Radioactivity and radiogenic heat production in the sediments of the São Francisco sedimentary basin, Central Brazil

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Appl. Radiat. lsot. Vol.48, No. 3, pp. 413-422, 1997 © 1997ElsevierScienceLtd Printed in Great Britain.All rights reserved PII: S0969-8043(96)00228-X 0969-8043/97 $17.00+ 0.00

Radioactivity and Radiogenic Heat Production in the Sediments of the Silo Francisco Sedimentary Basin, Central Brazil ARNALDO ROQUE and FERNANDO

BRENHA RIBEIRO*

Instituto Astronrmico e Geofisico, Universidade de S~o Paulo, Caixa Postal 9638, 01065-970, Sgo Paulo, SP, Brazil (Received 20 June; accepted 6 July 1996)

Uranium, thorium and potassium measurements, followed by radioactive heat generation calculations were made in Bambui Group neoproterozoic sedimentary rocks from the S~o Francisco Basin, Brazil. The measurements were made in drilling cuttings recovered from a deep well drilled near the city of Pirapora, central Minas Gerais State, and in Bambui Group outcrop samples from Minas Gerais. The obtained results allowed the construction of vertical profiles of concentration of the heat generating elements, of the heat generation rate and of the U/Th, U/K and Th/K ratios in these sediments. U, Th and K concentrations reflectmainly the lithology, whereas the variability of U/Th, U/K and Th/K, besides the lithology, also seem to indicate variations in sedimentation conditions in part of the Bambui Group Formations sampled by the deep well. The Bambui Group contribution to the local heat flow density is <0.37 mW/m2. © 1997 Elsevier Science Ltd. All rights reserved

Introduction The temperature distribution in a sedimentary basin is mainly determined by its formation and evolution process that includes the heat flow density in its basement, the heat transfer processes active in the sediments, thermal conductivity variations and transient effects such as those associated with the sedimentation rate and the eventual magmatic episodes inside the basin. Radiogenic heat generation in sediments may, depending on the prevailing conditions, significantly affect their thermal history and the heat flow in a basin (Rybach, 1986). The naturally occurring isotopes that significantly generate heat, 4°K and the members of the uranium and thorium series, are present in different Concentrations in sedimentary rocks and the heat generation varies widely with lithology due to U, Th and K concentration variations (Haack, 1982; Rybach and Cermak, 1982). The S~o Francisco Sedimentary Basin in central Brazil (Fig. 1) is part of the sedimentary cover of the S~o Francisco Craton (Inda et al., 1984; Dominguez, 1993). It has an approximate area of 216,800 km 2 and a maximum depth of about 2000 m (Braun et al., 1990). The basin has a roughly elongated form aligned to the upper course of the Sgo Francisco River and is filled by the neoproterozoic sediments *To whom all correspondence should be addressed.

that form the Bambul Group. Gas occurrences in water wells and in deep wells drilled in this basin by Petrobrfis, the Brazilian State Oil Company, and the natural gas seepage near the city of Pirapora (Minas Gerais State) indicates the presence of thermochemical gas in these sediments (Braun et al., 1990). In this paper, we present the concentration of heat generating elements, U, Th and K, measured in drilling cuttings from a deep bore hole and in outcrop samples of sediments from the S~o Francisco basin and calculate the corresponding heat generation rates.

The BambuiGroupStratigraphy The stratigraphy of the Bambui Group has been described by several authors (e.g. Branco and Costa, 1961; Braun, 1968; Dardenne, 1978; Inda et al., 1984; Braun, 1988 and Braun et al., 1990). Babinski (1993) recently presented a review of the main proposed stratigraphic columns for the Bambui Group. The Bambui Group is composed of the Tr~s Marias formation as its upper unit, followed by formations Serra da Saudade, Lagoa do Jacarr, Serra de Santa Helena and Sete Lagoas, which compose the Paraopeba Subgroup, and the Jequitai formation as its basal unit (Inda et al., 1984). The Trrs Marias formation is composed of siltstones and arkoses, whereas Serra da Saudade formation is composed of siltstones, argillites and shales, with the presence of

