jo ur nal home p ag e: www.elsev ier .com/ locate /precamres
eso-Neoproterozoic isotope stratigraphy on carbonateslatforms in the Brasilia Belt of Brazil
arlos J.S. Alvarenga ∗, Roberto V. Santos, Lucieth C. Vieira,arbara A.F. Lima, Luis H. Mancini
nstituto de Geociências, Universidade de Brasília, Campus Darcy Ribeiro, Brasília, DF 70910-900, Brazil
r t i c l e i n f o
rticle history:eceived 27 June 2013eceived in revised form 23 May 2014ccepted 9 June 2014vailable online 20 June 2014
eywords:esoproterozoic
sotope stratigraphyryogenianroterozoic basineoproterozoic glaciationambuí Group
a b s t r a c t
Carbonate platforms were present worldwide during the Proterozoic Eon, and variations in their C andSr isotope ratios are commonly used as a correlation tool particularly through the Neoproterozoic, whenrapid secular change in marine C and Sr isotope values permit distinction between Cryogenian glacialevents. In central Brazil, the late Mesoproterozoic and Neoproterozoic eras are represented by a thicksuccession of sedimentary rocks that were deposited, and later deformed, along the eastern margin of theSão Francisco Craton. These strata are divided into the three major groups: (1) the Paranoá Group, whichconsists of a succession of sandstone, siltstone, rhythmite, and selected intervals of carbonate, (2) theMacaúbas Group, which consists of a glacial diamictite (Jequitaí Formation), and (3) the Bambuí Groupwith an important carbonate-bearing succession that includes characteristic “cap carbonate” facies in itslower strata. Carbonate facies of Paranoá and Bambuí Groups typically occur in unconformable contact,and when the diamictite of Jequitaí Formation is absent, it can be difficult to determine the stratigraphicposition of these lithological similar groups. Furthermore, uncertainties in the age of the Bambuí Grouphas lead to several distinct interpretations regarding the age of the Jequitaí glacial diamictites.
We investigated the C, O, and Sr isotopes and chemical composition of carbonate rocks in five measuredsections, including both pre- and post-glaciation carbonate successions. The �13C (‰ pdb) values in theupper Paranoá Group occur in a narrow interval between +0.6 and +3.6, whereas the post-glacial BambuíGroup begins with substantially negative values (as low as −5.7‰) in cap dolomite facies and rises tovalues up to +11 permil in limestone of the upper Sete Lagoas Formation. Similarly, carbonate rocks ofthe Paranoá and Bambuí groups are distinct in terms of their 87Sr/86Sr ratio. Generally non-radiogenicratios between 0.7056 and 0.7068 are recorded in the upper Paranoá Group, and ratios between 0.7074and 0.7080, occurring within the Bambuí Group.
The stratigraphic pattern of the C and Sr ratios indicates distinctive isotopic characteristics for thesetwo carbonate successions. The isotopic data for the cabonates in the Paranoá Group are consistentwith a sedimentation age in upper Mesoproterozoic or lowermost Neoproterozoic, preceding the firstCryogenian glaciation. Carbonate facies and the isotopic data for the lower Bambuí Group suggest arelationship with the second Cryogenian glaciation.
The Brasiliano Orogeny exposes a broadly succession of Meso-eoproterozoic sedimentary rocks in central Brazil that is more
han 2.5 km thick. Although these rocks are folded near the edge ofhe São Francisco Craton, they are largely undeformed within theraton (Dardenne, 1978b; Alkmin et al., 2001).
The lithostratigraphic framework of the Paranoá and BambuíGroups, which are the main Meso-Neoproterozoic units in centralBrazil, were initially proposed by Braun (1968). The chronos-tratigraphic intervals are poorly constrained and were initiallyrecognized based on the report of different species of stromatolites,such as Conophyton metula Kirichenko (Cloud and Dardenne, 1973;Dardenne et al., 1976). These stromatolites are found in carbo-
nates of the Paranoá Group and display a restricted occurrence, thushampering the regional correlations. The distinction between theBambuí and Paranoá carbonates is well defined in areas in which theglacial Jequitaí Formation is present, and such distinctions may be
uite difficult to locate in places where these glacially derived rocksre absent because of the similarities of the two types of carbonate.
Among other regional features, the cap carbonates of the Seteagoas Formation overlay glacially derived deposits of the Jequitaíormation (Babinski et al., 2007; Alvarenga et al., 2007; Vieira et al.,007; Misi et al., 2007, 2011; Lima, 2011; Kuchenbecker, 2011;axito et al., 2012) as well as sedimentary rocks of the Paranoároup and older metamorphic rocks, granites, and gneiss found
n areas where the glacial diamictites of the Jequitaí Formationre absent (Guimarães, 1997; Lima, 2011; Alvarenga et al., 2007;axito et al., 2012). Various studies have shown that the �13C val-es of these cap carbonates range from −6.5‰ to −2.0‰ (Santost al., 2000, 2004; Martins, 1999; Babinski et al., 2007; Alvarengat al., 2007; Vieira et al., 2007; Misi et al., 2007, 2011; Lima,011; Kuchenbecker, 2011; Caxito et al., 2012). Their 87Sr/86Sratios range between 0.7074 and 0.7077 (Misi and Veizer, 1998;lvarenga et al., 2007, 2012; Lima, 2011; Caxito et al., 2012) andre comparable to those values commonly found in carbonatesverlying the Late Cryogenian glaciation (Halverson et al., 2007).
