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

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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. Map of southeast Brazil showing Parana Basin, outcrop of the Irati Formation, and sample locations. Roman numerals refer to tables of data. diagenesis. Murata and others (1969) have

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TABLE I. SC15, 80,S. AND STRATIGRAPHIC DATA ON SAMPLES FROM A also found dolomite and limestone of both STONE QUARRY LOCATED ALONG THE RIO CL ARO--PIRACICABA HIGHWAY

very heavy and very light isotopic composi- Stratigrophic outline A Sample 8C tions in the Miocene of California, and they A attributed the very light values to the A A influence of organically derived C02 during

diagenesis, and- the heavy values to the AS —14.4

influence of heavy C02 during diagenesis. AS -13.3 This heavy C02 was postulated to have been AS -12 00m + 146 - 7.6 £+2 AS - 10 + 12.4 - 8.2 m generated by the isotopic exchange between o o AS - 10 00m o. C02 and methane, which enriches the C02 13 AS - 9 OOm m "2 in C and depletes the methane in this AS - 6.

isotope (Bottinga, 1969). Murata and others AS - 7. (1969) did not find samples that were both AS - 6 very light and very heavy in the same AS - 5 outcrop, but we collected from the same + 2.9 - 7.9 outcrop both very light samples and extremely heavy samples. Figure 2 shows that the upper zone of the - 4,3 - 26 0 -2 -4 -6 -6 -10 -12 -14 two outcrops has heavy carbon, the AS - 0.5 - 3.4 AS - 0 0 - 4.9 10 intermediate zone has carbon of inter- SO PD8 (%o) 80s« of "Banco Figure 2. Plot of 8C'3 and SO18 of samples collected ill mediate composition, and the lower zone northern part of Parana Basin (Tables 1 and 2). Areas has carbon values that tend to be light in shown in rectangles and labeled marine and freshwater isotopic composition. The isotopic composi- are from Keith and Weber (1964); 85 percent of all the tion of these zones also corresponds to the marine limestones and 84 percent of all the freshwater limestones analyzed by Keith and Weber fall in areas change in lithology—the lower zone of the shown. Open triangle: Table 1, upper zone; filled outcrop (light to normal carbon) consists of TABLE 2, 8C . SO , AND STRATIGRAPHIC DATA triangle; Table 2, upper zone; cross in circle: Table 1, the more dense massive dolomite with small Sample SC13 SO18 Stratigraphie outline intermediate zone; cross: Table 2, intermediate zone; amounts of organic matter, whereas the Intermediate ione-- filled circle: Table 1, lower zone; open circle: Table 2, Infercoloted bitumi- upper zone (heavy carbon) consists of nous shale and lower zone. dolomite interbedded thin layers of organic-rich shale IN- 4.40 m Top of "banco"— IN - 4 20m - 3.8 ^ Lower lone-- and dolomite. We postulate that these IN - 2 20m - 7.2 R eipioitoble dolomitic IN- I.OOm - 7 3 ° "banco" with rare III upper, organic-rich layers were layers in IN-O 20m - 30 fossils Base of "bonco" which the reaction between C02 and methane occurred, whereas the lower, organic-poor layers were ones in which the

organic matter was oxidized directly to C02, exploitable aolomitic 13 L - 0.90m "banco" (black producing a C02 depleted in C . Possibly, in dolomite) L-0 40m -2 9 -2.6 £ these lower layers, the present isotopic composition of the carbon in the samples does not differ much from that originally

