Journal of Nepal Geological Society

Factors controlling the variation in physicochemical parameters of springs in Melamchi area, Nepal

Karishma Khadka — 2023-08-25

Understanding the factors controlling the spatial and seasonal variation of physicochemical parameters of groundwater are important for quick water quality assessment and for a better planning of the water quality management strategies. This study evaluated spring’s five physicochemical data (pH, EC, TDS, DO and temperature) for 75 springs in two seasons (pre and post monsoon). The aim of the research presented in this paper is to assess physicochemical parameters of spring water, understand their seasonal and spatial variation and the factors controlling the variation. Physicochemical parameters were correlated with each other to understand the processes and an attempt has been made to understand groundwater processes studying the physicochemical parameters.

Groundwater is slightly acidic to slightly alkaline with pH ranging from 5.5 to 7.5 and is weakly to very weakly mineralized with EC range of 21.6–234.0 μS/cm in pre-monsoon and 10.5–189.8 μS/cm in post-monsoon. TDS ranged between 5.3–116.9 mg/l and 10.8–94.9 mg/l, DO between 3.1–9.4 mg/l and 2.4–7.3 mg/l and groundwater temperature between 18.4–27.9°C and 19.2–27.4°C in pre-monsoon and post monsoon season respectively. TDS and EC are noted to be higher in pre-monsoon whereas pH, DO and temperature are comparatively higher in post-monsoon. The south-east region of the study area shows minimum groundwater temperature. Minimum concentration of EC and TDS were observed in south-east and north-west part of the study area with values increasing towards the river valley. Groundwater temperature shows inverse relation with elevation, pH and DO while correlated with EC and TDS of groundwater in the region. The measured parameters were seasonally and spatially variable.

The results indicate difference for measured physicochemical parameters mainly caused due to results generated by cumulative interactions of monsoonal rain, atmospheric temperature, rock-groundwater interaction, residence time of groundwater and seasonal influences. The change in parameters with respect to elevation and relation between different parameters shows shallow aquifer-related groundwater processes.

Evaluation of two (opx-cpx) pyroxene Geothermometer

Harel Thomas — 2023-08-25

In the last few decades, for estimation of the original equilibrium conditions of the mantle and deep crustal rocks, several empirical as well as synthetic thermometers have been proposed. Various types of rocks from earth, moon or meteorites contain two coexisting pyroxenes. Petrologists have long recognised the potential of coexisting high Ca and low Ca pyroxenes to yield thermometric calculations. Several models have been proposed for two pyroxene thermometers in the last few decades. The authors have compared ten models of two pyroxene thermometers proposed since 1973. Sixty one (61) sample data of granulites from the global literature were collected and processed through the “Opx-Cpx.EXE” software. We conclude that three models are the most valid and reliable of this kinds of thermometers: Kretz (1982), Bertrend and Mercier (1985) and Nickel and Green (1985).

An overview of copper ore mineralization in the Nepal Himalaya

Krishna Prasad Kaphle — 2023-08-25

Exploration of copper ore in the Nepal Himalayas has been carried out by different researchers at different times. The author conducted detailed geological studies and mineral exploration in some of the copper and polymetallic prospects and also reviewed the literature related to copper prospects in Nepal. The present research is able to delineate two distinct copper ore mineralization belts extending from west to east and identify the promising sites for copper ore and polymetallic mineralization that could be feasible for mining. The northern mineralization belt lies close to the Main Central Thrust where the mineralization occurs mostly in metamorphic rocks of garnet amphibolite facies in Darchula,
Bajhang, Gorkha, Dolakha, Solukhumbu and Ilam districts. The southern mineralization belt in the Lesser Himalaya occurs in low-grade metamorphic rocks of greenschist facies in Dadeldhura, Rukum, Baglung, Myagdi, Tanahun, Dhadhing, Makwanpur, Dolakha, Udaypur, Bhojpur and Ilam districts. Most of these prospects were locally exploited on a small scale by traditional mining before 1951. Almost all mineralization in both belts is related to hydrothermal dissemination, irregular multiple vein type, replacement and cavity fillings and only a few polymetallic mineralizations at the exo-contact zone of granite bodies. The ore minerals were concentrated during the processes of regional metamorphism at the time of Himalayan upheaval. In general, the copper content in the northern belt appears to be higher in grade and tonnage as compared to the southern belt. The frequent association of tungsten, molybdenum, bismuth, nickel, tin, lead, zinc ores and traces of gold suggests the urgent need for further detailed exploration and evaluation of selected prospects which could be economic deposits and feasible for mining.

