Kathmandu Basin Research Articles

398 Late Quaternary climate on the basis of pollen and diaspores from Kathmandu Basin, Nepal

398 Late Quaternary climate on the basis of pollen and diaspores from Kathmandu Basin, Nepal

Sedimentary facies at southern marginal part: an indicator of deposition environmental fluctuation in Kathmandu basin, Nepal

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Late Quaternary plant macrofossil assemblages from the Besigaon section of the Gokarna Formation, Kathmandu Valley, central Nepal

Plant macrofossil investigation was carried out on the Late Quaternary sediments of the Kathmandu Valley, central Nepal. The fluvial-deltaic to fluvio-lacustrine deposits of the Kathmandu Basin are found to be rich in plant macrofossil assemblages. A total of 66tax.a from 38 families were identified to genus and some of them up to species level from five different macrofossil bearing layers (BG- I Lo BG-5) from the 33 m thick Besigaon section belonging to the Gokarna Formation exposed at the northern part of the basin. The lower half of the exposed section is completely devoid of macrofossil horizons, however the upper half yielded a significant amount which could be used for climatic interpretation. The characteristic feature of the Besigaon section is the dominance of angiosperms in the lower and middle horizons (BG-I, BG-2, BG-3) but gymnosperms in the upper horizons (BG-4, BG-5). The dominance of angiosperms such as Carpinus, Alnus, Pyracantha, Quercus subgen. Cyclobalanopsis, Eurya and Zizyphus suggest the deposition of the lower and middle horizons in warm temperate climatic condition. The humid phase is documented during deposition of the BG-3 and BG-4 horizons with the findings of Selaginella remotifolia and wetland aquatic taxa such Carex, Schoenoplectus, Nymphoides indica. In contrast, the upper horizons dominated by gymnosperms such as Abies, Pinus, Picea smithiana, Tsuga dumosa and Taxus wallichiana represent cold and humid climate. The macrofossils from Besigaon section suggest minor fluctuation in climate from wam1to cold temperate phase with increasing humidity during the Late Pleistocene.

Seismites in the Kathmandu basin and seismic hazard in central Himalaya

Soft-sediment deformation structures have been analyzed at six sites of the Kathmandu valley. Microgranulometric study reveals that silty levels (60 to 80% silt) favor the development of soft-sediment deformation structures, while sandy levels (60 to 80% sand) are passively deformed. Nonetheless well sorted sand levels (more than 80% sand) generate over-fluid pressure during compaction if located beneath a silty cap, leading to fluidization and dike development. 3-D geometry of seismites indicates a very strong horizontal shearing during their development. Using a physical approach based on soil liquefaction during horizontal acceleration, we show that the fluidization zone progressively grows down-section during the shaking, but does not exactly begin at the surface. The comparison of bed-thickness and strength/depth evolution indicates three cases: i) no soft-sediment deformation occurs for thin (few centimeters) silty beds; ii) the thickness of soft-sediment deformation above sandy beds is controlled by the lithological contrast; iii) the thickness of soft-sediment deformation depends on the shaking intensity for very thick silty beds. These 3 cases are evidenced in the Kathmandu basin. We use the 30 cm-thick soft-sediment deformation level formed during the 1833 earthquake as a reference: the 1833 earthquake rupture zone extended very close to Kathmandu, inducing there MMI IX–X damages. A 90 cm-thick sediment deformation has therefore to be induced by an event greater than MMI X. From a compilation of paleo and historic seismology studies, it is found that the great (M ~ 8.1) historical earthquakes are not characteristic of the greatest earthquakes of Himalaya; hence earthquakes greater than M ~ 8.6 occurred. Kathmandu is located above one of the asperities that laterally limits the extent of mega-earthquake ruptures and two successive catastrophic events already affected Kathmandu, in 1255 located to the west of this asperity and in ~ 1100 to the east.