Carbonates of the upper part of the Paranoá Group have a nar-ow range of positive �13C values that rarely exceed +3.0‰. Thesearbonates contrast with the large range of �13C values observedor the Bambuí Group carbonates, which vary between −5‰ and14.0‰ (Santos et al., 2000, 2004; Alvarenga et al., 2007). The Srsotopic compositions of the carbonates of these two units arelso distinct. For instance, the 87Sr/86Sr ratios of two limestoneamples of the Paranoá Group, 0.70626 and 0.70683, (Alvarengat al., 2007) are much lower than those reported for carbonatesf the Bambuí Group, which vary between 0.70740 and 0.70760Alvarenga et al., 2007; Vieira et al., 2007; Misi et al., 2007; Caxitot al., 2012). This difference suggests that these carbonates wereeposited under rather different sedimentary conditions. Althoughhe age for glacial rocks below the Sete Lagoas Formation is poorlyonstrained, most papers agree that these materials represent theldest Cryogenian (Sturtian) glaciation based on Pb–Pb isochronBabinski et al., 2007). However, recent interpretation based onithostratigraphy, detrital zircons and chemostratigraphy has pro-osed the existence of a second Cryogenian (Marinoan) glaciationor the Jequitaí Formation (Caxito et al., 2012).
Here we present an integrated stratigraphic and isotopic analy-is of carbonate rocks found across six sections of five different areasf the Brasília Belt (BSB), that includes intervals below and abovehe glacial record of the Jequitaí Formation. These stratigraphic andsotopic data demonstrate that a large sedimentary gap exist alonghe unconformity between the Paranoá and Bambuí Groups, anmportant feature used to connect the Jequitaí Formation relatedo the first or second Cryogenian glaciations.
. Regional setting and stratigraphy
The western edge of the São Francisco Craton (SFC) includes auccession of siliciclastic and carbonate rocks deposited between.77 and 0.56 Ga (Pimentel et al., 2011). The BSB, which waseformed during the Brasiliano-Pan-African Orogeny between 790nd 540 Ma (Pimentel and Fuck, 1992), has been separated intohree main tectonics domains: (i) an unfolded domain that covershe SFC, (ii) a domain located on the outer BSB in which only thepper strata (∼2 km) are folded and faulted, and (iii) a domain fur-her west in which both the basement and the sedimentary coverre overprinted by deformation of the BSB.
Thick succession of sedimentary rocks was deposited along the
astern margin of the SFC. These strata are divided into the threeajor units (Table 1): (1) the Paranoá Group, (2) the glaciogenic
ocks of the Jequitaí Formation, and (3) the Bambuí Group. Thepper Paranoá Group includes Conophyton metulum Kirichenkio
esearch 251 (2014) 164–180 165
with a suggested age of 0.9–1.2 Ga (Dardenne et al., 1976). Stro-matolite and microfossils (stratifera undata) data of the ParanoáGroup suggest a narrower range of ages of 1170–950 Ma (Fairchildet al., 1996). The U–Pb data from detrital zircons along the type sec-tion of the Paranoá Group indicate a maximum depositional age of1.54 Ga; however, a maximum depositional Lu-Hf age of approxi-mately 1.04 Ga was obtained from diagenetic xenotime overgrowthon detrital zircon (Matteini et al., 2012). The age of the JequitaíFormation is not known, but U–Pb data of detrital zircons in diamic-tites suggest a maximum depositional age of 850 Ma (Pimentelet al., 2011). The lower portion of the Sete Lagoas Formation (Bam-buí Group) has an estimated age of 740 ± 22 Ma based on Pb–Pbisochron data in dolostone (Babinski et al., 2007). In contrast, theupper portion of this formation has a maximum age of 620 Mabased on detrital zircon U–Pb data (Rodrigues, 2008; Pimentel et al.,2011). If this Pb–Pb isochron is the sedimentation age an intervallarger than 100 Ma occur between the lower and the upper SeteLagoas Formation.
2.1. The Paranoá Group
The Paranoá Group consists of shallow-marine strata contain-ing mature siliciclastic rocks that include quartzites and siliciclasticrhythmites (i.e., layered intercalations of siltstones and quartzites).The rhythmite facies include occasional, stromatolite-bearing car-bonate lenses (Dardenne, 1979; Alvarenga and Dardenne, 1978).The Paranoá Group unconformably overlies the Araí Group, andbegins with a conglomerate (São Miguel conglomerate) followed bymore than 1000 m of predominantly siliciclastic rocks that includestratigraphic successions of sandstones and rhythmites (siltstonesand siltstones with fine layers of sandstones). The Paranoá Grouphas been subdivided into 12 lithofacies (Faria, 1995; Dardenneand Faria, 1985; Guimarães, 1997): São Miguel conglomerate (SM),rhythmite (R1), quartzite (Q1), rhythmite (R2), quartzite (Q2), silt-stone (S), slate (A), rhythmite (R3), quartzite (Q3), rhythmite (R4),feldspatic quartzite (QF), and pelitic-carbonate rocks (PC). Dolo-stones, limestones, and stromatolitic dolostone lenses that are tensof meters thick and hundreds of meters long occur within certainrhythmic units. The upper portions of the Paranoá Group (PC), inparticular, includes stromatolic dolostones, comprising forms thatbeen described as Conophyton metulum Kirichenkio.
2.2. Jequitaí Formation
The Jequitaí Formation is represented by a succession of diamic-tites that are 0–100 m thick and contain rare intercalations ofsandstone and siltstone (Table 1). The Jequitaí Formation occursunconformably above shallow marine strata of the Paranoá Group(Walde, 1978; Uhlein et al., 1999, 2011). The thickness of theJequitaí Formation varies between 0 and 20 m in the northernportion of the Bambuí Basin (Alvarenga et al., 2007; Lima, 2011),and can reach thicknesses of up to 180 m in the south portion ofthe basin near Cristalina town (Faria, 1985; Cukrov et al., 2005).The glacial origin of the Jequitaí Formation was first recognizedby Branner (1919) and confirmed by striated clasts and preserva-tion of striated pavements (Isotta et al., 1969; Walde, 1978). Thediamictite is mostly massive and contains clasts of granite, gneiss,quartzite, limestone, dolostone, and quartz in a matrix composedof silt and fine sand cemented by diagenetic carbonate. The pres-ence of tillites resting directly on striated pavements in this glacialdeposit has been interpreted to represent continental glacial envi-ronment (Isotta et al., 1969; Karfunkel and Hope, 1988). By contrast,
the scarcity of clasts, stratified diamictites, and fine-grained inter-calation and the absence of significantly thick out-wash facies hasbeen used to suggest a glaciomarine environment (Uhlein et al.,1999; Rocha-Campos et al., 1996).