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LEGEND Bambui Group Limits Sampling location ~ o City ~ Borehole ~=~Rivers Fig. 1. The S~o Francisco basin (Bambui Group) limits in Minas Gerais State (Central Brazil) and the sampling locations (from Parenti Couto et al., 1981). The outcrop samples for each sampling site are: (1) CYI, CY4, CY5, CY6; (2) R1, R3, R4; (3) J13; (4) DX2, DX3, DX4, DX6, M14; (5) M36; (6) J4; (7) AX2; J20; (8) AY3; AY4, AY6, AY10; (9) BZ2; (10) J24; (11) BX3, BX8, BX9, BX10; (12) BY8; (13) EY5; (14) EX2; and (15) VII.

Radioactivity and radiogenic heat production, central Brazil

415

Table 1. LithologicdescriptionU, Th and K concentrationsand heat generationrates in the outcrop samplesfrom the BambuiGroup. Field numbers and lithologicdescriptionfrom Parenti Couto et al. (1981) Formation or subgroup Fieldnumber Lithology K (%) + 7% U (ppm) ___16% Th (ppm) __.10% A (#W/m3) Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Tr~s Marias Trrs Marias Tr~s Marias Tr~s Marias Tr~s Marias Trrs Marias Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba Paraopeba

AX2 AY3 AY4 AY6 AY10 BX3 BX8 BX9 BXI0 BY8 BZ2 J20 J24 J4 J13 CY1 CY4 CY5 CY6 R1 R3 R4 DX2 DX3 DX4 DX6 EX2 EY5 MI4 M36 VII

siltstone siltstone siltstone siltstone siltstone arkosean siltstone arkosean siltstone arkosean siltstone arkosean siltstone arkosean siltstone arkosean siltstone arkosean siltstone arkosean siltstone argillite argilite siltstone siltstone siltstone siltstone siltstone siltstone siltstone chemical-detritic chemical-detritic chemical- detritic chemical-detritic chemical-detritic chemical-detritic chemical-detritic chemical-detritic tectonized siltstone

4.69 2.27 4.49 4.64 1.86 5.08 4.19 4.08 4.20 5.81 3.68 3.79 2.75 2.89 5.18 3.34 1.47 3.65 2.45 0.176 0.234 < 0.06 4.03 ' 3.96 3.35 3.15 2.11 0.888 4.50 6.45 4.67

localized limestone lenses. Lagoa do Jacar6 is composed of siltstones and marls intercalated with limestones. Shales, siltstones intercalated with fine grained sandstones and limestones are the predominant lithologies in the S e r r a de Santa Helena formation. Sete Lagoas formation is composed of intercalated limestones, dolomites and pelites. Pelites form the dominant lithology of this formation in the southeastern part of t h e basin (Braun et al., 1990). Organic matter rich pelitic and carbonatic levels occur both in the Sete Lagoas and Lagoa do Jacar6 formations (Braun et a l . ; 1990). The basal unit of the Bambui Group is the Jequitai formation, which is composed of conglomerates and paraconglomerates. Following the deposition of the Jequitai formation, during a continental scale glacial event at the beginning of the Neoproterozoic (Babinski, 1993) the Paraopeba subgroup w~8 deposited as a consequence of a marine transgression (Inda et al., 1984; Braun, 1988; Dominguez, 1993)i over the whole extension of the Silo Francisco Basin, The Tr~s Marias formation represents the final Sedimentation stage of the Bambui Group in a shallow marine to continental fluvio-deltaic environment.

Lithologic Description and Stratigraphic Position of the Samples Uranium, thorium and potassium were measured in 168 samples from the S~o Francisco Basin. The materials used were 137 samples of drilling cuttings, recovered from a Petrobrfis (Brazilian Oil Company) deep bore hole drilled near the city of Pirapora,

2.59 2.68 3.01 2.36 0.800 3.41 2.23 2.32 2.12 2.42 2.83 2.11 2.06 1.89 2.58 1.90 0.569 1.88 1.13 2.47 1.14 0.68 1.91 1.96 2.06 2.03 < 0.5 < 0.5 1.70 3.81 2.77

