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Gondwana Research 13

Isotope stratigraphy of Neoproterozoic cap carbonates in theAraras Group, Brazil

Carlos J.S. de Alvarenga a,⁎, Marcel A. Dardenne a, Roberto V. Santos a, Emanuele R. Brod a,Simone M.C.L. Gioia a, Alcides N. Sial b, Elton L. Dantas a, Valderez P. Ferreira b

a Instituto de Geociências, Universidade de Brasília, Campus Universitário, Asa Norte, CEP: 70910-900, Brasília, DF, Brazilb Departamento de Geologia, Universidade Federal de Pernambuco, Caixa Postal: 7852, CEP: 50732-970, Recife, PE, Brazil

Received 22 September 2005; accepted 7 May 2007Available online 23 May 2007

Abstract

The Neoproterozoic carbonate sequence on the southeastern border of the Amazon Craton is divided into three lithostratigraphic units: a basalcap dolomite, an intermediate limestone, limestone-mudstone unit, and an upper dolarenite-dolorudite unit. Sections of the cap-carbonate weremeasured from the inner shelf to the outer shelf. Carbon isotope ratios (relative to PDB) vary between −10.5 and −1.7‰ in cap dolomite, andbetween −5.4 and +0.1‰ in laminated limestone and mud-limestone. Limestones and mud-limestones exhibit 87Sr/86Sr ratios ranging from0.70740 to 0.70780. A comparative isotope stratigraphy between the inner-shelf and the middle-shelf basin shows differences in carbon isotoperatios: The cap dolomite and limestones have lower δ13C ratios on the border of the basin (inner shelf) than in the middle shelf of the basin. Theselower values can be related to shallower environmental conditions and to a stronger influence of the continental border. The 87Sr/86Sr ratios are thesame in both areas, and are consistent with seawater composition at around 600 Ma.© 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Neoproterozoic; Chemostratigraphy; Carbonate platform; Araras group; Glaciation

1. Introduction

Associations of glaciogenic and carbonate rocks are commonin Neoproterozoic sequences. As these successions are normallyunfossiliferous and poorly dated, carbon and strontium isotopedata have become powerful tools for correlation within asedimentary basin or even at a larger, global scale (Hoffmanet al., 1998; Kennedy et al., 1998, 2001; Melezhik et al., 2001;Hoffman and Schrag, 2002; Melezhik et al., 2005). Carbonisotope ratios of worldwide Neoproterozoic sequences placedjust at the end of the glacial event range from highly 13C-enriched to 13C-depleted values (Kaufman et al., 1997;Kennedy et al., 1998; Hoffman et al., 1998). The Sr isotope

⁎ Corresponding author. Tel./fax: +55 61 3347 4062.E-mail address: alva1@unb.br (C.J.S. de Alvarenga).

1342-937X/$ - see front matter © 2007 International Association for Gondwana Rdoi:10.1016/j.gr.2007.05.004

data are interpreted to reflect 87Sr/86Sr variation of seawater forthe different carbonate sequences in the Neoproterozoic. Someworkers have shown that the lowest 87Sr/86Sr values (0.7056)are consistent with 850–750 Ma seawater and higher values(0.7074 to 0.7087) with latest Neoproterozoic seawater(Jacobsen and Kaufman, 1999; Shields, 1999; Brasier andShields, 2000; Melezhik et al., 2001; Thomas et al., 2004).

Isotopic chemostratigraphic data in Neoproterozoic carbonatesuccessions in which shallow-water facies changes to deeperfacies have been considered in a number of papers (Hoffmanet al., 1998; Alvarenga et al., 2004; Cozzi et al., 2004; Frimmeland Fölling, 2004; Halverson et al., 2005). In the Paraguay BeltC, O, and Sr isotope data have been determined in three sectionsfrom the inner shelf to the foreslope (Alvarenga et al., 2004). Inthis paper we present new C isotope and Sr isotope data from thelower formations of the Araras Group (cap dolomite and GuiaFormation) and contribute to the isotope stratigraphic correlationbetween shallow-inner shelf facies on the border of a basin anddeep facies on the outer-shelf domain.

esearch. Published by Elsevier B.V. All rights reserved.

