Petrographic and geochemical characteristics of oncolites in the Ediacaran Dengying Formation of the Upper Yangtze Area, China: Implications for their Paleo-environment

Abstract

The Ediacaran Period represents a critical transition in the Earth's evolution, which could be potentially revealed by analyzing the oncolites formed during this period. This study investigates the genesis of oncolites in the Ediacaran Dengying Formation in the Yangba profile, Nanjiang County, Sichuan Basin. Moreover, efforts are made to analyze the effects of terrigenous input from perspectives of rare earth elements, probe into the variation of redox conditions via the microscopic analysis of trace elements, and exemplify the coupling relationship between the oxygen isotope, strontium isotope, paleosalinity (indicated by Z values), and sea level variation. The oncolites in the Yangba profile are generally featured by thin skins and dispersal grains and are highly differentiated in size and shape. A large number of microbial laminated fabrics and framboidal pyrite are developed in the dark laminae, and the former is yellow-green under the fluorescence microscope. According to the 13C-enrichment and petrographic analyses, oncolites and organic matter should be autochthonously formed by geochemical processes linked to microbial mats. Three cycles of sedimentary microfacies formed in subtidal low-energy zone and subtidal high-energy zone, and ten upward-coarsening cycles of oncolites are identified vertically, which, together with the vertical variations of δ18O and 87Sr/86Sr, indicate that the sea level has experienced six stages of variations during the deposition of the 2nd Member of the Ediacaran Dengying Formation (Deng-II Member): namely, transgression, lowstand fluctuating regression, transgression, continuous regression, transgression, and again lowstand fluctuating regression. Negative Ce anomalies (Ce/Ce*) and low V/Cr and low V/(V + Ni) ratios indicate the presence of molecular O2 in the water column, whereas characteristic Ni/Co, U/Th, and framboidal pyrite suggest the contemporaneous presence of anoxic water. The oncolites are suggested to be rarely impacted by terrigenous input, as is evident by the Er/Nd ratio of < 0.1, the average Y/Ho ratio of 35, the average La/Yb ratio of 36, and the Z value of > 130. To conclude, Upper Yangtze Deng-II Member oncolites are exemplified to be of microbial origins; the marine redox structure is featured by a temporarily stratified, oxygenated water body and a less oxygenated deeper water body during the Ediacaran, and frequent change of seawater energy, which are consistent with the Ediacaran seawater properties.