Fig. 2

Lithology and sedimentary facies of the Cambrian Miaolingian Series in the six studied sections (Peng et al. 2012; Xiao et al. 2017a, 2017b). Among these sections, Wuhai section has the thinnest succession, with a small thickness of limestone, and a large thickness of mudstone and dark-green organic muddy-sandy shale, which indicates deep-water deposition. The other five sections in the east reveal similar lithological variations and comparable boundaries. The Xuzhuang Formation shows a transition from red shale associated with tidal flats to oolitic limestone associated with grain banks (Ma et al. 2017; Xiao et al. 2017a, 2017b; Xiao et al. 2019a). Sedimentary facies of the Zhangxia Formation display cyclic changes from nongrain bank to grain bank and from deep-water to shallow-water, demonstrating shallowing-upward trends (Xiao et al. 2017a, 2017b). The oncoids mainly developed in the Xuzhuang and Zhangxia formations