166 C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180
Table 1Proterozoic lithoestratigraphic nomenclature in the eastern São Francisco Basin. Pl-C: pelitic-carbonate unit, Qzt: quartzite, Rht: rhythmite.
Era Mega-sequence Group Formation Lithology Ages
Neoproterozoic IIIBambuí
Três MariasSerra da SaudadeLagoa do Jacaré
Siltstone and arkose sandstoneSiltstoneclaystone, limestone
Serra de Sta. HelenaSete Lagoas
Siltstone, fine sandstoneDolostone, limestone, marls
U–Pb: <620 Ma*
Pb–Pb:740 ± 22 MaMacaúbas Jequitaí Diamictite and few claystone, sandstone U–Pb: <850 Ma*
Siltstone, claystone, lime lensesSandstone and rhythmiteConglomerate
PP IEspinhac o
AraíTraíras Siltstone, sandstone, lime lenses
al., 20
2
BSag(icMiFtdsdG1cbTi
3
sBfcGt
laasauhie
Arraias
* Ages taken from detrital zircons, or the maximum age for the unit (Pimentel et
.3. The Bambuí Group
The Bambuí Group is divided into five formations (Costa andranco, 1961): Sete Lagoas, Serra de Santa Helena, Lagoa do Jacaré,erra da Saudade and Três Marias. Dardenne (1978a) described
series of transgressive/regressive cycles through the strati-raphic succession of the Bambuí Group. The lowermost formationSete Lagoas) consists of dolostone, limestone, and claystone andncludes a distinct paleokarst horizon in its upper portion that out-rops on the border of the basin (Dardenne, 1978a; Lopes, 1981;artins and Lemos, 2007). This exposure horizon has not been
dentified in the inner basin. The overlying Serra de Santa Helenaormation consists predominantly of siliciclastic mudstone withhin layers and lenses of fine-grained sandstones. Above the Serrae Santa Helena Formation, a series of claystone, marl, and silt-tone containing lenses of dark gray limestone comprise the Lagoao Jacaré Formation. These three lower formations of the Bambuíroup have been referred to as the Paraopeba Subgroup (Braun,968). Uppermost strata of the Bambuí Group are composed of sili-iclastic mudstones of the Serra da Saudade Formation followedy mudstone intercalated with layers of arkose sandstone of therês Marias Formation, which were deposited in a storm and tidallynluences (Chiavegatto, 1992).
. Sampling and analytical methods
Stratigraphic sections of this study were systematically mea-ured and sampled at five sites of the middle-western portion of theSB (Figs. 1 and 2). Outcrop and drill core samples were selected
or C, O and Sr isotopic analyses intended to provide a detailedhemostratigraphic correlation between the Paranoá and Bambuíroup. Among the five sites, four are outcrop, and one occurs within
wo drill cores near Planaltina de Goiás (Fig. 2).In total, the study includes 213 samples of dolostones and
imestones of the Paranoá and Bambuí groups. All samples werenalyzed for major and minor chemical elements as well as Cnd O isotopes. 87Sr/86Sr ratios were obtained from a variety ofamples, with samples containing >400 ppm Sr showing the leastlteration. Prior to analysis, each rock specimen was investigated
nder a petrographic microscope to avoid fractures, veins, andeavily re-crystallized zones. The isotope data produced are shown
n Table 2, including the values previously published by Alvarengat al. (2007), in Portuguese.
Sandstone, conglomerate, vulcanic U–Pb: 1770 Ma
01, 2011, Babinski et al., 2007, Matteini et al., 2012).
Determinations of minor and major elements were performedusing a Rigaku model RIX 3000 XRF X-ray fluorescence unitequipped with a Rh tube at NEG-LABISE, Department of Geology,University of Pernambuco. The samples used for chemical analysiswere initially dried at 110 ◦C to eliminate excess humidity andheated to 1000 ◦C for 2 h to determine the percentage loss onignition.
C and O isotopes were obtained on a Delta V Advantage instru-ment connected to a Gas Bench II apparatus at the GeochronosLaboratory, Geosciences Institute, University of Brasilia in Brazil.Aliquots of each sample (approximately 300 �g) were placed inglass vials that were subsequently submitted to a He flush at 72 ◦C.All C and O isotopes are presented in VPD and were calibratedagainst NBS-18 and NBS-19 standards.
For 87Sr/86Sr analysis, 50 mg of carbonate powder samples wereweighted into Teflon beakers and dilute acetic acid (0.5 N) to dis-solve only the carbonate fraction and avoid leaching of radiogenic87Sr and Rb from the non-carbonate constituents of the samples.The 87Sr/86Sr ratios were measured using a Neptune Thermo MC-ICP-MS at the Geochronos Laboratory, Institute of Geosciences,University of Brasília in Brazil. Analysis of the NBS 987 standard dur-ing the course of this work yielded an average value of 0.710230 ± 8(1 s). The uncertainties in the individual analyses were lower than0.001% (2 s).
4. Lithologic and geochemical results
4.1. Serra de São Domingos – Area 1
Carbonate rocks in the upper Paranoá Group reach a total thick-ness of 110 m at the Serra de São Domingos site (Fig. 3). Here,carbonate rock occurs in sharp contact with heterolithic siliciclasticfacies of the underlying strata. The lower portion of the carbonateinterval consists of a purple laminated dolostone intercalated withlaminated mudstone that grades upward to dark gray limestonecontaining molar tooth structures (Fig. 4A; cf. Furniss et al., 1998;Pollock et al., 2006). The upper portion of the carbonate interval ischaracterized by stratified dolostone with intercalations of massivedolarenite (ic. grainstone) and dolostone layers containing domal
columnar, and stratiform stromatolites (Fig. 4B).
