Paleo-Kathmandu Lake Research Articles

Investigation of ground fissures at Kausaltar, Kathmandu by in-situ testing and spatial geographical mapping

On April 25th, 2015, the Gorkha earthquake jolted the central region of Nepal, causing extensive damage to buildings and grounds in the urban areas of Nepal. One embankment section of Kathmandu-Bhaktapur Road, crosses a small valley in the center of the Kathmandu Basin. The earthquake has caused this embankment to deform with its supporting soil. Investigating the mechanism of this ground deformation from the geotechnical and geological viewpoints was deemed necessary to examine possible countermeasures. For this purpose, we conduct several in-situ tests such as microtremor measurements, standard penetration tests, and Multichannel Analysis of Surface Waves. These investigations make two soft soil layers emerge as a causative factor. The estimated 3D soil profile shows that the deformed ground overlaps the area where the weak soil layers are below the groundwater level. The 3D soil profile also suggests that groundwater lowering using existing wells can reduce the water-saturated area by 81%. Carbon dating shows that the causative layer formed before the Paleo-Kathmandu Lake dried up.

Two times lowering of lake water at around 48 and 38 ka, caused by possible earthquakes, recorded in the Paleo-Kathmandu lake, central Nepal Himalaya

Sedimentary facies and micro-fossil analyses, and AMS14C dating were performed in order to reveal the water-level fall events and draining process of the lake (Paleo-Kathmandu Lake) that existed in the past in the Central Nepal Himalaya. The sedimentary facies change from the lacustrine Kalimati Formation to the deltaic Sunakothi Formation in the southern and central Kathmandu basin, and the abrupt and prominent increase of phytoliths Bambusoideae and Pediastrum, and contemporaneous decrease of sponge spicule and charcoal grains around 48 and 38 ka support the lowering of water level at these times. According to the pollen analysis, both events occurred under rather warm and wet climate, thus supporting that they were triggered by tectonic cause and not by climate change. The first event might be linked to a possible occurrence of a large earthquake with an epicenter in the vicinity of the Paleo-Kathmandu Lake. The occurrence of a mega landslide in Langtang area close to the north of the Kathmandu Valley producing pseudotachylite dated at 51 ± 13 ka could be linked to this earthquake. Finally, the water was completely drained out from the remnant lake at the central part of the Kathmandu basin by ca.12 ka.

Reexamination of changes in indian monsoon and paleoenvironment based on the composition analysis of microfossils in Paleo-Kathmandu Lake

Reexamination of changes in indian monsoon and paleoenvironment based on the composition analysis of microfossils in Paleo-Kathmandu Lake

中–後期更新世における古カトマンズ湖珪藻の古群集生態

中–後期更新世における古カトマンズ湖珪藻の古群集生態

135 Long climatic record during the last 700 kyr revealed by pollen and charcoal analyses on the lacustrine sediments of the Paleo-Kathmandu Lake, the central Himalaya

135 Long climatic record during the last 700 kyr revealed by pollen and charcoal analyses on the lacustrine sediments of the Paleo-Kathmandu Lake, the central Himalaya

過去60万年間にわたる古カトマンズ湖の浮遊性・底生珪藻群集の生産性・種多様性の比較研究

過去60万年間にわたる古カトマンズ湖の浮遊性・底生珪藻群集の生産性・種多様性の比較研究

Monospecific planktonic diatom assemblages in the Paleo-Kathmandu Lake during the middle Brunhes Chron: Implications for the paradox of the plankton

The almost monospecific assemblages of planktonic diatoms that occurred in ancient lakes during the middle Brunhes Chron are an important research issue for lake ecosystems because their existence implies that the famous paradox of the plankton was no paradox at that time. To investigate their cause, I examined the assemblage structure and diversity of planktonic diatoms in the Paleo-Kathmandu Lake at Nepal Himalaya over the past 600kyr. The assemblage structure changes frequently after 300ka but remains nearly stable before then. Additionally, planktonic species show a relatively high average diversity after 300ka but show an extremely low average before then. Those abnormal ecological conditions before 300ka reflect the formation of almost monospecific planktonic diatom assemblages of Cyclotella kathmanduensis and Puncticulata versiformis. The assemblage structure and species diversity of the planktonic diatoms were controlled by lake-level fluctuations (especially lake-level declines) as ecological disturbances. After 300ka, fluctuations in lake levels occur in about 20ka intervals, which probably helped to sustain diverse planktonic diatom assemblages. In contrast, much longer intervals occurred between lake-level fluctuations before 300ka, and these were of a frequency that was too low to sustain diverse planktonic diatom assemblages. This may have allowed C. kathmanduensis and P. versiformis to form monospecific assemblages.

