1.77 Ga and prior to diamictites Jequitaı´ For- mation at the base the Bambuı´ Group Pimentel
et al.,
1991. After
revising the
available geochronological and microfossils data from the
Paranoa´ Group, Fairchild et al. 1996 concluded that the best estimate of the age of this unit is
1170 – 950 Ma. This unit has been interpreted as a passive margin sequence that was deposited at the
western border of the Sa˜o Francisco craton Dar- denne, 1979; Pimentel et al., 1999. Petrographic,
chemical and isotopic
147
Sm
144
Nd studies indi- cate that the Paranoa´ sediments were derived
from a Paleoproterozoic sialic basement located in the Sa˜o Francisco craton Guimara˜es 1997;
Pimentel et al. 1999.
The Jequitaı´ diamictites consist of clasts sup- ported by a clay-rich matrix with siltstone and
sandstone lenses. The clasts include quartz, quartzite, granite, limestone and siltstone. The
age of this unit is poorly constrained and falls between 688 9 69 and 900 9 2 Ma. While the
lower age limit is based on Pb – Pb dating of the Sete
Lagoas SLA
formation limestones
Babinski et al. 1999, the upper age limit is based on U – Pb dating of zircon from basic dikes
that cut the underlying siliciclastic sequence Es- pinhac¸o Supergroup Uhlein et al. 1999. Hence,
the available data indicate that the Jequitaı´ glaci- ation event is probably Sturtian. This unit crops
out intermittently at the base of the Bambuı´ group and, depending of the region, overlies the
Paranoa´ sediments or the granite – gneiss base- ment Karfunkel and Hoppe, 1988; Uhlein, 1991;
Uhlein et al. 1999. The Bambuı´ group was de- posited during a transgressive phase and after the
deposition of the Jequitaı´ Formation Dardenne 1979.
The Bambuı´ Group covers an area of more than 300 000 km
2
over the Sa˜o Francisco craton. These sediments were deposited in an epiconti-
nental sea, commencing with a carbonate – pelite facies that was followed by siliciclastic sedimenta-
tion characterized by immature sediments Dard- enne, 1978; Dardenne 1979; Misi and Kyle,
1994. The base of this unit consists of laminated argillaceous limestones, dolostones, siltstones,
stromatolitic dolomite and marlstone SLA for- mation. Overlying are rhythmic interbeds of
mudstones, siltstones and very fine-grained sand- stones of the Serra de Santa Helena Formation,
followed by a second carbonate unit with argilla- ceous limestones, marlstones, shales and siltstones
with dark colored lime grainstone lenses Lagoa do Jacare´ formation. The upper portion of
the Bambuı´ group consists of siliciclastic rocks, mainly greenish siltstones and arkoses Serra da
Saudade and Treˆs Marias formations. In con- trast to the Paranoa´ group sediments described
above, the Bambuı´ group siliciclastic rocks are mineralogically and texturally immature sedi-
ments, which have been accumulated during the tectonic inversion of a foreland basin Guimara˜es
1997. Pb – Pb isochron ages obtained for un- deformed carbonate rocks from the SLA forma-
tion is 688 9 69 Ma and is considered as the minimum depositional age of this formation
Babinski et al., 1999. Evidence of the inversion also occurs in the southwestern part of the basin
west of SLA, Fig. 1 and Fig. 3, where conglom- erates outcrop associated to a fan delta system
that was also related to the tectonic evolution of the foreland basin Castro 1997. According
to Guimara˜es 1997 and Castro 1997 findings, the tectonic inversion of the basin was related
to eastward thrusting movements that, as has been pointed out, affected significantly the sedi-
mentological evolution of the basin. Nd isotope studies also give support for the inversion of the
Bambuı´ basin. They show that the Bambuı´ group pelites
present Nd
model ages
distinctively younger than those of the Paranoa´ group sedi-
ments, and thus indicate that its sediments were derived from a younger source region Pimentel
et al. 1999.
3. Sampling and analytical methods
We have analyzed oxygen and carbon isotopes along three stratigraphic profiles in central Brazil
Fig. 1: SSD and SLA, both in the State of Minas Gerais, and Sa˜o Domingos SD, in the
State of Goia´s.
