Chemostratigraphy of the Tamengo Formation (CorumbÃ¡ Group, Brazil… · 2010. 11. 1. · Brazil abstract The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil,

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Precambrian Research 182 (2010) 382–401

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Precambrian Research

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hemostratigraphy of the Tamengo Formation (Corumbá Group, Brazil):contribution to the calibration of the Ediacaran carbon-isotope curve

aulo C. Boggiania,∗, Claudio Gaucherb,c, Alcides N. Siald, Marly Babinskia, Cynthia M. Simona,laudio Riccominia, Valderez P. Ferreirad, Thomas R. Fairchilda

Instituto de Geociências, Universidade de São Paulo, Rua do lago 562, São Paulo, SP, BrazilDepartamento de Geología, Facultad de Ciencias, Iguá 4225,11400 Montevideo, UruguayInstitute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, DenmarkNEG-LABISE, Department of Geology, Federal University of Pernambuco, Recife, PE, Brazil

r t i c l e i n f o

rticle history:eceived 31 July 2009eceived in revised form 19 April 2010ccepted 2 June 2010

eywords:eoproterozoicloudinahemostratigraphyaraguay Beltrazil

a b s t r a c t

The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most completeEdiacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution insouthwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendo-taenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). TheTamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostrati-graphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the TamengoFormation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopicanalyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive ı13C excur-sion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated witha marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateaucharacterized by ı13C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upperpart of the ı13C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age (Babinskiet al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excur-sion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), theDengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained
between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negativeexcursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly,although ı13C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur justbeneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the ori-
glacio
gin of these breccias is aruled out.
. Introduction

The Neoproterozoic Era, and the Ediacaran Period (Knoll et al.,004) in particular, were characterized by severe biogeochemicalnd climatic oscillations (e.g. Gaucher et al., 2009a). Several iceges occurred in the Neoproterozoic (e.g. Hoffman, 2009), whichere characterized by coeval positive to negative ı13C excursions

n marine carbonates.
In the Ediacaran, the glacial record comprises the ca. 582 Ma
askiers glaciation (Bowring et al., 2003) and the ca. 547 Ma Vinger-reek glacial event (Germs, 1995; Kaufman et al., 2009), the lattereing possibly correlative to the Baykonurian glaciation in Central

∗ Corresponding author. Tel.: +55 11 3091 4202; fax: +55 11 3091 4207.E-mail address: [email protected] (P.C. Boggiani).

301-9268/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.precamres.2010.06.003

eustatic sea-level fall, but a tectonic interpretation cannot be completely

Published by Elsevier B.V.

Asia (Chumakov, 2009). A severe perturbation of the carbon cycleduring both glacial events is evidenced by negative ı13C excur-sions, much like in the older Cryogenian glaciations (Halverson etal., 2005, 2009; Kaufman et al., 2009). One of the most dramaticnegative ı13C anomalies (Shuram–Wonoka anomaly) is recordedin middle Ediacaran carbonates of Oman, Australia, southern China,Namibia, Norway and Siberia (Brasier et al., 2000; Walter et al.,2000; Amthor et al., 2003; Halverson et al., 2005; Fike et al., 2006;Le Guerroué et al., 2006; McFadden et al., 2008; Melezhik et al.,2008). The Shuram–Wonoka anomaly ended just before 551 ± 1 Maand reached its nadir before 555 ± 6 Ma in south China (Condon et
al., 2005; Zhang et al., 2005). The anomaly may be associated tothe Gaskiers glaciation (Halverson et al., 2005, 2009), or post-dateit (Sawaki et al., 2010), and is still a matter of debate regardingthe meaning of extremely negative ı13C values down to −12‰VPDB (e.g. Le Guerroué et al., 2006; Bristow and Kennedy, 2008;
dx.doi.org/10.1016/j.precamres.2010.06.003

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dx.doi.org/10.1016/j.precamres.2010.06.003

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e Guerroué and Cozzi, 2010). Precise documentation of the secu-ar variations of marine ı13C in the Ediacaran is thus important fornderstanding the causes of paleoclimatic perturbations, evolutionnd extinction of organisms in general and the advent of the Meta-oa in particular. Especially interesting are carbonate-dominated,ossiliferous sedimentary successions with radiochronometric ageonstraints, such as the Nama Group in Namibia (e.g. Germs et al.,009, and references therein).