13.5 14.7 11.8 12.2 7.02 20.2 11.6 13.1 12.4 15.9 13.9 9.44 9.86 15.0 13.1 10.3 3.07 9.16 6.67 < 1.0 < 1.0 < 1.0 7.55 8.19 6.96 6.42 3.65 1;38 17.6 9.73 12.3

2.05 4- 0.15 1.89 +_ 0.15 2.05 _+ 0.15 1.91 + 0.14 0.838 ___0.058 2.79 4- 0.20 1.81 _ 0.13 1.94 4- 0.14 1.83 4- 0.13 2.25 ___0.15 2.015 4- 0.15 1.57 ___0.11 1.46 4- 0.11 1.75 + 0.13 2.02 + 0.14 1.50 + 0.1 l 0.492 + 0.033 1.43 4- 0.10 0.916 4- 0.063 0.90 4- 0.17 0.319 4- 0.047 0.131 ___0.028 1.41 __. 0.10 1.48 ___0.10 1.36 __. 0.10 1.288 4- 0.099 0.551 _ 0.027 0.250 4- 0.011 2.05 4- 0.14 2.25 ___0.18 1.99 4- 0.15

central Minas Gerais State, and 31 samples from the Bambui Group outcrops i n Minas Gerais. Figure 1 shows the sampling points and well location. Outcrop samples were originally described by Parenti Couto et al. (1981) and belong to the Tr~s Marias formation and to the Paraopeba Subgroup. These authors do not identify the individual Subgroup formations to which the samples belong. The Tr~s Marias formation samples are mainly siltstones and there are two samples of argillite. Paraopeba Subgroup samples are siltstones and chemical-detritic sediments! Table 1 presents the lithologic description of each sample. These samples were collected for r a d i o m e ~ c dating purposes and do not present evidence of alteration or weathering (Parenti Couto et al., 1981)i The Pirapora well has a to~al depth of 1847 m. The lithology of the sediments cut by this well was initially described by Braun et al. (!990) and the following lithologic description and formation identification were given by Petrobr~is, :A simplified lithologic column of this well is presehted in Fig. 2. The initial 24 m of this well corresponds to the cretaceous sandstones f r o m the Urucuia formation that overlays part of the Bambui Group sediments. From this depth, the borehoie crosses about 280 m of shales from the Serra da Saudade formation. These shales are followed by an approx. 370 m thick layer of limestones of the Lagoa do Jacar6 formation intercalated with thin, < 25 m thick, shale layers. The Serra de Santa Helena formation is represented in this well by a thick layer (-280 m) of siltstones with

416

Arnaldo Roque and Fernando Brenha Ribeiro

few and thin, < 5 m, intercalated shales and limestones. Bellow Serra de Santa Helena formation sediments there is thick layer (-820 m) of shales of the Sete Lagoas formation with thin ( < 30 m) intercalated limestones and siltstones. The well finally crosses 7 m of conglomerate and 65 m of paraconglomerates from the Jequitai formation.

The Uranium, Thorium and Potassium Concentration Measurements Procedure Uranium, thorium and potassium concentrations were measured with a NaI(T1) detector gamma-ray spectrometer. Uranium and thorium concentrations 0

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were indirectly obtained from the gamma-ray activity of 214Bi (1.764 M e V , full energy peak) and 2°8T1 (2.164MeV, full energy peak), with the usual assumption that the -'38U and 2~2Th series are in radioactive secular equilibrium. Potassium concentrations were directly obtained by measuring the activity of the 4°K gamma-ray decay (1.460 MeV, full energy peak). These activities were time integrated in spectral windows around full energy peaks defined in Table 2. The uranium, thorium and potassium concentrations in the samples were calculated by comparing their activities with the activities of U, Th and K concentration standards (Adams and Gasparini, 1970; Rybach, 1988).

