Thrombolitic clots dominated by filamentous cyanobacteria and crusts of radio-fibrous calcite in the Furongian Changshan Formation, North China

Abstract

As a variety of non-laminated microbial carbonate or calcified microbialite, thrombolites commonly comprise diffusely clotted micrites as their basic structural unit. The current study is aimed at discussing the composition and forming mechanism of thrombolitic clots, which most likely represent the product formed by calcification of extracellular polymeric substances. It provides instances of sophisticated calcification of thrombolite-forming microbial mat in the calcite sea that is expressed by the distinctive composition of thrombolites. Thrombolites in the top part of the Furongian Changshan Formation are chiefly composed of chaotic large and small micritic clots as observed under low microscopic resolution. However, high density preservation of fossils of calcified sheaths of cyanobacteria is observed within most of the clots under higher resolution. Their distribution as groups or colonies suggests that they can be interpreted as Subtifloria of Girvanella group. These microbial components together with the enigmatic Epiphyton reflect fundamental features of microbial growth, and suggest a microbial origin of these clots. The general abundance of these calcified fossils within individual clots offers an instance for the formation of thrombolitic clots from sophisticated calcification of photosynthetic biofilms. It refutes the traditional view that both the thrombolites and the oncoids develop by accretion of microbial mats dominated by coccoid cyanobacteria. Aggregation of several individual clots to form millimetric agglutinated clots shows that their formation is genetically related to sophisticated calcification of extracellular polymeric substances making up multiple biofilms within relatively thick microbial mats. This inference is strengthened by their encrustation by radio-fibrous calcite, which leads to enlargement of the clots. Besides the clots, benthic ooids, pyrite and dolomite also developed in the current thrombolites, which are largely attributed to microbially mediated organomineralization.