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Unusual Isotopic Composition of Carbonates from the Irati Formation, Brazil

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Unusual Isotopic Composition of Carbonates from the Irati Formation, Brazil WAGNER FERRARESI DE GIOVANI 1 ENEAS SALATI F <--entro de Energia Nuclear na Agricultura, ESALQ-USP, Piracicaba, Brazil ONILDO J. MARINI Departmento de Geocièncias, Universidade de Brasilia, Brasilia, Brazil IRVING FRIEDMAN U.S. Geological Survey, Denver, Colorado 80225 Unusual Isotopic Composition of Carbonates from the Irati Formation, Brazil ABSTRACT easily identifiable lithology makes it a good were collected at the town of Sao Mateus do marker horizon. The formation was as- Sul. Near this outcrop, 5 more samples (Fig. Samples of dolomite and limestone from signed by MacGregor (in Mendes, 1967, p. 1, loc. II; Table 3, SMS samples) of the the Permian Irati Formation collected in the 139) to the Permian, in his study of the fossil intermediate zone were collected. Paraná Basin, southern Brazil, have been reptile Mesosaurus brasiliensis. Nine samples were collected from the analyzed for 8Cia and SO18. The 8C'3 ranges Carbon and oxygen isotopic composi- state of Santa Catarina: 5 from Papanduva from +18.3°/oo to -17.4°/oo PDB, while the tions have been used to differentiate (Fig. 1, loc. Ill; Table 4, P samples) and 4 SO18 ranges from -2.6°/oo to -12.5°/oo limestones deposited in marine environ- from Correia Pinto (Fig. 1, loc. IV; Table 4, PDB. In some quarries where the exposures ments from those originating from fresh CP samples). are especially good, a large variation in 8C13 water. For example, Clayton and Degens From the state of Rio Grande do Sul, 5 can be found. The lower, dense gray (1959) have shown that in 30 samples of samples were collected in Pantano Grande dolomite has light carbon ( —17to +2.6%o), limestone (mainly Pennsylvanian) the 8C13 (Table 5), in an outcrop of very poor quality, whereas the overlying intermediate zone of differs for samples from the two environ- containing clayey soil formed by decomposi- interbedded organic-rich shale and black ments. Keith and Weber (1964) carried out tion of light siltstone shale and by thin lenses dolomite has dolomite containing heavy a more detailed study by analyzing 504 of semidecomposed limestone. carbon (+4.8 to +14.9°/oo PDB). samples of all geologic ages from Precam- We believe that the dolomite represents brian through Quaternary, and they estab- ANALYTICAL PROCEDURE 13 diagenetically altered limestone, and that lished that the 8C of 85 percent of marine The samples were crushed and heated in a light C02 produced by decomposing organic limestone analyzed was above — 2°/oo PDB, stream of argon at 470°C for 20 min, to matter was involved in the diagenesis of the while 84 percent of the freshwater limestone release organic materials. The samples were 13 lower units, while heavy C02 produced by analyzed had 8C below this value. The then reacted with 100 percent phosphoric 18 the isotopic exchange between C02 and 80 of limestone deposited in fresh water acid (McCrea, 1950) to release carbon methane was involved in the alteration of also differed greatly from that in the marine dioxide, which was then analyzed for mass the upper, organic-rich layers. limestone. 45 and 46 in a Varian MAT mass The original environment of deposition In an attempt to see whether the 8C13 and spectrometer, model CH—4. Isotopic com- may be represented by the few limestone 8018 of samples of the Irati Formation could positions, the average of the two measure- outcrops from the southern part of the be used to determine the depositional ments carried out on different C02 prepara- basin, which have somewhat light (—2.6 to environment, 62 samples, mainly dolomite, tions, are expressed in the 8 terminology, as 3 — 7.6°/oo) carbon and light (—6.7 to were collected and analyzed for SC' and parts-per-thousand deviation from the C02 — ll.l°/oo) oxygen that may be the result of SO18.Of the 37 samples from the state of Sâo standard PDB Chicago and applying the 17 precipitation in a lake or inland sea. Paulo, 20 were collected in a stone quarry correction factors for O influence on the located along the Rio Claro—Piracicaba 8C13 and C13 influence on the SO18 (Craig, INTRODUCTION highway (Fig. 1, loe. I; Table 1); 5 (Fig. 1, 1957): One of the most controversial aspects of loe. I; Table 2, IN samples) were from the 8 = R (sample) - R (standard) x 1Q()0 the geological history of the Paraná Basin of dolomitic "banco" (lower zone) in another R (standard) southern Brazil is the environment of quarry, similar in stratigraphie situation, deposition of the Permian Irati Formation along this same highway; 4 (Fig. 1, loe. I; where R = C13/C12 or 018/016. (White, 