Glacial strata of the Jequitaí Formation is absent in this region,but a thin (<1.5 m) breccia occurs at the same stratigraphicposition and defines the unconformity that separates the Paranoá
C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180 167
Fig. 1. Major structural units in Brazil (after Almeida et al., 1981).
Fig. 2. Geological map of the studied sections area, showing the location sections: (1) Serra de São Domingos section, (2) Bezerra section, (3) JK section, (4) NW Planaltinasection, and (5) North Brasília section.
168 C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180
80
90
100
110
120
130
140
150
160
170
180
190
210
200
40
50
60
70
(m) 0
10
20
30
220
230
240
250
260
280
290
Bam
bu
í
Gro
up
Sete
Lag
oas
Form
atio
n
SHF
Para
no
á
Gro
up
U
pp
er
Car
bo
nat
e
Un
it
RHT
300
-15 -10 -5 0
-15 -10 -5 0δ18O
JJJ
JJJ JJJJ JJJJJJ JJJ
0 1000 2000 3000
HH
HH
HH
HH
HH
HH
HHHH
H
HHHH
0 1000 2000 3000Sr(ppm)
Sr/ Sr87 86
-10 -5 0 5 10 15
-10 -5 0 5 10 15δ13C
0.70782
0.707620.707680.70778
0.707730.70769
0.707840.70770
0.70762
0.70761
0.70760
0.70802
0.705620.705940.705830.70580
Upp
erLo
wer
Fig. 3. Isotope stratigraphy lithostratigraphy of the Paranoá and Bambuí Groups in the Serra de São Domingos area. See Fig. 5 for symbols and Table 2 for �13C, �18O, 87Sr/86Sr,and Sr (ppm) data. Field circles are dolomite and open circles are limestone.
C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180 169
F olar to( .
ai
∼al“hgttF
gphr
Gicdb�tgtlmSut
S
ig. 4. Serra de São Domingos area. Lithofacies in the upper Paranoá Group: (A) mC and D) aragonite pseudomorph in the basal contact of the Sete Lagoas Formation
nd Bambuí groups. This is composed of angular dolostone clastsn a coarse arkose matrix cemented by dolomite.
Strata directly overlying the thin boundary breccia consists of a1.5 m thick dolomite unit containing laminated dolostone layersnd layers of dolomite needle crystals ranging from 0.5 to 10 cmong (Fig. 4C and D). These large crystal fans are characterized ofcap carbonate” intervals in the Cryogenian (Hoffman, 2011), andave been interpreted as aragonite pseudomorph. The cap dolomiterades upward along a diffuse and transitional contact to a 90 mhick succession of purple to light gray limestone intercalated withhin laminae of green claystone that comprise the lower Sete Lagoasormation.
The upper carbonate Sete Lagoas Formation consists of darkray limestone and marls. The occasional presence of ooids andeloids in calcarenites and calcirudites beds indicates localizedigh-energy environments, suggesting deposition in a shelf envi-onment that shoaled upsection.
Carbon-isotope values (Table 2, Fig. 3) of the upper Paranoároup carbonate range between +0.6‰ and +3.4‰, with a slight
ncrease in the values from the base to the top of this unit. Theontact between the dolostone of the Paranoá Group and the capolostone is marked by an abrupt decrease in the �13C valuesy more than 3‰, and the thin cap dolostone exhibits negative13C values of −2.2‰. Negative C-isotope values continue upwardhrough the first 17 m of the lower Sete Lagoas Formation. Strati-raphically higher, a slight increase is observed in the �13C valueshat reach up to +3.5‰. The sharp contact between the lower clay-imestone lithofacies and the upper dark-gray limestone lithofacies
arks a major increase in the �13C values from +3.5‰ to +8.6‰.uch highly positive values occur through the remainder of the
pper Sete Lagoas Formation, and are similar to that observedhroughout the basin (Santos et al., 2004).
Oxygen-isotope composition of carbonates from the Serra deão Domingos site are variable and appear to vary according to
oth structures in limestone, (B) columnar stromatolite in dolomite. Cap dolomite:
the lithofacies (Fig. 3). For instance, limestone of the lower unitof the Paranoá Group displays �18O values ranging from −9.0‰to −6.3‰ with average values of −7.0‰, whereas dolostone faciesof the upper Paranoá Group record an abrupt increase to positive�18O. Similar to the �13C observations, the cap dolostone marksanother abrupt O-isotope change, which, in this case, is markedby negative �18O values that reach −8.5‰. Negative �18O valuesaveraging approximately −7‰ are recorded throughout the lowerSete Lagoas Formation, although values around −5‰ are found inshallower-water facies of the upper Sete Lagoas Formation (Table 2,Fig. 3).
Limestone containing molar tooth from the Paranoá Grouprecords 87Sr/86Sr ratios between 0.70562 and 0.70594 in sampleswith Sr concentrations ranging between 417 and 984 ppm. By con-trast, the 87Sr/86Sr isotope ratios for limestone of the Bambuí Grouprange from 0.70761 and 0.70802 and have high Sr concentrationsranging from 1000 to 3000 ppm (Table 2).
4.2. Bezerra-JK – Areas 2 and 3
Carbonate rocks in the upper Paranoá Group have beendescribed from a 28-m-thick succession of dololutite and lami-nated lime mudstone intercalated with clay laminae. These strataare succeeded by a 20-m-thick layer composed of lime mud-stone, marl, and occasionally calcarenite. Medium-coarse arkosewith a thickness of 40 m overlays the carbonate unit and rep-resents the top of the Paranoá Group in this section of the JK(Fig. 5).