Failure mechanism of large-scale landslide in the central Nepal Himalaya

Bikash Phuyal — 2023-08-25

The geomorphological and geological characteristics of central Nepal reflect the dynamic nature of tectonic and mass movement processes that lead to the occurrence of large-scale landslides (LSLs). The mechanism of the LSL failure process is not well understood due to the different deformation stages controlled by geological structures and causative/ triggering factors. Regional tectonic structures and local rock mass characteristics play a vital role in the formation of LSLs. Engineering geological characterization was performed to evaluate the rock/soil properties, and the associated state of danger (cracks, seepage, etc.). The rock mass rating (RMR) and geological strength index (GSI) methods were used to estimate rock mass quality and strength parameters. The estimated strength parameters together with laboratory test data were used to set up a two-dimensional (2D) landslide surface to simulate the failure scenarios. The simulation of landslide models was iterated under different loading conditions by incorporating geological heterogeneities. The results have demonstrated that the movement of the slope was affected by different stress conditions over time, which were verified using spatiotemporal landslide data. The landslide exhibited a three-stage failure mechanism with the development of tension-induced cracking at the rear and shearing failure of the critical locking section in the middle section. The tension shear and joint-step-path rupture along joint surfaces occurred during different stages of the landslide failure processes. The simulated results showed that the genetic development of LSL is mainly attributed to the geological structures and sudden changes in stress conditions. Thus, the research outcomes can be illustrative to evaluate the role of geological discontinuities, geotechnical parameters and triggering effects by the numerical simulation techniques for the failure mechanism of LSLs.

Facies association and Paleo depositional environment of the exposed Neogene succession in the Sitapahar Anticline of Rangamati Area, Chittagong Tripura Folded Belt (CTFB) region, Bengal Basin, Bangladesh

Minhazul Abedin Shakik — 2023-08-25

Neogene sedimentary rocks are exposed in the Chittagong Tripura Folded Belt (CTFB) Region in Bengal Basin. Sitapahar anticline is situated in the middle part of the CTFB Region. In this structure, eleven litho-facies have been identified in fluvial, overbank, shallow marine, deltaic, and deep marine depositional environments. Surma Group is composed of Bhuban and Boka Bil formation during Miocene time and was deposited in shallow marine, deltaic, and deep marine environments and characterized by the identification of interbedded shale facies (ShI), fine sandstone facies (SFs), lenticular laminated silty shale to shale facies (Zw), wavy bedded sandstone siltstone facies (Slsh), flaserbedded sandstone siltstone facies (Sxhs), ripple laminated sandstone facies (Sr), black Shale facies (Shb) and nodular shale facies (Zn). While, Pliocene Tipam group and Plio-Pleistocene Dupi Tila Formations were deposited in fluvial intervention and characterized by crossbedded sandstone facies (Sxbt), mudstone facies (Mf), interbedded mudstone and sandstone facies (MI).