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2. Geological setting

The Neoproterozoic Paraguay Belt is exposed along thesoutheastern border of the Amazon Craton and comprises a thicksuccession of glaciomarine turbidite, carbonate and siliciclasticsedimentary rocks that were deposited in a passive marginenvironment (Alvarenga and Saes, 1992; de Alvarenga andTrompette, 1992, 1993; Alvarenga et al., 2000). The thickness ofthe sedimentary rocks increases from the border of the craton(approximately 200 m thick) to the central part of the basin (morethan 3000 m thick). In the western cratonic area, the sedimentary

Fig. 1. Geological map of the Brasiliano Paraguay fold belt and Chiquitos–Tucavac1984, Litherland et al. (1986), Trompette (1994) and Trompette et al. (1998). A, Tercfrom João Santos Borehole at Bauxi; D, Camil-Emal Quarry at Cáceres; E, Cimento Tlocated here (A, B, C, D, E, F) are illustrated in Fig. 6.

rocks are sub-horizontal, in the central part of the belt they arefolded, whereas in the eastern part they are folded and meta-morphosed (Fig. 1). The extent of deformation andmetamorphismincreases from the western cratonic area to a low metamorphic-grade in the inner parts of the Paraguay Belt. The tectono-thermaloverprint was caused by the younger Brasiliano/Pan-AfricanOrogeny in late Neoproterozoic–early Cambrian times (Trompette,1994; Pimentel et al., 1996; Trompette, 1997; Alvarenga et al.,2000). The tectono-metamorphic event was followed by post-orogenic sub-alkaline granite magmatism at ca. 500 Ma (Almeidaand Mantovani, 1975).

a aulacogen at the southeastern part of the Amazon Craton. Based on Almeida,oni Quarry at Mirasol d'Oeste; B, Tangará Quarry at Tangará da Serra; C, Drillocantins at Nobres; F, Nossa Senhora da Guia Quarry at Cuiabá. Studied sections

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The Paraguay Belt stratigraphy was established on the basisof detailed sections across the southwestern border of theAmazon Craton (Alvarenga and Saes, 1992; Alvarenga andTrompette, 1992, 1993; Trompette et al., 1998; Alvarenga et al.,2004), where the widespread and continuous sequences areexposed across long distances (more than 1500 km). Thenorthern Paraguay Belt comprises, from bottom to top, fourmajor lithostratigraphic sequences that exhibit lateral faciesvariations, mainly between the western border and the centralpart of the basin (Fig. 2). The older lithostratigraphic unitsrepresent a passive margin and the younger ones a foreland basindevelopment that is related to the Brasiliano–Pan-Africanorogeny (Almeida, 1974).

The basal glacially-influenced sequence occurs in the cratoniccover (Puga Formation) aswell as within themetasedimentary foldbelt (Cuiabá Group; Alvarenga and Trompette, 1992; Alvarengaet al., 2004; Fig. 2). A correlation with the global Marinoanglaciation, dated at approximately 635Ma (Hoffmann et al., 2004;Condon et al., 2005), has been suggested for this glaciallyinfluenced sequence in the Paraguay Belt (Nogueira et al., 2003;Alvarenga et al., 2004; Allen and Hoffman, 2005). The PugaFormation consists of diamictite associated with conglomerate,sandstone, siltstone and shale. The widespread occurrence ofdiamictite indicates lateral transition from thin coarse-grained bedsclose to the Amazon Craton (Puga Formation), to thick fine-grained facies in the east (Cuiabá Group). The sedimentationmodel proposed for the glacially influenced unit involves threemain glacial depositional systems: platformal, slope and outerslope (Alvarenga and Trompette, 1992). The platformal deposi-tional systems, which cover the cratonic domain on the westerninner shelf, were reworked by gravity-flows on the outer shelf(Fig. 2). The deposits on the inner shelf show alternation ofdominant massive diamictite, sandstones and fine-grained sedi-ments with few dropstones. In the outer shelf, an association ofmassive diamictite, stratified diamictite and fine-grained sedimentsprogressively replaces the massive diamictite. Glaciomarinesediments reworked by gravity flows and related to submarinefan deposits are associated with the slope depositional system.Diamictite, conglomerate and sandstone intercalations withoccasional inverse and/or normal grading occur in the deeperparts of the fan. Sandstone and siltstone intercalations representinter-channel deposits formed by turbidity currents. Deposition onthe outer slope system was dominated by fine-grained sediments(phyllite and meta-siltstone) related to low-density turbiditycurrents, in which the decrease of the glacial influence is indicatedby the presence of a few isolated clasts or dropstones (Fig. 2). Thismodel of basin filling suggests that the Amazon Craton was themain source area of the sediments, which were later reworkedduring the glacial event, thus forming submarine channels andturbidites on the slope and outer slope (Alvarenga and Trompette,1992). Paleomagnetic studies on the cap carbonate covering thePuga Formation on the Amazon Craton indicate glacial sedimen-tation at low latitudes (Trindade et al., 2003).

