Petrographic and rare earth elemental characteristics of Cambrian Girvanella oncoids exposed in the North China Platform: Constraints on forming mechanism, REE sources, and paleoenvironments

Abstract

Oncoid-bearing Mioalingian strata in the North China Platform represent late highstand/forced regressive systems tract(s). In the current study, sequence stratigraphy, sedimentology, and microscopic studies were carried out to interpret the forming mechanism and paleoenvironment of these oncoids. Microscopic studies show the typical distribution of filamentous cyanobacteria (Girvanella) in the Cambrian oncoids. Based on these observations, the oncoids can be classified into three kinds, i.e., Girvanella-core oncoids, Girvanella-cortex oncoids, and Girvanella-full oncoids. Furthermore, the studied oncoids hold pyrite crystals and dolomite minerals that also indicate the influence of sulfate-reducing bacteria on their formation. The presence of Girvanella, pyrite, and dolomite in the cores and cortices of these distinctive oncoids depicts its microbial origin that flourished under relatively high-energy settings. The present work offers a reference example for the involvement of microbes, particularly Girvanella, in the development of microbial carbonates in the North China carbonate platform during the Cambrian period. Geochemistry of the Cambrian oncoids is interpreted in order to determine the involvement of detrital fractions, sources of rare earth elements (REEs), and paleoenvironmental settings. The results of slightly negative Eu anomalies, higher La/Yb (6.8–15.1), and the larger Y/Ho variations (30.17–42.08) in the studied oncoids advocate that these oncoids have been subjected to terrigenous input during their formation. Considering the absence of Ce anomaly, Gd/Gd* ratios (< 1), and lesser values of Y/Ho (30.17–33.93), we propose that these oncoids have been influenced by diagenetic activities. Relatively greater values of Er/Nd (0.09–0.23) suggest seawater signature preserved by the marine sediments. Meanwhile, observed weak negative Eu anomalies within the oncoids point out the retention of the original marine water characteristics, whereas lower values of the Y/Ho propose freshwater participation from the rivers in the seawater during precipitation of oncoids. Based on the ratios Er/Nd, (Nd/Yb)N, La/Yb, and depletion and variation of REEs, it can be confirmed that the sources of these REEs within the Cambrian oncoids are mainly from terrigenous input. However, the higher Er/Nd ratios are sourced from marine carbonates. Relatively lower values of Mo/U (1.10–2.46), V/Cr (< 2), V/(V + Ni) ratios (0.24–0.33), and deficiency of Ce anomaly in studied oncoids suggest their development under oxic environments. In addition, the variation of Sr/Cu (42–54) and Sr/Ba (35–79) indicates their growth under arid climatic conditions and classic marine settings. Considering these geochemical signatures, it can be concluded that the oncoids of the North China Platform flourished under oxic, arid, and classic marine conditions.