THE GENESIS AND TIME DISTRIBUTION OF 2 DISTINCTIVE PROTEROZOIC STROMATOLITE MICROSTRUCTURES

Stromatolites are commonly viewed as sedimentary proxies for microbial communities. In consequence, secular variation in stromatolite form h as been attributed to evolutionary change in mat organisms and/or thei r interactions with global environments. This interpretation requires that one be able to identify features of stromatolite macrostructure o r microstructure that are under direct biological influence and use th em to document the sedimentological or petrological consequences of mi crobial evolution. Well-preserved stromatolites in Proterozoic carbona tes of the Siberian Platform contain true distinctive types of microst ructure that enable us to address such issues. Preserved microstructur es in Baicalia lacera and Tungussia confusa from lower Neoproterozoic (lower Upper Riphean) platform carbonates of several widely separated regions clearly reflect the biology of underlying mat communities; mm- scale laminae of densely interwoven calcified. filaments alternate wit h filament-poor microspar. Comparable carbonate microstructures are kn own from a number of other Neoproterozoic stromatolites, but are as ye t unreported from older successions. Filamentous cyanobacteria formed mats throughout the Proterozoic Eon; thus, the temporal distribution o f this microstructure appears principally to reflect secular and envir onmental variations in carbonate cementation and diagenesis. Omachteni a omachtensis is the characteristic stromatolite of Mesoproterozoic (L ower and, occasionally, Middle Riphean) successions of the Siberian. P latform and elsewhere. Its distinctive microstructure consists of mech anically deposited event laminae separated by thin organic films that served as nucleation sites for micritic and, less commonly, fibro-radi ate carbonate precipitates. Microbial mats may have stabilized sedimen ts between events, but there is little evidence that mats played an ac tive role in the trapping and binding or precipitation of laminae. Thu s, physico-chemical factors must also be responsible for the distribut ion. of these stromatolites in time and space. Analysis of these two m icrostructures suggests that, in general, secular trends in stromatoli te microstructure may encrypt important information about environmenta l change through the Proterozoic Eon. Microbial evolution may also pla y a role in determining the stratigraphic distributions of particular Proterozoic stromatolites, but this remains to be demonstrated. Evolut ion. may be most important in driving the progressive environmental re striction of stromatolite-forming microbial communities through time.