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

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.