Paleoceanographic constraints on paleoproterozoic phosphorite and iron formation accumulation, Baraga Group, Michigan, USA

The Paleoproterozoic Baraga Group (ca. 1850 +/- 1 Ma) of northern Michigan is a ∼1,200 m thick sedimentary succession of marine clastic, iron formation, and phosphatic sedimentary rocks that accumulated at the peak of the world's first major phosphogenic episode. Lithofacies stacking patterns are interpreted to record the inundation and flooding of the Nuna continental margin during two sea-level cycles. The base of the first sequence is marked by a transgressive lag on Archean basement that is transitional into a highstand accumulation of sandstones deposited in peritidal environments. The bottom of the second sequence is characterized by a chert and carbonate unit with numerous subaerial exposure surfaces deposited in intertidal to supratidal environments. This chert grades upward into subtidal deposits composed of interbedded organic-rich mudstone and sandstone. The highstand and falling stage systems tracks are composed of progradational deltaic deposits.;Phosphorite is restricted to shallow water sediments of the first sequence and the lowstand of the second sequence. Precipitation is interpreted to be the result of Fe-redox pumping just below the sediment-water interface where photosynthetically produced, nearshore oxygen oases impinged on the seafloor. Such shallow-water phosphorite accumulation is in stark contrast to many Phanerozoic depositional systems in which phosphogenesis occurs across the shelf. This difference likely reflects the dissimilarity in the oxygenation state of the seafloor. In the Precambrian, Fe-redox pumping and thus, phosphogenesis, was restricted to shallow-water settings within a suboxic seafloor. In the Phanerozoic, phosphorite forms in the full spectrum of shelf environments because the entire seafloor is generally well oxygenated.