info:eu-repo/semantics/article
MEDIDAS ISOTÓPICAS DE CARBONO E OXIGÊNIO EM CARBONATOS DO GRUPO BAMBUÍ, NA REGIÃO CENTRO-NORTE DO ESTADO DA BAHIA
MEDIDAS ISOTÓPICAS DE CARBONO E OXIGÊNIO EM CARBONATOS DO GRUPO BAMBUÍ, NA REGIÃO CENTRO-NORTE DO ESTADO DA BAHIA
Autor
TORQUATO, JOAQUIM RAUL
MISI, AROLDO
Institución
Resumen
Analyses of 85 carbonate samples of the Bambuí Gröup from the Chapada de Irecê, Bahia, exhibit four distinct l3C and l8O isotopic ratio groupings. Based on these results the following sedimentation model for the Bambuí Group is proposed: l. A continental environment originating the basal dolomitic limestones and argillaceous dolomites. Isotopic ratios from these rocks fall within ranges considered typical of continental sedimentation: ô13C between -2.9 and -6.4%o, and ô18O from -5.3 to -10.7%.(PDB). 2. An alternating continental and restricted marine environment resulting in the deposition of laminated dolomitic limestones with thin interbeds of muddy sediments. ô13C ratios of +0.I to -4.9 /oo and a mean value of -8.2 + 0.37.. ô18O are compatible with such an environmenl. 3. A restricted marine environment dominated by sedimentation of the black, fetid limestones of the upper portion of the sequence. Isotopic ratios for l3C are all over-l.B 7oo,rP to about +7.syoo, and 18O ratios vary from -4.9 to -10.0/", A number of light-grey sileceous dolomites is interbeded with the lowermost black limestones of this sequence. Isotopic data for these rocks show a narrow range for ôl3C values (-0.5 to 1.01") and, a broader variation for ô18O ratios (0.0 to -8.3 /".). This latter may be interpreted as indicative of intense evaporation during deposition of the dolornitic facies. Analyses of 85 carbonate samples of the Bambuí Gröup from the Chapada de Irecê, Bahia, exhibit four distinct l3C and l8O isotopic ratio groupings. Based on these results the following sedimentation model for the Bambuí Group is proposed: l. A continental environment originating the basal dolomitic limestones and argillaceous dolomites. Isotopic ratios from these rocks fall within ranges considered typical of continental sedimentation: ô13C between -2.9 and -6.4%o, and ô18O from -5.3 to -10.7%.(PDB). 2. An alternating continental and restricted marine environment resulting in the deposition of laminated dolomitic limestones with thin interbeds of muddy sediments. ô13C ratios of +0.I to -4.9 /oo and a mean value of -8.2 + 0.37.. ô18O are compatible with such an environmenl. 3. A restricted marine environment dominated by sedimentation of the black, fetid limestones of the upper portion of the sequence. Isotopic ratios for l3C are all over-l.B 7oo,rP to about +7.syoo, and 18O ratios vary from -4.9 to -10.0/", A number of light-grey sileceous dolomites is interbeded with the lowermost black limestones of this sequence. Isotopic data for these rocks show a narrow range for ôl3C values (-0.5 to 1.01") and, a broader variation for ô18O ratios (0.0 to -8.3 /".). This latter may be interpreted as indicative of intense evaporation during deposition of the dolornitic facies.