Justin Burton HBSc thesis abstract

Thesis Title: 
Deposition and cementation of Paleoproterozoic Gunflint Formation carbonates: Implications for early hydrosphere chemistry

The carbonate sequence located in the upper most portion of the Paleoproterozoic Gunflint Formation was studied in detail, in an attempt to understand the ambient conditions that were present at the time of deposition and cementation.  The study included analyses of shore proximal sediments and structures using geochemical instrumentation, as well as qualitative interpretations of depositional environments based upon in-depth microscopic observations.  The carbonate sequence itself was located on the border between the sediments of the Gunflint Formation and the siliciclastic Rove Formation.  Moving up through the Gunflint Formation, through the carbonate and eventually into the Rove Formation one encounters major changes in the depositional environments in the surrounding area.  Indications of these major changes are drawn from the broad differences in compositions of the units.

Whole and partial rock geochemistries were acquired via the ICP-AES at Lakehead University (LUIL), as well as from the ICP-MS at the Ontario Geologic Survey, located in Sudbury.  Quantitative SEM analyses and qualitative SEM elemental mapping were conducted; as well the samples were subject to combustion reactions to determine H2O and CO2 content of samples via the LUIL.  Also, a vast amount of information was gained using a high-power Olympus petrographic microscope digital imaging system via the Lakehead University Geology Department.

Although the carbonate sequence throughout the area was discontinuous, those outcrops sampled contained similar units in comparable order and could be correlated.  The majority of the outcrops sampled contained strata that placed them in a shore proximal environment that may have been sub-aerially exposed at some point in the Paleoproterozoic.  These features included a stromatolitic layer with an adjacent scoured fine carbonate sand, as well as upper and lower coarse carbonate sands.  Analyses of the units indicated that they are composed mainly of Fe-rich chlorite grains bounded in a calcite cement.  As well, some layers contained abundant dissolution and silica replacement.  Abundant V enrichment was associated with the Fe-rich chlorites throughout the shore proximal sediments.

As a comparison, outcrops located at a stratigraphically lower level in the carbonate sequence were sampled.  These samples represented more off-shore sediments deposited in the submarine environment at the time.  Analyses of these sediments indicated a much different composition to the more shore proximal sediments.  Not only did the samples not contain any of the shore proximal features, they were composed almost entirely of ankerite.  The submarine sediments also contain rare, highly silicified stromatolite fragments, as well as pressure solution features, such as styloites characteristic of the deep burial environment.

The fact that the shore proximal sediments contain cements that do not reflect the chemistry of the ocean during the Paleoproterozoic suggests that the calcitic cements were more reflective of the meteoric environment.  The large V enrichments also suggested the presence of a redox boundary, as well as suggesting an oxic atmosphere.  REE curves for the ankeritic marine precipitates show Ce anomalies that reflect an anoxic ocean, whereas those of the meteoric calcite cements reflect an oxic atmosphere, further reinforcing conclusions from other geochemical data.