The South China Block During the Ordovician and Silurian: Facies, Stratigraphic Unconformities, and Orogenies

The Ordovician and Silurian systems in the South China Block are characterized by numerous stratigraphic unconformities, most of which exhibit significant differences in temporal extent across different regions. Precisely constraining and correlating these unconformities, as well as interpreting them in the context of orogenic events, remains one of the key scientific questions to be resolved. During the Early and Middle Ordovician, the South China Block displayed a contiguous “platform–slope–basin” facies pattern from northwest to southeast, which has been widely regarded as a continuation of Cambrian facies distribution. However, this pattern changed abruptly during the Ordovician–Silurian transition into a “land in the south and sea in the north” configuration. By the middle and late Silurian, the South China Block had evolved into an extensive subaerial landmass, with only restricted and intermittent deposition preserved. At least four major unconformities are recognized in the Ordovician succession, in ascending order: (1) the first occurs at the base of the Ordovician in the Upper Yangtze Region (e.g., northern Guizhou and eastern Yunnan provinces); (2) the second is marked by a hiatus spanning the Tremadocian–Floian transition; (3) the third corresponds to a stratigraphic gap at Darriwilian–Sandbian transition, identified in the Middle and Upper Yangtze Region; and (4) the fourth and most significant unconformity developed from late Katian to Hirnantian, characterized by the widespread absence of Hirnantian strata following the deposition of thick flysch-facies clastics in southern part of the South China Block. This major hiatus is generally attributed to the onset of the Kwangsian Orogeny. The Silurian stratigraphic record is dominated by clastic deposition during the Llandovery, followed by an extensive absence of mid to late Silurian strata across the South China Block. This gap has been attributed to the “Yangtze Uplift” orogeny.