UPPER PROTEROZOIC CARBONATE STRATIGRAPHY, DIAGENESIS, AND STROMATOLITIC PHOSPHORITE FORMATION, IRECE BASIN, BAHIA, BRAZIL

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece Basin of east-central Brazil. The Irece Bas in contains a basal siliciclastic sequence (the Bebedouro Formation) c omposed of diamictites dominantly of glacial origin that is overlain b y lacustrine carbonate sediments. These are succeeded by the Una Group , a marine carbonate sequence that totals several hundred meters thick . The marine units consist of a basal laminated limestone and dolomiti c limestone sequence that grades upward into dolostone and cherty dolo stone; a middle sequence dominated by gray argillaceous limestones, sh ales, and siltstones; and an upper sequence of black organic-rich lime grainstones. Stratabound phosphate- and sulfide-rich units are presen t within a 50-m-thick tidal flat sequence of dolomitic limestone and c herty dolostone in the lower Una Group. Shallow-water sedimentary stru ctures include stromatolitic laminae, columnar stromatolites, mudcrack s, teepee structures, collapse breccias, length-slow quartz nodules, a nd other pseudomorphs after evaporitic sulfates. Conventional petrogra phy supported by cathodoluminescence and scanning electron microscopy of the carbonate strata reveals a complex diagenetic history represent ing four major diagenetic environments: Marine, Reflux, Meteoric, and Burial. Carbon and oxygen isotope analyses of diagenetic carbonate gen erations yield distinct populations that support the environmental int erpretations. Concentrations of fine-grained carbonate fluorapatite ar e present in stromatolitic carbonate units in the Irece-Lapao area. Th ree types of primary phosphate concentrations have been defined: colum nar stromatolitic, laminar stromatolitic, and intraclastic. The presen ce of resedimented phosphatic clasts and of phosphatic units interbedd ed with barren dolostones suggest that the formation of stromatolitic phosphorite was an early-diagenetic process. Carbon and oxygen isotope data for calcite, dolomite, and carbonate fluorapatite, as well as su lfur isotope data from sulfates and sulfides, suggest the presence of an anoxic environment during the formation of the Irece phosphorites. Postglacial enrichment of the basin waters in phosphate is considered to be a critical aspect of phosphatization. Bacterial degradation of o rganic matter in the stromatolites probably was responsible for local phosphate enrichment of pore waters and resulted in concentration of c arbonate fluorapatite by direct precipitation or by replacement of cal cium carbonate.