Geostatistics and inverse distance weighing (IDW) multivariate interpolation methods for regional geochemical stream sediment survey for base metal prospecting around Wapsa, Solukhumbu District, Nepal

Dharma Raj Khadka — 2023-08-25

A systematic regional geochemical stream sediment survey for base metals copper, lead, and zinc over an area of 200 sq.km was carried out around Wapsa in Solukhumbu District. 200 stream sediment samples were collected from active streams and analyzed them using atomic absorption spectrometry (AAS) method. All chemical analysis data were interpreted with geostatistical analysis and inverse distance weighing (IDW) multivariate interpolation method using the geographical information system (GIS). The results have shown consistency with geology, homogeneity, and mineralization in the area which is verified with significant copper anomaly over an old working area around Wapsa. The log₁₀ transformed data of Cu, Pb, and Zn were used to estimate threshold and anomaly determination. The method is valid for regional geochemical stream sediment surveys for base metals prospecting.

Numerical modelling of cut-slope stability in the Lesser Himalayan terrain of west-central Nepal

Krishna Kumar Shrestha — 2023-08-25

Area development without due consideration of geological factors has led to recurrent slope failures within the Lesser Himalayan Zone thereby rendering socio-economic impacts. This issue is well exemplified by cut-slopes in major highways of Nepal traversing through a diverse range of rocks and soils. The study has focused on addressing the stability challenges posed by highly vulnerable excavated cut-slopes with particular attention given to a debris cut-slope considered as a model for this study is located in the Lesser Himalayan terrain of west-central Nepal. The cut-slope model is analyzed by numerical modellings through both the limit equilibrium method (LEM) and finite element method (FEM). The study further evaluated the underlying causes of these failures to recommend the options for remedial measures. Data acquired from two-dimensional electrical resistivity tomography (2D-ERT), multi-channel analysis of surface waves (MASW), and laboratory testing were integrated into numerical modelling and simulation. The results showed that the lower slope below the natural benching is stable whereas the upper failed slope is unstable due to reduced gravitational loading. In LEM, factors of safety (FoS) range from 0.77 to 0.89 for cut-slope angles of 70° and 60° whereas the FoS in modified slopes (50° and 40°) were slightly greater than 1 which ranged from 1.01 to 1.18 and indicated marginal stability. Application of 25 mm diameter soil nails, each measuring 6 m in length and spaced at 2 m intervals with tensile and plate capacity of 125.6 kN, bond strength of 75.4 kN/m, and 80% of nail length have produced FoS of 1.30 or greater in LEM and FEM which is acceptable values specified by EM 1110-2-1902 and IS 456 2000 for end-of-construction and multistage loadings.

An approach of geological mapping and geotechnical investigation: Case study from the Lalitpur Metropolitan City, Nepal

Sanjay Rizal — 2023-08-25

Urbanization is rapidly growing throughout the world, especially in the least developed and developing countries necessitating proper land use planning. However, the lack of sufficient study has created the possibility of unprecedented geohazards. In this study, a detailed geological and engineering geological mapping along with the geotechnical analysis of soils and sediments from the Bhainsepati–Pharsidol area of Lalitpur District has been carried out. Detailed geological maps and geological cross-sections were prepared on the scale of 1:25,000 following the most adopted stratigraphy of central Nepal for both hard rocks and Quaternary deposits. Two mappable geological units like the Tistung Formation and the Chandragiri Limestone of the Phulchauki Group are found within the hard rock succession at the study area’s hill sections. Similarly, the Quaternary succession is mapped into the Lukundol Formation and the Chapagaon Terrace deposits of Kathmandu Valley sediments from older to younger succession respectively. There is a distinct erosional unconformity between the rocks of the Phulchauki Group and the Quaternary Valley sediments. A kinematic analysis of discontinuities developed in the rocks was carried out in the rock slopes found in the area. Both field and laboratory analyses of soil samples were carried out for the representative samples. The laboratory tests include moisture content, specific gravity, particle size distribution, soil consistency, compaction, and consolidation properties. Finally, an engineering geological map was prepared on the same scale as geological mapping showing the details of the distribution of geo-materials. This study is expected to be valuable to make proper land-use decisions for town planning and developmental activities in the rapidly growing city of Nepal.

