THE END CRYOGENIAN AND EARLY EDIACARAN BOUNDARY AND THE ONSET OF PHOSPHOGENESIS IN A TRANSITIONAL SEDIMENTARY SUCCESSION: JACADIGO AND CORUMBA GROUPS, SOUTHWESTERN BRAZIL

The end-Cryogenian to Ediacaran (ca 630 Ma) transition was critical in geosphere and biosphere co-evolution. It was marked by an increase in oxygen, large perturbations in the global carbon cycle, diversification of eukaryotes, the appearance of multicellular animals, and emergence from the last Snowball Earth ice age. Associated with the Cryogenian ice ages was the return of iron formation deposition after nearly one billion years. This iron interval largely ended with the Cryogenian probably due to increase in oxygen and ocean mixing. The nature and timing of oxygen increase, however, are not well understood.

The environmental change at the Cryogenian-Ediacaran boundary appears rapid and extreme where cap carbonate units unconformably overlie the glaciogenic Marinoan. Rapid sea-level rise following melting of Marinoan ice caps and associated isostatic crustal adjustments make this transition appear short-lived and dramatic, but provides little information regarding the interval. To determine the nature of this interval, we examined continuous drill core through a sedimentary succession dominated by siliciclastic sediments in a rift basin, which recorded sedimentation during this transition. The Jacadigo Basin, near Corumbá, Brazil, evolved from a tectonically active aulacogen marked by hydrothermal input of Fe and Mn, seasonal input of eolian silt and sand, ice margin upwelling, periodic open-water, abundant ice-rafted debris, submarine mass wasting, and at least two major ice advances. These glaciogenic diamictite intervals constitute the Marinoan Puga Formation, which is interbedded with the iron formation of the Santa Cruz Formation.

We document the sedimentary transition from hematitic iron formation to onset of phosphogenesis. Phosphate (francolite) occurs in microbially laminated, hematitic sandstone and mudstone marine shelf facies of the Cerradinho Formation, which overlies outwash deposits of the Cadieus Formation and the underlying, uppermost diamictite of the Puga. Deposition of a regional “cap carbonate” marked the sea level highstand that followed clastic deposition. Phosphogenesis in this shelf environment suggests that redox gradients were established in sediments, and phosphate-concentrating, biochemical reactions were operating near the end of the Marinoan ice age.