present before diagenesis, and these precipi- M - 3 90 m -55 Upper lone-- M- 2.90 m - 7 5 tated in an environment—a lake or inland M-2 50m intermediate zone - - sea—that had somewhat light carbon M- 2.15 m intercolo'ed bitumi- nous shale and M-I 65m -72 compared to a marine environment, which \dolomite M- I - 79 Lower zone-- had a large contribution of dissolved e>pioitab'e dolo- mitic "banco" bicarbonate generated by the decomposition l8 CoMee'eO' f'om ' e sompies ol a second quarr oiong me Rio Cio'o-Pifoocobo ' + 2 of plant remains. The O composition of the Hcgftwoy, 'Ou' souples rom onothe' Quorry in Limi ) quarry oiong the Piro cobo-T.ei« Highwoy. 1 0 samples shown in Tables 1 and 2 is that which one would expect to find precipitated - 2 from a freshwater lake whose isotopic composition was somewhat different than modern-day sea water and also was 8c'5, 80ie, AND STPATiGRAPhiC DATA OF ii SAMPLES somewhat light in isotopic composition. If FROM THE STATE OF PARANA 18 13 the O of these samples is original, then the Som pi e Sc So Stratigrophic outline Serra Alto body of water in which they precipitated Formotion Upper zone-- was, in isotopic composition, like freshwa- pyrobituminous ter bodies in the same present-day geo- shale SM-II.OOm -4.0 -0 9 % intermediate rone intercalated bitumi- graphic position. On the other hand, if the S M -10 50 m -5.2 -9 9 o nous »hole and SO18 PDB (%o) SM- 9.90m -7.1 -8.9 Î dolomite oxygen values have been altered by SM- 9.00m -2.9 -0 1 ^ Figure 3. 8C13 and SO18 of samples collected in diagenesis in the same manner as the carbon SM - 8 90m -8.4 -6.0 SM- 8.40m -5 0 -9 3 " southern part of Paraná Basin (Tables 3,4, and 5). Areas values probably have, then all we can say is Lower zone-- shown in rectangles and labeled marine and freshwater pyrobituminoui that the isotopic composition of the water shale are from Keith and Weber (1964); 85 percent of all the marine limestones and 84 percent of all the freshwater involved in this alteration was probably not limestones analyzed by Keith and Weber fall in the areas too different from present-day meteoric SMS - 104 -12.3 -9.7 .2 intermedióte zone- SMS-103 - 88 -9 5 « •ntercoioted bitumi- shown. Open circle: samples SM, Table 3; cross: samples nous shoie ond water in the region. SMS-102 -12.3 -9.3 £ dolomite SMS, Table 3; open triangle: samples P, Table 4; filled SMS- 101 -154 -10.0 ^ circle: samples CP, Table 4; filled triangle: samples RS, The southernmost part of the basin SMS - TOO +18.3 -10.1 ° Table 5. (represented in Tables 3, 4, and 5) shows

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TABLE 8C15.SO10, AND STRATI GRA PMLC DATA FROM TWO QUARRIES those in the water of the more northern Nuclear (Brazilian Nuclear Energy Com- samples. The water involved in the mission) for financial assistance, and to the diagenesis of the southern samples precipi- International Atomic Energy Agency (Vi- tated at a much colder temperature than did enna), which funded Friedman's visit to the more norther.y samples. Brazil.