1908), the lower formation of the Table 2, L samples), containing abundant For dolomite samples, the C02 released Passa Dois Group, which is made up of organic matter, were from the "banco" during the first 4 hr of reaction was pumped pyrobituminous shale and dark siltstone (lower zone) in Limeira, where its black, away, and only the C02 released during the interspersed with dolomite or limestone and laminated dolomitic layer is 2.40 m thick; interval between 4 and 72 hr was utilized lenses of chert. and 8 (Fig. 1, loe. I; Table 2, M samples) (Epstein and others, 1964). The O18 values The formation is distributed all over the were collected along the Piracicaba-Tiete of dolomite have been corrected by a 0.8%o basin, cropping out in Brazil in a narrow highway, where the "banco" (lower zone) is factor (Sharma and Clayton, 1965). S-shaped strip in a general northeast- composed of stratified dolomite 2.50 m southwest direction (Fig. 1), which goes thick. RESULTS AND DISCUSSION from the north part of Sao Paulo state to From the state of Paraná, 6 samples (Fig. The results are shown in Tables 1 through south of the state of Rio Grande do Sul, and 1, loe. II; Table 3, SM samples) from the 5. The sample numbers correspond to the continues into Uruguay. Its unusual and Petrobras pyrobituminous shale deposit elevation above the base of the exposure in Geological Society of America Bulletin, v. 85, p. 41-44,3 figs., January 1974 41 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/1/41/3429147/i0016-7606-85-1-41.pdf by guest on 23 September 2021 42 DE GIOVANI AND OTHERS which the samples were collected, except for 2 and 3, which also show the areas where the these samples as a guide to the original the SMS and RS samples, which were data of Keith and Weber (1964) on Permian depositional environment of the Irati, we numbered at random because the outcrop limestone, both freshwater and marine, must first decide whether the isotopic did not allow satisfactory measurement. We would plot. composition of the samples is the original have plotted the SO18 versus SC13 in Figures In order to use the isotopic composition of depositional isotopic composition or whether these samp es have undergone diagenesis or other changes in composition 58° 54° SC)0 46° with time. All the samples, exc:pt for those in Table 5, are dolomitic. At present, no primary dolomites appear to se forming, with the exception of a few in rather peculiar environments, such as the Coorong of Australia and Lake Magadi in Africa, where we are dealing with hypersaline brines. The normal marine environment does not seem to be conducive to farmation of primary dolomite. This tends to rule out a marine origin for the majori ry of the samples we have examined, if the dolomite is a primary product. The southern part of the basin in the state of Rio Grande do Sul contains calcium carbonate rather than dolomite, and perhapis there was a complex environ- ment in which the sourhern part of the basin precipitated calcite directly, and the north- ern part precipitated primary dolomite in a hypersaline environment. Alternatively, the dolomite can be attributed to diagenesis, or supergene alteration. Supergene alteration would tend to homogenize the isotopic composition, and we Jo not believe that the present extreme inho:tiogeneities in C13/C12 can be explained by supergene alteration. As can be seen from Figures 2 and 3, the samples collected in the northern part of the basin (Tables 1 and 2) seemed to differ isotopically from samples collected from the south (Tables 3, 4, and 5). In the northern part of the basin, the samples varied widely in SO'8 as well as in 8C'3. In the southern part of the basin, th<; samples varied even more widely in C13, but had a very narrow range of O18 values. The range in 8C13 of the samples would rule out their primary character. If the samples had been precipitated from marine bicarbonate, their 8C13 values should fall in a rather restricted range of 0 ± 2°/oo PDB. If, however, they had been precipitated under equilibrium conditions from a freshwater lake, the 8C13 would be significantly lighter since the bicarbonate in such a freshwater lake would normally be depleted in C13, because much of the bicarbonate would be formed from decomposing organic matter, which has a low 8C13 value. On this basis, it is very difficult to der.ve limestone with 8C'3 as high as +4 to +18. It is also difficult to get carbon values lighter than about —10 or — 12 from these "nomal" environments. As the tables and the diagrams show, a significant number of samples have 8C'3 3 0 500 KILOMETERS values above +4 and below —12. These 8C' 1 i i I i J values can only be explained by alteration in the original 8C13 of the limestone during Figure 1.
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