The Miocene volcanic ash layer at the Kekeya section, in the southwestern Tarim Basin (NW China), has a debated origin and stratigraphic importance. To clarify whether these deposits are primary air-fall tephra or reworked sediments, we conducted detailed petrographic, mineralogical, geochemical, and zircon UPb dating analyses. The ash layer contains abundant angular K-feldspar, plagioclase, and volcanic glass fragments with minimal rounding, suggesting direct pyroclastic deposition. Zircon UPb dating yields a weighted mean Pb-206/U-238 CA-ID-TIMS age of 10.997 +/- 0.027/0.042/0.044 Ma, consistent with published Ar-40/Ar-39 ages of K-feldspar. The uniform Pb isotopic compositions of K-feldspar (Pb-206/Pb-204 = 17.62-19.03) and Sr isotopic ratios of plagioclase (Sr-87/Sr-86 = 0.70968-0.71047) indicate a single volcanic source rather than heterogeneous provenance. Comparison with Miocene volcanic rocks from the eastern Pamir reveals geochemical and isotopic affinities to the Dunkeldik volcanic belt. The excellent preservation of primary K-feldspar and other mafic phases suggests limited post-depositional alteration under hyper-arid conditions. These results confirm that the Kekeya ash layer represents primary fallout tephra derived from coeval (similar to 11 Ma) alkaline volcanism in the eastern Pamir, providing a robust marker horizon for regional Miocene stratigraphy.