The Jequitaí Formation in these two sections (Bezerra and JK)is between 6 and 15 m thick and overlies, in sharp contact, arkose
sandstone (Fig. 5). The massive diamictite includes angular clastsof limestone, dolomite, quartz, and feldspar immersed in a clay-siltmatrix cemented by calcite (Fig. 6A). Clasts within the diamictiteare similar to rocks found in the underlying stratigraphy of the
170
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lvarenga et
al. /
Precambrian
Research
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164–180Table 2C, O, Sr-isotope ratio and elemental data for samples from Serra de São Domingos (Area 1), Bezerra-JK (Areas 2 and 3), NW Planaltina (Area 4) and North Brasília (Area 5).
No. Area Sample Group For. Height m Lithology �13C ‰pdb
Ref.: 1 = This paper, 2 = Alvarenga et al. (2007), 1*–2 = Only 87Sr/86Sr from this paper.For.: Formation, LSL: Lower Sete Lagoas Formation, USL: Upper Sete Lagoas FormationHeight: Datums for stratigraphic height.Clay dolo: Dolostone intercalated with mudstone. MT lime: Molar tooth limestone. Dolo breccia: Dolostone breccia. Clay lime: limestone intercalated with mudstone. Oo-lime: oolite limestone. Lime breccia: limestone breccia.
174 C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180
F : (A) Bf
Pbd
ao
Ff
ig. 5. Isotope stratigraphy and lithostratigraphy of the Paranoá and Bambuí groupsor �13C, �18O, 87Sr/86Sr, and Sr (ppm) data.
aranoá Group. The diamictite in the Bezerra section is succeededy a 0.5-m-thick layer of feldspar sandstone underlying the capolomite.
The 7-m thick cap dolomite of the Sete Lagoas Formationbruptly overlies diamictites of the Jequitaí Formation and consistsf laminated gray-pink dolostone (Fig. 6B). This dolostone is
ig. 6. Glaciogenic diamictite and cap dolostone in Bezerra and JK sections: (A) glacial
eldspar immersed in a clay-silt matrix cemented by calcite and (B) laminated gray-pink
ezerra section and (B) JK section (modified from Alvarenga et al., 2007). See Table 2
then sharply overlain by intercalated limestone and purple-greenclaystone (Fig. 7) of the lower Sete Lagoas Formation. Thesestrata include thin-bedded peloidal limestones that often display
low-angle cross-lamination. The lower Sete Lagoas Formation thengrades upward to a gray-colored 200-m-thick succession of clayeylimestone, followed by a 40-m-thick dark gray limestone layer and
diamictites of the Jequitaí Formation with angular clasts of dolomite, quartz, andcap dolomite, that occurs in the first 7 m of the Sete Lagoas Formation.
C.J.S. Alvarenga et al. / Precambrian R
Ft
aleF
Fi
ig. 7. Intercalated limestone and purple claystone of the lower Sete Lagoas Forma-ion.
few meters of marls (Fig. 5). The presence of ooids and peloids inocalized calcarenites and calcirudites facies indicates high-energynvironments and suggests gradual shoaling of the Sete Lagoasormation.
ig. 8. Isotope stratigraphy and lithostratigraphy of the upper Paranoá Group and the lown two drill cores from Cimento Tocantins. See Table 2 for �13C, �18O, 87Sr/86Sr, and Sr (pp
esearch 251 (2014) 164–180 175
Isotope data in the Bezerra-JK area were previously publishedby Alvarenga et al. (2007), in Portuguese. �13C values of carbonatein the Paranoá Group range between +0.74‰ and +2.6‰ (Table 2,Fig. 5). The �18O values vary from −7.3‰ to −12.3‰ near the baseof the section to −4.7‰ near the top of the Paranoá Group. The capdolomite displays �13C values between −3‰ and −5‰, decreasingupward in the section by 2‰ (Fig. 5). The limestone and clayeylimestone facies above the cap dolostone continue to preserve neg-ative �13C values over approximately 70 m of stratigraphic sectionbefore and increasing to highly positive �13C values near 9 permilin the dark gray limestone of the upper Sete Lagoas Formation. Thecap dolomite presents a range of �18O values between −7.6‰ and−5.5‰ (Fig. 5, Table 2). Toward the top of the succession, the O iso-topic value exhibits an abrupt drop, which is also accompanied byan abrupt increase in the SiO2 and Al2O3 contents.
The 87Sr/86Sr ratio of the entire Bezerra-JK profile rangesbetween 0.70626 and 0.70797. Samples with high and variable87Sr/86Sr are associated with a substantially lower Sr content(<148 ppm) and may have been affected by post-depositionalalteration related to diagenetic fluid reactions. Limestone samples
with a Sr concentration of greater than 450 ppm and a low Mn/Srratio (<0.4), are believed to have preserved the original Sr isotopiccomposition display 87Sr/86Sr ratios varying between 0.70626 and0.70683 for the limestone of the Paranoá Group and 0.70745 and
er Sete Lagoas Formation in the NW-Planaltina de Goiás. Samples were collectedm) data.
ation of the upper Paranoá Group in the NW-Planaltina de Goiás and (B) Gray,icro-crystalline dolomite intercalated with fine breccia in the first few meters of
he Sete Lagoas Formation. See Fig. 8 for stratigraphic location.
.70758 for the limestone of the Sete Lagoas Formation (Table 2,ig. 5).
.3. NW Planaltina – Area 4
In the NW Planaltina de Goiás (Fig. 2) profile, whole-rock sam-les were collected from two drill cores (Fig. 8). The dolostone andtromatolitic dolostone described in the lower portion of these twoections is similar to that found in outcrops in the region. The stro-atolitic intervals contain convex laminae and cylinder-conical
aminae identified as Conophyton Metula Kirichenko (Fig. 9A)Dardenne et al., 1976). Dolostone breccia (approximately 1-mhick), including dolostone with angular clasts, marks the uncon-ormity between the Paranoá and Bambuí Groups.
