The Geobiology of the Paleoproterozoic Belcher Group, Nunavut, Canada
The Belcher Group Microbiota (BGM) are a collection of microbial organisms from the Paleoproterozoic (~2.0 to 1.8 Ga) Belcher Group from the Belcher Islands, Nunavut, Canada. The Belcher Group is exceptionally well-preserved, having avoided deep burial or strong deformation during the Trans-Hudson orogen. The 6-9 km thick Belcher Group includes a variety of shallow marine depositional environments, two distinct periods of volcanism, and a prograding submarine fan system. The rock types range from dolomitic carbonate platforms to flood and pillow basalts to granular iron formations to sandstones. The BGM are found primarily in chert-replaced stromatolitic layers in dolostones of the lower and middle Belcher Group, including the Kasegalik and McLeary formations, and show dominantly filamentous (i.e., segmented) and coccoid (i.e., spherical) morphologies. The Belcher Group was first studied over 100 years ago, with the focus initially on the iron formation rocks and stromatolitic dolostones. Research on the putative microfossils and the geological history of the Belcher Group became the focus of later studies during the 1960s through the 1980s. The BGM were first described by Hans Hofmann as a collection of benthic prokaryotic organisms consisting of up to 24 taxa with filamentous and coccoid morphologies. However, due to degradation and diagenesis effects, Hofmann determined that the 24 different taxa could be as few as 10 distinctive taxa. The taxa of the BGM are contemporaneous with the Gunflint Microbiota but share more similarities with the microbiota of the Bitter Springs Formation. The main objective of this thesis is to confirm the biogenicity of the BGM. Previous characterization of the BGM utilized transmitted light microscopy. This thesis further investigates the BGM by utilizing other methods including Raman spectroscopy, X-ray photoelectron spectroscopy, and both electron microprobe and scanning electron microscopy, in addition to petrography and field work. This work shows that the microbiota likely represent cyanobacteria that built stromatolite structures in peritidal carbonate rocks, consistent with previous work. New data from Raman and XPS show that organic carbon is present in the spherules from the Kipalu Formation, which suggests microbiota could have facilitated the deposition of the granular iron formation. Pseudofossils were discovered during the re-examination of McLeary Formation samples, which show that abiotic structures exist as well as biogenic ones. Nevertheless, data from Raman and XPS show evidence for organic matter preservation in the Belcher Group rocks associated with stromatolites and microbial mats. The discovery of pseudofossils does not invalidate the interpretation of the BGM as cyanobacteria, however it shows that extraordinary preservation of the BGM is not universal.
Geobiology, Paleoproterozoic, Belcher Islands, Microbiota
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