Sophie Kurucz HBSc honours thesis

The Giant Dome Lithofacies of the Mosher Carbonate is located within the Steep Rock Group, 5 km north of Atikokan in northwestern Ontario. The Mosher carbonate has been studied for over a century and is one of the most well preserved Archean carbonate sequences in the world (Grotzinger, 1989). The giant domes that constitute the uppermost 70m of the 500m thick carbonate sequence are referred to as the Elbow Point Member. The giant domes are meter-sized, elongate in shape, and are internally composed of alternating crystal fan fabric and cuspate and net-like microbialite fabric. Crystal fan fabric consists of centimeter to decimeter tall radiating fans that have been argued to be originally aragonite that precipitated directly on the seafloor. Microbialites have been described by Sumner (1997) as being composed of draping, mat-like laminae, vertically oriented microbial support structures, and cement-filled voids. While both microbialite and crystal fan fabrics are common in the late Neoarchean to early Proterozoic, their occurrence with one another, and even as alternating lithologies, may be isolated to the Neoarchean. Therefore, the environmental factors that controlled the development of the giant domes, and their unusual internal composition, is not well understood.

The giant domes have been interpreted to have formed in a rimmed platform environment, where a fluctuating redox boundary resulted in the alternation in precipitation of aragonite and calcite (Fralick and Riding, 2015). Major element geochemical data suggests that the crystal fan fabric contains lower concentrations of Mn and Fe than adjacent fenestrate microbialite fabric, while Mg concentrations do not show any change in concentration. This trend may be a good indication that the primary mineralogy of the crystal fan fabric and fenestrate microbialite fabric is different. Similarly, cements that are interstitial and mantle the crystal fans contain relatively higher concentrations of Mg than the crystal fans themselves, indicating that the deposition of the fans and void-filling cements was not synchronous.

There are also features that suggest periodic subaerial exposure of the giant domes. A well-preserved desiccation surface with a typical polygonal crack pattern can be seen on a sample that in cross-section displays cuspate fenestrate fabric. The association of the support structures with the cracks that are expressed on the surface, leads to consideration of a desiccation related process in their formation. While there is an otherwise complete lack of preserved desiccation surfaces, Fe-rich red-brown surfaces that intervene between the interbedded layers of the giant domes at periodic intervals may represent desiccation or subaerial exposure surfaces that were later destroyed. Lastly, Fe- and carbon-rich dissolution surfaces separate some adjacent lithologies. These surfaces are continuous and can be seen to mark the boundary between crystal fan fabric and fenestrate microbialite fabric within certain samples. They are composed of microcrystalline quartz and zoned dolomite. The dissolution surfaces may represent hardgrounds that formed as a result of a hiatus in sedimentation and/or as result of early cementation and provided preferential fluid pathways for later silicification.