Overlying the dolostone breccia there is a cap dolostone upo 6-m thick dolarenite, and dolorudite described in these tworill cores (Fig. 8). The cap dolomite appears as gray and thin-edded, micro-crystalline dolomite intercalated with fine brecciaith angular clasts (Fig. 9B). Purple to light gray limestone interca-
ated with green–purple claystone overlies the cap dolostone andrades upward to limestone with rare mud cracks and to ooliticnd intraclastic calcarenite at the top of the section, indicating ahallow tidal environment.
Carbonate strata of these two cores record �13C values ran-ing between −1.7‰ and +4.2‰, with the negative �13C values areestricted to the cap dolomite. The �18O values are similar to thether sections, with values ranging from −3.0 to −7.8‰ (Fig. 8).
87Sr/86Sr ratios vary between 0.70730 and 0.70800 for sam-les with a total Sr content ranging from 450 to 3100 ppm andn/Sr value of less than 0.2 (Table 2). Partly dolomitized samples
Mg/Ca > 0.25) with a significant amount of terrigenous detritusSiO2 + Al2O3 > 20%) exhibit more radiogenic Sr compositions of upo 0.71015 (Table 2).
.4. North Brasília – Area 5
The studied sections in northern Brasília (Fig. 2) were describedrom the Cimento-Ciplan and Engexplo quarries (Fig. 10A) as wells from a quarry of the Contagem Mining Company (Fig. 10B). Dolo-tones and stromatolitic dolostone of the upper Paranoá Groupccur in the lower section (Fig. 10A) followed by a thin brecciaayer (0.30-m thick), marking the unconformity preceding Bam-uí Group sedimentation. In this section, the cap dolomite of theete Lagoas Formation is restricted to a thin layer of dolostone<0.5 m) placed a few centimeters above of the breccia that marks
he unconformity. Above the cap dolomite is a 80-m-thick succes-ion of purple–light gray limestone intercalated with thin-laminaelaystone with a sharp contacts that shift upward to a gray-pinkolostone. This dolostone displays an irregular and rippled upper
esearch 251 (2014) 164–180
surface followed by a transgressive surface that is subsequentlyoverlain by a 1.5-m thick shale layer that grades upward to lime-stone and clay limestone (Fig. 10B).
�13C values at the base of the 0.5-m thick breccia and dolo-stone layer in Sete Lagoas Formation range between −3.6‰ and+2.6‰, with the negative values found only in the first 3 m abovethe unconformity. Toward the upper portion of the succession,the �13C values become increasingly more positive reaching morethan 10‰ to top of the succession (Fig. 10). Seven limestonesamples with high Sr contents (>810 ppm) and low Mn/Sr ratios(<0.2) Exhibit 87Sr/86Sr values between 0.70742 and 0.70782. Themost radiogenic ratio (0.71750) recorded occurs within dolomi-tized limestone. Samples with Sr isotope ratios between 0.70815and 0.70839 or greater have a significant detrital component andmay have not preserved primary isotopic ratios; these samples alsoexhibit a high content of SiO2 and Al2O3 (Table 2).
5. Discussion
The stratigraphic and isotopic data presented in this paper sug-gest significant differences between rocks from the Paranoá andBambuí Goups. The Paranoá Group is composed mainly of siliciclas-tic rock, and its carbonate units are restricted to a few stratigraphicintervals. By contrast, the Bambuí Group includes two formationscomposed predominantly of carbonate (Sete Lagoas and Lagoa doJacaré). The Paranoá Group is, furthermore, dominated by dolo-stone, with limestone restricted to a few intervals, such as themolar tooth limestone. This unit is also characterized by abundantstromatolites, exemplified by Conophyton metula Kirichenko andothers (Dardenne, 2000). By contrast, the Bambuí Group is dom-inated by limestone, with dolostone restricted to the lower SeteLagoas Formation. Stromatolites within the Sete Lagoas Formationare also dominated by columnar and stratiform, rather than conical,morphologies.
Although glacial rocks of the Jequitaí Formation are absent inmost of the studied sections, the contact between the Paranoá andBambuí Groups is consistently marked by an unconformity overlainby a thin cap dolostone interval that contains negative �13C values(Figs. 3, 5, 8 and 10). The negative �13C values, cap dolomite, and thepresence of glacial diamictites in certain sections provide evidenceof the glacial record prior to the Sete Lagoas Formation sedimenta-tion. The thickness of the diamictite is variable and ranging between0 and 15 m in the northern Bambuí Basin. In the southern portionof the basin, diamictites of the Jequitaí Formation have thicknessesof 80 m, and can reach up to 180 m in the Cristalina region (Cukrovet al., 2005; Uhlein et al., 2011). This observation suggests a shal-lower glaciomarine environment or a glaciocontinental domain inthe northern basin, and potentially more extensive erosion relatedto post-glacial uplift.
5.1. C and O isotopes
C and Sr isotopes record the temporal variations through theMesoproterozoic and Neoproterozoic (Kah et al., 1999, 2012;Halverson et al., 2007, 2010). In general Mesoproterozoic to earlyNeoproterozoic (Tonian) samples are characterized by moderatelyvariable �13C values, that rarely exceeds outside the −4‰ to +4‰range (Gorokhov et al., 1995; Kah et al., 1999, 2012; Bartley et al.,2001, 2007; Santos et al., 2000, 2004; Semikhatov et al., 2002; Guoet al., 2013). By contrast, in the Cryogenian and Ediacaran �13Cvalues may vary between −10‰ to +10‰ (Brasier and Lindsay,1998; Santos et al., 2000; Xiao et al., 1997; Halverson et al., 2005,
2010). Particularly large isotopic excursions described in pre- andpost-glacial time, may further help refine the chronostratigraphicposition of units deposited during this time (Halverson et al., 2005;Halverson and Shields-Zhou, 2011).
C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180 177
A
250
200
150
100
50
0
-50
Bam
buí
G
roup
Sete
Lago
as
Fo
rmat
ion
Para
noá
Gr.
