Carbon- and Oxygen-Isotopic Behavior of Carbonate Rocks of the Phosphatic Gramame Formation, Pernambuco-Paraíba Coastal Basin, Northeastern Brazil

Abstract

In the basal section of the carbonates of the Maastrichtian Gramame Formation in the Pernambuco-Paraiba coastal basin of northeastern Brazil, major phosphate concentrations (P2O5 > 10%) were deposited in shallow-marine environments (inner shelf). In contrast, dolomite-associated phosphates are characterized by low P2O5 concentrations (<10%) and were deposited in the relatively deep water of a platform ramp, under conditions of limited oxygen availability. Calcite is the main carbonate phase remaining after the diagenesis that affected the phosphorite. A positive δ13C (up to +2%PDB), coupled with a positive MgO-δ180 correlation in the phosphate-enriched carbonates, suggests that upwelling currents were the early phosphogenic vectors during marine transgression, in contrast to warm superficial seawater that prevailed during the Maastrichtian elsewhere. The major phosphate concentrations are related to reworking and diagenesis in a shallow shelf environment during a regression pulse of sea level, followed by a dramatic drop of δ13C to negative values (down to −6%PDB). This study suggests that carbon and oxygen isotopes can be used as potential tools for phosphorite prospecting elsewhere.