The Neoproterozoic carbonate sequence on the southeastern border of the Amazon Craton is divided into three lithostratigraphic units: a basal cap dolomite, an intermediate limestone, limestone-mudstone unit, and an upper dolarenite-dolorudite unit. Sections of the cap-carbonate were measured from the inner shelf to the outer shelf. Carbon isotope ratios (relative to PDB) vary between − 10.5 and − 1.7‰ in cap dolomite, and between − 5.4 and +0.1‰ in laminated limestone and mud-limestone. Limestones and mud-limestones exhibit 87Sr/86Sr ratios ranging from 0.70740 to 0.70780. A comparative isotope stratigraphy between the inner-shelf and the middle-shelf basin shows differences in carbon isotope ratios: 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. These lower values can be related to shallower environmental conditions and to a stronger influence of the continental border. The 87Sr/86Sr ratios are the same in both areas, and are consistent with seawater composition at around 600 Ma.

1. Isotope stratigraphy of Neoproterozoic cap carbonates in the
Araras 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, Brazil
b
Departamento de Geologia, Universidade Federal de Pernambuco, Caixa Postal: 7852, CEP: 50732-970, Recife, PE, Brazil
Received 22 September 2005; accepted 7 May 2007
Available online 23 May 2007
Abstract
The Neoproterozoic carbonate sequence on the southeastern border of the Amazon Craton is divided into three lithostratigraphic units: a basal
cap dolomite, an intermediate limestone, limestone-mudstone unit, and an upper dolarenite-dolorudite unit. Sections of the cap-carbonate were
measured from the inner shelf to the outer shelf. Carbon isotope ratios (relative to PDB) vary between −10.5 and −1.7‰ in cap dolomite, and
between −5.4 and +0.1‰ in laminated limestone and mud-limestone. Limestones and mud-limestones exhibit 87
Sr/86
Sr ratios ranging from
0.70740 to 0.70780. A comparative isotope stratigraphy between the inner-shelf and the middle-shelf basin shows differences in carbon isotope
ratios: The cap dolomite and limestones have lower δ13
C ratios on the border of the basin (inner shelf) than in the middle shelf of the basin. These
lower values can be related to shallower environmental conditions and to a stronger influence of the continental border. The 87
Sr/86
Sr ratios are the
same 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 common
in Neoproterozoic sequences. As these successions are normally
unfossiliferous and poorly dated, carbon and strontium isotope
data have become powerful tools for correlation within a
sedimentary basin or even at a larger, global scale (Hoffman
et al., 1998; Kennedy et al., 1998, 2001; Melezhik et al., 2001;
Hoffman and Schrag, 2002; Melezhik et al., 2005). Carbon
isotope ratios of worldwide Neoproterozoic sequences placed
just at the end of the glacial event range from highly 13
C-
enriched to 13
C-depleted values (Kaufman et al., 1997;
Kennedy et al., 1998; Hoffman et al., 1998). The Sr isotope
data are interpreted to reflect 87
Sr/86
Sr variation of seawater for
the different carbonate sequences in the Neoproterozoic. Some
workers have shown that the lowest 87
Sr/86
Sr 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 and
Shields, 2000; Melezhik et al., 2001; Thomas et al., 2004).
Isotopic chemostratigraphic data in Neoproterozoic carbonate
successions in which shallow-water facies changes to deeper
facies have been considered in a number of papers (Hoffman
et al., 1998; Alvarenga et al., 2004; Cozzi et al., 2004; Frimmel
and Fölling, 2004; Halverson et al., 2005). In the Paraguay Belt
C, O, and Sr isotope data have been determined in three sections
from the inner shelf to the foreslope (Alvarenga et al., 2004). In
this paper we present new C isotope and Sr isotope data from the
lower formations of the Araras Group (cap dolomite and Guia
Formation) and contribute to the isotope stratigraphic correlation
between shallow-inner shelf facies on the border of a basin and
deep facies on the outer-shelf domain.
Available online at www.sciencedirect.com
Gondwana Research 13 (2008) 469–479
www.elsevier.com/locate/gr
⁎ 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 Research. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.gr.2007.05.004

2. 2. Geological setting
The Neoproterozoic Paraguay Belt is exposed along the
southeastern border of the Amazon Craton and comprises a thick
succession of glaciomarine turbidite, carbonate and siliciclastic
sedimentary rocks that were deposited in a passive margin
environment (Alvarenga and Saes, 1992; de Alvarenga and
Trompette, 1992, 1993; Alvarenga et al., 2000). The thickness of
the sedimentary rocks increases from the border of the craton
(approximately 200 m thick) to the central part of the basin (more
than 3000 m thick). In the western cratonic area, the sedimentary
rocks are sub-horizontal, in the central part of the belt they are
folded, whereas in the eastern part they are folded and meta-
morphosed (Fig. 1). The extent of deformation and metamorphism
increases from the western cratonic area to a low metamorphic-
grade in the inner parts of the Paraguay Belt. The tectono-thermal
overprint was caused by the younger Brasiliano/Pan-African
Orogeny 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 (Almeida
and Mantovani, 1975).
