Bulletin of the Department of Geology

Rockmagnetic Characterization of the Quaternary Fluviolacustrine Sediments of the Thimi Section, Kathmandu Valley, Nepal

Pitambar Gautam — 2022-12-31

The fine-grained silt/clay sediments sampled at 38 levels of a 21 m thick segment at the Thimi outcrop in the Kathmandu valley have been studied for their rockmagnetic properties. These sediments have yielded consistently normal magnetic polarity and ¹⁴C ages determined by various workers between 20 and 45 kyrs or even more. They were measured for a set of rock magnetic parameters: low-field magnetic susceptibility, parameters derived from isothermal remanent magnetization (IRM) acquisition and demagnetisation (IRM_0.3T, IRM_3T, MDF_IRM), anhysteretic remanent magnetization (ARM) (ARM_tot, ARM(30 mT)/ARM_tot, χ_ARM), hysteresis parameters, etc. Several relationships between these parameters have been suggested for the Kathmandu valley sediments. Four rockmagnetic zones, based on the minor variations in magnetic mineral types and grain size parameters, are proposed and their possible relationship to the published pollen data has been discussed.

Engineering Properties of Aggregates for Railway Ballast, Rapati Nadi, Central Nepal Sub-Himalaya

Nirjal A.K. Pokharel — 2022-12-31

Nepal has proposed various railway projects, such as the East-West Railway, Kathmandu Metro Railway, and Raxaul-Kathmandu Railway. Good ballasts are always sought in the projects to diminish maintenance cost. Sediments from the Rapati Nadi (River) in central Nepal are among probable sources of ballasts as those sediments are rich in quartzite clasts. To meet the ballast requirements for railway projects, the study assessed the physical, mechanical and durability properties of alluvial deposit aggregates.

The compositional analysis yielded maximum percentage of quartzite followed by sandstone and others. The aggregates yielded uniformity in gradation. Flakiness and Elongation Indices of the tested samples ranged from 11.18 to 24.10% and from 13.07 to 42.77%, respectively. Specific gravity exceeded 2.4. Aggregate Impact Value, Aggregate Crushing Value and Los Angeles Value of the tested samples were respectively 4.4-13.2%, 10.20-17.67% and 15.75-34.25%. Similarly, the point load strength index ranged from 3.69 to 6.57 MPa, and the Sodium Sulphate Soundness Values ranged from 0.96 to 2.06%. All the test results satisfied the conditions based on IS specification in the context of aggregate shape, crushing strength, impact strength, and durability against abrasion. When the results of different indices and values are rated based on their higher, intermediate and lower ranges, all the samples fall in the range of the high rating. Thus it shows suitability of aggregates of the Rapati Nadi and sediment aggregates deposited between Basantapur and Bastipur seem better compared to other locations.

Geology, Structure, Metamorphism and Deformation of Botechaur-Chaurjhari Area of Western Nepal

Sushma Kadel — 2022-12-31

The Lesser Himalayan unit of the Nepal Himalaya, especially the western and central parts comprise numerous metamorphic crystalline thrust sheets occupying the cores of large synclinorium. Despite having complex geological settings, some areas in western parts of Nepal lack detailed geological mapping with credible information. This study aims to explore one least explored section; with emphasis on lithostratigraphy, structural setting, deformation, metamorphism, and magmatism. Detailed geological mapping along the Botechaur-Chaurjhari section revealed three tectonostratigraphic units separated by the Main Boundary Thrust (MBT) and the Mahabharat Thrust (MT); the Siwaliks (Middle Siwalik), autochthonous units of the Lesser Himalaya (Dandagaon Phyllite, Nourpul Formation and Dhading Dolomite) and the Lesser Himalayan Crystalline units (Kalagaon Formation and Chaurjhari Formation) from south to north respectively. From the microstructural analysis and measurements of shear sense indicators, the MT shows a top-to-the-south sense of shear indicated by asymmetric boudins, parasitic folds, and thrusting indications in blasto-mylonitic augen gneiss and asymmetric porphyroblast of garnet and the shearing in the garnet. The area also displays an inversion of metamorphism, supporting the development and existence of thrust. The metamorphic grade ranges from chlorite grade to kyanite grade having metamorphic facies assemblage of zeolite to amphibolite facies. Five phases of deformation (D₁ to D₅) are prevalent within the area, two of which (D₁ and D₂) belong to the pre-Himalayan (pre-Tertiary) orogeny and the rest of the deformation syn-to post-Himalayan.

Landslide Susceptibility Assessment Using GIS-Based Weights-of-Evidence Model in the Khimti Khola Watershed, Eastern Nepal

Milan Kumar Rai — 2022-12-31

The Khimti Khola watershed of the Eastern Nepal experiences numerous landslides each year, pose a significant natural hazard that can cause property damage and fatalities. In mountainous areas like Nepal, an accurate evaluation of landslide hazards may be an essential tool for disaster risk reduction and the strategic planning of development initiatives. To address this concern, a landslide susceptibility assessment was conducted in the Khimti Khola watershed, which covers an area of 440 km². The purpose of this assessment is to locate areas that are susceptible to landslides and to provide light on the factors that affect them. A total of 415 landslides were identified, covering 1.715 km² or 0.39% of the entire study area. The assessment derived the landslide susceptibility map using the Weights-of-Evidence (WoE) model from selected conditioning factor classes that were processed in ArcGIS and categorized into five classes: Very High, High, Moderate, Low, and Very Low. Each of these classes covers 7.12%, 16.63%, 25.23%, 30.24%, and 20.78% of the total study area, respectively. According to the findings, the Khimti Khola watershed has a 33.2% landslide occurrence percentage in the high susceptibility zone, out of the total area. Additionally, the very high, moderate, low, and very low susceptibility zones cover 24.35%, 20.75%, 17.08%, and 4.62% area of the landslide occurrence, respectively. In this assessment, the Area Under Curve (AUC) is used to validate the model and is plotted using ArcSDM. Identified a total of 415 landslides, out of which 311 (75%) were used as training sample for susceptibility mapping, while the remaining 104 (25%) were used to validate. The AUC for the landslide susceptibility map is 83%, indicating a very good degree of accuracy and satisfaction. The resulting susceptibility map provides information for predicting landslide-prone areas. It can be used to minimize the risk of potential landslides, safeguard lives, and prevent property damage. It will be a vital tool for disaster preparedness, future construction planning and development projects in this area.