As yet undefined.
The base is currently undefined but provisionally dated at ~129 ka relating to significant warming at the beginning of the Last Interglacial.
The term Upper Pleistocene, at the rank of subseries, was ratified by the Executive Committee of the IUGS on 30 January 2020. The selection of a GSSP for the Upper Pleistocene Subseries and its corresponding stage is in progress.
Head, M.J., Pillans, B., Zalasiewicz, J.A., and the ICS Subcommission on Quaternary Stratigraphy, 2021. Formal ratification of subseries/subepochs for the Pleistocene Series/Epoch of the Quaternary System/Period. Episodes 44(3): 241-247 https://doi.org/10.18814/epiiugs/2020/020084