The Araras Group (carbonate sequence) on the southeasternborder of Amazon Craton overlies the Puga Formation andreaches a thickness of about 100 to 150 m on the western border(inner shelf) of the basin. Towards the east, this sequence gives

way to a 1300 m thick carbonate sequence on the middle shelfdomain, and continuing to deeper sequences occur mud-richlimestone and laminated meta-siltstone successions towards theslope depositional system of the basin (Fig. 2).

The Serra Azul Formation includes discontinuous outcrops ofdiamictite and siltstone above post-Marinoan carbonates of theAraras Group (Fig. 2), and represents the record of a secondglaciation in the Paraguay Belt (Figueiredo et al., 2004; Figueiredo,2006; Alvarenga et al., 2007). This diamictite is approximately70 m thick and is composed of massive diamictite with abundantclay-silty matrix, followed by a 200 m thick succession oflaminated siltstone. Evidence of a glacial setting in the Serra Azuldiamictite is provided by striated clasts, and this glacial unit hasbeen related to the Gaskiers glaciation (Alvarenga et al., 2007) withan age of ca. 580 Ma (Bowring et al., 2003; Knoll et al., 2004).

The upper siliciclastic unit known as the Alto Paraguay Group(Almeida, 1964) consists of two siliciclastic formations: the basalRaizama Formation, comprising cross-bedded sandstones (fine tovery coarse-grained sub-arkose), and the upper DiamantinoFormation, which consists of red shale, siltstone and arkose(Fig. 2).

3. Materials and methods

Three new sections were studied in the Paraguay belt in orderto detail isotopic data across the lower carbonate sequence ofthe Araras Group. One of the new sections is placed in the inner-shelf region of the basin (western cratonic sub-horizontal coverrocks) and has isotope data (Tangará Quarry) that correlate withthose of the cap dolomite of the Terconi Quarry (Nogueira et al.,2003; Alvarenga et al., 2004; Allen and Hoffman, 2005).

The other sections are located in the middle-shelf regionwhere the cratonic cover is folded. The thinner section wasintersected by drill core that crosscuts the contact between capdolomite and glacial diamictite at Bauxi (Grupo João Santosborehole, BX-10-120/8). The thicker section includes almost300 m of the Guia Formation as well as the lower part of theNobres Formation (Fig. 2).

Thirty six bulk samples of carbonate were analyzed for C andO isotopes and 12 were analyzed for 87Sr/86Sr ratios (Table 1).Previously published data of three sections from Terconi, Cáceres(Camil-Emal) and Guia quarries (Alvarenga et al., 2004) areincluded to complete the stratigraphic and isotopic correlation forthe Araras Group. Prior to analysis each rock specimen wasinvestigated under a petrographic microscope in order to avoidfractures, veins and heavily recrystallized micro-samples.

Carbon and oxygen isotope ratios were obtained on a SIRA IItriple collector, dual inlet, VG Isotech mass spectrometer at theNEG-LABISE, Department of Geology, University of Pernam-buco, Brazil. Powdered carbonate samples were reacted individ-ually with 100% H3PO4 at 25 °C for at least 12 hours for calciteand for over 3 days for dolomite (McCrea, 1950). The standarderror of isotope measurements was 0.2‰ during the period ofanalysis. All C and O isotope ratios are reported relative to thePDB standard.