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.

Middle to late Pleistocene climatic and depositional environmental changes recorded in the drilled core of lacustrine sediments in the Kathmandu Valley, central Nepal

Since October 2000, we have undertaken core-drilling for pure academic purposes at Kathmandu, Nepal, in order to clarify the continuous paleoclimatic records in the southern slope of the Himalaya and to reconstruct the depositional environmental changes of the ancient lake in Kathmandu (Paleo-Kathmandu Lake) during the last one million years. In this project, we also aimed to clarify the uplift history of the Himalaya and its linkage to paleoclimatic and paleo-environmental changes. We could have penetrated black muddy lacustrine sediments, called the Kalimati Formation, at western central part of the valley (RB core at Rabibhawan) in 2000 (Sakai et al 2001) and southern part of the valley (CP core at Champi and JK core at Jorkhu) in 2003 (Figures 1 and 2). We also could have penetrated the lower part of the fanglomerate, called Itaiti Formation and underlying Lukundol Formation which shows marginal lacustrine facies comprising of fluvial and swamp deposits in 2001 (Sakai 2001a). The longest core drilled at Rabibhawan is 218 m in length and ranges in age from about 700 ka to 10 ka on the base of

Reconstruction of environmental and climatic changes in the Paleo-Kathmandu Lake during the last 700 ka: An approach from fossil-diatom study

Terrestrial paleoclimatic and paleoenvironmental records on the Indian monsoon are extremely limited, though many investigations have been done on the deep-sea sediments in the Indian Ocean during the last few decades. In order to clarify the terrestrial monsoonal climatic records during the Quaternary, we undertook core-drilling of the basin-fill sediments of the Kathmandu Valley, which is located under Indian monsoon zone. In this paper, we attempt to reconstruct the environmental changes of the Pakeo-Kathmandu Lake, on the basis of studies on the fossil-diatom collected from a 218-m-long core drilled at Rabibhawan, western central part of Kathmandu. Except the uppermost thin cover of fluvial sediments (Patan Formation), the core is continuous and mainly composed of muddy lacustrine sediments (Kalimati Formation) containing abundant fossil-diatoms, which document ecological responses to climatic and environmental changes (Sakai 2001, Hayashi et al. 2002). Mud samples were collected at 50 cm interval from 7 to 45.5 m in depth and at 2 m interval from 47.5 to 218 m in depth. We identified each species and counted the number of valves by means of a scanning electron microscope (JSM-5600) at x5,000 magnification, because dominant species in the RBcore are very small, ranging from 5 to 30 μm. For each sample, at least 300 diatom valves were counted. On the basis of relative abundance and the number of principal diatoms, eight fossil zones and three subzones in zone 2 were defined in the Kalimati Formation (Figure 1). Zone 8 at the basal part is characterized by variety of diatom assemblages and relatively low number of diatom valves, which reflect marsh or very shallow-water environments. Based on water-level indicators (ratio of planktonic diatoms to benthic diatoms and frequency of genus Aulacoseira), the water-level seems to have deepened gradually from zone 8 to zone 7, and retained deep condition from zone 6 to zone 4. Zones 6 to 4 are characterized by monodominance of Cyclotella: Cyclotella sp.1 is a characteristic in zone 5, and Cyclotella sp.2 is in zone 6 and zone 4. Number of their valves is very abundant. On the other hand, in zone 3 and zone 2, there are several dominant species and number of total diatom valves decreases. Furthermore, ratio of planktonic diatoms to benthic diatoms periodically rises and falls, which probably indicates water-level fluctuations. Especially in zone 2A, fluvial and marsh environments were expanded in marginal area of the lake, because relative abundance and number of Staurosira construens and Pseudostaurosira brevistriata, indicative species for marsh environment, become high. In zone 1, ratio of planktonic diatoms increases, which indicates the water-level rose again. But after zone 1, number of total diatom valves drastically reduces. It demonstrates that the lake was drained during a short period at about 12 ka. Comparison of environmental changes of the PaleoKathmandu Lake with δO record from a core MD900963 collected in the Indian Ocean (Bassinot et al. 1994) for the past 630 ka, shows that the water-level of the Paleo-Kathmandu Lake seems to have fallen at glacial age (MIS 14, 10, 8, 6, 2). We recognized following five major events expressed by high ratio of benthic diatoms, which indicate lowering of water-level. According to the time-scale derived from paleomagnetic study and AMS 14C dating of the core, the following stages correspond to marine isotope stages (MIS) from MIS 14 to MIS 2. Zone 5 ~ zone 4 = MIS 14 Early stage of zone 3 = MIS 10 Later stage of zone 3 = MIS 8 Later stage of zone 2C = MIS 6 Later stage of zone 2A = MIS 2 It is likely that these environmental changes of the PaleoKathmandu Lake mainly caused by fluctuations of Indian monsoon related to global climatic changes (glacial-interglacial cycle).