Fig. 1. Geological map of the Brasiliano Paraguay fold belt and Chiquitos–Tucavaca aulacogen at the southeastern part of the Amazon Craton. Based on Almeida,
1984, Litherland et al. (1986), Trompette (1994) and Trompette et al. (1998). A, Terconi Quarry at Mirasol d'Oeste; B, Tangará Quarry at Tangará da Serra; C, Drill
from João Santos Borehole at Bauxi; D, Camil-Emal Quarry at Cáceres; E, Cimento Tocantins at Nobres; F, Nossa Senhora da Guia Quarry at Cuiabá. Studied sections
located here (A, B, C, D, E, F) are illustrated in Fig. 6.
470 C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

3. The Paraguay Belt stratigraphy was established on the basis
of detailed sections across the southwestern border of the
Amazon Craton (Alvarenga and Saes, 1992; Alvarenga and
Trompette, 1992, 1993; Trompette et al., 1998; Alvarenga et al.,
2004), where the widespread and continuous sequences are
exposed across long distances (more than 1500 km). The
northern Paraguay Belt comprises, from bottom to top, four
major lithostratigraphic sequences that exhibit lateral facies
variations, mainly between the western border and the central
part of the basin (Fig. 2). The older lithostratigraphic units
represent a passive margin and the younger ones a foreland basin
development that is related to the Brasiliano–Pan-African
orogeny (Almeida, 1974).
The basal glacially-influenced sequence occurs in the cratonic
cover (Puga Formation) as well as within the metasedimentary fold
belt (Cuiabá Group; Alvarenga and Trompette, 1992; Alvarenga
et al., 2004; Fig. 2). A correlation with the global Marinoan
glaciation, dated at approximately 635 Ma (Hoffmann et al., 2004;
Condon et al., 2005), has been suggested for this glacially
influenced sequence in the Paraguay Belt (Nogueira et al., 2003;
Alvarenga et al., 2004; Allen and Hoffman, 2005). The Puga
Formation consists of diamictite associated with conglomerate,
sandstone, siltstone and shale. The widespread occurrence of
diamictite indicates lateral transition from thin coarse-grained beds
close to the Amazon Craton (Puga Formation), to thick fine-
grained facies in the east (Cuiabá Group). The sedimentation
model proposed for the glacially influenced unit involves three
main glacial depositional systems: platformal, slope and outer
slope (Alvarenga and Trompette, 1992). The platformal deposi-
tional systems, which cover the cratonic domain on the western
inner shelf, were reworked by gravity-flows on the outer shelf
(Fig. 2). The deposits on the inner shelf show alternation of
dominant massive diamictite, sandstones and fine-grained sedi-
ments with few dropstones. In the outer shelf, an association of
massive diamictite, stratified diamictite and fine-grainedsediments
progressively replaces the massive diamictite. Glaciomarine
sediments reworked by gravity flows and related to submarine
fan deposits are associated with the slope depositional system.
Diamictite, conglomerate and sandstone intercalations with
occasional inverse and/or normal grading occur in the deeper
parts of the fan. Sandstone and siltstone intercalations represent
inter-channel deposits formed by turbidity currents. Deposition on
the outer slope system was dominated by fine-grained sediments
(phyllite and meta-siltstone) related to low-density turbidity
currents, in which the decrease of the glacial influence is indicated
by the presence of a few isolated clasts or dropstones (Fig. 2). This
model of basin filling suggests that the Amazon Craton was the
main source area of the sediments, which were later reworked
during the glacial event, thus forming submarine channels and
turbidites on the slope and outer slope (Alvarenga and Trompette,
1992). Paleomagnetic studies on the cap carbonate covering the
Puga Formation on the Amazon Craton indicate glacial sedimen-
tation at low latitudes (Trindade et al., 2003).
The Araras Group (carbonate sequence) on the southeastern
border of Amazon Craton overlies the Puga Formation and
reaches 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 shelf
domain, and continuing to deeper sequences occur mud-rich
limestone and laminated meta-siltstone successions towards the
slope depositional system of the basin (Fig. 2).
The Serra Azul Formation includes discontinuous outcrops of
diamictite and siltstone above post-Marinoan carbonates of the
Araras Group (Fig. 2), and represents the record of a second
glaciation in the Paraguay Belt (Figueiredo et al., 2004; Figueiredo,
2006; Alvarenga et al., 2007). This diamictite is approximately
70 m thick and is composed of massive diamictite with abundant
clay-silty matrix, followed by a 200 m thick succession of
laminated siltstone. Evidence of a glacial setting in the Serra Azul
diamictite is provided by striated clasts, and this glacial unit has
been related to the Gaskiers glaciation (Alvarenga et al., 2007) with
an 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 basal
Raizama Formation, comprising cross-bedded sandstones (fine to
very coarse-grained sub-arkose), and the upper Diamantino
Formation, which consists of red shale, siltstone and arkose
(Fig. 2).
3. Materials and methods
Three new sections were studied in the Paraguay belt in order
to detail isotopic data across the lower carbonate sequence of
the Araras Group. One of the new sections is placed in the inner-
shelf region of the basin (western cratonic sub-horizontal cover
rocks) and has isotope data (Tangará Quarry) that correlate with
those 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 region
where the cratonic cover is folded. The thinner section was
intersected by drill core that crosscuts the contact between cap
dolomite and glacial diamictite at Bauxi (Grupo João Santos
borehole, BX-10-120/8). The thicker section includes almost
300 m of the Guia Formation as well as the lower part of the
Nobres Formation (Fig. 2).
