We report here the discovery of a new end-Cryogenian glacial diamictite and an overlying basal Ediacaran cap carbonate within the Tsagaan Oloom Formation of southwestern Mongolia. The identification of the Cryogenian-Ediacaran boundary, coupled with new δ13C chemostratigraphic profiles, facilitates the integration of the Neoproterozoic stratigraphy of Mongolia with records elsewhere. These correlations indicate that a previously unrecognized, −16‰ Cryogenian δ13C anomaly is present in the newly defined Tayshir member (informal) of the Tsagaan Oloom Formation. Furthermore, chemostratigraphic and lithostratigraphic relationships suggest an ~35 m.y. depositional hiatus below the phosphorite-bearing Zunne Arts member (informal) of the upper Tsagaan Oloom Formation and that subsidence renewed in the latest Ediacaran–Early Cambrian. We propose that the lower ~1500 m of the Tsagaan Oloom Formation was deposited on a thermally subsiding passive margin after Rodiniaage rifting, whereas the Zunne Arts member and the overlying ~1600 m of Early Cambrian strata were deposited in a foredeep basin that formed as the southern margin of the Dzabkhan terrane was subducted beneath the Khantayshir-Dariv arc.