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Radioactivity and radiogenic heat production, central Brazil Isotope 4°K 214Bi ~STl

417

Table 2. Spectralwindowlimitsfor 4°K,2~4Biand 2°STl Full energypeak (keV) Spectralwindowlimits(keY) 1460 1349-1569 1760 1685-1887 2614 2465-2726

The gamma-ray spectrometer has a NaI(T1) scintillation detector with a 7.6 cm (3") dia. and a 348 cm 3 vol. Its resolution (FWHM/full energy peak x 100%) is 8% for the 137Cs662 keV full energy peak and 4.8% for the 2°8T12.614 MeV full energy peak. The critical detection level and the limits of qualitative detection and quantitative determination were obtained from the background radiation in each spectral window, following Currie (1968). The background radiation was integrated for 24 h. The value of 5% was adopted for type I and type II errors, that define the critical detection level and the limit of qualitative detection, respectively. For the definition of the limit of quantitative detection, the maximum allowed relative error in the net activity in each spectral window was 20%. Table 3 summarizes the laboratory background radiation and the three limits of detection. The laboratory mean limits of quantitative detection limits, expressed in concentrations, are (0.059 + 0.002)°/0 for K, (0.53 ___0.02) ppm for U and (1.19 ___0.04) ppm for Th (the indicated errors correspond to 1 SEM). The relative precision of the concentration measurements were estimated from the reproducibility of a series of measurements with fourteen aliquots of the same sand sample with U, Th and K concentrations (2.5 ppm, 16.4 ppm and 2.8% respectively) similar to concentrations of the S~o Francisco Basin samples. The mean relative precisions obtained were 16% for U, 10°/0 for Th and 7% for K. Heat generation rates were calculated using the relation (Rybach, 1988) A = 10-s(9.52Cu + 2.56Crh + 3.48CK)

(1)

where A is the heat generation rate in ,ttW/m 3, p is the sample density in kg/m 3, Cu is the uranium concentration in ppm, Crh is the thorium concentration in ppm and CK is the potassium concentration in %.

Vertical Distributions of Uranium, Thorium and Potassium Concentrations and Heat Production Rates in the Pirapora Region Figure 3 presents the vertical distributions of U, Th and K concentrations measured in the drill cuttings

from the Pirapora well and the corresponding heat generation rate profile calculated using equation (1). In spite of the variable concentration profiles, which reflect lithologic variations inside each formation, it is possible to distinguish the Serra da Saudade formation shale layer in the upper part of the profile with concentrations around 10 ppm of Th, 2.2 ppm of U and 2.8% of K. The depth interval between 307 and 673 m (Lagoa do Jacar6 formation) corresponds to a limestone layer with low concentrations of U, Th and K with intercalations of shale dearly indicated by concentration peaks. The siltstone layer between 673 and 956 m corresponds to the Serra de Santa Helena formation and present a small variation in the concentrations of Th (-11 ppm), U (-2.5 ppm) and K (-2.5%). Between 956 and 1250 m the concentration values are variable, reflecting the presence of intercalations of limestones, siltstones and shales. In the depth interval from 1250 to 1650 m, shale is the predominant lithology and concentration values present little variation around about 11 ppm for Th, 2.5 ppm for U and 2.8% for K. The depth interval between 1650 and 1775 m presents lower concentration values (-4 ppm for Th, 1.5 ppm for U and 1.5% for K) reflecting the presence of limestone intercalations. The whole interval from 956 to 1775 m corresponds to the Sete Lagoas formation. Finally the Jequitai formation conglomerates and paraconglomerates present concentrations of U, Th and K around 1.5 ppm, 6.5 ppm and 3.2%, respectively. Figure 4 presents the vertical distributions of U/Th, U/K and Th/K. The vertical distributions of the U/K and U/Th relations, except for some isolated peaks, do not present great variations through the whole profile, whereas the Th/K profile presents more variable values. In the Serra da Saudade formation the U/Th ratio presents small variations around a geometric mean value of 0.23 whereas the U/K ratio profile presents small variations around a geometric mean value of 7.80 x 10 -5, with small but noticeable peaks at depths of 27, 74 and 101 m. These peaks, however, do not correspond to any special feature in the lithologic column. The Th/K profile is more variable around a mean value of 3.67 × 10 -4 but without prominent peaks.

