NEOPROTEROZOIC PERITIDAL PHOSPHORITE, SETE LAGOAS FORMATION, BRAZIL: IMPLICATIONS FOR THE PRECAMBRIAN P CYCLE

The Neoproterozoic Sete Lagoas Formation (ca. 610 Ma) of central Brazil is a succession of epeiric sea siltstone, limestone, and phosphorite. Phosphatic sedimentary rocks are restricted to peritidal deposits in a ca. 150-m-thick unconformity bounded sequence of siltstone. Lithofacies stacking patterns indicate deposition occurred during a marine transgression punctuated by higher order fluctuations in relative sea level that produced three parasequences. Parasequences are 40 to 50-m-thick and defined by a basal flooding surface that is generally overlain by interbedded carbonate mudstone and wavy laminated siltstone that grades to flaser and parallel bedded silty sandstone containing francolite-rich microbial laminae. Each cycle is interpreted to record progradation of phosphatic intertidal flats over subtidal deposits as accommodation filled. The presence of beach facies, mudcracks, absence of coarse terrigenous clastics, and the abundance of silt with fine, abraded quartz grains indicate deposition along an arid coastline dominated by eolian input.

The shallow nature of phosphorite in the Sete Lagoas Formation is unlike younger, larger late Neoproterozoic-Phanerozoic phosphorites. These giant phosphorites generally formed in distal shelf environments in association with coastal upwelling. The increasing size of phosphatic deposits through the Neoproterozoic is interpreted to reflect the expansion of oxygenated environments across shelves that were favourably positioned for upwelling; deposition of the first true phosphorite giants occurred after ocean ventilation at ca. 580 Ma. This codependence of seafloor oxygenation and phosphogenesis highlights the importance of ocean redox state on the Precambrian P cycle. The Sete Lagoas Formation demonstrates that redox-controlled phosphogenic processes such as bacterial sulphate reduction and Fe-redox pumping were still restricted to nearshore oxygen oases until the distal shelf became fully oxygenated, permitting for the first time the production of Phanerozoic-style phosphorites.