For the 87Sr/86Sr analysis, 50 mg of carbonate powdersamples were weighed into Teflon beakers and digested in weak

Fig. 2. Schematic stratigraphic cross section along the southeaster edge of the Amazon Craton including neoproterozoic depositional sequences on western platform and its eastern foreslope. Studied sections located here(A, B, C, D, E, F) are illustrated in Fig. 6.

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Table 1Carbon, O, and Sr isotope ratios as well as major element concentrations of carbonates from the Tangará Quarry at Tangará da Serra, João Santos Borehole at Bauxi,and Cimento Tocantins at Nobres

Sample Lithology Height (m) SiO2 (%) Al2O3 (%) δ13CPDB δ18OPDB Mg/Ca Mn (ppm) Sr (ppm) Mn/Sr 87Sr/86Sr

Tangará QuarryTG-24 Dolostone 0 7.94 2.51 −7.5 −2.54 0.72 1084 44 24.64TG-23 Dolostone 10 3.11 1.30 −7.5 −1.3 0.74 828 44 18.82 0.71126TG-21 Dolostone 18 4.25 1.21 −6.5 −1.91 0.73 803 41 19.59TG-19 Dolostone 26 7.40 2.01 −6.4 −1.98 0.74 1423 27 52.70TG-25 Limestone 36 5.76 1.52 −4.9 −6.8 0.01 153 771 0.20 0.70740TG-26 Limestone 43.5 7.33 1.95 −5.1 −6.6 0.04 286 318 0.90TG-28 Limestone 55 7.54 1.77 −5.4 −6.4 0.01 151 191 0.79

Cimento Tocantins NobresNB-24 Limestone 66.5 0.34 0.11 0.1 −8.4 0.01 54 1016 0.05 0.70767NB-02 Limestone 93.5 14.64 2.42 −0.7 −7.7 0.12 141 717 0.20 ndNB-04 Limestone 120.5 7.18 0.98 −0.1 −7.7 0.05 55 1010 0.05 ndNB-06 Limestone 147.5 10.40 1.18 −0.5 −8.5 0.11 96 384 0.25 ndNB-08 Limestone 174.5 17.89 2.99 −0.9 −8.1 0.13 131 494 0.26 ndNB-10 Limestone 201.5 15.09 2.30 −0.9 −8.4 0.04 138 858 0.16 0.70776NB-11A Limestone 215 29.90 7.60 −1.0 −7.9 0.22 270 599 0.45 ndNB-12 Limestone 228 7.33 1.11 −0.7 −8.5 0.05 55 1010 0.05 0.70780NB-13A Mud-limestone 240 37.2 2.39 −1.6 −9.0 0.07 76 669 0.11 ndNB-14 Limestone 255 11.42 2.09 −0.4 −8.0 0.09 122 1075 0.11 0.70775NB-16 Limestone 282 7.37 1.04 −0.6 −8.3 0.11 163 2244 0.07 ndNB-18 Limestone 309 nd nd −1.0 −9.8 nd nd nd nd ndNB-19 Lime-dolostone 322.5 19.09 1.19 −0.9 −6.4 0.61 52 110 0.47 0.70871NB-20 Dolostone 335 4.64 0.81 −0.8 −11.4 0.70 51 31 1.64 ndNB-21B Dolostone 347.5 1.65 0.05 −0.7 −5.3 0.69 29 43 0.67 ndNB-21A Dolostone 350 5.21 0.79 −0.7 −5.1 0.59 47 113 0.42 0.70871NB-22 Dolostone 363 0.12 0.03 −0.4 −5.1 0.72 59 30 1.97 nd

