Recalibrating the Zavkhan: A Revised Stratigraphic Framework for Southwestern Mongolia

The Ediacaran–Cambrian transition marks a fundamental restructuring of the Earth's biosphere, yet global correlation of this interval remains hindered by endemic faunas, facies-dependent stratigraphic markers, and a lack of precise radioisotopic dates. The Zavkhan Terrane in southwestern Mongolia preserves a thick, continuous mixed carbonate-siliciclastic succession, spanning the Zuun-Arts, Bayangol, Salaanygol, and Khairkhan formations, that serves as a critical archive for this interval. Historically, the chronostratigraphy of this basin has been compromised by the reliance on composite stratigraphic columns stitched across tectonically complex blocks, leading to decoupled bio- and chemostratigraphic datasets and conflicting age assignments. Here, we present a revised, high-resolution multi-proxy stratigraphic framework utilizing paired small shelly fossil (SSF) and carbon isotope (δ13C) data derived from continuous stratigraphic sections across the Zavkhan Terrane. We confidently place the Ediacaran–Cambrian boundary in the upper Zuun-Arts Formation. This placement is defined by the nadir of the Basal Cambrian Carbon Isotope Excursion (BACE), which occurs in close stratigraphic association with the first appearance datum (FAD) of the early Cambrian protoconodont Protohertzina anabarica. The overlying Bayangol Formation records a distinct Fortunian (Nemakit-Daldynian) faunal succession, progressing from the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone into the Purella Zone. A dramatic increase in taxonomic richness and morphological disparity characterizes the mid-upper Bayangol Formation with the FAD of the globally significant index fossil Watsonella crosbyi in the uppermost Bayangol Formation, occurring in tandem with a sequence of positive δ13C excursions, providing a crucial biostratigraphic marker approximating the base of global Cambrian Stage 2 (Tommotian equivalent). The Salaanygol Formation, marked by extensive microbial (transitioning to archaeocyath-microbial) reef development coincides with a high-amplitude positive δ¹³C excursion and preserves in its uppermost strata a diverse skeleltal fauna (assigned to the Camenella mongolica Assemblage Zone). The absence of trilobites, in combination with observed shelly fossils and our chemostratigraphic age model collectively support a Terreuvian, Age 2 (Tommotian) age for the upper Salaanygol and Khairkhan formations. This is in contrast to the previously suggested Epoch 2, Age 3–4 (Atdabanian–Botomian) age for these formations, arguing against previously proposed unconformities of significant duration within this interval. This revised framework resolves longstanding chronostratigraphic ambiguities within the Zavkhan Terrane, provides a precisely calibrated record of early Cambrian biotic and carbon cycle evolution, and strengthens the case for the Zavkhan succession as a key reference section for global correlation of the E–C transition.