Architecture and Source of Alluvial Fan deposits of the Gish and Lish Rivers, Sikkim-Darjeeling Himalaya, India

Abstract

The Gish and Lish rivers are flowing through Higher Himalaya, Lesser Himalaya, Sub Himalaya and Quaternary Alluvium of the Sikkim- Darjeeling region. These thrust bounded tectonostratigraphic zones are comprised of different lithologies. Comparative sedimentological studies of the sediments of these rivers were undertaken to understand landforms developed by these rivers and source to sink relationships of these sediments. The Gish River fans are broad and wider as compared to the narrow and linear shaped fans of the Lish River. Textural analysis of these fan sediments revealed high energy conditions of deposition within these rivers. The petrographic analysis of the Gish River sediments shows dominance of Quartz (Qt63%F4% L33%) in the framework components, while the Lithic metamorphic fragments (Lm62% LV0% Ls 38%) show dominance over Lithic sedimentary fragments. This is also supplemented by the dominance of Quartz Polycrystalline grains (Qp62% Lv0% Ls38%) over Lithic sedimentary fractions. The Lish river sediments show dominance of Lithic fragments (Qt39% F2% L59%) in the framework components, while the Lithic sedimentary fragments (Lm44% Lv0% Ls56%) show dominance over lithic metamorphic fragments. This is also supplemented by Lithic sedimentary fractions (Qp41% Lv0% Ls59%) dominance over the Quartz Polycrystalline grains. These results of Quartz dominance in the Gish River sediments indicate higher contribution is from gneissic and metamorphic sources. While, Lithic dominance in the Lish River sediments infers higher contribution from sedimentary and metamorphic sources.