Thirty six bulk samples of carbonate were analyzed for C and
O isotopes and 12 were analyzed for 87
Sr/86
Sr ratios (Table 1).
Previously published data of three sections from Terconi, Cáceres
(Camil-Emal) and Guia quarries (Alvarenga et al., 2004) are
included to complete the stratigraphic and isotopic correlation for
the Araras Group. Prior to analysis each rock specimen was
investigated under a petrographic microscope in order to avoid
fractures, veins and heavily recrystallized micro-samples.
Carbon and oxygen isotope ratios were obtained on a SIRA II
triple collector, dual inlet, VG Isotech mass spectrometer at the
NEG-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 calcite
and for over 3 days for dolomite (McCrea, 1950). The standard
error of isotope measurements was 0.2‰ during the period of
analysis. All C and O isotope ratios are reported relative to the
PDB standard.
For the 87
Sr/86
Sr analysis, 50 mg of carbonate powder
samples were weighed into Teflon beakers and digested in weak
471C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

4. 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.
472C.J.S.deAlvarengaetal./GondwanaResearch13(2008)469–479

5. acetic acid to dissolve only the carbonate fraction and avoid
leaching of radiogenic 87
Sr and Rb from the non-carbonate
constituents of the samples. Chemical procedures described by
Derry et al. (1989), Asmeron et al. (1991), Kaufman et al.
(1993) were used. 87
Sr/86
Sr ratios were determined using a
Finningan MAT 262 thermal ionization mass spectrometer in
static mode at the geochronology laboratory of the University of
Brasilia. Analyses of NBS 987 standard carried out during the
course 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 at
110 °C in order to eliminate excess humidity and then heated to
1000 °C for 2 hours in order to determine percentage loss on
ignition values. Determinations of minor and major elements
were 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: the
lower Mirassol d'Oeste Formation, the cap dolomite to the Puga
Formation (Nogueira et al., 2003); the middle Guia Formation
consists of laminated limestone, mud-limestone and mudstones;
and the upper Nobres Formation, comprising shallow-water
dolostone (Almeida, 1964; Alvarenga et al., 2004; Boggiani and
Alvarenga, 2004). Sedimentary rocks of these three units reflect
shallow facies in the west and deep-water facies in the east. For
this study we sampled sedimentary rocks from different parts of
the carbonate platform including one new section in the inner-
shelf basin on cratonic sub-horizontal cover rocks and two
Table 1
Carbon, 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 (%) δ13
CPDB δ18
OPDB Mg/Ca Mn (ppm) Sr (ppm) Mn/Sr 87
Sr/86
Sr
Tangará Quarry
TG-24 Dolostone 0 7.94 2.51 −7.5 −2.54 0.72 1084 44 24.64
TG-23 Dolostone 10 3.11 1.30 −7.5 −1.3 0.74 828 44 18.82 0.71126
TG-21 Dolostone 18 4.25 1.21 −6.5 −1.91 0.73 803 41 19.59
TG-19 Dolostone 26 7.40 2.01 −6.4 −1.98 0.74 1423 27 52.70
TG-25 Limestone 36 5.76 1.52 −4.9 −6.8 0.01 153 771 0.20 0.70740
TG-26 Limestone 43.5 7.33 1.95 −5.1 −6.6 0.04 286 318 0.90
TG-28 Limestone 55 7.54 1.77 −5.4 −6.4 0.01 151 191 0.79
Cimento Tocantins Nobres
NB-24 Limestone 66.5 0.34 0.11 0.1 −8.4 0.01 54 1016 0.05 0.70767
NB-02 Limestone 93.5 14.64 2.42 −0.7 −7.7 0.12 141 717 0.20 nd
NB-04 Limestone 120.5 7.18 0.98 −0.1 −7.7 0.05 55 1010 0.05 nd
NB-06 Limestone 147.5 10.40 1.18 −0.5 −8.5 0.11 96 384 0.25 nd
NB-08 Limestone 174.5 17.89 2.99 −0.9 −8.1 0.13 131 494 0.26 nd
NB-10 Limestone 201.5 15.09 2.30 −0.9 −8.4 0.04 138 858 0.16 0.70776
NB-11A Limestone 215 29.90 7.60 −1.0 −7.9 0.22 270 599 0.45 nd
NB-12 Limestone 228 7.33 1.11 −0.7 −8.5 0.05 55 1010 0.05 0.70780
NB-13A Mud-limestone 240 37.2 2.39 −1.6 −9.0 0.07 76 669 0.11 nd
NB-14 Limestone 255 11.42 2.09 −0.4 −8.0 0.09 122 1075 0.11 0.70775
NB-16 Limestone 282 7.37 1.04 −0.6 −8.3 0.11 163 2244 0.07 nd
NB-18 Limestone 309 nd nd −1.0 −9.8 nd nd nd nd nd
NB-19 Lime-dolostone 322.5 19.09 1.19 −0.9 −6.4 0.61 52 110 0.47 0.70871
NB-20 Dolostone 335 4.64 0.81 −0.8 −11.4 0.70 51 31 1.64 nd
NB-21B Dolostone 347.5 1.65 0.05 −0.7 −5.3 0.69 29 43 0.67 nd
NB-21A Dolostone 350 5.21 0.79 −0.7 −5.1 0.59 47 113 0.42 0.70871
NB-22 Dolostone 363 0.12 0.03 −0.4 −5.1 0.72 59 30 1.97 nd
J. Santos Borehole, Bauxi
BX-10-120/8-1 Dolostone 2 4.17 0.77 −1.7 −5.1 0.61 13229 68 194.54 0.71381
BX-10-120/8-3 Dolostone 3 4.10 0.63 −4.8 −6.1 0.63 7462 112 66.62 nd
