Stromatolites and Precipitates
Early diagenetic chert in the upper Kotuikan and Yusmastakh formations, northeastern Siberia, preserves an exceptional record of carbonate textures and microfossils in an early Mesoproterozoic peritidal carbonate platform. Silicified lithologies include carbonate precipitates that formed at or near the sediment-water interface, as well as micritic event laminae that appear to have lithified more slowly. Precipitated textures include (1) radial-fibrous laminae nucleated on organic horizons and locally forming botryoids that stack vertically to produce microdigitate structures; and (2) micron-scale carbonate laminae. Both radial-fibrous carbonates and microlaminites contain abundant microfossils, some of which are preserved as uncompressed casts and molds that retain cellular detail. These indicate that lithification preceded microbial decay; actualistic taphonomy experiments on filamentous cyanobacteria suggest that lithification occurred on a timescale of days to weeks. In other silicified textures, microfossils show evidence of extensive post-mortem decay and compression, suggesting less rapid lithification. Papier-mâché carbonate sedimentation, characterized by essentially instantaneous lithification, appears to have been locally common in restricted tidal-flat environments during the Mesoproterozoic and earlier eras but uncommon in Neoproterozoic and later times.
Figures & Tables
Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.