Controlling weathering and erosion intensity on the southern slope of the Central Himalaya by the Indian summer monsoon during the last glacial

This paper reports the results of clay mineral analysis (the amount of clay fraction, clay mineral assemblages, illite crystallinity) of samples collected from a drilled core (Rabibhawan (RB) core) located in the west-central part of the Kathmandu Basin on the southern slope of the Central Himalaya. The amount of clay fraction in the core sediments between 12 m and 45 m depth (corresponding to ca. 17–76 ka), which belong to the Kalimati Formation, is variable and shows three clay-poor zones (19–31 ka, 44–51 ka, and 66–75 ka). The variations correspond with those of illite crystallinity index (Lanson index (LI) and modified Lanson index (MLI)) and kaolinite/illite ratio as well as the fossil pollen and diatom records reported by previous workers. These data reveal the following transformations occurring during the weathering process in this area: micas ( mainly muscovite ) → illite ( → illite − smectite mixed layer mineral ( R = 1 ) ) → kaolinite The sedimentation rate (~ 50 cm/kyr) of clay-poor zones that correspond to dry climate intervals is only half that of clay-rich zones (~ 120 cm/kyr) that correspond to wet climate intervals, indicating weakened chemical weathering and erosion and low suspended discharge during dry climate intervals. The clay-poor zones commonly show unique laminite beds with very fine, authigenic calcite, which was probably precipitated under calm and high calcite concentration conditions caused by low precipitation and run-off. The variations between dry and wet conditions in this area as deduced from clay minerals appear to follow the Indian Summer Monsoon Index (ISMI) (30°N–30°S, 1 July) and northern hemisphere summer insolation (NHSI) signals (30°N) at 1 July, especially during the dry climate zones, whereas the wet maxima of the wet climate zones somewhat deviate from the strongest NHSI. On the other hand, the dry–wet records lead markedly the SPECMAP stack (by about 5000 years). These results suggest that the Indian summer monsoon precipitation was strongly controlled by the NHSI or summer insolation difference between the Himalayan–Tibetan Plateau and the subtropical Indian Ocean, showing a major fluctuation on the 23,000 years precessional cycle, and that it was not driven by changes in high-latitude ice volume, although the records of clay mineral indices during the wet intervals leave a question that other factors, in addition to insolation forcing, may play important roles in weathering, erosion, and sedimentation processes.

Stratigraphy and depositional environment of late Pleistocene Sunakothi Formation in Kathmandu Basin, central Nepal

Thick sandy sequences were recognised between the central and southern part of the basin, it was named as the Sunakothi Formation. We designate the type locality of this formation at Sunakothi, a village lying 3.0 km south of Patan. This formation is extensively distributed in the Nakhu, Kodku, and Godavari Kholas ranging in altitude from ca 1420 m in the southern margin to 1300 m in the central part. The average thickness of this formation ranges from 34 to 60 m. The sedimentary strata are gently inclined towards the north. On the basis of geological mapping this formation is located between the muddy part of the Kalimati Formation of the ancient lake and the terrace gravel deposits. It is further divided into the following four parts: (1) muddy rhythmic basal part, (2) sandy lower part, (3) muddy, sandy, and gravelly middle part, and (4) laminated silty upper part. The basal part shows a transition from the lacustrine to fluvial environment in the south and the prodeltaic environment towards the basin centre. Its lower part contains sandy fluvial to lacustrine delta front deposits whereas the middle part comprises sandy bar, muddy floodplain, and gravelly channel-fill deposits. The upper part of this formation is restricted only to the southern end of the basin and shows marginal shallow lacustrine environments. The sedimentological evidence indicates that the Palaeo-Kathmandu Lake was drained out from the south due to tectonic activities as well as late Pleistocene climatic changes.

Ecological variations in diatom assemblages in the Paleo-Kathmandu Lake linked with global and Indian monsoon climate changes for the last 600,000 years

Variations in fossil diatom assemblages and their relationship with global and Indian monsoon climate changes for the last 600,000 yr were investigated using a core of ancient lake (Paleo-Kathmandu Lake) sediments drilled at the Kathmandu Basin, Nepal Himalaya. Chronological scales of the core were constructed by tuning pollen wet and dry index records to the SPECMAP δ 18O stack record. Examinations of biogenic silica contents and fossil diatom assemblages revealed that variations in productivity and compositions of diatom assemblages were closely linked with global and Indian monsoon climate changes on glacial and interglacial time scales. When summer monsoonal rainfall increased during interglacials (interstadials), diatom productivity increased because of increased inputs of terrestrial nutrients into the lake. When summer monsoonal rainfall reduced and/or winter monsoonal aridification enhanced during glacials (stadials), productivity of the diatoms decreased and lake-level falling brought about changes in compositions of diatom assemblages. Monospecific assemblages by unique Cyclotella kathmanduensis and Puncticulata versiformis appeared during about 590 to 390 ka. This might be attributed to evolutionary fine-tuning of diatom assemblages to specific lake environmental conditions. Additionally, low-amplitude precessional variations in monsoon climate and less lake-level changes may have also allowed both species to dominate over the long periods.

