TIMING OF GLACATION AND ALLUVIAL TERRACE DEPOSITION IN THE NORTHERN TIAN SHAN, KYRGYZSTAN, FROM CORRELATION OF 10BE AND 36CL EXPOSURE AGES

Formation of valley-filling alluvial terraces requires a significant source of sediment input into fluvial systems. Transitions between glacial- interglacial periods could potentially provide a source of these sediment pulses. Here, we present data from the Kungey Range in the northern Tian Shan, Kyrgyzstan, which correlates timing of glacial advances with exposure-ages of fluvial fill terrace sequences. ³⁶Cl exposure ages of boulders contained in moraines within the Chon-Koi-Suu valley show two major generations at ~90 ka and 20 ka. A widespread fill terrace designated Q32 flanking the southern Kungey Range has an exposure age of 86 ± 8 ka calculated using ¹⁰Be. The younger moraine sequence correlates with the LGM in the Tian Shan, and widespread deposition of broad Pleistocene- Holocene alluvial aprons designated Q4. From these ages, we hypothesize a sequence of late Quaternary deposition within the northern Tian Shan: (1) The penultimate glaciation at ~90 ka in the Kungey Range is a potential source for the large sediment flux into the adjacent basin that formed the widespread Q32 fill terrace; (2) A hiatus of deposition between ~80 and 25 ka; and (3) LGM at ~20 ka followed by deposition of broad alluvial aprons and younger valley fill. The most recent moraine sequence at ~20 ka is coincident with the LGM throughout the Tian Shan, and ubiquitous deposition of Q4 terraces.