Responses of Indian Summer Monsoon Dynamics and Late Quaternary Fluvial Development: Records from Yamuna River Valley, NW-Himalaya

Abstract

In the NW Himalaya, Late Quaternary sedimentary sequences along with the glacial fed Yamuna River deposits offer an opportunity to understand the feedback mechanism of Glacial-Interglacial cycles to Indian Summer Monsoon (ISM) dynamics and resultant fluvial development since MIS-5e (Interglacial). Fluvial terraces and alluvial fans contribute to major landforms along the River valley from Higher Himalaya (source area) to Sub-Himalayan region and comprise ~10 m to ~220 m thick gravel deposit carved into distinct terrace levels (T-1 to T-7; in order of ascending height from modern Yamuna R.). In the present work, the geomorphological and sedimentological studies and OSL based absolute chronology of these deposits constraints multiple phases (Phase-I to IV) of sediment dispersal and accretion in the pre-existing valley punctuated by shorter periods of incision. In the narrow upper reaches of the valley (near present glacier) and in the middle reach (Lakhamandal area), oldest fluvial aggradation (Phase-I) recorded during >105 ka to ~80 ka (MIS-5) is represented by distinct remnant of fluvial deposits (T-7). This older aggradation is indicative of limited glacial advance and fluvial (meltwater) sediment transport and deposition of gravel facies followed by subsequent incision. Phase-II aggradation continued from ~50 ka to ~41 ka that coincides with MIS-3 (Inter-glacial stage) representing warm and humid climatic conditions and the deposits are preserved as remnant patch of fluvial deposits (T-6) followed by rapid incision owing to lower sediment- water ratio. During ~37 ka and upto ~23 ka, a major valley filling phase (Phase-III) is recorded in the wider parts of valley (identified between MBT and MCT) in the form of ~120 m thick clast supported fluvial gravel. This phase coincides with transitional MIS-3 & MIS-2 (Interglacial-Glacial transition). The sedimentary facies and its accretion geometry is suggestive of deposition under braided river environment. Glacier retreat and release of sediment might have been responsible for such an extensive aggradation under more sediment-water ratio and less sediment transporting capacity of Yamuna River. The aggradation phase is carved into distinct degradational terrace levels (T-3 to T-5). Another younger aggradation phase (Phase-IV) represented by discontinuous terrace deposits (T-1 and T-2) is recorded across MCT. The Late Quaternary sedimentary archives in the valley across MCT and correlation of aggradation and incision phases correlates well with ISM dynamics (δ¹⁸O record), and profound control of glacial-interglacial cycles and ISM variation in the area since MIS-5e.