Tectono-lithostratigraphy of the outer Lesser Himalaya in the Barahakshetra-Tribeni area, Arun valley section, eastern Nepal

Drona Adhikari — 2023-08-25

Geological mapping was carried out in the outer part of the Lesser Himalaya in the Barahakshetra – Tribeni area of east Nepal along the Arun River valley. A revised and updated tectono-lithostratigraphic scheme of the area along with detailed description of each unit and their comparison with central and west Nepal has been presented in this work. The area is divided into three tectonic units named as the Chimra Thrust Sheet, the Dharapani Thrust Sheet and the Tribeni Paraautochthon from north to south, respectively, separated by the Chimra Thrust and the Dharapani Thrust. The rocks of the Chimra Thrust Sheet consist the Pre-Cambrian Phongsawa Group and is divided into the Mulghat Formation, Okhre Formation, Jyamire Gneiss and Belhara Formation, from bottom to top, respectively. The Jyamire Gneiss is comparable to the Ulleri Augen Gneiss and other formations are equivalent to the lower part of the Kuncha Formation. The Dharapani Thrust Sheet, comprising Pre-Cambrian Bhendetar Group, is subdivided into the Chiuribas Formation, Sangure Quartzite and Karkichhap Formation, from bottom to top, comparable with the upper part of the Kuncha Formation, Fagfog Quartzite and Dandagaon Phyllite, respectively. The Tribeni Paraautochthon is divided into the Late Pre-Cambrian Lukuwa Dolomite unconformably overlain by the Gondwana and post-Gondwana rocks of the Barahakshetra Group. The Barahakshetra Group is divided into the Kokaha Diamictite and Baraha Volcanics, Sapt Koshi Formation and Tamrang Formation, from bottom to top, comparable with the Sisne Formation, Taltung Formation and the Dumri Formation, respectively.

Flood hazard modelling using HEC-RAS in the Pathariya Khola, far-western Nepal

Bala Ram Upadhyaya — 2023-08-25

Flood hazards are a common problem in flat-lying areas, particularly in southern Nepal due to heavy precipitation and the consequent large transportation of sediments from hilly regions. Bank erosion, flooding, inundation, and sedimentation are the major problems in the plain area of the Kailali District along the riverbanks of Pathariya Khola, which is located on the southern flanks of the Churia Range. The study has been focused on flood hazards and vulnerability evaluation. The river discharge was calculated by WECS/DHM method and flood frequency in various return periods was analysed using HEC-RAS, RAS-Mapper, and geographic information system (GIS) for hazard modelling to delineate the flood-prone areas. The flood hazard zones were categorized as moderate (26.29%), high (39.59%), and very high (34.12%) based on the computed water depths of peak floods. The agricultural land is the most vulnerable to flooding followed by built-up areas. The results have revealed that the flood hazard areas are ever-expanding with the increased discharge levels of different return periods.

Ground motion characteristics of the 2015 central Nepal (Gorkha-Dolakha) Earthquake and 2023 SE Turkey Earthquake doublets

Dilli Ram Thapa — 2023-08-25

The Alpine-Himalayan orogenic belt, the well-identified seismically active region in the world, has been evidenced by the occurrence of recent two devastating earthquake doublets in 2015 in central Nepal (M_W 7.8 and M_W 7.3) and in 2023 in southeastern Turkey(M_W 7.8 and M_W 7.5). In this study, seismic ground motion characteristics of two damaging earthquakes namely the 2015 central Nepal earthquake doublet that occurred in the central Himalaya and the 2023 southeastern Turkey earthquake doublet that occurred in the eastern Mediterranean has been investigated. The observed strong-motion recordings indicate that the peak ground acceleration (PGA) values are about 152.61–154.96 cm/s² of the 2015 central Nepal earthquake doublet at KATNP station and about 451.834–842.924 cm/s² of the 2023 southeastern Turkey earthquake doublet at station 3137. The seismic ground motion of both the 2015 central Nepal and the 2023 SE Turkey earthquakes doublet shows that the 2015 central Nepal doublet produced much lower peak ground acceleration than the 2023 southeastern Turkey earthquake doublet.