CONCLUSIONS REFERENCES CITED We conclude that none of the samples, Bottinga, Yan, 1969, Calculated fractionation with the possible exception of those listed in factors for carbon aid hydrogen isotope Table 5, are primary products, but their exchange in the system calcite-carbon isotopic composition and possibly their dioxide — graphite - methane - hydrogen — chemistry were altered during diagenesis. water vapor: Geochim. et Cosmochim. The limestone samples listed in Table 5 Acta, v. 33, no. 1, p. 4!>—64. Clayton, R. N., and Degen:;, E. T., 1959, Use of might be original products with very little carbon isotope analyses of carbonates for subsequent isotonic alteration. If so, these differentia ring fresh-water and marine sed- could have precipitated in a lake or inland iments: Am. Assoc. I'etroleum Geologists sea. All the other materials have been altered Bull., v. 43, p. 890—89'.'. in a manner that altered their carbon Craig, H., 1957, Isotopic standards for carbon even greater variability in C'3, having both isotopic composition and their oxygen and oxygen and correction factors for mass some of the heaviest and some of the lightest 13 isotopic composition as well. The SC s have spectrometric analysis of carbon dioxide: samples. The calcium carbonates shown in changed in both directions; some of them Geochim. et CosmocHm. Acta, v. 12, p. Table 5 have oxygen and carbon values that have been enriched, possibly due to the 133-149. do not scatter too widely, and these may not influence of heavy C0 generated by Epstein, S., Graf, D. L., and Degens, E. T., 1964, have been much altered from their original 2 Oxygen isotope studies on the origin of isotopic exchange between C0 and composition. If so, they were formed in a 2 dolomites, in Craig, H., and others, eds., methane, and sorre have been altered, with freshwater lake. Isotopic and cosmic chemistry: Amsterdam, 18 light C02 formed by the decomposition of North Holland Pub. Cc., p. 169-180. Note, also, that the O composition of 13 organic matter which is depleted in SC . Keith, M. L., and Weber, J. K., 1964, Carbon and the samples in Tables 3,4, and 5 and plotted The existence of both the heavy and light oxygen isotopic composition of selected on Figure 3 is in general lighter in O18 than 8C'3 materials in the same outcrop has limestones and fossils: Geochim. et Cos- the samples shown in Figure 2. If the O18 in important implications in the formation of mochim. Acta, v. 28, p. 1787-1816. these materials has been exchanged, just as 3 M.cCrea, J. M., 1950, On the isotopic chemistry petroleum. The 8C' and chemical composi- carbon probably was during diagenesis, of carbonates and a palt otemperature scale: tion of the organic matter in both the upper then the water involved in this metamor- Jour. Chem. Physics, v. 18, p. 849-857. and lower zones of these outcrops should be phosis of the southern samples would have a Mendes, J. C., 1967, The Pi ssa Dois Group, in examined to determine why the upper zone Bigarella, J. J., and others, eds., Problems in much lighter isotopic composition than 13 altered to heavy SC , and the lower zone is Brazilian Gondwana geology, first interna- either very little altered or is altered in a tional symposium on the Gondwana stratig- direction to give lighter 8C13 values. In raphy and paleontology: Curitiba, Brazil, summary, then, it s difficult to make very Instituto de Geologia, p. 119-166. TABLE 5. RELATIVE STRATIGRAPHIC POSITION AND 13 18 definite statements regarding the environ- Murata, K. J., Friedman, I., £nd Madsen, B. M., 6C AND 60 RESULTS FOR THE LIMESTONE FROM 1969, Isotopic composition of diagenetic PANTANO GRANDE, RIO GRANDE, DO SUL STATE ment of deposition of the Irati limestone and carbonates in marine Miocene formations of dolomite. However, apparently at least (Location V, Fig. 1) California and Oregon: U.S. Geol. Survey some of these materials were deposited in a Prof. Paper 614-B, p. B1-B24. freshwater lake or inland-sea environment, Sharma, T., and Clayton, R. N., 1965, Measure- and possibly all of them have had the same ment of 018/016 ratios oi total oxygen from Sample ÓC13 SO18 depositional environment. Subsequent al- carbonates: Geochim. et Cosmochim. Acta, teration during diagenesis has obviously v. 29, p. 1347-1353. changed some of them isotopically and White, I. C, 1908, Relatoiio sobre as Coal RS-6 -6.8 -6.7 possibly chemically. Measures e rochas associadas do sul do RS-4 -2.7 -9.8 Brasil, in Final Report Comissao Estudos ACKNOWLEDGMENTS Minas de Carvao de Pedra do Brasil, Pt. 1: RS-3 -4.0 -10.9 Rio de Janeiro, p. 1-300. We are indebted to the Fundajao de RS-2 -7.6 -11.1 Amparo a Pesquisa do Estado de Sao Paulo MANUSCRIPT RECEIVED BY THE SOCIETY APRIL RS-1 -2.6 -8.6 (State Foundation for Support to Research) 26,1973 and to the Comissao Nacional de Energia REVISED MANUSCRIPT RECEIVUD JUNE 29,1973

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