摘要
Conophyton(Maslov)is a cylindroidal stromatolite form-genus characterized by nested conical laminae.Well-preserved Conophyton,up to 4 m tall and with basal diameters of up to 50 cm,are exposed in the Proterozoic Atar Formation of Mauritania,where many occur together,in growth position,as fields of individual columns spaced between 5 and 70 cm apart.The uniformity of these forms and their regular distribution suggest that they grew in quiet-water environments below wave base.Evidence for their penecontemporaneous organomineralization is indicated by nearby toppled examples of undeformed Conophyton forms alongside eroded lithified Conophyton fragments in carbonate breccias.Two characteristics of Conophyton have been used to classify the structures-the form of the lamination and the nature of its axial structure.A mathematical/physical model provides an explanation for the growth pattern of Conophyton.It predicts that coniform structures with thickened axial zones form when upward organic growth of a biofilm moderately exceeds the rate of its mineralization.The varying characteristics of these features between different forms of Conophyton are thought to reflect biomineralization of the decaying biofilm rather than differences in the composition of microbial communities.A modern example of a syngenetic mineralization process capable of producing similar structures has been observed in the contemporary sediments of Lake Preston,Western Australia,where benthic microbial mats are being transformed into coniform lithified crusts.The initial biomineralization of the coniform mat forms magnesium silicate that first coats and permineralizes web-like microbial extracellular polymeric substances(EPS)and then coalesces into a uniform mass that provides mechanical strength to the cones.At a later stage,massive carbonate crystal growth occurs that over-prints much of this texture,leaving only small,remnant areas of the magnesium silicate phase.Many fossil Conophyton are composed of dolomite,and the remains of the microbial communities responsible for their construction are rarely found,except in areas of chert within the Conophyton.It is suggested that Proterozoic Conophyton were constructed in a tranquil environment through the accretion of microbial mats that were syngenetically permineralized by a magnesium silicate such as a smectite.Later,much of the unstable smectite would be susceptible to diagenetic replacement by either dolomite,or chert in which remnants of microbes that had been coated or permineralized could,potentially,be preserved.