Maximum extent of the Paleo-Kathmandu Lake in the late Pleistocene on the basis of piedmont gentle slope formation and lacustrine distribution in the Kathmandu basin, Nepal

The Kathmandu basin is an intradeep located in the southern slope of the Nepal Himalaya. The basin, surrounded by ridges hanging around 2000 m asl, extends between the altitudes of 1200 and 1400 m asl and is filled by a thick sequence of lacustrine sediments deposited in the Paleo-Kathmandu lake during the Pliocene and Pleistocene. Several terrace surfaces, which occupy the basin floor, have been formed in response to the disappearance of the Paleo-Kathmandu lake, that was drained due to southerly erosion by the Bagmati river. This paper attempts to estimate theage and paleogeography in which the PaleoKathmandu lake was maximized and drained in terms of geomorphological investigation of the marginal area of the basin. In particular, characteristics of piedmont gentle slopes, lacustrine distribution, and the relationship between them are examined. The piedmont gentle slope is well developed at the junction of the basin floor and the surrounding mountains. The piedmont gentle slope, dipping 7 to 15 degree towards the basin, shows a smooth surface with a slightly concave or almost linear vertical section. This slope coincides with alluvial and talus cones by Yoshida and Igarashi (1984), colluvial slope by Saijo (1991), and alluvial cone by Sakai et al. (2001). The piedmont gentle slope successively changes into the Gokarna surface in the direction of the basin in many places, and fades into narrow valleys in the back slopes. Surficial part (mostly less than 10 m deep) of the piedmont gentle slope is composed mainly of poorly sorted subrounded/subangular gravel or coarse sand with angular gravel. These sediments, tentatively called piedmont gentle slope deposit in this paper, are regarded as being of colluvial or fluvial origin. On the basis of their morphological characteristics and geomorphic settin integrated with the piedmont gentle slope deposit, it was determined that the piedmont gentle slope had been formed by debris supplied from the surrounding mountain slopes. The organic clayey sediments were found at both northern and southern margins of the basin, and the piedmont gentle slope deposit overlies or interfingers with them. It is estimated, therefore, that these organic clayey sediments are considered to be lacustrine deposited near the shoreline of the lake at the foothills in a rising trend of lake level. Such deposits are recognized up to an altitude of between 1420 and 1440 m asl Several radiocarbon dates obtained from these lacustrine indicate that the uppermost part of them was deposited between 37100 and 29200 yr B.P. We can therefore conclude that the maximum level of the Paleo-Kathmandu lake reached at least 1420 to 1440 m asl and the lake occupied almost all of the basin around 30000 yr B.P. Judging from the relationship between the piedmont gentle slope deposit and the lacustrine, the debris derived from the surrounding mountains was frequently transported and flowed into the lake. These estimates suggest a possibility that the PaleoKathmandu lake was drained not by the Bagmati river but another river. However, the cols on the divide of the surrounding mountains occur at the altitudes between 1465 and 1500 m asl. If we ignore tectonic movement, it is apparent that the altitudes of the cols were higher than the estimated maximum level of the Paleo-Kathmandu lake. In addition, the red weathered bedrocks are observed in topsoil of the cols on the divide. This also suggests that those cols were not covered by the lake water and overflow from the cols did not occur. It is estimated that the drainage of the lake by the Bagmati river occurred just after 30000 yr BP and the lake has been gradually reduced. The Gokarna surface emerged in conjunction with the drainage of the lake. Supply of the debris from the surrounding mountains continued even after the recession of the lake and resulted in the piedmont gentle slope formation.

