Unit IX: Geology of Nepal(3hrs)
9.1 Geological subdivision of the Nepal Himalaya
The geological subdivision of the Nepal Himalaya is complex and can be broadly divided into several tectonic units. Here are the main subdivisions of the Nepal Himalaya:
1. Sub-Himalaya (Siwalik Group): The Sub-Himalaya, also known as the Siwalik Group, is the outermost tectonic unit in the Nepal Himalaya. It consists of sedimentary rocks, including sandstones, conglomerates, and mudstones, which were deposited in ancient river systems and form the foothills of the Himalayas.
2. Lesser Himalaya (Molasse Group): The Lesser Himalaya lies northward of the Sub-Himalaya and consists of a sequence of sedimentary rocks that were deposited during the collision between the Indian and Eurasian plates. It includes various formations such as sandstones, shales, and limestones, often folded and faulted.
3. Higher Himalaya (Greater Himalaya Crystalline Complex): The Higher Himalaya represents the core zone of the Himalayan mountain range and consists mainly of crystalline rocks, including gneisses, schists, and granites. These rocks were formed as a result of intense metamorphism and represent the deeper levels of the Earth's crust.
4. Tethyan Sedimentary Sequence (Kathmandu Complex): The Tethyan Sedimentary Sequence is located north of the Higher Himalaya and consists of a series of sedimentary rocks that were deposited in a deep marine environment during the closure of the Tethys Ocean. This unit includes shale, limestone, and sandstone formations.
5. High Himalayan Crystalline Sequence (High Himalayan Crystalline Belt): The High Himalayan Crystalline Sequence lies immediately north of the Tethyan Sedimentary Sequence. It comprises highly metamorphosed rocks, including gneisses, migmatites, and crystalline schists. This belt represents the zone of intense crustal deformation and high-grade metamorphism.
These subdivisions reflect the diverse geological history and structural complexities of the Nepal Himalaya. They are a result of the ongoing collision between the Indian and Eurasian plates, which has led to the uplift and formation of the Himalayan mountain range. The geology of the region has significant implications for geological hazards, such as earthquakes and landslides, as well as for natural resource exploration and management.
9.2 Engineering geological problem of each geological division of Nepal
While the geological divisions of Nepal present a wide range of engineering geological challenges, here are some typical problems associated with each division:
1. Sub-Himalaya (Siwalik Group):
- Slope Stability: The Sub-Himalaya region is characterized by steep slopes and loose sedimentary deposits, making it prone to slope instability, landslides, and erosion.
- Foundation Conditions: The heterogeneous nature of the Sub-Himalayan sedimentary rocks poses challenges for building foundations due to variable load-bearing capacities and potential settlement issues.
2. Lesser Himalaya (Molasse Group):
- Rock Mass Stability: The complex folding and faulting in the Lesser Himalaya can lead to unstable rock masses, requiring careful engineering design and support measures for infrastructure development.
- Landslide Risk: The presence of weak rock layers and intense monsoon rainfall in the Lesser Himalaya increases the susceptibility to landslides, necessitating slope stabilization and hazard mitigation measures.
3. Higher Himalaya (Greater Himalaya Crystalline Complex):
- Rock Strength and Stability: The Greater Himalaya consists of high-grade metamorphic rocks, which may exhibit variable strength and structural discontinuities, requiring rock support systems and engineering measures during excavation and tunneling projects.
- Glacial Hazards: The Higher Himalaya is home to numerous glaciers, and the retreat of these glaciers can lead to glacial lake outburst floods (GLOFs) and other glacial hazards, which need to be considered in infrastructure planning and risk assessment.
4. Tethyan Sedimentary Sequence (Kathmandu Complex):
- Soil Liquefaction: The Tethyan Sedimentary Sequence includes loose and poorly consolidated sediments, making it susceptible to soil liquefaction during earthquakes, requiring proper geotechnical assessment and foundation design for structures.
- Groundwater Conditions: The presence of aquifers and the potential for groundwater inflow can affect the stability of excavations and underground structures, requiring appropriate dewatering measures.
5. High Himalayan Crystalline Sequence (High Himalayan Crystalline Belt):
- Complex Geology: The highly metamorphosed rocks of the High Himalayan Crystalline Belt present challenges in characterizing rock properties, understanding structural discontinuities, and assessing rock mass behavior for engineering projects.
- Seismic Hazards: This region is seismically active, and the presence of faults and geological complexities can contribute to increased earthquake hazards, requiring seismic design considerations for infrastructure.
It is important to note that these are general engineering geological challenges associated with each geological division, and site-specific investigations and assessments are necessary for a comprehensive understanding of the engineering geology and proper design and construction practices.
9.3 Major rock type, Soil type, construction material found at the different zones of Nepal
The different zones of Nepal exhibit variations in major rock types, soil types, and construction materials. Here's a general overview of each zone:
1. Terai/Madhesh Zone:
- Major Rock Types: The Terai zone mainly consists of alluvial deposits, including sand, gravel, and clay, as well as sedimentary rocks of the Siwalik Group.
- Soil Types: The soil types in this zone include sandy soils, loamy soils, and clayey soils, which are suitable for agriculture.
- Construction Materials: Common construction materials in this region include sand, gravel, and locally available clay for bricks and tiles.
2. Hill/Mid-Hill Zone:
- Major Rock Types: The Hill/Mid-Hill zone comprises a mix of rock types, including sedimentary rocks, such as sandstone and shale in the Lesser Himalaya, and metamorphic rocks, including gneiss and schist in the Higher Himalaya.
- Soil Types: The soil types in this zone vary from fertile loamy soils to clayey soils and weathered rocks.
- Construction Materials: Construction materials in this region include locally available stones, such as sandstone and slate, as well as bricks made from clay or shale.
3. Mountain/High Himalaya Zone:
- Major Rock Types: The Mountain/High Himalaya zone consists primarily of metamorphic rocks, including gneiss, schist, and quartzite, which are part of the Greater Himalaya and High Himalayan Crystalline Belt.
- Soil Types: The soil types in this zone are typically thin and composed of weathered rock materials, often mixed with glacial deposits.
- Construction Materials: Construction materials in this region include locally available stones, such as quartzite and gneiss, as well as timber from forests.
The specific rock types, soil types, and construction materials within each zone can vary depending on local geology, topography, and environmental conditions. Site-specific geological investigations are necessary to determine the exact composition and suitability of construction materials for specific projects within each zone of Nepal.
