Stratigraphy and depositional evolution of the terminal Ediacaran platform in the central to northern Sichuan Basin, Southwest China

Abstract

The upper Ediacaran Dengying Formation in the Sichuan Basin provides well-preserved sedimentary records for investigating Precambrian microbialites and platform evolution. However, as previous studies have mainly focused on the broad platform-basin architecture, the depositional evolution of the platform has not been fully illuminated in the central to northern Sichuan Basin, where an intraplatform rift trough developed. Therefore, based on drilling wells and outcrops in the area, 20 lithofacies were identified and grouped into eight lithofacies associations indicating peritidal flat, shallow subtidal flat, lagoon, mixed tidal flat, slope, basin, coast and shallow sea environments. Moreover, two transgressive-regressive depositional cycles (Cycle 1 and Cycle 2) were recognized based on the five shallowing-upward (types A–E) and three deepening-upward (types F–H) cycle types. A ramp-like epeiric platform with a gentle slope developed in Cycle 1. The platform is dominated by shallow subtidal grainstones and peritidal microbialites, and the trough is in a slope environment without deep-water basin facies. In the transgression of Cycle 2, a siliciclastic-dominated environment comprising the coast and shallow sea developed on the shelf. Then a rimmed carbonate platform developed in the regression of Cycle 2, with the accumulation of shoal grainstone and peritidal microbialite in the platform margin, lagoonal dolomudstone in the platform interior, and steep slope rhythmite and basinal chert in the distal rift trough. Eustatic sea-level fluctuation and climatic change controlled the lithological transitions between carbonate and siliciclastic deposits. The platform configuration change is closely related to the evolution of the rift trough, which is the result of intense differential subsidence and sedimentation rates between the trough and platform under the effect of pre-existing basement rifts and coeval growth fault activities. A detailed depositional history of the Ediacaran Dengying Formation around the intraplatform rift trough was finally obtained, providing a good template for the study of the Precambrian microbialite-dominated platform with a similar tectonic setting.