Changes in mineral composition and depositional environments recorded in the present and past basin-fill sediments of the Kathmandu Valley, central Nepal

Surface and drill core sediments from the Kathmandu Basin were investigated to decipher their capability to record changes in the mineral composition and depositional environment. Spatial distribution of mineral assemblages in the terrigenous sand and mud reflects different provenance of the past and present basin filled sediments. Bulk mineral assemblages in the modern sediments are controlled by supply of terrigenous detritus from the source rocks in the adjacent surrounding mountains. This suggests the occurrence of granitic and gneiss source in the north and metasediments in the eastern, western and southern part of the Kathmandu Basin. X-ray diffraction (XRD) experiment was done for the mineralogical study of the present river sediments and RB core sediments. RB core was drilled at Rabhibhawan in the western part of the Kathmandu Basin (Sakai et al. 2001). All samples were prepared on the basis of rule adapted by Kuwahara (2001). XRD measurement were done by Rigaku X-ray Diffractometer RINT 2100V, using CuKα radiation monochromatized by a curve graphite crystal in a step counting time of 2 second. The profile fitting of obtained XRD patterns was performed with an Apple Power Macintosh computer and a scientific graphical analysis program XRD MacDiff (Petschik 2000). The result of the individual minerals obtained from the profile-fitting method was used for quantitative analysis. Relative amount of minerals in the sediments were determined by calibration curve obtained from integrated intensity ratio of the standard mineral to internal standard zincite. X-ray diffraction (XRD) analysis of greater than 2 mm fraction of the present river sediments from the Kathmandu Basin shows quartz, K-feldspar, plagioclase and mica to be the dominant bulk minerals (accounting over 80%). It shows the presence of low amounts of chlorite and calcite. Chlorite is relatively higher in the northern part than the east, west and south. Based on the study done, the mineral assemblages of the present river sediments within the Kathmandu Valley are divided into two groups. The first group is rich in mica, poor in quartz and has presence of K-feldspar, plagioclase and chlorite in all samples. This group of mineral originated from Shivapuri Lekh of the granite and gneiss complex. The second group is rich in quartz and poor in mica and has presence of plagioclase, K- feldspar and very poor chlorite. This group probably originated from metasediments in the eastern, western and southern part of the Kathmandu Valley. Some samples of this group also contain very low percentage of calcite (less than 5 %). Mineral assemblages in the RB core sediments from 7 to 40 m depth indicate the same as those encountered in the present river sediments. XRD-analysis of the greater than 2 mm fraction shows the presence of quartz (10-60%), K-feldspar (2-37%), plagioclase (2-8%), mica (4-19%), chlorite (2 -14%) and calcite (0-18%). Relatively high percentages of these minerals are

Variations of paleoclimate and paleoenvironment during the last 40 kyr recorded in clay minerals in the Kathmandu Basin sediments

...... The Asian monsoon system and its evolution are known to be closely linked to the Himalayan-Tibetan orogen. The Kathmandu Basin is the best target for clarifying the variability of the Asian monsoon climate and its linkage to the uplifting of the Himalayan-Tibetan orogen, since it is located on the southern slopes of the central Himalayas and filled with thick sediments from late Pliocene to Quaternary (Sakai 2001). Unfortunately, previous studies could not critically and completely resolve on the paleoclimatic and paleoenvironmental changes in the Kathmandu Basin because of samples from discontinuous surface exposures. Our group has proceeded on a Japan-Nepal collaborative project “Paleo-Kathemandu Lake (PKL) Project”. In the project, we have carried out core drilling within the Kathmandu Basin and have investigated the cores and surface exposures from various viewpoints and methods. The reconstruction of paleoclimatic and paleoenvirnmental variations recorded in the Kathmandu Basin sediments is one of many purposes of our project. We have already reported the results of fossil pollen analysis and characteristics of sediments for a drilled core sample obtained from the Kathmandu Basin and surface geological survey in the southern part of the Kathmandu Basin. Clay minerals represent useful markers of successive climates, since they formed through weathering or hydrolysis processes during successive periods of the geological history and basically express the intensity of weathering or hydrolysis in the land masses adjacent to sedimentary basin. The information provided from such clay minerals fundamentally integrates the combined effects of temperature and precipitation. Detrital clay minerals can also be used as tracers of sediment transport processes, dispersal and provenance. Clay minerals in the Kathmandu Basin sediments do contain good information on the paleoclimate and paleoenvironment in this area, because they are directly formed through the weathering or hydrolysis process only from the parent minerals, feldspars and micas, in both gneiss and granite of the Shivapuri injection complex and weakly metamorphic rocks of the Phulchauki Group. In order to reconstruct the paleoclimatic and paleoenvironmental changes during the last 40 kyr recorded in the Kathmandu Basin, we examined the estimation of the amount of the clay size fraction, the relative amounts of individual clay minerals, and the crystallinity of illite in the drillcore sediments by using the decomposition procedure of X-ray diffraction (XRD) patterns and the mineral reference intensity (MIF) method. In this paper, we report the paleoclimatic and paleoenvironmental information in the Kathmandu Basin deduced from the clay mineral data. For clay mineral analysis, we used core sediment samples collected at 10 cm interval between 7 m and 40 m in depth of the RB core, which was drilled at Rabibhawan in the western central part of the Kathmandu Basin and is 218 m long (Sakai et al. 2001). The topmost part of the RB core from 7 m to 12.15 m in depth is generally composed of medium-to very coarse-grained micaceous granitic sand. The core sediments between 12.15 m and 40 m in depth are organic black or dark gray mud called “Kalimati Clay”. A 14 C age of the Kalimati clay at 38.3 m in depth of the RB core is 44690±360 yr. B.P. and the mean sedimentation rate between 7 m and 40 m in depth of the RB core is about 900 mm/kyr.

