Fortunian small skeletal fossils and carbon isotope stratigraphy of the Bayangol Formation type section at Bayan Gol, southwestern Mongolia

Abstract

The Zavkhan Basin of southwestern Mongolia preserves one of the most complete mixed carbonate–siliciclastic archives of the Ediacaran–Cambrian transition globally, yet the precise chronostratigraphy of the Bayangol Formation at its type locality of Bayan Gol has remained contentious. Conflicting age interpretations, ranging from entirely Nemakit–Daldynian (Fortunian in the global scheme) to partially Tommotian (approximately Stage 2), have persisted largely as a consequence of composite section construction across tectonically complex, faulted terrain and the decoupling of biostratigraphic and chemostratigraphic datasets. Here we present a high–resolution, integrated small shelly fossil (SSF) biostratigraphy and δ¹³C chemostratigraphy derived from a single, continuous stratigraphic section measured through 359 m (true thickness) of the Bayangol Formation. Two successive SSF biozones are recognised: the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone in the uppermost Zuun–Arts and basal Bayangol formations, directly above the Basal Cambrian Carbon Isotope Excursion (BACE), and the Purella antiqua Zone, defined by the lowest occurrence of Purella antiqua and positioned between positive carbon isotope excursions 3p and 4p. A markedly abundant and diverse micromollusc assemblage in the uppermost member BG4, including Ilsanella compressa, Barskovia mongolica, and Nomgoliella sinistrivolubilis, is correlated with the Fortunian Stage in the global scheme (upper Nemakit–Daldynian of the Siberian Platform and broadly with Zone II of the Yangtze Platform of the South China Block). The absence of Watsonella crosbyi and positive excursion 5p (ZHUCE) from the measured succession demonstrates that members BG2–BG4 are entirely Fortunian in age, and that the Fortunian–Stage 2 boundary lies either within the overlying member BG5 or the Salaanygol Formation. These results resolve a long–standing chronostratigraphic controversy and provide a robust, multi–proxy framework for correlating the Mongolian record with global Terreneuvian reference successions.