Table3. Meanbackground,criticalleveland detectionand determinationlimits,as definedby Currie(1968),for the gamma-rayspectrometer used. The presentedvalues,expressed in counts/hour(c/h), correspondto the means of five differentspectrometercalibrations Heat generating Background (B) Critical level( L c ) Detection limit(Ld) Determinationlimit(Lq) element (c/h) (c/h) (c/h) (c/h) K 663 __.22 675 _+22 688 _+23 701 _+23 U 257 _+7 265:1:7 273 _ 7 281 _+7 Th 171 _ 3 177 + 3 183 + 3 190 _ 3

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Fig. 3. Vertical distributions of U, Th and K concentrations and of heat generation rate in the Pirapora deep well. The relative uncertainties in the concentrations are 16% for U. 10% for Th and 7% for K.

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Table 4. Mean values and variances of the natural logarithm of the U/Th, U/K and Th/K ratios for the five Bambui Group formations sampled by the Pirapora deep well. The hypothesis of log-normality of these ratios in each formation was tested by aplication of g2 test (see text). The critical U" for the log-normality rejection with 0.05 significancelevel is, in this case, 3.84 Bambui Group (U/Th) (U/K) (Th/K) Formation Log (U/Th) Zz n Log (U/K) Z2 n Log (Th/K) ~(2 n Serra da Saudade (1) - 1.547 _+0.079 1.73 30 - 9.459 +_0.097 8.93 30 - 7.91 +_0.11 2.27 30 Serra da Saudade (2) - 1.547 _+0.079 1.73 30 - 9.481 ___0.071 2.48 27 - 7.91 _.+0.11 2.27 30 Lagoa do Jacar6 (I) - 1.43 _+0.32 7.54 26 - 9.20 ___0.48 12.29 28 - 7.89 ___0.25 8.70 27 Lagoa do Jacar6 (2) 1.51 ___0.21 3.61 23 -- 9.31 ___0.27 6.20 25 -- 7.87 + 0.20 6.72 22 Serra de Santa Helena -- 1.56 _+0.13 1.67 24 - 9.31 + 0.19 3.66 24 - 7.75 _+0.13 0.64 24 Sete Lagoas (1) -- 1.32 +_0.35 44.93 59 - 9.22 + 0.35 42.20 59 -- 7.90 +__0.17 30.02 59 Sete Lagoas (2) .- 1.43 _+0.13 9.63 51 -- 9.30 + 0.15 17.60 47 -- 7.86 _+0.10 6.30 52 Jequitai - 1.52 _+0.12 2.0 6 - 10.02 _+0.12 0.67 6 - 8.503 _+0.092 0.67 6 n indicates the number of data points. (1) indicates all data points used. (2) indicates outliers removed (see text).

T h e L a g o a do Jacar6 f o r m a t i o n presents the m o s t p r o m i n e n t peak in the whole U / K profile at a d e p t h o f 432 m, which c o r r e s p o n d s to a limestone layer n e a r the interface with a 7 m thick shale layer just below. The very low T h content, below the quantitative d e t e r m i n a t i o n limit (Lq) a n d the low (0.11%) K c o n t e n t confirms t h a t this peak is observed in the limestone. D u e to the very low T h c o n c e n t r a t i o n s T h / K a n d U / T h ratios were n o t calculated at this depth. T w o peaks in the U / K ratio profile are also observed in the limestone layers at d e p t h s o f 397 a n d 450 m. Again, for the peak at 397 m, T h c o n c e n t r a t i o n is below Lq a n d T h / K a n d U / T h ratios were n o t calculated. The small peak observed in the U / T h profile at 362 m c o r r e s p o n d s to a limestone whereas the peak between 400 a n d 432 m, in the same profile, corresponds to limestones with a thin (-1 m) shale intercalations. These two peaks seem to be reflected as negative variations (values noticeably below m e a n ) in the T h / K ratio profile. Finally the peak between 450 a n d 459 m in the T h / K ratio profile c o r r e s p o n d s to a limestone layer. In the Serra de S a n t a Helena the U / K a n d U / T h ratios present small variations a r o u n d geometric m e a n values o f 7.63 × 10 -5 a n d 0.21, respectively, whereas the T h / K profile is more variable a r o u n d a geometric m e a n value of 4.31 × 10 -4, with only a p r o m i n e n t peak at 949 m. This peak corresponds to a siltstone in layer near the base o f the formation. In the Sete Lagoas f o r m a t i o n U / K a n d U / T h ratio profiles present peaks at 1003 m, c o r r e s p o n d i n g to a thin (~2 m) limestone intercalation in a shale layer, a n d between 1036 a n d 1045 m, c o r r e s p o n d i n g to a limestone layer with a 1 m siltstone intercalation. A n o t h e r peak, in b o t h U / K a n d U / T h , is observed at 1218 m c o r r e s p o n d i n g to a limestone layer. Two p r o m i n e n t peaks in the U / K profile occur near the base o f the Sete Lagoas f o r m a t i o n at 1646 a n d 1673 m. The first peak c o r r e s p o n d s to a 10 m limestone intercalation in a shale layer, b u t it does not seem to be m u c h influenced by the shale immediatly a b o v e since t h a t rock has U / K ratios t h a t are significantly smaller. It seems instead to reflect the low K c o n t e n t (0.79%) in the limestone. The peak at 1673 m, which can also be observed in the T h / K ratio