Fluctuation of Indian monsoon during the last glacial period revealed by pollen analysis of Kathmandu Basin sediments, Nepal Himalaya

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 133-4

Plio-Pleistocene rapid uplift process of the Nepal Himalaya revealed from fission-track ages

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 98-99

Streambank erodibility and lateral instability hazard in the Manahara River, Kathmandu basin, Nepal

The Manahara River, a large tributary of the Bagmati River, is located in the northeast of the Kathmandu basin. Recently, the river (especially, its lower reach) has been excessively mined for sand and gravel. Such anthropogenic activities together with the natural processes have led to rapid and haphazard shifting of its channel. As a result, its banks and adjacent areas are subject to failures and other mass movements. In this paper, the river course is evaluated in terms of its bank erosion and lateral instability (BELI) hazard by considering bank erodibility hazard index, near-bank stress index, lateral instability hazard index, and anthropogenic disturbances.
 The downstream reaches of the fourth-order stream belong to a moderate BELI hazard, whereas the fifth-order stream in the middle reach, near Mulpani, falls under a very high hazard. The rest of its fifth-order stretch exhibits a high hazard. An increase in the degree of anthropogenic disturbances increases the BELI hazard in the downstream stretch of the river. Poor riparian vegetation seems to be one of the factors aggravating the bank erodibility hazard in the river. Hence, the restoration of bank vegetation and establishment of riparian vegetation buffer zones can significantly reduce the BELI hazard.

A fossil freshwater Thalassiosira, T. inlandica sp. nov. (Bacillariophyta), with semicontinuous cribra and elongated marginal fultoportulae

T. Hayashi, Y. Tanimura and H. Sakai. 2007. A fossil freshwater Thalassiosira, T. inlandica sp. nov. (Bacillariophyta), with semicontinuous cribra and elongated marginal fultoportulae. Phycologia 46: 353–362. DOI: 10.2216/06-60.1A fossil freshwater diatom species, Thalassiosira inlandica sp. nov., is described from Pleistocene lacustrine sediments of the Kathmandu Basin on the southern slope of the Nepal Himalaya. Thalassiosira inlandica is characterized by (1) a nonplicated (flat) valve face, (2) networks of ribs and costae, (3) semicontinuous cribra composed of the aggregate of many pore rows, (4) absence of valve face fultoportula, (5) elongated marginal fultoportulae, and (6) multiple rimoportulae in the mantle area. This new species is compared with allied taxa based on several morphological characters.