J. Santos Borehole, BauxiBX-10-120/8-1 Dolostone 2 4.17 0.77 −1.7 −5.1 0.61 13229 68 194.54 0.71381BX-10-120/8-3 Dolostone 3 4.10 0.63 −4.8 −6.1 0.63 7462 112 66.62 ndBX-10-120/8-5 Dolostone 5 0.56 0.18 −3.9 −6.5 0.63 5496 96 57.25 0.71435BX-10-120/8-9 Dolostone 9 1.94 0.55 −3.7 −6.1 0.58 9787 266 36.79 ndBX-10-120/8-12 Dolostone 12 4.70 0.95 −4.8 −8.0 0.61 7891 111 71.09 ndBX-10-120/8-14 Dolostone 14 3.20 1.20 −4.2 −6.6 0.55 3049 162 18.82 ndBX-10-120/8-16 Dolostone 16 5.98 1.1 −3.9 −6.9 0.64 2698 86 3137 0.71153BX-10-124/4-1 Limestone 20 8.83 1.55 −3.8 −6.3 0.04 214 282 0.76 ndBX-10-124/4-6 Limestone 25 6.13 0.80 −2.3 −8.2 0.02 68 4351 0.01 ndBX-10-124/4-11 Limestone 30 10.97 1.13 −1.9 −7.8 0.02 60 4326 0.01 0.70763BX-10-124/4-16 Limestone 37 14.50 2.31 −1.9 −7.7 0.13 160 469 0.34 ndBX-10-124/4-21 Limestone 42 0.59 0.13 −1.1 −7.5 0.01 141 336 0.42 nd

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acetic acid to dissolve only the carbonate fraction and avoidleaching of radiogenic 87Sr and Rb from the non-carbonateconstituents of the samples. Chemical procedures described byDerry et al. (1989), Asmeron et al. (1991), Kaufman et al.(1993) were used. 87Sr/86Sr ratios were determined using aFinningan MAT 262 thermal ionization mass spectrometer instatic mode at the geochronology laboratory of the University ofBrasilia. Analyses of NBS 987 standard carried out during thecourse of this work yielded an average value of 0.710230±8(1σ). Uncertainties in individual analyses are better than 0.01%(2σ).

Samples used for chemical analysis were initially dried at110 °C in order to eliminate excess humidity and then heated to1000 °C for 2 hours in order to determine percentage loss onignition values. Determinations of minor and major elementswere performed using a Rigaku model RIX 3000 XRF X-ray

fluorescence unit equipped with an Rh tube at the NEG-LABISE, Department of Geology, University of Pernambuco.

4. Sedimentology of the Araras Group

The Araras Group can be subdivided into three main units: thelower Mirassol d'Oeste Formation, the cap dolomite to the PugaFormation (Nogueira et al., 2003); the middle Guia Formationconsists of laminated limestone, mud-limestone and mudstones;and the upper Nobres Formation, comprising shallow-waterdolostone (Almeida, 1964; Alvarenga et al., 2004; Boggiani andAlvarenga, 2004). Sedimentary rocks of these three units reflectshallow facies in the west and deep-water facies in the east. Forthis study we sampled sedimentary rocks from different parts ofthe carbonate platform including one new section in the inner-shelf basin on cratonic sub-horizontal cover rocks and two

Fig. 3. Sharp contact between Puga diamictite and cap dolomite from TerconiQuarry at Mirassol d'Oeste. Hammer for scale is 32 cm long.

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sections in the middle-shelf basin on folded cratonic cover rocks(Figs. 1 and 2).

4.1. Inner-shelf (Western sections)

In the western part of the basin two sections (Tangará andTerconi) were studied from the lower part of the Araras Group(Fig. 2). The cap dolomite of the Araras Group is in sharp contactwith glacial diamictites of the Puga Formation. This contact hasbeen plastically deformed (Fig. 3) as result of a loading during thevery fast transition between icehouse and greenhouse conditions(Nogueira et al., 2003). All rocks in the Terconi Quarry, atMirassol d'Oeste include a post depositional bitumen occurrence,filling a diagenetic porosity (Alvarenga et al., 2004; Faulstich,2005).

4.1.1. Cap dolomite (Mirassol d'Oeste formation)The cap carbonate basal lithofacies association is 20 to 32 m

thick and is found in outcrops in two sections (Terconi and Tangará

Fig. 4. Thin section photomicrographs of dolomite columnar crystals, plane light, infrom Faulstich, 2005).

Quarries) that are about 100 km apart from each other. In theTerconi Quarry, where the underlying Puga Formation is wellexposed, the first few metres of cap dolomite consist of laminatedpinkish dolostone that grades upward along a diffuse andtransitional contact to a grey laminated dolostone. These dolomitesshow variable degree of recrystallization from microcrystallinefabric-preserve to a completely fabric-destructive dolomite mosaictexture with 10 to 200 μm dolomite crystal size (Alvarenga et al.,2004). The latter is more evident in the upper part of the capdolomite in which sphalerite and fan-like dolomite crystalsinterpreted as aragonite pseudomorphs are commonly found(Fig. 4; Faulstich, 2005). Primary sedimentary structures includestromatolites, breccias and giant wave ripples (Allen and Hoffman,2005), alternatively interpreted as tepee-like structures (Nogueiraet al., 2003), are typically founded in this cap dolomite (Fig. 5). Thecap dolomite at the Tangará Quarry consists of 32 m of amonotonous laminated pink dolostone sequence.

