Geochemical and Petrographic Analyses of the Cambrian Oncoids of the North China Platform: Implications for Their Paleogeography and Paleoenvironment

After the global extinction event at the end of the Cambrian Epoch 2, widely spread oolitic bank dominated the North China Carbonate Platform during the Cambrian Miaolingian Epoch. To better understand the influence of paleogeographic and paleoenvironmental factors on microbial carbonate particles during this period, carbonate oncoids of the Cambrian Miaolingian Series were selected to reconstruct the paleogeography and paleoenvironment. The study samples were collected from four different sections: Wuhai, Diaoquan, Xiawidian and Sandaogou (from west to east in the North China Platform). Stratigraphic, sedimentological, petrological and geochemical studies including analysis of trace and rare earth elements were carried out to define the stratigraphic location, and morphological and geochemical characteristics of oncoids. Geochemical analysis of oncoids reveals that Er/Nd and Y/Ho values are relatively low, signifying that the formation of oncoids was influenced by terrestrial inputs. Meanwhile, Sr/Cu, Sr/Ba, V/Sc and V/Cr values indicate that the oncoids were developed in a shallow marine environment under oxidizing conditions. The low content of total rare earth elements, low LREE/HREE ratios and LREEs, negative anomalies of Ce/Ce* and Eu/Eu* as well as (La/Sm)N suggest that the oncoids were less influenced by late diagenetic processes. More importantly, morphological differentiations of oncoids in the study area coincide with the changing trend of Y/Ho and Sr/Ba. The results of this study show that oncoids with regular morphology mainly formed at offshore area, while those with irregular shape and preserving rough laminae mostly occurred at nearshore area. From the comparison made between the paleogeographic locations of the study sections, it is proposed that the paleosalinity of marine depositional environment and the transportation distance are the prime controls for morphological differentiation of oncoids.