Una Group Research Articles

Upper Proterozoic Carbonate Stratigraphy, Diagenesis, and Stromatolitic Phosphorite Formation, Irece Basin, Bahia, Brazil

ABSTRACT Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece Basin of east-central Brazil. The Irece Basin contains a basal siliciclastic sequence (the Bebedouro Formation) composed of diamictites dominantly of glacial origin that is overlain by 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 dolomitic limestone sequence that grades upward into dolostone and cherty dolostone; a middle sequence dominated by gray argillaceous limestones, shales, and siltstones; and an upper sequence of black organic-rich lime grainstones. Stratabound phosphate- and sulfide-rich units are present within a 50-m-thick tid l flat sequence of dolomitic limestone and cherty dolostone in the lower Una Group. Shallow-water sedimentary structures include stromatolitic laminae, columnar stromatolites, mudcracks, teepee structures, collapse breccias, length-slow quartz nodules, and other pseudomorphs after evaporitic sulfates. Conventional petrography supported by cathodoluminescence and scanning electron microscopy of the carbonate strata reveals a complex diagenetic history representing four major diagenetic environments: Marine, Reflux, Meteoric, and Burial. Carbon and oxygen isotope analyses of diagenetic carbonate generations yield distinct populations that support the environmental interpretations. Concentrations of fine-grained carbonate fluorapatite are present in stromatolitic carbonate units in the Irece-Lapao area. Three types of primary phosphate concentrations have been defined: columnar stromatolitic, laminar stromatolitic, and intraclastic. The presence of resedimented phosphatic clasts and of phosphatic units interbedded 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 sulfur 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 phosphatizat on. Bacterial degradation of organic matter in the stromatolites probably was responsible for local phosphate enrichment of pore waters and resulted in concentration of carbonate fluorapatite by direct precipitation or by replacement of calcium carbonate.

Origin of Sulfide and Phosphate Deposits in Upper Proterozoic Carbonate Strata, Irece Basin, Bahia, Brazil

Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals include pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values.more » However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.« less

Radiochronological age and correlation of proterozoic sediments in Brazil

A review of available Rb—Sr and K—Ar datings obtained on sedimentary sequences, metamorphosed or not, interbedded volcanics and cross-cutting intrusives of the Precambrian of Brazil yields the following conclusions: 1.(1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age.2.(2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age.3.(3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma.4.(4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian.5.(5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician. (1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age. (2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age. (3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguaçu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma. (4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian. (5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic—Phanerozoic boundary and the Ordovician.