古カトマンズ湖堆積物から抽出された多環式芳香族炭化水素類(PAHs)を用いた乾湿変動の復元_---_最終氷期極大期前後のPAHsの濃度変化_---_

古カトマンズ湖堆積物から抽出された多環式芳香族炭化水素類(PAHs)を用いた乾湿変動の復元_---_最終氷期極大期前後のPAHsの濃度変化_---_

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.

The Kathmandu Basin: an archive of Himalayan uplift and past monsoon climate

The basin-fill sediment of the Kathmandu Valley is one of the best archives of both Himalayan uplift and past monsoon climate. Thick lacustrine sediments, mainly comprised of black clay, locally called as Kalimati clay, are the deposits of the Paleo-Kathmandu Lake during the last 2.5 million years, and the underlying fluvial sediments represent the deposits by the Proto-Bagmati River of the valley. The clay-predominant Kalimati Formation is rich in proxies of paleoclimate, such as fossil pollen and diatom as well as vertebrate and invertebrate fossils. In addition, crystallinity, chemical composition and stable isotope of clay minerals have a potential to provide a continuous paleoclimatic and paleoenvironmental records in the Central Himalaya. The sedimentological study of the fluvial and deltaic deposits that rest on the Kalimati Formation would reveal recent changes of tectonic and depositional environments in the Central Himalaya.

Paleoclimatic changes during the last 2.5 myr recorded in the Kathmandu Basin, Central Nepal Himalayas

Palynological and sedimentological studies of a series of slimes collected from a 284m-long drill-well from the Kathmandu Basin reveal paleoclimatic records and environmental changes within the Kathmandu Valley during the last 2.5myr. The slimes are composed of fluvio-deltaic and lacustrine sediments comprising sand beds of 66.3m and mud beds of 218m in length. Pollen analyses show Quercus and Cyclobalanopsis are predominant, with frequencies exceeding 70%. Pinus, Alnus and Gramineae are the next dominant taxa. Three fossil pollen zones were discriminated; each zone reflects major climatic change: Zone I, the oldest stage, indicates a cool and rather wet climate during 400kyr from ca. 2.5 to 2.1Ma; Zone II, the middle stage, reflects a warm and relatively dry climate without remarkable fluctuation; Zone III is characterized by seven cycles of warm-and-wet and cold-and-dry climate, which reflect the alternation of glacial and interglacial periods. The last cold maximum, 11m deep, corresponds to the last glacial age around 20kyr bp, judging from the 14C dating of the uppermost part of the lacustrine sediments.The Paleo-Kathmandu Lake is likely to have been initiated at around 2.1Ma and to have been filled with black organic mud, the Kalimati Clay. The top of the Kalimati Clay is eroded and was overlain by fluvial sand after the last glacial age. The abrupt appearance of a 4m-thick fossiliferous sand bed at the top of the middle member suggests a lowering of water level at around 1Ma.

Changes in the depositional system of the Paleo-Kathmandu Lake caused by uplift of the Nepal Lesser Himalayas

