A. Misi et Jr. Kyle, UPPER PROTEROZOIC CARBONATE STRATIGRAPHY, DIAGENESIS, AND STROMATOLITIC PHOSPHORITE FORMATION, IRECE BASIN, BAHIA, BRAZIL, Journal of sedimentary research. Section A, Sedimentary petrology and processes, 64(2), 1994, pp. 299-310
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.