SHALLOW-WATER PHOSPHORITE ACCUMULATION IN THE PALEOPROTEROZOIC 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 during the world's first major phosphogenic episode. Vertical and lateral lithofacies stacking patterns are interpreted to record the inundation and flooding of the Nuna continental margin during two sea-level cycles that resulted in two corresponding depositional sequences. 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 chert 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. Deltaic sediments comprise the highstand and falling stage systems tracks.

Sedimentary apatite is restricted to chert and organic-rich mudstone in clastic-poor facies in the lowstand systems tract of the second sequence. Precipitation is interpreted to be the result of iron-redox pumping and microbially-mediated organic matter degradation just below the sediment-water interface within photosynthetically produced, nearshore oxygen oases. Such shallow-water phosphorite accumulation is in stark contrast to many Phanerozoic depositional systems where phosphogenesis occurs in a spectrum of shelf environments.