Sedimentological and palynological studies on a series of slimes taken from a drill-well in the central part of the Kathmandu Basin and the Lukundol Formation at the southern margin of the basin indicate that the depositional environments of the Paleo-Kathmandu Lake changed at around 1Ma. In the central part of the basin, the abrupt appearance of a fossiliferous 4m thick sand bed, containing abundant fish teeth and gastropod opercula, and shell fragments, in an otherwise open-lacustrine mud sequence, suggests that a lowering of the water level occurred at about 1Ma. The common occurrence of the green alga Pediastrum in the overlying mud beds implies that the lake remained shallow after the deposition of the sand bed. Changes in the depositional system of the Paleo-Kathmandu Lake at about 1Ma are also recorded in the Lukundol Formation. Granitic gravel and detrital muscovite flakes, which are common in the Lower and Middle Members, disappear from the Upper Member. Paleocurrent directions in the Lower and Middle Members show flow from the north and east, whilst in the Upper Member they change to flow from the south. Sedimentary facies change from marginal lacustrine in the Middle Member, to a braided river facies in the Upper Member. These changes occurred at around 1Ma, at the base of the Upper Member. They seem to have been caused by the initiation of rapid uplift of the Mahabharat Lekh, which was due to faulting and underthrusting along the Main Boundary Thrust System.

Climatic Changes and Tectonic Events Recorded in the Paleo-Kathmandu Lake Sediments

Climatic Changes and Tectonic Events Recorded in the Paleo-Kathmandu Lake Sediments

Facies association and depositional environment of fan-delta sequence in southwest Kathmandu Basin, Nepal

Lake marginal sedimentation prevailed around the Paleo-Kathmandu Lake. Owing to the difference in local basin conditions; tectonics, source rock types and river systems therein, the lake marginal environments and sedimentary facies associations differ around the Paleo-Kathmandu Lake. In this study, the basin-fill sediments of southwestern margin of the Kathmandu Basin were studied for the sediments recorded in vertical sequences at various localities and facies analysis was made. Mainly eight facies were recognised. They were matrix-supported massive gravel (Gmm), matrix-supported graded gravel (Gmg), gravelly fine or mud (GF), massive silt (Fsm), massive mud (Fm), ripple-laminated silt or laminated silt/mud/clay (Fl), carbonaceous clay (C), and incipient soil with roots (Fr). Four facies associations that were identified were proximal fan-delta facies association (FA1), mid fan-delta facies associaiton (FA2), distal fan-delta facies association (FA3), and gravelly sinuous river facies association (FA4). Remarkably, these facies associations do not contain any sandy facies and foreset bedding of Gilbert-type. The fan-delta region was characterised by flood-dominated flows and vertical accretion of fines in the flood basins, and vegetated swamps rich in organic sediments. The distribution of facies associations suggests extensive lake transgression followed by rapid lake regression. The recent river system then incised the valley against local upliftment due to faulting or lowering of base level of the main river in the Kathmandu Basin probably related to draining out of the lake water. doi: Bulletin of the Department of Geology, Vol. 12, 2009, pp. 1-16

Stratigraphy and depositional environments of basin-fill sediments in southern Kathmandu Valley, Central Nepal

Quatenary fluvio-lacustrine basin-fill sediments in the southern part of the Kathmandu Basin was studied in order to clearify the stratigraphy and reconstruct the sedimentary environment during the initial stage of the Paleo-Kathmandu Lake. Six stratigraphic units; Tarebhir, Lukundol, Itaiti, Kalimati, Sunakothi Formations and Terrace gravel deposits, have been described based on field observation of lithology and sediment distribution. The Tarebhir Formation is the basal unit which is overlained by alluvial fan of the Itaiti Formation in the southern part and by the marginal lacustrine deposit of the Lukundol Formation towards the northern part. Further 3 km toward the north from the basin margin at Jorkhu the the Lukundol Formation is overlain by the open lacustrine facies of the Kalimati Formation. At the same locality the latter is overlain by fluvio-lacustrine facies of the Sunakothi Formation. Moreover, the Terrace gravel deposits erosionally cover the Sunakothi Formation. The Kalimati Formation thickens northward, while the Sunakothi Formation thickens between the central and southern part of the basin. The study shows that the Sunakothi Formation is of fluvio-lacustrine (fluvial, deltaic and shallow lacustrine) origin and extends continuously from the southern margin (~1400m amsl) to the central part (~1300m amsl) of the basin. It also indicates that sediments of this formation were deposited at the time of lake level rise and fall. Thick gravel sequence in the southern margin represents the alluvial fan before the origin (before 1 Ma) of the Paleo-Kathmandu Lake, while thick gravel sequence situated above the Sunakothi Formation is the Terrace gravel deposits of the late Pleistocene age (14C method), deposited during and after the shrinkage of the Paleo-Kathmandu Lake from south to north. doi: 10.3126/bdg.v11i0.1544 Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 11, 2008, pp. 61-70