74 / 2022-01-23 17:30:49

Upper Ediacaran carbonate tempestites in the eastern Yangtze Gorges area, South China

Shibantan Member, carbonate ramp, sedimentary facies, storm deposit, hummocky cross-stratification

The upper Ediacaran Dengying Formation in the eastern Yangtze Gorges area, South China is well known for hosting diverse Ediacara-type macrofossils and trace fossils. However, the depositional processes of this carbonate succession are still poorly known. Here we present detailed sedimentological data on the Dengying carbonates in four sections across the eastern Yangtze Gorges area, with an emphasis on tempestites in limestones of the Shibantan Member of Dengying Formation that have largely been overlooked. The Shibantan limestones in the studied sections contain extensive autochthonous microbial mats and storm-related deposits, including calcirudite, individual swaley cross-stratified calcarenites, hummocky cross-stratified fine- to very fine-grained calcarenites, normally graded calcarenites, calcarenite-calcisiltite beds in discrete units, and soft-sediment deformed beds. The spatial variation of facies suggests that the Shibantan Member was deposited on a carbonate ramp system with an intra-ramp lagoon. With the upward shallowing of water depth and progradation of storm-generated sediments, the ramp gradually become steeper, as characterized by the occurrence of soft-sediment deformation structures and graded bedding in the middle of the Member. The abundance of tempestites in the Shibantan limestones suggests that the Shibantan Basin was located in low latitude regions frequently affected by tropical storms, consistent with previous paleogeographic reconstructions. Frequent storm events have probably brought nutrients into the environment and contributed to the rapid burial and preservation of Ediacara-type fossils. The Shibantan limestones thus provide another example of mass-flow related taphonomic window for the Ediacaran organisms and is comparable to the preservation style of the Avalon assemblage.