The Zavkhan terrane in western Mongolia preserves thick, fossiliferous, carbonate-rich strata that span the Ediacaran-Cambrian transition. Measured stratigraphic sections and geological mapping of these strata, which include the Zuun-Arts, Bayangol, Salaagol, and Khairkhan Formations, reveal large lateral facies changes over short distances that necessitate revisions to previous lithostratigraphic correlations and biostratigraphic range charts. Here, we integrate new geological mapping and measured stratigraphic sections across the Zavkhan terrane with high-resolution carbon isotope (δ13C) chemostratigraphy, sequence stratigraphy, and biostratigraphy. Using these data, we revise correlations within the Zavkhan terrane and place small shelly fossil and ichnofossil horizons in a refined temporal and spatial framework. Finally, we integrate these data from Mongolia with absolute ages and chemostratigraphy from sections in Morocco and Oman. Our revised age model suggests that deposition of the Bayangol Formation and the lower part of the Salaagol Formation was limited to the Nemakit-Daldynian and Tommotian Stages.
With the new correlations and age model, we place the small shelly fossil first appearance datum in the basal Bayangol Formation instead of the basal Zuun-Arts Formation, which moves this horizon hundreds of meters higher in the stratigraphy, above the large negative excursion in the Zuun-Arts Formation. The first appearance datum of Treptichnus pedum is ∼275 m above the large negative excursion in the Zuun-Arts Formation and ∼250 m above the first appearance datum of small shelly fossils, highlighting the rarity and facies dependence of its preservation.
We shift the first appearance datums of tommotiids, orthothecimorphs, hyolithelminths, cap-shaped fossils, protoconodonts, and Salanacus to just below the positive peak 3p. This interpretation differs from previous chronostratigraphic placements of first appearance data of genera between positive excursions 1p and 2p. Using this level as the first appearance datum in Mongolia for these genera, we replot global fossil first appearances. With this new compilation, there are still three distinct pulses of fossil first appearances, as was suggested in previous compilations; however, we suggest that this pattern is controlled largely by regional sedimentation and taphonomy rather than the rate of taxonomic origination.