4.1.2. Guia formationRocks of this unit represent a transgressive sequence of

limestone that begins with a grey laminated limestone lithofacies,about 8 m in thickness, that grades upward to an intercalation ofhomogeneous and laminated fine-grained limestone and laminat-ed mudstone (Alvarenga et al., 2004). Fan-like calcite crystalsinterpreted as aragonite are commonly found in both sections(Terconi and Tangará Quarries; Fig. 4). Microprobe analysisperformed on the fan-like crystals from the Terconi Quarryindicates the presence of both dolomite and calcite (Faulstich,2005).

4.2. Middle shelf basin (Província Serrana hills)

The Araras Group in Provincia Serrana consists of 1300 mthick, unmetamorphosed folded rocks conformably overlyingdiamictite of the Puga Formation (Luz and Abreu Filho, 1978;Alvarenga et al., 2004). While the lower 200 to 300 m of the

terpreted as aragonite pseudomorph. Cap dolomite from Terconi Quarry. (Photo

Fig. 5. Enigmatic sedimentary structure described at the Cap Dolomite fromTerconi Quarry. It was interpreted as giant wave ripples by Allen and Hoffman(2005) and was also described as tepee-like structures by Nogueira et al. (2003).

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sequence are made up of limestone (Guia Formation), theremaining 1000 to 1100 m are mainly dolomitic (Nobres For-mation). A cap dolomite has been described in a drill core (BX-10-120/8) with 18 m thick white dolostone at the base of theGuia Formation, overlying in sharp contact the diamictite of thePuga Formation. This sequence is correlated with the cap do-lomite from Mirassol d'Oeste Formation. Two sections throughthe base of the Araras Group were studied and sampled in Bauxiand in Nobres (Fig. 2).

The lower contact of the cap dolomite is rarely exposed andwas only found in a borehole in theBauxi area (Fig. 6C). The 18mthick overlying the diamictite is a laminated white dolostone withmicrocrystalline peloids, 0.05 to 0.5 mm in diameter. Thislaminated dolostone consists of macro-peloid layers (N0.2 mm)intercalatedwith wispy layers of micrite. This dolomite is overlainby a thick sequence (200–300 m) of dark-grey, laminatedlimestone intercalated with mudstone layers. The middle toupper part of this unit has 1 to 5 cm thick grainstone layers. Thinsections studied show layers of micrite and peloid with sparse silt-size detrital grains (quartz and feldspar), and local concentration ofquartz-silt wispy layers.

4.3. Outer shelf — Foreslope basin (folded and metamor-phosed rocks)

The carbonate sequence (Araras Group) in the outer shelfforeslope has been found at Nossa Senhora da Guia Syncline,30 km NW of Cuiabá (Figs. 1 and 2), overlying glacial massivediamictite. This carbonate sequence was affected by low-grademetamorphism, and is composed predominantly of limestonewith intercalations of black–grey siltstone of the Guia Formationwith the final 20 m consisting of light grey oolitic and intraclasticdolostone (Alvarenga et al., 2004). In the outer shelf foreslope,this formation has alternation of packstones-grainstones withfrequent hummocky cross stratification that suggests movementby storm waves.

5. Chemostratigraphy

The chemical composition of the samples (Table 1) was usedmainly to evaluate the post-depositional alteration based on theirMn/Sr ratios and Sr content. The degree to which the carbonaterock samples were affected by post-depositional modifications isparticularly important to evaluate if the primary depositional valueswere preserved. The isotopic and chemical data were used toproduce composite isotopic curves for the Araras Group carbonatesthat may be used for regional isotopic correlation (Fig. 6).

