Origin of ooids in ooidal‐muddy laminites: A case study of the lower Cambrian Qingxudong Formation in the Sichuan Basin, South China

Abstract

Ooids have been considered the most intriguing component of carbonate rocks, but the formation mechanism of carbonate ooids remains a matter of intense discussion. The lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin contains abundant ooidal‐muddy laminites. Petrographic and sedimentological analyses revealed that these laminites formed in a shallow‐marine environment (preferably along the windward side of the reef‐shoal). Changing energy conditions caused variable supplies of grainy and muddy accumulation. A wide variety of ooids developed in grainy intervals. The normal ooids (<2 mm in diameter) in the lower portion of the laminites developed under high‐energy shallow‐water conditions, whereas the giant ooids (>2 mm in diameter) in the upper portion of the laminites developed in a deepening and moderate‐energy subtidal environment but were influenced by episodic hydrodynamic events. Petrographic and geochemical analyses suggest that microbes played an important role in ooid formation. High carbonate‐supersaturated and alkaline conditions that were mediated by microbial activity (e.g., bacterial sulfate reduction and photosynthesis) and agitated seawater conditions were the most important factors that mediated the formation of the Qingxudong ooids. This study provides new insights into paleooceanographic conditions during the Early Cambrian and offers a perspective on the formation of ancient normal and giant ooids.

Paleo-trade wind directions over the Yangtze Carbonate Platform during the Cambrian–Ordovician, Southern China

“…Previous studies reported on the general microfacies analysis of Cambrian–Ordovician carbonate facies in the YCP (e.g. Zou et al 2017; Tan et al 2018; Zhang-SC et al 2018; Zhai et al 2019; Fu et al 2020; Gao et al 2021). The present study focused on oolitic and intraclastic grainstones with the aim of identifying sedimentary differentiation due to prevailing wind directions.…”

“…Previous studies reported on the general microfacies analysis of Cambrian-Ordovician carbonate facies in the YCP (e.g. Zou et al 2017;Tan et al 2018;Zhai et al 2019;Fu et al 2020;Gao et al 2021). The present study focused…”

Reconstruction of a Ross lost CambrianSeries 2mixed siliciclastic–carbonate platform from carbonate clasts of the Shackleton Range, Antarctica

“…The lower Cambrian Qingxudong Formation, Sichuan Basin, could be a counterexample for giant ooids developed in deepening and moderate-energy subtidal environments affected by episodic hydrodynamic events (Tan et al . 2018).…”

“…(Duguid et al 2010). Some Cambrian ooids show different types of microbial activity as well, such as encrusting filamentous cyanobacteria (e.g., Girvanella-cortex ooids; Liu & Zhang 2012) or the presence of microbial microborings, filaments and extracellular polymeric substances or EPS (Tan et al 2018).…”

“…The most common examples of giant ooids come from Precambrian, Cambrian, Lower Triassic and Jurassic platforms (Lehrmann et al 2012 and references therein). The lower Cambrian Qingxudong Formation, Sichuan Basin, could be a counterexample for giant ooids developed in deepening and moderate-energy subtidal environments affected by episodic hydrodynamic events (Tan et al 2018).…”