Testing the Use of Anisotropy of Magnetic Susceptibility (AMS) in Determining Genetic Origins of Paleoproterozoic Diamictites

Abstract

The Huronian Supergroup (2.4-2.1 Ga) in Ontario, Canada is widely accepted as an important stratigraphic interval for interpreting Paleoproterozoic climate. This is because it contains some of the oldest glaciogenic rocks on the planet. However, massive and poorly-stratified diamictites in the Gowganda Formation of the Huronian Supergroup have varying depositional interpretations among sedimentologists (subglacial, rainout, sediment gravity flow etc.). Diamictites can occur from a variety of processes and, therefore, proper depositional interpretation is essential for unraveling detailed environmental conditions at the time of deposition. Anisotropy of magnetic susceptibility (AMS) looks at the orientation of magnetic particles within a rock and coupled with sedimentary investigation, can help interpret depositional processes. Rock magnetic data show that magnetism is carried by multi-vortex (titano)magnetite in fine-grained facies and includes the addition of a higher- coercivity contribution (potentially diagenetic goethite) in some sandstone facies. Most magnetic fabrics iii are oblate in shape and oriented transverse to flow, although vertical fabrics were found in sites that exhibited substantial deformation or dewatering. Results and observations from this project suggest that Gowganda sedimentation was dominated by sediment gravity flows, deposited on a marine post-glacial slope with a southwestern transport direction. Interpretation of depositional processes through a combination of AMS and sedimentologic observation provide a more comprehensive understanding of the environmental conditions controlling deposition, and in this case, painting a more elaborate picture of Paleoproterozoic climate transitions. In contrast with subglacially derived diamictites, which are deposited directly under glacial ice, those produced by sediment gravity flows suggest a more glacially distal to non- glacial environment. The presence of bedded diamictites, water escape structures, quarter structures around clasts, a general lack of shear horizons and striated/faceted clasts as well as an abundance of flow-transvers magnetic fabrics observed in most Gowganda facies suggest this more distal environment. However, subglacial deformation and deposition cannot be entirely ruled out for one oriented and striated boulder bed horizon producing flow-aligned magnetic fabrics; both characteristics of subglacial processes.