The Shuram Excursion and Lithostratigraphic Framework of the NW Yangtze Doushantuo Formation

The northwestern margin of the Upper Yangtze Craton was significantly influenced by the rifting of the Kangdian and Bikou-Hannan belts during the Ediacaran Period, resulting in a sedimentary succession characterized by rapid facies transitions and substantial thickness variations. Establishing a robust stratigraphic correlation between this region and the classic Yangtze Gorges sections has remained challenging. This study presents a systematic lithostratigraphic and carbon isotope (δ13Ccarb) chemostratigraphic analysis of four sections Tuanbaoshan (TBS), Qingping (QP), Qiujiahe (QJH), and Majiawan (MJW) to track the Shuram Excursion (SE/EN3) and establish a regional stratigraphic framework. Our results show that the basal Ediacaran in all four sections is marked by a prominent unconformity with the underlying basement, characterized by the universal absence of the typical post-Marinoan cap dolomite (Member I/DST I). This indicates that the northwestern margin was situated at a tectonic high or experienced delayed transgression during the early Doushantuo stage. Above this unconformity, Members II-IV exhibit good comparability with the integrated South China stratigraphic framework: the basal sequences in the TBS, QP, and QJH sections correlate with DST II, where TBS preserves transgressive lag deposits and QJH records a thick (>146.5m) mudstone succession. The MJW section likely lacks DST II due to persistent tectonic highs. As sea level rose, the region entered a unified evolutionary phase during DST III, dominated by carbonate deposition, eventually transitioning into the organic-rich black shales of DST IV (characterized by distinct carbonate nodules in the TBS section). Integrated elemental geochemical data support the primary nature of the isotopic signals. All four sections record a pronounced negative excursion that is consistent in both trend and character with the global Shuram Excursion (DOUNCE). By integrating litho- and chemostratigraphic constraints, this study establishes a refined stratigraphic correlation for the northwestern Yangtze Platform, providing a foundation for understanding tectonic controls on sedimentary evolution and the regional response to global carbon cycle perturbations.