Depositional environment and mechanism of preservation of microfossils, upper Proterozoic Bitter Springs Formation, Australia

Microfossils preserved in black chert nodules from the upper Proterozoic Bitter Springs Formation of Australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds. Groundwaters of halite salinity “pickled” the microorganisms and thus inhibited their early bacterial degradation. These same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions, nodules, and bipyramidal quartz crystals. Where the chert nodules formed in areas of buried cyanobacterial mats, further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved.