Thrombolites and ichnofossils in the Middle Cambrian of south North China Block: Implications for the environmental controls on the evolution of microbes and metazoans

Abstract

Thrombolites and ichnofossils are well‐preserved in a carbonate succession of the Cambrian (Miaolingian Series) Zhangxia Formation, south North China Block. This provides a chance to investigate the factors controlling the distributions of thrombolites and metazoans. Three types of thrombolites are identified, on the basis of clot shape and their array styles, namely, bushy, columnar, and blocky thrombolites. Bushy thrombolites occur in the lower part of the study unit, columnar thrombolites in the middle, and blocky thrombolites in the upper. Thrombolitic intervals alternate with bioturbated limestones, with the latter including two ichnogenera: Balanoglossites triadicus Mägdefrau, 1932 and Sulcolithos variabilis Knaust, 2019. Both thrombolites and bioturbated limestones are covered by oolitic limestones. The facies analysis indicates depositional environments from deep to shallow subtidal, then to oolitic shoals upwards. Such a sequence represents an overall shallowing‐upward trend. Thrombolites generally flourished during transgressions, whereas bioturbated and oolitic limestones represent regressive systems. Thus, relative sea level was a main factor controlling the alternations of thrombolites and ichnofossils in the Zhangxia Formation. Although there is no clear evidence that bioturbation can directly destroy thrombolites, exploration of new habitats by benthic animals and the increasing mixing degree of substrates could have destroyed the previous firmground substrate required for thrombolite development. The alternations of thrombolites and bioturbated limestones demonstrate that ecological conditions frequently fluctuated in the neritic environment during the Middle Cambrian.