Study On Microbialites From The Third Member Of The Mantou Formation(Cambrian Series 3),Lushan,Western

The third member of the Mantou Formation(Cambrian Series 3)occurs superbly in Lushan Areas of western Henan and is characterized by oolitic limestones, micrites and varies of microbialites. The analysis of the macroscopic morphological characters, microscopic cortex characters, microbial fossils and their calcification, and the formation and controlling factors of microbialites such as oncoids and thrombolites can provide evidences for the study on the spatial distribution and evolution of Cambrian microbial carbonates.There are three types of thrombolites and two types of oncoids can be recognized in the upper carbonates of the third member of the Mantou Formation(Cambrian Series 3),Lushan area,western Henan. The Spherical and axiolitic oncoids develop regular dark-light cortex and are formed in moderate-energy subtidal environment; the long olivary oncoids develop irregular dark-light cortex and are preserved in low- to moderate-energy subtidal environment; the irregular oncoids develop unclear dark-light cortex and are dominant in low-energy subtidal environment. The massive thrombolites are characterized by irregular massive mesoclots absorbed by microbes and formed in low-energy subtidal environment; the dendritic thrombolites display the divergent dendritic mesoclots with tree-like growing pattern and are formed in deep low-energy environment under the normal wave base. The genetic analysis on microbialites shows that barren of metazoan is the determining factor for the formation of microbialites in the Mantou Formation of western Henan; the diversified shapes of microbialites are associated to different water energy; the high-energy condition of sustained agitation and influx of terrigenous clays are the direct controlling factors for the ultimate demise of microbialites. A large number of calcified cyanobacteria fossils are found in microbialites. Girvanella is the main cyanobacteria fossil in oncoids. Girvanella and Epiphyton widely occur in thrombolites, but the spherical Renalcis can locally be seen.Five microbialite cycles can be recognized in the carbonates based on the evolutionary relationship among thrombolites, oncoids and ooids. The thrombolites are always located in the bottom of every cycle and are formed in low energy subtidal environments. The oncolites are more common in the middle part of every cycle and are preserved in subtidal environments with periodic changes between low and high energy. Ooilitic limestones occur in the top of every cycle and are formed in agitated high energy shallow subtidal environments. The sea water dynamic condition and its periodic change are the main controlling factors for the variation of microbialites and the evolution of microbialite cycles.