BX-10-120/8-5 Dolostone 5 0.56 0.18 −3.9 −6.5 0.63 5496 96 57.25 0.71435
BX-10-120/8-9 Dolostone 9 1.94 0.55 −3.7 −6.1 0.58 9787 266 36.79 nd
BX-10-120/8-12 Dolostone 12 4.70 0.95 −4.8 −8.0 0.61 7891 111 71.09 nd
BX-10-120/8-14 Dolostone 14 3.20 1.20 −4.2 −6.6 0.55 3049 162 18.82 nd
BX-10-120/8-16 Dolostone 16 5.98 1.1 −3.9 −6.9 0.64 2698 86 3137 0.71153
BX-10-124/4-1 Limestone 20 8.83 1.55 −3.8 −6.3 0.04 214 282 0.76 nd
BX-10-124/4-6 Limestone 25 6.13 0.80 −2.3 −8.2 0.02 68 4351 0.01 nd
BX-10-124/4-11 Limestone 30 10.97 1.13 −1.9 −7.8 0.02 60 4326 0.01 0.70763
BX-10-124/4-16 Limestone 37 14.50 2.31 −1.9 −7.7 0.13 160 469 0.34 nd
BX-10-124/4-21 Limestone 42 0.59 0.13 −1.1 −7.5 0.01 141 336 0.42 nd
473C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

6. 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 and
Terconi) were studied from the lower part of the Araras Group
(Fig. 2). The cap dolomite of the Araras Group is in sharp contact
with glacial diamictites of the Puga Formation. This contact has
been plastically deformed (Fig. 3) as result of a loading during the
very fast transition between icehouse and greenhouse conditions
(Nogueira et al., 2003). All rocks in the Terconi Quarry, at
Mirassol 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
Quarries) that are about 100 km apart from each other. In the
Terconi Quarry, where the underlying Puga Formation is well
exposed, the first few metres of cap dolomite consist of laminated
pinkish dolostone that grades upward along a diffuse and
transitional contact to a grey laminated dolostone. These dolomites
show variable degree of recrystallization from microcrystalline
fabric-preserve to a completely fabric-destructive dolomite mosaic
texture with 10 to 200 μm dolomite crystal size (Alvarenga et al.,
2004). The latter is more evident in the upper part of the cap
dolomite in which sphalerite and fan-like dolomite crystals
interpreted as aragonite pseudomorphs are commonly found
(Fig. 4; Faulstich, 2005). Primary sedimentary structures include
stromatolites, breccias and giant wave ripples (Allen and Hoffman,
2005), alternatively interpreted as tepee-like structures (Nogueira
et al., 2003), are typically founded in this cap dolomite (Fig. 5). The
cap dolomite at the Tangará Quarry consists of 32 m of a
monotonous laminated pink dolostone sequence.
4.1.2. Guia formation
Rocks 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 of
homogeneous and laminated fine-grained limestone and laminat-
ed mudstone (Alvarenga et al., 2004). Fan-like calcite crystals
interpreted as aragonite are commonly found in both sections
(Terconi and Tangará Quarries; Fig. 4). Microprobe analysis
performed on the fan-like crystals from the Terconi Quarry
indicates 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 m
thick, unmetamorphosed folded rocks conformably overlying
diamictite of the Puga Formation (Luz and Abreu Filho, 1978;
Alvarenga et al., 2004). While the lower 200 to 300 m of the
Fig. 3. Sharp contact between Puga diamictite and cap dolomite from Terconi
Quarry at Mirassol d'Oeste. Hammer for scale is 32 cm long.
Fig. 4. Thin section photomicrographs of dolomite columnar crystals, plane light, interpreted as aragonite pseudomorph. Cap dolomite from Terconi Quarry. (Photo
from Faulstich, 2005).
474 C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

7. sequence are made up of limestone (Guia Formation), the
remaining 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 the
Guia Formation, overlying in sharp contact the diamictite of the
Puga Formation. This sequence is correlated with the cap do-
lomite from Mirassol d'Oeste Formation. Two sections through
the base of the Araras Group were studied and sampled in Bauxi
and in Nobres (Fig. 2).
The lower contact of the cap dolomite is rarely exposed and
was only found in a borehole in the Bauxi area (Fig. 6C). The 18 m
thick overlying the diamictite is a laminated white dolostone with
microcrystalline peloids, 0.05 to 0.5 mm in diameter. This
laminated dolostone consists of macro-peloid layers (N0.2 mm)
intercalated with wispy layers of micrite. This dolomite is overlain
by a thick sequence (200–300 m) of dark-grey, laminated
limestone intercalated with mudstone layers. The middle to
upper part of this unit has 1 to 5 cm thick grainstone layers. Thin
sections studied show layers of micrite and peloid with sparse silt-
size detrital grains (quartz and feldspar), and local concentration of
quartz-silt wispy layers.
