Facies and platform development of a microbe‐dominated carbonate platform: The Zhangxia Formation (Drumian, Cambrian Series 3), Shandong Province, China

Abstract

Microbial carbonate has been increasingly recognized as an important player for sediment production and building up of organosedimentary deposits. The integrated spatio‐temporal pattern of microbial facies is important for understanding carbonate production processes in a carbonate platform. This paper focuses on the sedimentary facies and development of a microbe‐dominated platform in the Cambrian North China Platform (the Zhangxia Formation of the Cambrian Series 3) in Shandong Province, China. In the entire Zhangxia platform, deposition started with oolite‐dominated shoal‐lagoon system with rare microbialites. The shoal‐lagoon system in the western part of the study area kept up with sea‐level rise and formed a flat‐topped platform, whereas that in the eastern part failed to keep up and thus drowned. The platform top then changed to a microbe‐dominated carbonate platform consisting of thrombolite, stromatolite, dendrolite, and Epiphyton framestone. The initial significant microbialites were biohermal thrombolite and dendrolites, which subsequently changed into wider biostromal forms. The morphologic change was largely due to initial rapid rise in sea level and the following deceleration. Extensive biostromes of laterally coalesced thrombolite–Epiphyton bioherms occur in the shale‐dominated eastern part. These microbialites then perished by platform‐wide drowning. The vast initial development of microbialite facies of the Zhangxia Formation in the Shandong area was important for the establishment of an extensive microbialite‐dominated carbonate factory made by combination of an absence of reef‐building metazoans and possible transition of seawater chemistry from aragonite sea to calcite sea in the Cambrian Series 3. It is comparable to the other carbonate environments lacking dominant metazoan reef builders after the mass extinctions.