Samples from SSD are fresh limestones, dolo- stones and marlstones, sampled along a 2000 m
profile that includes both the Paranoa´ and Bam- buı´ sediments. The Paranoa´ sediments consist
Fig. 3. Geological cross section of southwestern border of the Bambuı´ foreland basin during the tectonic inversion after Castro, 1997.
mainly of siliciclastic sediments and minor dolo- stones with up to 30 m thick stromatolite-bearing
dolomite lenses Fig. 2A. These rocks are over- lain by sediments of the Bambuı´ group, which
includes from base to top: limestones, dolostones with stromatolites and marlstones of the Sete
Lagoas formation, siltstones of the Serra de Santa Helena formation, siltstones, marlstones
and fetid and dark colored limestones of the Lagoa do Jacare´ formation. The redish-lime-
stones and dolostones from the base of the SLA formation cap carbonate have also been de-
scribed in other parts of the Bambuı´ basin and are believed to represent a lithostratigraphic
marker which lies directly above glaciogenic di- amictites or a narrow karstified and brecciated
carbonate horizon Dardenne 1979; Montes et al. 1981; Guimara˜es 1996; Alvarenga et al. 1998. In
SSD the sediments are progressively more de- formed from the top to the base eastwards of
the sequence due to the SD fault Alvarenga, 1978. The rocks exhibit no sign of metamor-
phism or fluid percolation Fig. 2. Limestones and dolostones of the SD region
overly unconformably Archean ? granitic and gneissic
rocks, as
well as
Paleoproterozoic ? metapelites and intrusive rocks of the SD
volcano-sedimentary sequence Fig. 2B. In this area, the base of the Bambuı´ consists of stroma-
tolitic dolostones and limestones SLA forma- tion
superposed by
siltstone with
minor limestone lenses Serra de Santa Helena forma-
tion. Higher up in the stratigraphy are fetid and dark colored limestones of the Lagoa do Jacare´
formation. Samples
were collected
along a
140 m-thick sedimentary sequence that dips 30° westward and presents no signs of metamor-
phism. Samples from SLA consist of dolostones, lime-
stones and pelitic rocks that overly granite and gneiss basement rocks. In contrast to previous
areas, the lower contact of the Bambuı´ group is tectonic and marked by intense shearing close to
the fault zone, the latter with centimetric veins of calcite and quartz that indicate local carbonate
remobilization.
R .V
. Santos
et al
. Precambrian
Research
104 2000
107 –
122
113 Table 1
Carbon and oxygen isotopes and elemental composition of samples from the SSD profile
a
d
18
O
PDB
MgCa100 Sr ppm
Fe ppm Mn ppm
Ba ppm Si ppm
Height m Sample
d
13
C
PDB
Lithology Nd
Nd SD-73
Nd Dolostone
Nd Nd
0.00 0.9
− 5.6
Nd Nd
Nd Nd
Nd Nd
Nd Dolostone
SD-75 −
7.8 0.4
13.00 Nd
Dolostone Nd
Nd Nd
Nd Nd
20.00 0.4
− 6.7
SD-76 −
8.8 Nd
Nd Nd
Nd Nd
Nd SD-78
30.00 Dolostone
1.3 Nd
Nd Nd
Nd Nd
Nd 1.0
− 9.9
SD-80 Dolostone
40.00 Nd
Dolostone Nd
Nd Nd
Nd Nd
50.00 0.1
− 10.4
SD-81 Nd
Dolostone Nd
Nd Nd
Nd Nd
55.00 0.8
− 11.3
SD-82 Nd
Nd Nd
Nd Nd
Nd SD-83
− 10.3
0.9 65.00
Dolostone Nd
Nd SD-86
Nd Dolostone
Nd Nd
905.00 0.6
− 4.3
Nd Nd
Nd Nd
Nd Nd
Nd Dolostone
SD-87 −
4.3 1.4
915.00 Nd
Dolostone Nd
Nd Nd
Nd Nd
925.00 1.4
− 5.5
SD-88 14
1299 204
30 125
SD-17 Dolostone
1096.00 2.7
− 2.6
28 19
1505 203
31 87
28 Dolostone
SD-19 −
2.7 2.9
1105.70 28
Dolostone 30
814 107
32 110
1115.40 4.1