4.3. Outer shelf — Foreslope basin (folded and metamor-
phosed rocks)
The carbonate sequence (Araras Group) in the outer shelf
foreslope has been found at Nossa Senhora da Guia Syncline,
30 km NW of Cuiabá (Figs. 1 and 2), overlying glacial massive
diamictite. This carbonate sequence was affected by low-grade
metamorphism, and is composed predominantly of limestone
with intercalations of black–grey siltstone of the Guia Formation
with the final 20 m consisting of light grey oolitic and intraclastic
dolostone (Alvarenga et al., 2004). In the outer shelf foreslope,
this formation has alternation of packstones-grainstones with
frequent hummocky cross stratification that suggests movement
by storm waves.
5. Chemostratigraphy
The chemical composition of the samples (Table 1) was used
mainly to evaluate the post-depositional alteration based on their
Mn/Sr ratios and Sr content. The degree to which the carbonate
rock samples were affected by post-depositional modifications is
particularly important to evaluate if the primary depositional values
were preserved. The isotopic and chemical data were used to
produce composite isotopic curves for the Araras Group carbonates
that may be used for regional isotopic correlation (Fig. 6).
5.1. Carbon and oxygen isotopes
Carbon and Oxygen isotope ratios are shown across six different
sections of the lower part of the Araras Group. In the inner shelf
(western border of the basin), δ13
C 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 mud
limestone at the top of this section (Table 1, Fig. 6A, B). The δ18
O
ratios range from −6.0 to −1.3‰ in two sections of this inner-shelf
cap dolomite (Tab. 1, Fig. 6A, B). Middle-shelf cap dolomite
immediately above the Puga Formation diamictite, found only in
the samples of drill cores from the João Santos Borehole, have δ13
C
between −4.8 and −1.7‰, whereby the highest value was found
next to the contact with the diamictite (Table 1, Fig. 6C). The δ18
O
values in cap dolomite range between −8.2 and −5.1‰ (Table 1,
Fig. 6C). Middle-shelf limestone and mud-limestone unit above the
cap dolomite starts with increasing δ13
C values for the first 20 m
(Table 1, Fig. 6C) and towards the top shows a narrow range
between −1.6 and +0.1‰ (Table 1, Fig. 6E), whereas δ18
O ranges
from −9.8 to −6.3‰. The firsts dolostones of the Nobres
Formation above the Guia Formation display a narrow range of
δ13
C values from −1.0 to −0.4‰ (Fig. 6D, E), whereas δ18
O
values abruptly change from −11.4 to −0.8‰ (Fig. 6D,E).
5.2. Strontium isotopes
The 87
Sr/86
Sr ratios in carbonates above the glacial Puga
Formation show a similar pattern in all sections. While isotopic
ratios 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 87
Sr/86
Sr ratios of the cap dolomite are
associated with lower Sr content; 27 to 74 ppm at inner-shelf basin
and 68 to 266 ppm at middle-shelf basin. In both parts of the basin
these dolostones have high Mn/Sr (between 7.3 and 71.1 with an
extreme of 194.5). This is indicative of diagenetic fluids that
transported continentally derived Sr from meteoric water. The
limestone and mud-limestone above the cap dolomite with high
Sr concentration (750–4351 ppm) and a low Mn/Sr ratio (0.01–
0.20) has 87
Sr/86
Sr ratios that are close to the original Sr isotopic
composition 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. 5. Enigmatic sedimentary structure described at the Cap Dolomite from
Terconi 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).
475C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

8. Fig. 6. Measured stratigraphic sections across the platform (inner-shelf to outer-shelf) and slope and variations of δ13
CPDB, δ18
OPDB and 87
Sr/86
Sr of Cap carbonate and Guia Formation. Locations sections: A, Terconi
Quarry at Mirasol d'Oeste (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from Alvarenga et al., 2004); B, Tangará Quarry at Tangará da Serra (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from this paper); C, Drill from João
Santos Borehole at Bauxi (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from this paper); D, Camil-Emal Quarry at Cáceres (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from Alvarenga et al., 2004); E, Cimento Tocantins
at Nobres (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from this paper); F, Nossa Senhora da Guia Quarry at Cuiabá (δ13
C, δ18
O, 87
Sr/86
Sr data and stratigraphy from Alvarenga et al., 2004).
476C.J.S.deAlvarengaetal./GondwanaResearch13(2008)469–479

9. isotopes between inner and outer shelf — foreslope basin (Fig. 6).
Samples from the lower part of the sections present more negative
and variable δ13
C values than samples from the upper part of the
sections. The cap dolomite shows two δ13
C trends: very low ratios
at the inner-shelffrom −10.5 to −4.1‰, and slightly higher ratios at
the middle-shelf (from −4.8 to −1.7‰). The δ13
C ratios in the
limestone sequence above the cap dolomite, considered to be
primary, show also two trends: at the inner shelf with the ratio is
lower, between −5.4 and −2.7‰, whereas higher ratios, −2.7 to
+0.1‰, were recorded from the middle-shelf. Only very low δ13
C
values (−10.5 and −9.6‰) obtained for the Terconi Quarry cap
dolomite are most likely not primary but related to a fabric
destructive feature and post-diagenetic fluids with relative high
carbon content (Alvarenga et al., 2004). Moreover, other variations
are also observed: dolomite-bearing rocks present higher oxygen
isotopes values compared to calcite-bearing rocks (Tab. 1, Fig. 6B)
and more variable isotopic values (C and O) are observed near the
contact between the carbonate rocks and the diamictites.
