Table 1 Field observation characteristics of various oncoids in the Cambrian Miaolingian Series, North China Platform
Section name | Strata | Diameter of oncoids (cm) | Facies | Field observation characteristics |
|---|---|---|---|---|
Wuhai section | Middle Xuzhuang Formation | 0.3–1.2 | Ooid shoal | The 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 section | Top Zhangxia Formation | 0.9–2.1 | Ooid shoal | The 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 section | Top Xuzhuang Formation | 0.8–1.7 | Ooid shoal | The 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 section | Top Zhangxia Formation | 0.5–1.8 | Ooid shoal | The 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 section | Top Zhangxia Formation | 0.7–2.3 | Ooid shoal | The 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 section | Top Zhangxia Formation | 0.7–1.9 | Ooid shoal | The 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. |