PUNCTICULATA VERSIFORMIS SP. NOV. AND CYCLOTELLA KATHMANDUENSIS SP. NOV. (BACILLARIOPHYTA), NEW FOSSIL SPECIES FROM MIDDLE PLEISTOCENE LACUSTRINE SEDIMENTS, KATHMANDU, NEPAL HIMALAYA1

New fossil species, Puncticulata versiformis sp. nov. and Cyclotella kathmanduensis sp. nov., are described from lacustrine sediments in the Kathmandu Basin on the southern slope of the Nepal Himalaya. They were dominant in the Middle Pleistocene. Both LM and SEM observations reveal their unique morphological features. Puncticulata versiformis is characterized by (1) various valve outlines (circular, elliptical, or oval), (2) a tangentially undulate central area, (3) a complex alveolar structure composed of three kinds of costae (thick costae, thin costae, and thin and short costae), (4) well‐developed spines (Y‐shaped and tapering) located on one side of the valve face/mantle area junction, and (5) a valvocopula with an extremely undulate margin. The species‐specific feature of C. kathmanduensis is the presence of two kinds of alveolate zones in a single valve: type‐1 zone composed only of normal costae, and type‐2 zone composed of both normal costae and recessed costae bearing fultoportulae. Changes in valve ornamentation occur in these two species from initial valves to vegetative valves. In P. versiformis, the arrangement of areolae with internal domed cribra and fultoportulae in the central area changes from radial rows in the initial valve to groups in the vegetative valve. In the initial valve of C. kathmanduensis, the type‐1 alveolate zone is generally absent.

The lacustrine section at Lukundol, Kathmandu basin, Nepal: Dating and magnetic fabric aspects

The fluvial and lacustrine sediments of Kathmandu basin in central Nepal are a good archive to study the past environmental changes and the history of development of the SW Indian monsoon. A sequence of about 170 m exposed at the southern part of the basin is divided into two lithological units: Lukundol Formation and Itaiti Formation. The Lukundol Formation mainly comprises fine-grained sediments whereas gravel beds dominate the Itaiti Formation. In total 750 oriented samples were collected all along the accessible part of the sequence in 10 cm 3 plastic boxes with a sampling interval of ∼10 cm. Additional 128 samples were collected for pollen analysis with a sampling interval of ∼50 cm. Magnetostratigraphy is established based on four magnetic transitions indicating that the sampled section spans 0.75–1.1 Ma. Amino-acid dating on a fossil found in a lignite layer at 88 m yields an age of ∼0.8–1.0 Ma is consistent with the age derived from magnetic polarity stratigraphy. Anisotropy of magnetic susceptibility (AMS) measured in 250 samples spanning the whole section reveals a sedimentary fabric, with NW–SE oriented maximum directions. Lithological changes are significant at 67.5 m, above which thick gravel beds appear and imply significant change in the depositional regime. Mainly two magnetic components are identified: Magnetite – dominant all along the whole sequence, and hematite – relatively more important at depths with lower susceptibility.

Bank instability and erosion problems in Bishnumati River, Kathmandu, Nepal

The Bishnumati River, a major tributary of the Bagmati River in the Kathmandu basin, suffers from bank erosion, river instability, and environmental degradation. These processes are responsible for the loss of sediments, modification of river morphology, loss of vegetative buffer zone, and deterioration of stream environment. Bank erosion hazard in the Bishnumati River was assessed at ten different reaches using bank height ratio, ratio of riparian vegetation rooting depth to bank height, rooting density percentage, bank slope, and bank surface protection. Vertical and lateral stability of the river was assessed at four reference segments, namely at Bishnumatigaun, Okhaltar, Mahadevtar, and Tamsipakha, respectively from upstream to downstream.
 The riverbanks in the Bishnumatigaun segment show fluctuating trends of bank erosion potential from upstream to downstream. All the four segments of the Bishnumati River are vertically unstable and all the segments except the Okhaltar are entrenched. The Tamsipakha and Okhaltar segments have a high risk of lateral shifting, as they possess high values of bank erosion hazard index, meandering width ratio, and width-depth ratio. The Bishnumati river is in a degrading condition and capable of eroding sediments. The main causes of river instability are the excavation of river sediments, encroachment on riverbanks, clearing of riparian vegetation, and canalization.