The carbon isotopic values are mostly negative across the
studied sections, although more negative values are commonly
found in the inner shelf samples. We argue that these rocks were
formed under a stronger influence of the continental margin. A
similar feature was observed across sections of the Tsumeb
Subgroup and the Karibib Formation in Namibia (Halverson et al.,
2005), where isotopic differences were also observed from platform
to foreslope.
The δ13
C versus δ18
O cross-plot of all samples studied (Fig. 7)
shows that inner-shelf samples record enrichment in δ18
O 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 isotope
enrichment for the inner shelf at the margin of the basin can be
related to the restriction and evaporation from shallow expansive
environments. A similar pattern of oxygen isotopic variation has
been suggested for dolomitic rocks of the Society Cliffs Formation
(Kah, 2000).
The 87
Sr/86
Sr data show variations with rock composition. The
cap dolomite has highly radiogenic 87
Sr/86
Sr ratios in excess of
0.71, which is interpreted as a post-depositional effect. The
87
Sr/86
Sr ratios (0.70740–0.70780) from the limestone above the
cap dolomite with low Mn/Sr ratios and high Sr content (up to
4350 ppm) suggest an aragonitic limestone precursor that can be
interpreted as reflecting the primary seawater isotopic values.
Based on Neoproterozoic seawater 87
Sr/86
Sr data (Jacobsen and
Kaufman, 1999; Shields, 1999; Walter et al.,2000; Melezhik et al.,
2001; Thomas et al., 2004), carbonates of the Nobres Formation
maybe correlated with other carbonates elsewhere that were
deposited at around 600 Ma.
7. Conclusions
Correlations of inner-shelf and middle-shelf facies can be
possible with more accuracy with the help of C and O isotope
stratigraphy. An important time line is the contact between the cap
dolomite and glacial diamictite, which has been interpreted as a
transgressive system tract at the end of a glacial period, with a
maximum flooding surface at the clay-limestone interface above
the cap dolomite. A comparison of C isotope values between
sections from the inner shelf to the outer shelf along the basin
shows different absolute isotopic values but comparable isotopic
trends. The carbonates from the inner-shelf section are depleted in
13
C compared to those from the more distal platform, probably
due to a shallower environment and to a stronger influence of the
continental margin.
The 87
Sr/86
Sr ratios follow a similar trend in all sections,
suggesting that these sections might be coeval. The Sr isotopic
composition of cap dolomite samples is highly radiogenic in all
the sections, indicating that the primary isotopic values were
modified by post-depositional diagenetic alteration. All samples
placed above the cap dolomite have calcite as the main carbonate,
high Sr concentration and 87
Sr/86
Sr ratios (0.70740–0.70780)
that are near-primary. These 87
Sr/86
Sr values characterize the cap
carbonate 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 Formation
in the Paraguay Belt as suggested previously (Alvarenga and
Trompette, 1992; Nogueira et al., 2003; Alvarenga et al., 2004;
Allen and Hoffman, 2005).
Acknowledgements
Field work has been supported by CNPq (Conselho Nacional
de Desenvolvimento Cientifico e Tecnológico) grants (Projects:
461482/2001-0 and 550860/2002-9). We thank Francisco E. C.
Fig. 7. Cross-plots of δ13
C vs. δ18
O of all studied samples in this work and from
Alvarenga et al., 2004.
477C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

10. Pinho, E. Marechal Tagliarini and Fanio T. Guimarães for
assistance in the field. The text benefited greatly from reviews
by Galen P. Halverson, Paul Knauth and Hartwig Frimmel. This
is a contribution to the International Geological Correlation
Programme (IGCP) Project 478 “Neoproterozoic–Early Paleo-
zoic Events in SW-Gondwana”.
References
Allen, P.A., Hoffman, P.F., 2005. Extreme winds and waves in the aftermath of a
Neoproterozoic glaciation. Nature 433, 123–127.
Almeida, F.F.M. de, 1964. Geologia do centro-oeste matogrossense. Brazil,
Ministério Minas Energia, Departamento Nacional Produção Mineral,
Boletim Divisão Geologia e Mineralogia, vol. 215, pp. 1–137.
Almeida, F.F.M. de, 1974. Sistema Tectônico Marginal do Cráton do Guaporé.
Anais 28° Congresso Brasileiro Geologia, Sociedade Brasileira Geologia,
Porto Alegre, vol. 4, pp. 09–17.
Almeida, F.F.M. de, 1984. Província Tocantins. Setor sudoeste. In: de Almeida,
F.F.M., Hasui, Y. (Eds.), O Precambriano do Brasil. Edgard Blücher Ltd.,
São Paulo, pp. 265–281.
Almeida, F.F.M. de, Mantovani, M.S.M., 1975. Geologia e geocronologia do
granito de São Vicente, Mato Grosso, vol. 47. Academia Brasileira Ciências,
Anais, pp. 451–458.
Alvarenga, C.J.S. de, Saes, G.S., 1992. Stratigraphy and sedimentology of the
Middle and Late Proterozoic in the southeast of the Amazonian Craton.
Revista Brasileira de Geociências 22, 493–499.