5.1. Carbon and oxygen isotopes

Carbon andOxygen isotope ratios are shown across six differentsections of the lower part of the Araras Group. In the inner shelf(western border of the basin), δ13C ratios vary between −10.5 and−4.1‰ in cap dolomite, and show a slight rise to values between−5.4 and −2.7‰ towards the laminated limestone and mudlimestone at the top of this section (Table 1, Fig. 6A, B). The δ18Oratios range from −6.0 to −1.3‰ in two sections of this inner-shelfcap dolomite (Tab. 1, Fig. 6A, B). Middle-shelf cap dolomiteimmediately above the Puga Formation diamictite, found only inthe samples of drill cores from the João Santos Borehole, have δ13Cbetween −4.8 and −1.7‰, whereby the highest value was foundnext to the contact with the diamictite (Table 1, Fig. 6C). The δ18Ovalues in cap dolomite range between −8.2 and −5.1‰ (Table 1,Fig. 6C).Middle-shelf limestone andmud-limestone unit above thecap dolomite starts with increasing δ13C values for the first 20 m(Table 1, Fig. 6C) and towards the top shows a narrow rangebetween −1.6 and +0.1‰ (Table 1, Fig. 6E), whereas δ18O rangesfrom −9.8 to −6.3‰. The firsts dolostones of the NobresFormation above the Guia Formation display a narrow range ofδ13C values from −1.0 to −0.4‰ (Fig. 6D, E), whereas δ18Ovalues abruptly change from −11.4 to −0.8‰ (Fig. 6D,E).

5.2. Strontium isotopes

The 87Sr/86Sr ratios in carbonates above the glacial PugaFormation show a similar pattern in all sections. While isotopicratios for the cap dolomite range between 0.70848 and 0.71435,isotopic ratios for the limestone and mud-limestone (9 samples)above the cap dolomite are between 0.70763 and 0.70780 (Table 1).

The high and variable 87Sr/86Sr ratios of the cap dolomite areassociatedwith lower Sr content; 27 to 74 ppm at inner-shelf basinand 68 to 266 ppm atmiddle-shelf basin. In both parts of the basinthese dolostones have high Mn/Sr (between 7.3 and 71.1 with anextreme of 194.5). This is indicative of diagenetic fluids thattransported continentally derived Sr from meteoric water. Thelimestone and mud-limestone above the cap dolomite with highSr concentration (750–4351 ppm) and a low Mn/Sr ratio (0.01–0.20) has 87Sr/86Sr ratios that are close to the original Sr isotopiccomposition of the inferred aragonite precursor (Fig. 6).

6. Discussion

The correlation scheme presented here suggests that paleogeo-graphical differences may explain variations of carbon and oxygen

Fig. 6. Measured stratigraphic sections across the platform (inner-shelf to outer-shelf) and slope and variations of δ13CPDB, δ18OPDB and 87Sr/86Sr of Cap carbonate and Guia Formation. Locations sections: A, Terconi

Quarry at Mirasol d'Oeste (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from Alvarenga et al., 2004); B, Tangará Quarry at Tangará da Serra (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from this paper); C, Drill from JoãoSantos Borehole at Bauxi (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from this paper); D, Camil-Emal Quarry at Cáceres (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from Alvarenga et al., 2004); E, Cimento Tocantinsat Nobres (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from this paper); F, Nossa Senhora da Guia Quarry at Cuiabá (δ13C, δ18O, 87Sr/86Sr data and stratigraphy from Alvarenga et al., 2004).

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isotopes between inner and outer shelf— foreslope basin (Fig. 6).Samples from the lower part of the sections present more negativeand variable δ13C values than samples from the upper part of thesections. The cap dolomite shows two δ13C trends: very low ratiosat the inner-shelf from−10.5 to−4.1‰, and slightly higher ratios atthe middle-shelf (from −4.8 to −1.7‰). The δ13C ratios in thelimestone sequence above the cap dolomite, considered to beprimary, show also two trends: at the inner shelf with the ratio islower, between −5.4 and −2.7‰, whereas higher ratios, −2.7 to+0.1‰, were recorded from the middle-shelf. Only very low δ13Cvalues (−10.5 and −9.6‰) obtained for the Terconi Quarry capdolomite are most likely not primary but related to a fabricdestructive feature and post-diagenetic fluids with relative highcarbon content (Alvarenga et al., 2004). Moreover, other variationsare also observed: dolomite-bearing rocks present higher oxygenisotopes values compared to calcite-bearing rocks (Tab. 1, Fig. 6B)and more variable isotopic values (C and O) are observed near thecontact between the carbonate rocks and the diamictites.