The geochemical study of fluvio-lacustrine aquifers in the Kathmandu Basin (Nepal) and the implications for the mobilization of arsenic

The geochemical analyses of fluvial-lacustrine aquifer sediments of the Kathmandu Valley have been made as a step in assessing the environment for the mobilization of arsenic in groundwater. Elements measured by X-ray fluorescence (XRF) include 4 major oxides (Fe2O3, TiO2, CaO, P2O5) and 14 trace elements (As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc and TS). Elution tests of 15 selected core samples were also carried out to determine the potential leaching of arsenic from the sediments. The XRF results show that average bulk concentrations of the major oxides and trace elements are similar to modern unconsolidated sediments and average upper continental crust. However, the abundance of elements varies with grain size, with higher concentrations in finer-size grades. Variations in elemental abundances within the basin are strongly controlled by sediment facies. The elution tests show that greater amounts of arsenic are generally eluted from the fine sediments, although the rates are variable. The results overall suggest that As concentrations in the bulk sediments are not a controlling factor for elevated As in the Kathmandu Basin groundwater, and the roles of other factors such as redox conditions and organic matter contents are likely to be more significant.

Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins

Core-drilling project carried out in the southern margin of the Kathmandu Basin revealed that muddy debris flow deposits dammed up the Proto-Bagmati river to form the Paleo-Kathmandu Lake during the Jaramillo subchron from 1.07 to 0.97 Ma. Subsequent deposition of the alluvial fanglomerate, derived from the uplifting Mahabharat Range to the south, raised the dam deepening the lake-water. After 1 Ma, in the southern part of the basin, palaeo-current directions changed from southward to northward and deposition of gneissose and granitic detritus are replaced by meta-sediments derived from the Mahabharat Range. During the same time, at about 1 Ma, the boulder conglomerates were deposited on top of the Siwalik Group as piggy-back basins in front of an intra-basinal high, along the Main Dung Thrust in Nepal and NW India. Onset of movement of the Main Dung Thrust is dated back to 3 to 2.4 Ma [Mugnier, J.L., Huyge, P., Leturmy, P., Jouanne, F., 2003, Episodicity and rates of thrust-sheet motion in the Himalayas, Western Nepal. Am. Assoc. Petrol. Geol., Mem. 82, 1–24]. The Main Frontal Thrust is most active at present suggesting that imbricated structure of the Siwalik Group was formed by convergence of the Indian plate during the last 3 myr. The accretionary wedge of the Siwalik Group, stacked beneath the Main Boundary Thrust, might have started to jack up the frontal range of the Lesser Himalaya since 1 Ma. Coeval uplift and erosion of the frontal range of the Lesser Himalaya and the intra-basinal high in the Siwalik since 1 Ma are possible causes of an abrupt increase in both sedimentation rate and grain size of detrital quartz, and changes in composition of clay minerals, recorded in the sediments of the Bengal Deep Sea Fan at 0.9 Ma.

An Overview of Younger Kathmandu Lake, Nepal, during the Late Quaternary, with Special Reference to Ferruginous Structures in Carbonaceous Sediments

The Kathmandu Basin in Central Nepal is located in the Lesser Himalayas. Older Kathmandu Lake evolved during the Pliocene and Early Pleistocene; Younger Kathmandu Lake with its widespread carbonaceous rocks and ferruginous biogenic structures silted up during the Late Quaternary. Sedimentological analyses supplemented with palynological and chemical studies of carbonaceous sediments formed the basis of the present paleogeographic and paleoclimatic studies. The basin was covered by a perennial freshwater lake before 30,000 yr. B.P. It was silted up with alluvial and fluvial sediments. Diatomaceous earths, and carbonaceous mud- and siltstones were deposited in the center of the lake and small relict ponds. Toward the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. The most proximal parts of this alluvial-fluvial sedimentary wedge also have debris flows interfingering with fine-grained floodplain deposits. Three lake highstands (> 30,000 yr. B.P., 28,000-19,000 yr. B.P., 11,000-4000 yr. B.P.(?)) have been recognized in the sedimentary record of Younger Kathmandu Lake. Second-order waterlevel fluctuations were triggered by damming as a result of tectonically induced landslides. Firstorder water-level fluctuations were due to climatic changes. During the last highstand, favorable conditions occurred for vertical to subvertical tubular ferricretes to develop. The tubular structures are likely to have been created by bottom dwellers. Three different types of mineralization have evolved during diagenesis, each of which is characterized by different Fe-bearing marker minerals. The overall silting-up process in Kathmandu Basin is manifested on a microscopic scale in the textural and mineralogical variations of Fe mineralization in these biogenic sedimentary structures.