Alvarenga, C.J.S. de, Trompette, R., 1992. Glacial influenced turbidite
sedimentation in the uppermost Proterozoic and Lower Cambrian of the
Paraguay Belt (Mato Grosso, Brazil). Palaeogeography, Palaeoclimatology,
Palaeoecology 92, 85–105.
Alvarenga, C.J.S. de, Trompette, R., 1993. Brasiliano tectonic of the Paraguay
Belt: the structural development of the Cuiabá Region. Revista Brasileira de
Geociências 23, 18–30.
Alvarenga, C.J.S. de, Santos, R.V., Dantas, E.L., 2004. C–O–Sr isotopic
stratigraphy of cap carbonates overlying Marinoan-age glacial diamictites in
the Paraguay Belt, Brazil. Precambrian Research 131, 1–21.
Alvarenga, C.J.S. de, Moura, C.A.V., Gorayeb, P.S.S., Abreu, F.A.M. de, 2000.
Paraguay and Araguaia belts. In: Cordani, U.G., Milani, E.J., Thomaz Filho,
A., Campos, D.A. (Eds.), Tectonic Evolution of South America. 31st
International Geological Congress, Rio de Janeiro, Brazil, pp. 183–193.
Alvarenga, C.J.S. de, Figueiredo, M.F., Babinski, M., Pinho, F.E.C., 2007.
Glacial diamictites of Serra Azul Formation (Ediacaran, Paraguay Belt):
evidence of the Gaskiers glacial event in Brazil. Journal of South American
Earth Sciences 23, 236–241.
Asmeron, Y., Jacobsen, S.B., Knoll, A.H., Butterfield, N.J., Swett, K., 1991.
Strontium isotopic variations of Neoproterozoic seawater: implications for
crustal evolution. Geochimica et Cosmochimica Acta 55, 2883–2894.
Boggiani, P.C., Alvarenga, C.J.S. de, 2004. A Faixa Paraguai. In: Mantesso Neto,
V., Bartorelli, A., Carneiro, C.D.R., Neves, B.B.B. (Eds.), O Desvendar de um
continente: a moderna geologia da América do Sul e o legado da obra de
Fernando Flávio Marques de Almeida. Beca, São Paulo, pp. 113–120.
Bowring, S., Myrow, P., Landing, E., Ramezani, J., Grotzinger, J., 2003.
Geochronological constraints on terminal Neoproterozoic events and the rise
of metazoans. Journal of Geophysical Research Abstracts 5, 13219.
Brasier, M.D., Shields, G., 2000. Neoproterozoic chemostratigraphy and
correlation of the Port Askaig glaciation, Dalradian supergroup of Scotland.
Geological Society of London, Journal 157, 909–914.
Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., Jin, Y., 2005. U–Pb
ages from the Neoproterozoic Doushantuo Formation, China. Science 308,
95–98.
Cozzi, A., Allen, P.A., Grotzinger, J.P., 2004. Understanding carbonate ramp
dynamics using δ13
C profiles: examples from the Neoproterozoic Buah
Formation of Oman. Terra Nova 16, 62–67.
Derry, L.A., Keto, L.S., Jacobsen, S.B., Knoll, A.H., Swett, K., 1989. Sr isotope
variations in Upper Proterozoic carbonates from Svalbard and East
Greenland. Geochimica et Cosmochimica Acta 53, 2331–2339.
Faulstich, F.R.L., 2005. Dolomitização e sulfetos (Zn) dos carbonatos
neoproterozóicos da formação Araras, MT. Master Thesis, Instituto de
Geociências, Brasília University.
Figueiredo, M.F., 2006. Quimioestratigrafia das rochas ediacaranas no extremo
norte da Faixa Paraguai, Mato Grosso. Master Thesis, Instituto de Geociências,
São Paulo University.
Figueiredo, M.F., Babinski, M., Alvarenga, C.J.S. de, Pinho, F.E.C., 2004.
Diamictites overlying Marinoan-age carbonates of Araras Formation,
Paraguay Belt, Brazil: evidence of a new glaciation? Symposium on
Neoproterozoic–Early Paleozoic Events in SW-Gondwana, 1, Extended
Abstracts, IGCP Project 478, Second Meeting, São Paulo, Brazil, pp. 18–19.
Frimmel, H.E., Fölling, P.G., 2004. Late Vendian closure of the Adamastor
Ocean: timing of tectonic inversion and syn-orogenic sedimentation in the
Gariep Basin. Gondwana Research 7, 685–699.
Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A., Rice, A.H.N., 2005.
Toward a Neoproterozoic composite cabon-isotope record. Geological
Society of American, Bulletin 117, 1181–1207.
Hoffman, P.F., Schrag, D.P., 2002. The snowball Earth Hypothesis: testing the
limits of global change. Terra Nova 14, 129–155.
Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. A
Neoproterozoic snowball earth. Science 281, 1342–1346.
Hoffmann, K.H., Condon, D.J., Bowring, S.A., Crowley, J.L., 2004. U–Pb
zircon date from the Neoproterozoic Ghaub Formation, Namibia: constraints
on Marinoan glaciation. Geology 32, 817–820.
Jacobsen, S.B, Kaufman, A.J., 1999. The Sr, C and O isotope evolution of
Neoproterozoic seawater. Chemical Geology 161, 37–57.