0
5
10
15
20
25
30
B
Low
erUp
per
(m)
(m)
Limestone
Intraclast dolostone
Dolostone
Clay limestone
Siltstone
Stromatolite
Dolomite
Limestone
-10 -5 0
0.70758
0.70796
Sr/ Sr87 86
10 15 5
B
0 20001000 3000
BB
B
B
B
B
Sr (ppm)-15 -10 -5 0
δ1313 C C δ1818 O O
-10 -5 0 10 15 5 -15 -10 -5 0 0 20001000 3000
-10 -5 0 5 10 15
0.70815
0.70782
0.70760
0.707490.70742
0.708390.70754
Sr/ Sr87 86
δ1313 C C0 20001000 3000Sr (ppm)
-15 -10 -5 0
δ1818 O O
-10 -5 0 5 10 15 -15 -10 -5 0 0 2000 1000 3000
Fig. 10. Isotope stratigraphy and lithostratigraphy of the upper Paranoá Group and Sete Lagoas Formation in northern Brasília. See Table 2 for �13C, �18O, 87Sr/86Sr, andS ries anF in text
oam2cBGM
dnLvlit1Cvmrtci2st
r (ppm) data. (A) Section described from the Cimento-Ciplan and Engexplo quarormation from the Contagem Quarry. (For interpretation of the references to color
The �13C values of the Paranoá Group in the northern portionf the BSB present a narrow range of isotope values (+0.6‰nd +3.6‰) that is similar to the isotope range observed in theesoproterozoic units of the Espinhac o Group, Brazil (Santos et al.,
004). This narrow isotope range is observed, and appears to beharacteristic of late Mesoproterozoic rocks including those of theylot Supergroup in Canada (Kah et al., 1999), the Turukhanskroup of Siberia (Bartley et al., 2001), and the Atar Group ofauritania (Kah et al., 2012).In sharp contrast to Paranoá Group strata, the Bambuí Group
ata preserve C-isotope values between −5‰ and +11‰. Stronglyegative �13C values always occur along the first 4 m of the Seteagoas Formation, which can be diagnosed as a cap dolomite inter-al, but can extend up to 75 m upsection as in the JK and Bezerraocalities (Fig. 5). These negative values are consistent with thosenterpreted as a primary marine signal characteristic of the Neopro-erozoic post-glacial oceans (Kaufman et al., 1997; Hoffman et al.,998; Hoffman and Schrag, 2002; Halverson et al., 2005). Negativearbon isotope values in the lower Sete Lagoas Formation rise toalues of +4.9‰ in the upper portion of the lower Sete Lagoas For-ation (Martins and Lemos, 2007). However, the �13C values can
each +5.6‰ in other localities (Alvarenga et al., 2012). Strata ofhe upper Sete Lagoas Formation is marked by an abrupt lithologi-al contact between dolostone and limestone in Area 5 (Fig. 2) and
n other regions of the Bambuí Group (Dardenne, 1978a; Santana,011; Tonietto, 2010; Lima, 2011; Alvarenga et al., 2012). Thisharp stratigraphic contact coincides with an abrupt increase ofhe �13C values, typically with values above +6.0‰ and reaching up
d (B) section showing the contact between the lower and the upper Sete Lagoas near the reference citation, the reader is referred to the web version of this article.)
to +10.9‰ as reported in this paper, and +14.5‰ in other reports(Santana, 2011).
In contrast to the C-isotopes, the �18O values of the ParanoáGroup present a large variation range, with the lowest values asso-ciated with the limestone and molar tooth structures (−5‰) and thehighest values associated with the dolostone together with stroma-tolites and algal mats (Fig. 11). The �18O values between 0‰ and−4‰ can be interpreted as related to the primary seawater records(Knauth and Kennedy, 2009).
Strongly negative O-isotope values associated with Cryogenianglaciation have also been interpreted to result from meteoric orburial diagenesis (Knauth and Kennedy, 2009). Oxygen isotopevalues for the Paranoá and Bambuí groups range from −12.5‰to −1.5‰, with the majority of samples falling within the rangeof −8‰ to −3‰ (Fig. 11). O-isotope compositions reflect distinctfacies intervals within the stratigraphic succession, with generallyopen marine carbonate facies of both the Paranoá and Bam-buí groups, recording O-isotope values near −7 permil (Fig. 11).These values are consistent with O-isotope values from a widerange of open marine carbonate facies in the Proterozoic (Kah,2000; Kah et al., 2012). Two exceptions are the dolomitic faciesof the Paranoá Group, with O-isotope values ranging from −5‰to −2‰, and shallow-marine (oolitic, etc.) facies of the upperSete Lagoas Formation, with O-isotope values near −5‰. Slightly
heavier O-isotope values, such as these, are consistent with thatwell-preserved restricted to evaporitic marine facies elsewhere(Kah, 2000; Kah et al., 1999, 2012). Thus O-isotope compositionsrecorded here are consistent with marine values of the Proterozoic
178 C.J.S. Alvarenga et al. / Precambrian Research 251 (2014) 164–180
-10
-5
0
5
10
15
-15 -10 -5 0
Upper Paranoá GroupLimestone
Molar tooth limestone
Clay limestone
Dolomite
Bambuí Group
Dolomite
Clay limestone
limestone
limestone
Cap dolomite
δ18O (VPDB)
δ13 C
(VP
DB)
Upper Sete Lagoas Formation
Lower Sete Lagoas Formation
Clay limestone
Fig. 11. C vs. O isotope cross-plot for carbonates from the Paranoá and Bambuí Groups: (a) dolomites from the upper Paranoá Group exhibit the highest �18O values and canb nd low �18O values, that can be interpreted as result from meteoric or burial diagenesis,(
ar
5
t1S0swb2a02NS(zom(
cceHrMDtc0itpD
Fig. 12. Cross-plots of 87Sr/86Sr vs. Sr(ppm) content for carbonates from the Para-noá and Bambuí Groups. Dolostone samples with low Sr content concentration(<300 ppm) have high 87Sr/86Sr ratios, and are related to radiogenic Sr or post-
e related to a primary seawater record, (b) cap carbonates with low �13C values ac) limestones with high C-isotope values and slightly heavier O-isotope values.
nd do not support a diagenetic origin for the preserved C-isotopeecord.