The carbon isotopic values are mostly negative across thestudied sections, although more negative values are commonlyfound in the inner shelf samples. We argue that these rocks wereformed under a stronger influence of the continental margin. Asimilar feature was observed across sections of the TsumebSubgroup and the Karibib Formation in Namibia (Halverson et al.,2005), where isotopic differenceswere also observed fromplatformto foreslope.

Fig. 7. Cross-plots of δ13C vs. δ18O of all studied samples in this work and fromAlvarenga et al., 2004.

The δ13C versus δ18O cross-plot of all samples studied (Fig. 7)shows that inner-shelf samples record enrichment in δ18O from−7.9‰ to −1.3‰ compared to mid-shelf (from −11.4 to −5.1‰),outer-shelf and fore slope (from −14.9 to −7.1‰). The O isotopeenrichment for the inner shelf at the margin of the basin can berelated to the restriction and evaporation from shallow expansiveenvironments. A similar pattern of oxygen isotopic variation hasbeen suggested for dolomitic rocks of the Society Cliffs Formation(Kah, 2000).

The 87Sr/86Sr data show variations with rock composition. Thecap dolomite has highly radiogenic 87Sr/86Sr ratios in excess of0.71, which is interpreted as a post-depositional effect. The87Sr/86Sr ratios (0.70740–0.70780) from the limestone above thecap dolomite with low Mn/Sr ratios and high Sr content (up to4350 ppm) suggest an aragonitic limestone precursor that can beinterpreted as reflecting the primary seawater isotopic values.Based on Neoproterozoic seawater 87Sr/86Sr data (Jacobsen andKaufman, 1999; Shields, 1999;Walter et al., 2000;Melezhik et al.,2001; Thomas et al., 2004), carbonates of the Nobres Formationmaybe correlated with other carbonates elsewhere that weredeposited at around 600 Ma.

7. Conclusions

Correlations of inner-shelf and middle-shelf facies can bepossible with more accuracy with the help of C and O isotopestratigraphy. An important time line is the contact between the capdolomite and glacial diamictite, which has been interpreted as atransgressive system tract at the end of a glacial period, with amaximum flooding surface at the clay-limestone interface abovethe cap dolomite. A comparison of C isotope values betweensections from the inner shelf to the outer shelf along the basinshows different absolute isotopic values but comparable isotopictrends. The carbonates from the inner-shelf section are depleted in13C compared to those from the more distal platform, probablydue to a shallower environment and to a stronger influence of thecontinental margin.

The 87Sr/86Sr ratios follow a similar trend in all sections,suggesting that these sections might be coeval. The Sr isotopiccomposition of cap dolomite samples is highly radiogenic in allthe sections, indicating that the primary isotopic values weremodified by post-depositional diagenetic alteration. All samplesplaced above the cap dolomite have calcite as the main carbonate,high Sr concentration and 87Sr/86Sr ratios (0.70740–0.70780)that are near-primary. These 87Sr/86Sr values characterize the capcarbonate overlying Marinoan-age glacial rocks at about 600 Ma(Jacobsen and Kaufman, 1999; Shields, 1999; Walter et al., 2000;Melezhik et al., 2001) confirming this age for the Puga Formationin the Paraguay Belt as suggested previously (Alvarenga andTrompette, 1992; Nogueira et al., 2003; Alvarenga et al., 2004;Allen and Hoffman, 2005).

Acknowledgements

Field work has been supported by CNPq (Conselho Nacionalde Desenvolvimento Cientifico e Tecnológico) grants (Projects:461482/2001-0 and 550860/2002-9). We thank Francisco E. C.

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Pinho, E. Marechal Tagliarini and Fanio T. Guimarães forassistance in the field. The text benefited greatly from reviewsby Galen P. Halverson, Paul Knauth and Hartwig Frimmel. Thisis a contribution to the International Geological CorrelationProgramme (IGCP) Project 478 “Neoproterozoic–Early Paleo-zoic Events in SW-Gondwana”.

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