Tectono-depositional pattern and evolution of the middle Yangtze Platform (South China) during the late Ediacaran

Abstract

Abstract The depositional environments and evolution of the Yangtze Block during the Ediacaran–Cambrian transition remain uncertain, preventing a thorough understanding of the coeval ocean stratification, redox heterogeneity and organism distribution. In this study, detailed facies and sequence stratigraphic analyses were carried out on the Dengying Formation and its correlatives to better constrain the tectono-depositional pattern and evolution of the middle Yangtze area in the late Ediacaran. Twenty-five lithofacies were identified in nineteen sections and grouped into six depositional facies: intrashelf basin, intrashelf shoal, restricted platform interior, platform margin, upper-middle slope, and lower siliceous slope-basin. The spatiotemporal distributions of these depositional facies reveal an isolated, rimmed middle Yangtze carbonate platform embayed with a westward-opening intrashelf basin. This intrashelf basin gradually became wider and deeper westwards, linking to a NNE-trending basinal seaway that separated the middle Yangtze Platform from its upper Yangtze counterpart. The enclosure of the marginal shoal-reef complex with an intrashelf shoal chain formed a large-scale restricted lagoon and associated tidal flat in the platform interior. Based on key sequence boundaries and cycle stacking patterns, two and a half transgressive-regressive sequences were distinguished in the Dengying Formation and its correlatives in the twelve representative sections. Furthermore, these sequences can be correlated well with those identified in other upper Ediacaran successions elsewhere in upper Yangtze, Namibia, India and Oman, indicating a eustatic origin for these sequences. However, the complex platform-basin configuration, with significant facies and thickness variations, reflects a transtensional tectonic regime in the middle Yangtze area during the late Ediacaran. The abrupt changes in lithofacies and thickness across the southern principal platform margin and those from sandy shoals/banks to storm-influenced shelf along the northern portion of the intrashelf basin in the Three Gorges (western Hubei) suggest synthetic extensional faulting across the margins during deposition. The two synthetic extensional fault zones might have been induced by NNE-trending transtensional faulting along the eastern margin of the upper Yangtze Platform. This structural pattern fundamentally controlled the depositional differentiation of the middle Yangtze Platform during the late Ediacaran and also laid the foundation for the subsequent facies distribution and depositional evolution during the early Cambrian Terreneuvian Epoch.