− 1.5
SD-23 28
Dolostone 22
996 113
37 118
1125.10 4.0
− 2.2
SD-24 21
1212 121
39 159
27 2.3
− 2.5
SD-25 Dolostone
1134.80 28
Dolostone 52
596 78
43 99
1144.50 1.9
− 0.2
SD-26 2
Limestone 183
1592 140
59 425
1154.20 0.8
− 7.5
SD-27 Nd
Nd Nd
Nd Nd
Nd MG98-14H
− 7.7
− 0.6
1155.00 Limestone
− 7.7
Nd Nd
Nd Nd
Nd Nd
MG98-14G 1155.80
Limestone −
0.6 Nd
Nd Nd
Nd Nd
Nd −
0.7 −
7.7 MG98-14F
Limestone 1156.60
Nd Limestone
Nd Nd
Nd Nd
Nd 1157.40
− 2.8
− 6.5
MG98-14E Nd
Nd Nd
Nd Nd
MG98-14D Limestone
1158.20 −
3.7 −
6.4 Nd
Nd Nd
Nd Nd
Nd Nd
Limestone MG98-14C
− 2.4
2.3 1159.80
Nd Limestone
Nd Nd
Nd Nd
Nd 1160.60
2.2 −
2.9 MG98-14B
Nd Limestone
Nd Nd
Nd Nd
Nd 1161.40
− 2.4
− 5.7
MG98-14A 18
8828 594
31 519
27 0.9
− 5.9
SD-08 Dolostone
1163.90 27
Dolostone 11
7418 460
29 470
1173.60 1.2
− 7.5
SD-09 27
Dolostone 12
8820 578
31 537
1183.30 1.2
− 8.6
SD-10 187
1026 155
43 120
1 SD-11
− 6.6
1.7 1193.00
Limestone 3
Limestone 312
851 87
51 327
1202.70 2.4
− 7.6
SD-02 188
1449 71
39 966
16 Dol-limestone
SD-03 −
5.6 1.6
1212.40 7
Limestone 417
764 52
46 341
1222.10 2.5
− 6.3
SD-04 4
Limestone 427
350 31
36 159
1231.80 2.2
− 6.7
SD-O5 216
1438 70
53 1753
19 SD-06
− 2.5
2.7 1241.50
Dol-siltstone 26
973 52
33 184
SD-07 Dolostone
1251.20 2.8
− 2.7
28 26
1003 52
39 1027
27 3.1
− 3.9
SD-13B Dolostone
1260.90 27
Dolostone 41
1058 51
32 495
1270.60 2.4
− 2.3
SD-14 28
Dolostone 23
1246 59
34 256
1280.30 2.1
− 1.9
SE-15 17
805 50
32 99
28 SE-16
− 1.7
3.3 1290.00
Dolostone
R .V
. Santos
et al
. Precambrian
Research
104 2000
107 –
122
Table 1 Continued Lithology
MgCa100 Sr ppm
Fe ppm Mn ppm
Ba ppm Si ppm
Height m d
13
C
PDB
Sample d
18
O
PDB
− 11.3
4 557
6241 223
107 3417
1300.00 SD-62
7.9 Cal-siltstone
425 7597
398 154
3476 3
SD-61 −
9.4 10.4
1310.00 Cal-siltstone
− 6.9
1 897
999 74
50 240
1390.00 SD-59A
10.7 Limestone
1553 2357
158 83
949 1
− 7.0
SD-58 Cal-siltstone
1395.00 12.1
1921 969
69 85
648 SD-56
Limestone 1405.00
13.5 −
4.6 347
5991 588
34 2101
2 SD-31A
− 10.1
8.3 1630.00
Cal-siltstone 954
4423 1190
62 SD-33
1609 Cal-siltstone
1665.00 9.9
− 8.8
1 1139
559 29
52 157
1 Limestone
SD-36 −
6.9 10.6
1700.00 1
Limestone 1677
1454 217
69 149
1735.00 10.5
− 7.9
SD-38 1
Limestone 1370
491 43
77 173
1770.00 9.3
− 7.9
SD-39 1173
4376 1234
56 910
1 SD-43
− 7.3
9.9 1805.00
Limestone SD-46
1358 Limestone
657 51
63 188
1840.00 10.5
− 5.2
1557 4141
203 62
1174 8.9
− 5.4
SD-49 Cal-siltstone
1875.00 Limestone
1573 1313
87 60
509 1910.00
9.5 −
4.5 SD-52
− 7.3
1398 1109
107 67
182 SD-54B
Limestone 1945.00
6.9
a
Nd = not determined.
Table 2 Carbon and oxygen isotopes of samples from the SD profile
Lithology Sample
Height m d
13
C
PDB
d
18
O
PDB
5 DG-1A
− 2.6
Dolostone −
7.2 7
DG-1B −
1.5 Dolostone
− 4.0
10 −
2.2 Dolostone
− 4.4
DG-02 Dolostone
DG-19 20
− 1.0
− 4.6
Pinkish DG-20B
24 0.2
− 2.9
dolomite 38
DG-21 10.2
Limestone −
10.1 DG-22
45 Limestone
9.8 −
2.7 125
11.6 Limestone
− 7.5
DG-27 Limestone
DG-28B 130
12.8 −
4.6
factors 1.01025 and 1.01111. The uncertainties of the isotope measurements were 0.2‰ during the
period of analyses. Trace element data were ob- tained after partially reacting the samples with
HCl 10 for 24 h and then analyzing the filtered solution by ICP-AES at the University of Brası´lia.
These analyses are qualitative considering that only a fraction of the samples was dissolved by
the acid solution.
4. Results