IRSL ages and significance of Late Pleistocene surficial sand deposits in the lower Khumbu region, Nepal

Abstract:

Several exposures of well-bedded sand deposits, recently recognized in the lower Khumbu region of the Nepal Himalaya, near the Namche, Phortse, and Thyangboche villages, were examined in an effort to understand the history of deposition or lake formation and the related geomorphological processes involved in deposition and subsequent erosion. Six sediment samples were collected from selected sections and were analyzed by utilizing the infrared stimulated luminescence (IRSL) dating of feldspar grains from sand to constrain the depositional ages. Coupling the analytical results with the field stratigraphic descriptions and sedimentology, we propose that lakes existed from 42 ka to at least 18 ka and possibly 14 ka. Landslides and glacial moraines likely played a role in damming the rivers or forming small slump basins to form these lakes though evidence for the dams has been eroded. There is a large landslide complex, the Khumjung Rockslide Complex (KRC), that is cut by two younger landslides we refer to as the Namche landslides. Our new data indicates that the Namche landslides are younger than 24 ka and the Khumjung Rockslide Complex is older than 26 ka. Towards the northeast, sedimentation on the strath terrace of Phortse began at around 42 ka marking the end of the Thyangboche Stage I glacial stage (Finkel et al., 2003), which carved the terrace. Since 14-18 ka, the rate of river incision may have been as high as 21 mm/yr, while average exhumation rates calculated over longer time periods from thermochronology in the area are closer to 1.0 mm/yr (Hubbard et al., 1991; Streule et al., 2012).

References

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Streule, M. J., Carter, A., Searle, M. P., & Cottle, J. M. (2012). Constraints on brittle field exhumation of the Everest‐Makalu section of the Greater Himalayan Sequence: Implications for models of crustal flow. Tectonics, 31(3).