.2. Sr isotope
Low 87Sr/86Sr ratios < 0.7065 has been reported in late Mesopro-erozoic strata in southern Urals and Siberia, Russia (Gorokhov et al.,995; Bartley et al., 2001, 2007; Kuznetsov et al., 2006) and in Bylotupergroup, Canada (Kah et al., 2001). 87Sr/86Sr ratios between.7055 and 0.7070 are common in the Tonian and Early Cryogenianuccessions that precede the first Cryogenian glaciation (Sturtian),hereas 87Sr/86Sr ratios that remain below 0.7075 are common
efore the Late Cryogenian glaciation (Marinoan) (Halverson et al.,007, 2010; Sawaki et al., 2010). The 87Sr/86Sr data in carbonatesfter the Late Cryogenian glacial age (Marinoan) increase from.7072 to 0.7085 in the Ediacaran Period (Melezhick et al., 2001,009; Halverson et al., 2007). During the Mesoproterozoic andeoproterozoic Eras, carbonates from the Turukhansk region iniberia present 87Sr/86Sr values that decrease from 0.7060 to 0.7055Bartley et al., 2001). Similar data can be found in the Mesoprotero-oic (approximately 1.2 Ga) rocks from the Society Cliff Formationf Canada, for which the 87Sr/86Sr ratios decrease from approxi-ately 0.70600 to 0.70550 in the lower portion of this formation
Kah et al., 2001).The chemical composition should be included in the pro-
ess of identifying whether post-sedimentary processes may havehanged the primary 87Sr/86Sr ratios of the limestone (Fairchildt al., 2000; James et al., 2001; Alvarenga et al., 2007, 2008;alverson et al., 2007, 2010; Shields, 2007). The high 87Sr/86Sr
atio in dolomite with low Sr concentrations (<400 ppm) and highn/Sr ratios (>0.45) has been related to post-depositional effects.olomite is the dominant rock in the Paranoá Group, which reduces
he success of the Sr isotope. Nevertheless, limestone with a Sr con-entration of grater than 400 ppm Exhibits 87Sr/86Sr values from.70626 to 0.70683 in the JK section and from 0.70562 to 0.70594
n the molar tooth structures from the Serra de São Domingos sec-ion. These data are interpreted as primary values of the seawaterresented during the Mesoproterozoic sedimentation (Fig. 12). Capolomite samples with high Mn/Sr ratios, low Sr concentrations
depositional effects. Limestone with a Sr concentration of grater than 400 ppmExhibits 87Sr/86Sr ratios from 0.7056 to 0.7068 in the upper Paranoá Group. In theBambuí Group (Sete Lagoas Formation) 87Sr/86Sr ratios range from 0.7074 to 0.7080.
(<300 ppm), and a terrigenous content of more than 15% are relatedto radiogenic Sr or post-depositional effects (Fig. 12). In the SeteLagoas Formation 87Sr/86Sr ratios from 0.7074 to 0.7080 show avery clear range of least-radiogenic values (Fig. 12). These ratios
can be interpreted as a marine Sr isotope record related to theEarly Ediacaran Period (Halverson et al., 2007, 2010), suggestingsedimentation after the late Cryogenian glaciation, as previouslyproposed by Caxito et al. (2012).
brian R
6
BottsSta
i(St0
uNtKtLMm
orh
A
d3mrflvttfLt
R
A
A
A
A
A
A
B
B
C.J.S. Alvarenga et al. / Precam
. Conclusion
The unconformity between the carbonates of the Paranoá andambuí groups is marked by sedimentary breccia with a thicknessf up to 1.5 m or glacial diamictites with a thickness of up to 15 m;hese layers separate two domains of distinct isotopic patterns athe studied locations. Despite the absence of glacial rocks, threeections with negative �13C values in the first few meters of theete Lagoas Formation provide evidence of a glacial trace, similaro the aragonite pseudomorphs overlying the unconformity brecciand the coeval cap carbonates elsewhere.
The �13C values for the Bambuí Group range from −5‰ to +11‰,n contrast with the narrow range of the Paranoá Group limestone+0.6‰ to +3.6‰). These isotopic differences are also noted for ther isotope, with low values ranging between 0.7056 and 0.7068 forhe Paranoá Group and high values ranging between 0.7074 and.7080 for the Bambuí Group.
The low ratios of 87Sr/86Sr and the constant positive �13C val-es suggest an age between the Late Mesoproterozoic and Earlyeoproterozoic for the Paranoá Group, which is consistent with
he previous age interpretation based on the Conophyton metulairichenko stromatolite. When compared with published Neopro-
erozoic 87Sr/86Sr marine record, the Sr isotope data for the Seteagoas Formation suggest a sedimentation age linked to the post-arinoan glaciation. Similar age can be suggested based on theore positive �13C that may reach more than 10‰.The unconformity between these two stratigraphic successions
f different ages may be marked by the absence of Early Cryogenianocks because either they were eroded or not deposited due to theigh paleo-topography along the northwester edge of the basin.
cknowledgments
Research for this study was supported by Conselho Nacionale Desenvolvimento Científico e Tecnológico (CNPq, grant no.06627/2011-6, 476484/2007-3). We thank M.A.Dardenne (inemoria) for the enthusiastic discussions for years. Valderez Fer-
eira e Alcides N. Sial are thanked for determinations by X-Rayuorescence and the stable isotope data measured at LABISE Uni-ersidade Federal de Pernambuco, Brazil. We also would like tohank Jessica Bogossian and André Cadamuro for the field assis-ance as well as Cimento Tocantins and Wiliam Marcelino Coelhoor access to company drillcore. Finally, we would like to thankinda Kah and an anonymous reviewer for helpful comments onhe manuscript.
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