Facilitation of microbialite development by continental weathering in the Cambrian Zhangxia Formation, southern North China Block

Abstract

We report geochemical data for carbonate rocks from the Cambrian Zhangxia Formation, southern North China Block, including trace-element contents and C, O, and Sr isotopic compositions, to determine the controls on the development of microbialites in this formation. The sedimentary environment of the Zhangxia Formation evolved from carbonate-slope through platform-margin-reef to ooid-shoal, and then to open-platform settings, corresponding to the development of micrites, microbialites, and oolitic limestones. Geochemical data show that δ¹³C values decrease upwards through the Zhangxia Formation, from 0.99‰ to −1.59‰, whereas δ¹⁸O values increase from −9.91‰ to −7.10‰. Total rare earth element (REE) contents decrease upwards through the stratigraphy, and the contents of Th and Sc show a similar trend. ⁸⁷Sr/⁸⁶Sr ratios reach a maximum of 0.710544 in the horizons of microbialite development, Member 2 of the Zhangxia Formation, above which the ratios are uniform, with a mean value of 0.709251. Combining these geochemical data with the inferred sedimentary evolution, the Zhangxia Formation is interpreted to have formed in a depositional environment that changed from transgression to regression. Continental weathering played a crucial role in promoting the input of terrigenous materials, which in turn favored the development of microbialites during the early stage of deposition of the Zhangxia Formation.