profile seem to be associated with a thin (-1 m) shale intercalation in a limestone. In the U / T h ratio profile two p r o m i n e n t peaks occur at 1646 a n d 1682 m, b o t h in limestones. The U / K ratio profile also presents low values between a b o u t 1700 a n d 1750 m which c o r r e s p o n d to a shale layer. These low values, which also occur in the T h / K ratio profile, do n o t seem to have a correspondence in the U / T h ratio profile. Finally, the p r o m i n e n t peaks in the U / K , U / T h a n d T h / K ratio profiles at 1769 m occur in a 5 m thick shale layer at the base o f the Sete Lagoas formation. The Jequitai f o r m a t i o n is characterized by U / K , U / T h a n d T h / K ratio profiles with small variations a r o u n d geometric m e a n values o f 4.45 × 10 -5, 0.22 a n d 2.03 × 10 -4 respectively. In order to establish the similarities a n d differences between the U / T h , U / K a n d T h / K ratios in the formations, the hypothesis o f log-normality o f these ratios was verified in each formation. The s t a n d a r d ized histograms o f the n a t u r a l l o g a r i t h m o f these ratios were divided in four equal 2 5 % probability categories a n d the observed n u m b e r o f d a t a points in each category were c o m p a r e d with the expected n u m b e r of data points in a standardized log-normal distribution (Davis, 1973). The g 2 test, with t h e null hypothesis t h a t the m e a s u r e d d a t a are extracted from a l o g - n o r m a l distribution a n d with a significance level o f 0.05, was applied. Table 4 presents the results o f these tests. In the case o f the Serra d a S a u d a d e f o r m a t i o n the log-normality was rejected for the U / K ratio distribution. R e m o v i n g the observed peaks in t h a t profile as possible outliers reduces the data dispersion a r o u n d the m e a n a n d leads to a log-normal distribution. In the case o f the L a g o a do Jacarr, the log-normality was reject for the three element ratios. T h e elimination o f the observed peaks as possible outliers reduces the dispersion a r o u n d the m e a n values b u t only in the case o f the U / T h ratio does lead it to a log-normal distribution. Serra de S a n t a Helena is characterized by log-normal distributions in the three element ratios. As in the case of Lagoa d o Jacarr, the log-normality was rejected for the three element ratios o f the Sete Lagoas formation, a n d in this case, elimination o f the observed peaks as

t

Radioactivity and radiogenic heat production, central Brazil

possible outliers still does not lead to log-normal distributions. Finally, the Jequitai formation is characterized by log-normal distributions for the U/K, U/Th and Th/K ratios.

