TRANSGRESSIVE-REGRESSIVE SEDIMENTATION IN THE LATE EDIACARAN OF SOUTH CHINA AND ITS IMPACT ON METAZOAN COMMUNITIES

At the northern margin of the Yangtze Platform (South China), the Gaojiashan Member represents a distinctive siliciclastic-rich interval in the dolostone-dominated Dengying Formation (late Ediacaran, ca. 551-542 Ma). This unit yields several fossil assemblages that bear little resemblance to other Ediacaran communities. Much interest has been paid to the paleobiology and taphonomy of these peculiar organisms collectively referred to as the Gaojiashan biota. However, comparatively little is known about the sedimentary paleoenvironments where they thrived.

We report here on the sedimentary facies and structures found in the Gaojiashan Member at its type-locality in the southern Shaanxi province. There, mixed carbonate-siliciclastics sediments deposited in foreshore to upper offshore settings record a two-step transgressive-regressive cycle. After the initial backstepping of the carbonate platform, prevailing peritidal processes led to subaerial brecciation and mass-accumulation of Shaanxilithes fossils. A second deepening phase allowed benthic tube-shaped Metazoa to settle close to or below the storm wave base. The final regressive phase is marked by a resumption of the carbonate production and well-developed biomats. Overall, the sedimentary succession points to a protected, tide- then storm-dominated shallow marine paleoenvironment.

This transgressive-regressive cycle can be traced platformward into coeval strata of the adjacent Sichuan and Hubei provinces. However, it is still unclear whether the related event is of regional or global significance. A glacial-eustatic origin can not be ruled out given the growing evidence for late Ediacaran glaciations, e.g. in Namibia at ca. 547 Ma. Whatever their causes, such transient ecological changes likely impacted the extent and diversity of ecological niches available to metazoan life in the shallow marine realm, thereby acting as an evolutionary push while opening new taphonomic windows.