Chronology of relict lake deposits in the Spiti River, NW Trans Himalaya: Implications to Late Pleistocene–Holocene climate-tectonic perturbations

Abstract

The Spiti River that drains through the arid Trans-Himalayan region is studied here. The relict deposits exposed along the river provide an opportunity to understand the interaction between the phases of intense monsoon and surface processes occurring in the cold and semi arid to-arid Trans-Himalayan region. Based on geomorphological observation the valley is broadly divided into the upper and lower Spiti Valley. The braided channel and the relict fluvio-lacustrine deposits rising from the present riverbed characterize the upper valley. The deposits in the lower valley occur on the uplifted bedrock strath and where the channel characteristics are mainly of meandering nature. Conspicuous is the occurrence of significantly thick lacustrine units within the relict sedimentary sequences of Spiti throughout the valley. The broad sedimentary architecture suggests the formation of these palaeolakes due landslide-driven river damming. The Optically Stimulated Luminescence (OSL) dating of quartz derived from the bounding units of the lacustrine deposits suggests that the upper valley preserves the phase of deposition around 14–6 ka and in the lower valley around 50–30 ka. The review of published palaeoclimatic palaeolake chronology of Spiti Valley indicates that the lakes were probably formed during the wetter conditions related to Marine Isotope Stage III and II. The increased precipitation during these phases induced excessive landsliding and formation of dammed lakes along the Spiti River. The older lacustrine phase being preserved on the uplifted bedrock strath in the lower valley indicates late Pleistocene tectonic activity along the Kaurick Chango normal fault.

Introduction

Collision of the northward drifting Indian plate with Eurasia resulted in the formation of Himalayan mountain systems. Since then the continuous oblique underthrusting of the Indian plate has recorded movements along various fault/thrust systems viz. Indus Suture Zone (ISZ), Karakoram Fault, Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Fault (HFF), that has not only compartmentalized the entire Himalayan system into four tectono-climatic–geomorphic sectors namely Trans-Himalaya, Higher Himalaya, Lesser Himalaya and Outer Himalaya but also modified the precipitation regime globally. The interaction of the varying climate and tectonic evolution of this orogen has been responsible for an enormously increased oceanic sediment budget. Since the rivers in the Himalaya serve as a conduit of sediment erosion and unloading, it is desirable to understand and determine the phases of excessive erosion and sedimentation in the Himalayan River valleys.

The Spiti Valley is located above the tree line (3000 m asl), between 31–33°°E; 77–79° S; (Fig. 1, Fig. 2A), with little vegetation restricted to the valley floor. The prevailing climate is cold, semi-arid to arid and the valley remains covered with snow for nearly six months. Because of the orographic barrier in the south the valley remains in a rain shadow zone with respect to the Indian summer monsoon and thus experiences ~ 50 mm of rainfall annually and about 200 cm of snowfall (in the winters). The temperature ranges between − 25 °C and 30 °C in winter and summer respectively. The instrumental data of the past ten years show a marked change in temperature and snowfall patterns, e.g., 1990's have experienced temperatures as low as − 49 °C and snow fall > 200 cm in winters. It is interesting to note here that during Abnormal Monsoon Years (AMYs), rainfall penetrates ~ 75 km further north into the valley and causes extensive landslides and erosion (Bookhagen et al., 2005a). The large temperature contrasts and thin vegetation cover increases the vulnerability of landsliding and channel damming in such areas during the excessive rainfall events. Recent research involving cosmogenic radionuclide dating of palaeolandslides in the arid Trans-Himalayan region indicated that 14 of the 16 dated landslides occurred during periods of intensified monsoons (Dortch et al., 2008). The sedimentary sequences exposed along the Spiti River therefore bear a signature of past interactions between climate and surface processes in the cold and arid regions of Himalaya.

In the present study we show the phases of lake formation in the Spiti River and its implications for geomorphic evolution of the region. The main emphasis is on the spatial distribution and timing of the lacustrine deposits. The chronology is established using radiocarbon dating and Optically Stimulated Luminescence (OSL) dating techniques on the bulk sediment. We observed discrepancies in the two sets of ages and the cause for the discordant ages is discussed.