Kah, L.C., 2000. Depositional δ18
O signatures in Proterozoic dolostones:
constraints on seawater chemistry and early diagenesis. In: Grotzinger, J.P.,
James, N.P. (Eds.), Carbonate Sedimentation and Diagenesis in the Evolving
Precambrian World. SEPM Society for Sedimentary Geology, Special
Publication No.67, Tulsa, pp. 345–360.
Kaufman, A.J., Jacobsen, S.B., Knoll, A.H., 1993. The Vendian record of Sr and
C isotope variations in seawater: implications for tectonics and paleoclimate.
Earth and Planetary Science Letters 120, 409–430.
Kaufman, A.J., Knoll, A.H., Narbonne, G.M., 1997. Isotopes, ice ages, and
terminal Proterozoic earth history. Proceedings of the National Academy of
Sciences 94, 6600–6605.
Kennedy, M.J, Runnegar, B., Prave, A.R., Hoffmann, K.-H., Arthur, M.A.,
1998. Two or four Neoproterozoic glaciations? Geology 26, 1059–1063.
Kennedy, M.J., Christie-Blick, N., Sohl, L.E., 2001. Are Proterozoic cap
carbonates and isotopic excursions a record of gas hydrate destabilization
following Earth's coldest intervals? Geology 29, 443–446.
Knoll, A.H., Walter, M.R., Narbonne, G.M., Christie-Blick, N., 2004. A new
period for the geologic time scale. Science 305, 621–622.
Litherland, M., Annells, R.N., Appleton, J.D., Berrangé, J.P., Bloomfield, K.,
Burton, C.C.J., Darbyshire, D.P.F., Fletcher, C.J.N., Hawkins, M.P., Klinck,
B.A., Llanos, A., Mitchell, W.I., O'Connor, E.A., Pitfield, P.E.J., Power, G.,
Webbs, B.C., 1986. The geology and mineral resources of the Bolivian
Precambrian shield. British Geological Survey, Overseas Memoir 9 153 pp.
Luz, J.S., Abreu Filho, W., 1978. Aspectos geológico-econômico da Formação
Araras do Grupo Alto Paraguai- MT. Anais 30° Congresso Brasileiro
Geologia, Sociedade Brasileira Geologia. Recife, vol. 4, pp. 1816–1826.
McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotempera-
ture scale. Journal of Chemical Physics 18, 849–857.
Melezhik, V.A., Gorokhov, I.M., Kuznetsov, A.B., Fallick, A.E., 2001.
Chemostratigraphy of Neoproterozoic carbonates: implications for ‘blind
dating’. Terra Nova 13, 1–11.
Melezhik, V.A., Roberts, D., Fallick, A.E., Gorokhov, I.M.,Kusnetzov, A.B., 2005.
Geochemical preservation potencial of high-grade marble versus dolomite
marble: implication for isotope chemostratigraphy. Chemical Geology 216,
203–224.
Nogueira, A.C.R., Riccomini, C., Sial, A.N., Moura, C.A.V., Fairchild, T.R.,
2003. Soft-sediment deformation at the base of the Neoproterozoic Puga cap
carbonate (southwestern Amazon craton, Brazil): confirmation of rapid
icehouse to greenhouse transition in snowball Earth. Geology 31, 613–616.
Pimentel, M.M., Fuck, R.A., Alvarenga, C.J.S. de, 1996. Post-Brasiliano (Pan-
African) high-K granitic magmatism in central Brazil: the role of late
Precambrian–early Paleozoic extension. Precambrian Research 80, 217–238.
478 C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479

11. Shields, G., 1999. Working towards a new stratigraphic calibration scheme for
the Neoproterozoic–Cambrian. Eclogae Geologicae Helvetiae 92, 221–233.
Thomas, C.W., Graham, C.M., Ellam, R.M., Fallick, A.E., 2004. 87
Sr/86
Sr
chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and
Ireland: constraints on depositional ages and time scales. Geological Society
of London, Journal 161, 229–242.
Trindade, R.I.F., Fonta, E., D'Agrella-Filho, M.S., Nogueira, A.C.R.,
Riccomini, C., 2003. Low-latitude and multiple geomagnetic reversals in
the Neoproterozoic Puga cap carbonate of Amazonia. Terra Nova 15,
441–446.
Trompette, R., 1994. Geology of Western Gondwana (2000–500 Ma). Pan-
African–Brasiliano Aggregation of South America and Africa. Balkema,
Rotterdam.
Trompette, R., 1997. Neoproterozoic (∼600 Ma) aggregation of Western
Gondwana: a tentative scenario. Precambrian Research 82, 101–112.
Trompette, R., Alvarenga, C.J.S. de, Walde, D., 1998. Geological evolution of
the Neoproterozoic Corumbá graben system (Brazil). Depositional context
of the stratified Fe and Mn ores of Jacadigo Group. Journal of South
American Earth Sciences 11, 587–597.
Walter, M.R., Veevers, J.J., Calver, C.R., Gorjan, P., Hill, A.C., 2000. Dating the
840–544 Ma Neoproterozoic interval by isotope of strontium, carbon, and
sulfur in seawater, and some interpretative models. Precambrian Research
100, 371–433.
479C.J.S. de Alvarenga et al. / Gondwana Research 13 (2008) 469–479