systems.
River regulation by dams and embankments drastically reduce/alter flow, which affects the natural channel pattern. Existing braiding indices have seldom incorporated the effects of diurnal flow ...variations caused by hydropeaking, leading to over/underestimation of the braiding intensity. These indices consider only the visible wet channels, ignoring the existence of dry channels that are activated only episodically during phases of water release from hydropower dams. We have extracted the dry channels (those that are periodically wet) coursing across the channel belt of the highly regulated River Tista from Landsat images between 1977 and 2014, using Normalized Difference Wetness Index values. These were combined with existing wet channel widths and numbers to formulate the Regulated Braiding Index (RBi) for characterizing channel braiding in the Tista's course over the Himalayan foothill plains. Overall, the widths and numbers of wet channels decreased by 63% and 25%, respectively, during the regulated years (2003 and 2014) as compared to pre‐dam years (1977 and 1995) due to the collective operation of 14 upstream dams/barrages (having cumulative ~89 million m3 reservoir capacity), whose operations reduced the braiding intensity (eliciting lower RBi values). Further, the number of high braiding reaches decreased by half while low braiding stretches rose by 10% in comparison to the pre‐dam period due to reduced/regulated flow. Comparative analysis of derived RBi values with three existing braiding indices revealed that RBi values consistently decreased near barrages, bridges, and within embankment‐confined reaches, unlike the other indices, thus providing a better framework for assessing expected river regulation effects.
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