Uranium, Thorium and Potassium Concentrations in the Outcrop Samples of the Bambui Group The uranium, thorium and potassium concentrations measured in the Bambui outcrop samples and the corresponding heat generation rates calculated using equation (1) are summarized in Table 1. Field identification numbers and the lithological description were given by Parent± Couto et al. (1981). The Tr6s Mar±as formation analysed sediments include five siltstone samples, with mean concentrations of (3.6 + 0.6)% of K, (2.3 ± 0.4) ppm of U and (11.8 ± 1.3) ppm of Th (the presented precisions correspond to 1 SEM), eight samples of arkosean siltstones, with (4.2 ± 0.3)% of K, (2.4 4- 0.2) ppm of U and (13.3 ± 1.2)ppm of Th, and two argillite samples, with (4.0 ± 1.2)% of K, (2.2 ± 0.4) ppm of U and (14.1 ± 1.0) of Th. The analysed sediments of the Paraopeba Subgroup included seven siltstone samples, eight chemical-detfific samples and one sample of a tectonized siltstone. Siltstone samples showed a great variability in their concentrations. The standard deviation of the potassium concentration in these samples is equal to the mean value (1.6%), whereas the standard deviation of the uranium concentrations (4.2 ppm) is comparable to its mean value (4.5 ppm). Thorium concentrations present a smaller scatter around the mean value, with a standard deviation of 0.7 ppm for a mean value of 1.4ppm. The chemical-detritic sediments from the Paraopeba Subgroup present a smaller scatter around its mean values, with a 0.6% SD for a mean potassium concentration value of 3.6%, 0.4 ppm SD for a mean uranium concentration of 1.7 ppm and a 1.7 ppm SD for a mean value of 7.7 ppm for thorium concentration.

Heat Production in the Bambui Group The heat generation rates calculated from the U, Th and K concentrations measured in the drilling cuttings from the Pirapora well are low, < 1.9 #W/ m 3. The mean heat generation rate calculated for the siltstone samples from the Tr~s Mar±as formation outcrops is (1.7± 0.2)/~W/m3, whereas for the arkosean siltstones from the same formation the mean is (2.0 ___0 . 2 ) # W / m r The argillite samples from this formation have a mean heat generation rate of (1.9 ± 0.1) #W/m 3. In the case of the Paraopeba Subgroup outcrop samples, the mean heat generation rates calculated for the siltstones is (0.8 ± 0.2) #W/ m 3 and for the chemical-detritic sediments the mean is (1.3 ± 0.2)/~W/mt

421

Conclusions The obtained results allowed the construction of vertical profiles of U, Th and K concentrations and heat generation rates in the Bambui Group sediments near the city of Pirapora (Minas GerMs State, central Brazil). The heat generating elements and heat generation rate profiles essentially reflect the lithology of the sediments sampled by the bore hole. The variability of U/Th, U K and Th/K ratios in part reflects changes in the lithology but they also indicate variations in sedimentation conditions m part of the Bambui Group formations sampled by the deep well. In the case of Serra de Santa Helena and Jequitai formations the data seem to indicate relatively homogeneous local sedimentation conditions, whereas in the Serra da Saudade formation the presence outliers indicate small local enrichment of U relative to K which may reflect changes in the oxidizing potential during the sediment deposition. Lagoa do Jacar6 and Sere Lagoas present more variable element ratios. The prominent peaks observed in these formations indicate a large U enrichment in relation to K and Th at the corresponding depths which also seems to reflect changes in the oxidizing potential during sediment deposition. Lithologic descriptions do not mention the presence of organic matter in these sediments. Measurements in the Bambui outcrop samples allowed an estimate to be obtained of the U, Th and K concentrations and heat generation rates in the Tr6s Mar±as formation. On the other hand, the large scatter of values obtained for the Paraopeba Subgroup reflects the lithologic variability of the samples. The calculated heat generation rates lead to an upper limit of 0.37 mW/m 2for the contribution of the Bambui Group sediments to the heat flow density in that region. There is no heat flow density measurement in the $5o Francisco Basin but the upper limit of the Bambui Group sediments' contribution to the local heat flow is less than 1% of the estimated mean heat flow density for the Silo Francisco Craton (Vitorello et al., 1980). Acknowledgements--The authors wish to thank Petrobr~ts-Petr61eo Brasileiro S.A.--and in particular to Dr Celso Fernando Lucchesi, for permitting the use of the drill cuttings recovered from the Pirapora well and the access to the well information. Cristiano Leite Sombra, Hung Kiang Chang and Mariela Martins from Petr6bras are thanked for their assistance. Dr Koji Kawashita from Instituto de Geoci~ncias da Universidade de S~o Paulo (IG/USP) furnished the outcrop samples. Drs Wilson Teixeira and Marl± Babinski are thanked for useful discussions and suggestions.

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