Brittany Ramsay HBSc thesis abstract
The 2310 Ma Kona Dolomite of Marquette, MI lies within the Chocolay Group of the Marquette Range Supergroup and has previously been correlated to the Cobalt Group of the Huronian Supergroup. The dolomitic units of the Gordon Lake Formation, Cobalt Group, are equivalent to the Kona Dolomite in Marquette and this study refers to the two dolomite units as a whole. These formations, termed the Kona Dolomite (KD) were deposited on what was a passive continental margin after the last Huronian glaciation and the Great Oxidation Event, making the KD an interesting suspect for analysis.
In order to better understand how free oxygen entered our atmosphere, sedimentological, geochemical, and isotopic techniques were utilized. It’s important to have a good understanding of the sedimentology and lithofacies before looking at the geochemistry in order to see localized depositional trends with the ocean chemistry. Thus, nine lithofacies associations (LA) have been identified in the KD based on their morphological and mineralogical differences. Each LA is indicative of its own depositional setting on a tidal flat.
The Ripples and Low Domes LA is composed of soft, centimetric, irregular, and wavy laminations of varying pink and gray carbonates- representing climbing ripples, wave ripples, and low-domal stromatolites. Lenticular and wavy bedding show bi-directional cross-stratification, a feature common in intertidal flat settings. Small Domal to Stratiform Stromatolite LA (<4cm tall) are low pustular stromatolites that have laterally continuous lamination and tend to gradually change upwards into stratiform stromatolites. These likely formed in an intertidal flat setting as they are associated with the bi-directional ripples.
Smooth to crinkly, millimetric, and relatively isopachous laminations comprise the Parallel Laminated LA. Laminations are wispy in places and contain low domes that are indicative of microbial activity and were likely situated within a pond on the intertidal flat.
Hummocky Cross-stratified (HCS) LA is composed of gray carbonate sands that have low-angle truncation surfaces and likely formed in a tidal channel environment that feed the tidal flat system. The stunning outcrop of Large Columnar to Domal Stromatolites (LCDS) stands 5 to 6m tall, and are composed of laterally continuous columnar lamination joined by cuspate depressions that gradually shift into domal stromatolites. The LCDS grew on top of the sandy HCS within the tidal channels.
Fenestral Microbialite LA show good, fairly isopachous banding with dark reddish elongated fenestrae that are discontinuous on the microscale but show low domes on a macroscale. The fenestral microbialite is similar in colour to the Bedded Siltstone and Mudstone LA as they are both composed of similar reddish-brown siliciclastics and carbonate. The latter is composed of intercalated siliciclastics, iron-rich, and iron-poor carbonate mud, and contain sub-euhedral rectangular carbonate clasts. The clasts were deposited by storm events that ripped up the fenestral microbialite in the upper intertidal flat, flooded the supratidal flat and deposited them on a sabkha.
The Subaqueous Gypsum Pseudomorph LA is composed of laterally continuous chevron textured carbonates replacing vertical gypsum crystals, while the Subaerial Gypsum Pseudomorph LA is composed of euhedral, diamond shaped pseudomorphs that lie within a very fine-grained dark purplish siliciclastic- and carbonate-rich groundmass. The former would have formed in an open water supersaturated environment, such as a pond on the sabkha, whereas the latter would have formed within the sediment in the sabkha itself.
Geochemically, these LAs are enriched in middle rare earth elements (REEs), but do not have pronounced Eu anomalies. Prior to the 2310 Ma KD, ocean chemistry was controlled by hydrothermal activity that created Eu enrichment compared to the other REEs. This contrasts with today’s oceans that are controlled by continental inflow. The absent Eu anomaly is illustrated in the REE concentrations of the LCDS and HCS LAs and represents what seems to be a turnover event from an ancient ocean system to a modern system.
Carbon isotopic ratios show an interesting trend when compared to their respective lithofacies. The δ13C increases from lower tidal flat to sabkha environments. This probably represents increasing evaporative loss of light CO2. This local trend is superimposed on the beginning of the Lomagundi Event which is known as the most prolonged and largest period of carbon-13 enrichment in Earth’s history.