Munira Afroz MSc thesis abstract
The Mesoarchean Red Lake carbonate platform is the oldest (~2.93 Ga) known carbonate platform on Earth. It is comprised of a variety of chemical sedimentary rocks including: limestone, dolostone, chert, oxide iron formation, and sulfide iron formation. This study deals with its five different types of deeper water lithofacies, including four chemical sedimentary rocks and siliciclastic black slate, to delineate Mesoarchean ocean chemistry and comprehend the localized depositional settings. Geochemical analysis of selected elements in the chemical sediments were used to constrain depositional processes. Of these positive Eu anomalies in REE patterns suggests that the ocean was heavily influenced by hydrothermal activity and positive Ce anomalies in oxide iron formation indicate that a redoxcline existed in the depositional environment. Zirconium and hafnium in oxide facies iron formation were mostly derived from seawater, and their sorption was dependent on the rate of precipitation of iron hydroxide. Multiple sulfur isotope analyses revealed that mass-independent fractionation of sulfur was operating along with bacterial sulfate reduction, and the source of sulfur was diverse. Organic carbon isotopes increase from ~ -27 ‰ to ~ -20 ‰ up-section towards the shallow portion of the carbonate shelf, possibly reflecting the presence of anoxic phototrophs in the shallows. Inorganic carbon isotopic ratios averaging ~ -1.5 ‰ fall within the range of Archean carbonate and reflect seawater values. Also, δ18O data reveals that dolomitization occurred in a freshwater influenced environment. Evidence such as increasing Mn concentration towards the carbonate platform and positive Ce anomaly in oxide iron formation, as well as redox-sensitive element enrichment in the chemical sedimentary rocks, suggests that bacterial O2 production was somewhat active in the Mesoarchean sea. The interlayering of various types of chemical sediments at differing stratigraphic scales indicates that localized ocean chemistry changed repeatedly over both short and long time intervals. Carbonates were perhaps formed in the slightly oxic shallow water environment, while magnetite and chert were precipitated in a suboxic environment distal to any venting fluids and iron sulfides accumulated in a reducing, anoxic environment during intervals of intense venting of hydrothermal fluids. Shale was deposited as background sediment and accumulated during cessation of chemical sedimentation. Finally, the deeper water sediments repeatedly interlayered with the shallow water stromatolitic carbonate successions, which suggests multiple events of carbonate platform flooding.
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