HOW EXCEPTIONAL IS EXCEPTIONAL FOSSILIZATION? THE CHARACTERIZATION OF SILICIFIED MESOPROTEROZOIC MICROBIAL MATS OF THE ANGMAAT FORMATION, BAFFIN ISLAND, CANADA (Invited Presentation)

Exceptionally preserved microfossil assemblages are common in Proterozoic chert deposits. These deposits occur as beds, lenses, and nodules in ancient peritidal carbonate environments and represent precipitation of silica phases from seawater during early diagenesis. Combined, petrographic observations of microfossils, observations of modern microbial communities, actualistic studies of microbial degradation, and artificial fossilization experiments, indicate that silicification occurred penecontemporaneously with microbial growth and decomposition to preserve the observed variation in microbial morphologies caused by taphonomic alteration, and to halt further decomposition of the preserved microbial morphologies.

Late Mesoproterozoic carbonate strata of the Angmaat Formation, Bylot Supergroup, records an intertidal to supratidal microbial flat. Black chert provides exceptional insight into microbial growth and decomposition within both filamentous and coccoidal microbial mats, across a range of environments. Physical and chemical variation within these mats provide the opportunity to investigate the extent to which microbial organisms and assemblages are preserved during silicification, and to determine the environments and mechanisms of silicification.

Here we use taphonomic assessment of the Angmaat microfossils and overall mat fabrics to describe the nature of microbial preservation (Manning-Berg et al., 2019), and petrographic analysis (Dunham, 2018) to decipher the silicification process. We also present organic geochemical data show that, despite their exceptional morphological preservation, microbial lipids are not preserved. We propose that the silicification process itself may explain our observations, wherein initial formation of a silica gel played a critical role in protection of mat elements from degradation, but also compelled migration of filamentous cyanobacterial trichomes, thus creating a fundamental bias against lipid preservation.