Facies and Depositional Model of an Alluvial Fan: An Example from Pokhara Valley, Nepal

ABSTRACT An alluvial fan is a fan-shaped feature formed by sporadic flood events caused by heavy rainfall or rapid snow/ice melting upstream. Floodwaters carry sediments like gravel, sand, and silt from higher elevations and deposit them as water velocity decreases, shaping the alluvial fan. The Pokhara Fan provides valuable insights into the study of alluvial fans. We identified fifteen lithofacies and nine facies’ associations and categorised them into three depositional stages: lobe building, channel building, and abandonment. In the lobe-building phase, sedimentary layers transition from debris flow deposits to deep, narrow channels to widespread sheet-like floods and unrestricted stream deposits, moving from proximal to distal areas of the fan. During the channel building stage, gravelly braided river systems are divided into ‘main’, ‘intermediate’, and ‘minor’ discharge areas across the fan. Erosional lags formed by overland flows mark the abandonment stage. These changes are influenced by alternating climate cycles and long-term responses to neotectonic forces. The initial lobe-building phase experiences sporadic floods due to higher precipitation and elevated water and sediment levels. The channel building stage sees reduced precipitation intensity and flood discharge, leading to the formation of lower water flow channels. The abandonment stage may have no floods. Neo-tectonic activity enhances erosion capacity, creating a trench or incision at the fan’s leading edge. Overall, the research contributes to our understanding of alluvial fans, particularly the Pokhara Fan, and provides insights into the geological archives related to tectonics, flood and climate conditions they experienced.