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Table 1 Field observation characteristics of various oncoids in the Cambrian Miaolingian Series, North China Platform

From: Morphology and features of Cambrian oncoids and responses to palaeogeography of the North China Platform

Section nameStrataDiameter of oncoids (cm)FaciesField observation characteristics
Wuhai sectionMiddle Xuzhuang Formation0.3–1.2Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 0.3 m. There is no clear boundary between oncoids and ooids. The individual oncoids have small diameters and appear relatively dispersed. It is overlain by massive micrite limestone of the upper Xuzhuang Formation, and underlain by green muddy-sandy shale interbedded with thin oolitic limestone of the lower Xuzhuang Formation.
Diaoquan sectionTop Zhangxia Formation0.9–2.1Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 2.2 m. There is no obvious boundary between the oncoids and ooids. The individual oncoids have bigger diameters than those in the Wuhai section and show local concentration. The limestone is overlain by calcareous mudstone of the lower Gushan Formation and underlain by massive micritic limestone interbedded with thin oolitic limestone.
Xiaweidian sectionTop Xuzhuang Formation0.8–1.7Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 3.75 m. The individual oncoids have relatively big diameters and are locally concentrated. The limestone is overlain by calcareous mudstone of the lower Zhangxia Formation and underlain by massive micritic limestone interbedded with thin oolitic limestone.
Sandaogou sectionTop Zhangxia Formation0.5–1.8Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 4.5 m. There is no clear boundary between the oncoids and ooids. Vertically, individual oncoid sizes at the bottom of the bed are small and gradually increase upwards. The limestone is overlain by calcareous mudstone from the lower Gushan Formation and underlain by massive micritic limestone interbedded with thin oolitic limestone.
Fuzhou Bay sectionTop Zhangxia Formation0.7–2.3Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 5.2 m. The oncoids vary in type and size and are widely horizontally distributed. It is overlain by calcareous mudstone from the lower part of the Gushan Formation, and underlain by massive micritic limestone interbedded with thin oolitic limestone.
Jinzhou Bay sectionTop Zhangxia Formation0.7–1.9Ooid shoalThe thickness of oncoid-bearing oolitic limestone beds is about 4.9 m. The oncoids vary in type and size and are widely horizontally distributed. It is overlain by calcareous mudstone from the lower part of the Gushan Formation, and underlain by massive micritic limestone interbedded with thin oolitic limestone.