Sequence stratigraphy and tectono-depositional evolution of a late Ediacaran epeiric platform in the upper Yangtze area, South China

Abstract

The tectono-depositional evolution of the Yangtze Block during the Ediacaran–Cambrian transition is controversial, leading to uncertainties in understanding the life-environment co-evolution and in petroleum exploration. This study investigated multiple upper Ediacaran (Dengying Formation) outcrops in the upper Yangtze area to reconstruct the carbonate platform evolution and underlying controls. Detailed sedimentological observations allow the identification of nineteen carbonate-dominated and five terrigenous siliciclastic-dominated lithofacies. Based on the spatiotemporal distributions of lithofacies, two and a half transgressive–regressive sequences (the regressive systems tract of S1 and the full sequences S2 and S3 in ascending order) are distinguished. The Dengying Formation mainly consists of carbonate-dominated lithofacies, which can be grouped into four depositional facies: semi-restricted platform, marginal shoal, middle-outer ramp/slope and siliceous basin. The broad platform interior with a flat-topped topography represents an epeiric platform, which can be subdivided into two variants: one dominated by tidal islands in the platform interior mainly during middle-late regressive systems tracts, and another dominated by lagoons in the platform interior mainly in the condensed section of S2 and early regressive systems tracts. Terrigenous siliciclastic-dominated lithofacies occur only in the transgressive systems tract of S3 and are grouped into two depositional facies: coast and shallow sea, representing a terrigenous material-dominated epeiric marine setting formed by a drowning event. These transgressive–regressive sequences can be roughly correlated with the sequences and glaciations documented in coeval successions worldwide, reflecting the control of eustasy. Across the eastern margin of the upper Yangtze Platform, the incompleteness (or absence) of S2 and S3 and lower Cambrian Terreneuvian strata along the platform margin and the extremely condensed Ediacaran–Cambrian transitional successions in the lower slope and basin may have been caused by differential uplift and subsidence induced by extensional faulting. The fault zone and the two reported synthetic extensional fault zones in the middle Yangtze area largely followed the pre-existing horst and graben configuration, indicating the activation of the pre-existing major faults likely associated with a transtensional tectonic regime.