OCEAN STRUCTURE AND ECOLOGY OF THE ANIMIKIE BASIN IRON-FORMATIONS

The Paleoproterozoic Animikie Basin contains exceptionally well preserved microfossils, iron-formations, and stromatolites in the ~1.85 Ga Biwabik, Gunflint and Ironwood iron-formations of Minnesota, Ontario, and Wisconsin. Through field study, petrography, rare earth element systematics and Fe isotope work we characterized an oxygen chemocline within the iron-formations. The oxygen mixing zone in a shallowing-upward succession comprises hummocky cross-stratified grainstone, jaspillite, siliceous and hematitic stromatolites, manganese-rich oncoids, and mixed siliceous, carbonate, and hematitic stromatolites. The oxygen chemocline is characterized by a transition into facies with negative cerium anomalies and the transition out of facies with authogenic iron deposition. The sedimentary features within the chemocline suggest a surprisingly limited, shallow oxygenic zone. The Gunflint and Biwabik stromatolites, oncoids, and microfossils strongly suggest that a widespread and facies-variant iron- and manganese-oxidizing microbial community existed in the oxygen mixing zone. The spatial distribution and facies variability of the stromatolites, microfossils and oncoids suggests that Fe and Mn based chemotropic metabolisms were widespread, and that chemotropic bacteria were the predominant primary producers of the Animikie Basin. The presence of similar stromatolites and microfossils in the nearly coeval Chuanlingou Iron-Formation in China and the Frere Iron-Formation in Australia suggests that our observation on the Animikie Basin may have global applicability. The absence of similar signatures of an oxygen mixing zone in preserved shallowing-upwards successions in Archean Superior-type iron-formations suggests disparate modes of deposition.