TAPHONOMIC GRADE FOR SILICIFIED MICROBIAL MATS OF THE ANGMAAT FORMATION, BYLOT SUPERGROUP, BAFFIN ISLAND

Our understanding of the Proterozoic fossil record derives largely from the permineralization of benthic microbial mats. The very nature of growth and decomposition in microbial mats, however, results in a broad range in the quality of microbial preservation. Such taphonomic variability further complicates both taxonomic classification and interpretation of organic geochemistry. Here we provide a first stage of analysis aimed at correlating the preserved morphology of benthic microbial mats to the chemical and molecular structure of ancient organic matter within microfossils (via Raman and FTIR spectroscopic techniques) to help understand the extent to which of biological and biochemical information is lost or retained during the preservation process.

The late Mesoproterozoic (~1.1 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island, records undeformed, intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification records microbial growth and decomposition across a range of environments from more restricted and exposed to more persistently submerged. The distribution of mat-building and mat-dwelling populations varies along this environmental transect, and is recorded by four distinct microfossil assemblages (Knoll et al., 2013). Furthermore, mats are preserved across a range of taphonomic states, from pristine mats silicified during active growth, to highly degraded and compacted mats that represent preservation during later stages of biological decomposition. Secondary silicification events also overprint original silicification, resulting in markedly poorer morphological preservation. Here we present a petrographic evaluation of these samples, and assign a taphonomic grade to preserved microbial mats. Taphonomic grade is assigned using a modified classification that takes into account both the taphonomic preservation state (good, fair, poor) of individual microfossils (Bartley, 1996), as well as the degree of compaction of the overall mat, and effects of secondary silicification. Combined, this information will provide a new look at the preservational process of ancient microbial systems.