Microfossils in stromatolitic cherts from the upper proterozoic Min'yar formation, southern Ural Mountains, USSR.

A diverse assemblage of exceptionally well-preserved microorganisms, including several previously unknown taxa, has been discovered in stromatolitic black chert from the ca. 680-790 Ma-old Min'yar Formation (Suite) of the southern Ural Mountains, USSR. Like most ancient and modern stromatolitic communities, the Min'yar microflora is dominated by filamentous and unicellular cyanobacteria. Geologic evidence indicates that the microbial community inhabited a shallow water, presumably marine environment. The microfossils occur in two interlaminated and thinly interbedded sedimentary fabrics: 1, fact to wavy-laminated Stratifera-like stromatolitic laminae that presumably were deposited during periods of little wave action; and 2, intraclast grainstone that formed as a result of desiccation and (or) wave agitation. Microfossils are both better preserved and more abundant in the intraclasts than in the Stratifera-like laminae. The occurrence of probable pseudomorphs after replacement of sulfate minerals provides additional evidence for a shallow water, periodically emergent depositional environment for the Min'yar microbial mats. Kerogenous microfossils are three-dimensionally preserved, permineralized in fine-grained silica of primary or early diagenetic origin. In many aspects the Min'yar assemblage is comparable to that of the well-known ca. 850 Ma-old Bitter Springs Formation of central Australia. The following taxa are herein described: Division? Schizomycophyta or ? Cyanophyta, Biocatenoides sp.; Family Oscillatoriaceae, Eomycetopsis robusta Schopf emend. Knoll and Golubic, Rhicnonema antiquum Hofmann, Entosphaeroides? sp., Palaeolyngbya? sp., Siphonophycus capitaneum n. gen., n. sp.; Family? Oscillatoriaceae or? Rivulariaceae, Caudiculophycus? sp.; Family? Scytonemataceae or? Stigonemataceae, Ramivaginalis uralensis n. gen., n. sp.; Family Chroococcaceae, Sphaerophycus medium Horodyski and Donaldson, Eosynechococcus amadeus Knoll and Golubic, Gloeodiniopsis lamellosa Schopf emend. Knoll and Golubic, Gloeodiniopsis magna n. sp., Eoaphanocapsa oparinii n. gen., n. sp.; Division? Chlorophycophyta or? Rhodophycophyta, Glenobotrydion majorinum Schopf and Blacic. Also discussed in the taxonomic section, but not formally described, are intermediate-diameter tubular sheaths (Oscillatoriaceae), small tubular sheaths enclosed by larger tubular sheaths (Oscillatoriaceae?), and undifferentiated spheroidal unicells.