Coated Grains

pp 276-283

Cyanoliths (Cyanoids): Oncoids Formed by Calcified Cyanophytes

  • Robert RidingAffiliated withDepartment of Geology, University College

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A wide variety of calcareous nodules have been regarded as being probably formed by the activity of cyanophytes. Stromatolites too, are generally assumed to be mainly of cyanophyte origin. But it is only possible to be confident about these interpretations if the deposits contain direct internal evidence of the algae involved. In the Recent this can be provided by the presence of the living algae themselves, but in ancient material the evidence must be in the form of mineralized fossil remains or distinctive petrographic fabrics. In many cases it must be admitted that a cyanophyte origin for stromatolites and oncoids is inferred only from general similarities in form and structure between them and Recent cyanophyte mats which have trapped and bound particulate sediment. Specific evidence for the type of algae (or other microorganisms) involved is usually lacking. However, some fossil examples of stromatolites and oncoids contain convincing evidence of their cyanophyte origin due to the presence of mineralized algal remains within them. Many of the Precambrian silicified microfloras, which provide valuable information about the early history of life on Earth, occur within stromatolites. Examples include the microfloras from the Transvaal Dolomite, Gunflint Iron Formation, Belcher Group, Paradise Creek Formation, Bungle Bungle Dolomite, Beck Spring Dolomite, Bitter Springs Formation, and many others (Schopf 1977, Table 2).