In South America, one of the units providing the most completeecord of Ediacaran climatic, biogeochemical and biotic evolutions the Corumbá Group and the older Jacadigo Group and Puga For-

ation, which crop out in the southern Paraguay Belt (Boggiani,998; Gaucher et al., 2003; Fig. 1). The most salient features ofhese units are the occurrence of probable glacial deposits asso-
iated to Rapitan-type banded iron formation (Puga Formation andacadigo Group, e.g. Urban et al., 1992; Alvarenga et al., 2009), thickarbonates (Bocaina and Tamengo formations), abundant micro-nd megafossils and phosphorites. The bio- and chemostratigraphicecord of the Tamengo Formation, in the middle-upper Corumbá
Fig. 1. Simplified geological map of the Paraguay Bel

esearch 182 (2010) 382–401 383

Group, is especially rich and thus central to our understandingof the overall environmental evolution in the Paraguay Belt andassociated cratonic cover. In this work, we revisit the geographi-cal distribution of the Corumbá Group and associated units in theParaguay Belt, and report new carbon-isotope analyses of carbon-ates. Emphasis is given to the correlation of the isotopic excursionsrecorded in the Corumbá Group, as well as their environmentalsignificance.

2. Geological setting and age: Paraguay Belt and CorumbáGroup

2.1. Geological setting

The Paraguay Belt, a late Pan-African-Brasiliano age fold belt insouthwestern Gondwana, is characterized by its convex, arc shape(Fig. 1). It comprises a southern and a northern branch with distinc-tively different lithostratigraphy. The main lithological similaritybetween the southern and northern Paraguay Belt is the presence

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f extensive diamictite exposures at the base, generally assigned tohe Puga Formation and interpreted as of glacial origin (Alvarengand Trompette, 1992). Overlying carbonate units in the southernCorumbá Group) and northern (Araras Group) Paraguay Belt areithologically and biostratigraphically different, and probably ofifferent age, as will be explained below.

The most expressive diamictite exposures have been describedn the northern Paraguay Belt, and initially defined as Jangada For-

ation (Almeida, 1964a,b; Rocha Campos and Hasui, 1981) andater correlated to the Puga Formation in the southern part ofhe belt (Corrêa et al., 1979; Araújo et al., 1982; Alvarenga andrompette, 1992). In the northern part, the Puga Formation exhibitsvidence of glacially influenced sedimentation, such as faceted andtriated pebbles and blocks. The unit is interpreted as depositedn a glaciomarine environment, with turbiditic flows occurring inastern, more distal portions (Alvarenga and Trompette, 1992).

Diamictites in the northern Paraguay Belt are overlain by well-haracterized cap carbonates belonging to the Araras Group, whichresent typical sedimentary features observed in other cap carbon-tes worldwide (Nogueira et al., 2003, 2007; Nogueira & Riccomini,006; Alvarenga et al., 2004; Riccomini et al., 2007). Glaciogeniciamictites of the Serra Azul Formation overlie the Araras Group,nd were probably deposited during the Gaskiers glacial eventFigueiredo, 2006; Alvarenga et al., 2007, 2009; Figueiredo et al.,008). The Serra Azul Formation is in turn overlain by the Altoaraguai Group, interpreted as a molasse succession (Almeida,968, 1984; Alvarenga et al., 2000), possibly deposited in a fore-

and basin related to the Paraguay Belt (Dantas et al., 2009; Gauchert al., 2009b).

The lithostratigraphy of the southern Paraguay Belt differsarkedly from that of the northern part (Fig. 1; for a detailed com-

arison see Alvarenga et al., 2009). At the base, diamictites of theuga Formation interpreted as of glacial origin occur in the Morroo Puga hill, where the unit was first described (Maciel, 1959). Inhe same section red, laminated limestones of the lower Bocainaormation overlie the diamictites (Boggiani and Coimbra, 1996;oggiani et al., 2003), yielding homogeneous ı13CPDB values around5‰. Stromatolitic “tubestones” occur in carbonates in the vicin-

ty of this occurrence (Porto Morrinhos; Fig. 10), which are identicalo other examples of cap dolostones worldwide (e.g. Hegenberger,987, 1993; Corsetti and Grotzinger, 2005). According to their iso-opic and sedimentological features, these carbonates have beennterpreted as a cap carbonate (Boggiani et al., 2003). The Puga For-

ation is interpreted as glacial, although typical glacial featuresuch as facetted or striated clasts, have not been properly docu-ented. However, Rapitan-type BIF with granitic dropstones are

ssociated to the Puga Formation (Piacentini et al., 2007), renderingupport for a glaciogenic origin of the diamictites.

The predominantly terrigenous Cadiueus and Cerradinho for-ations were deposited at an early stage, in a context of

ift initiation and climax (Prosser, 1993), before the onset ofidespread carbonate deposition (Gaucher et al., 2003). Theocaina Formation comprises stromatolitic dolostones and sub-rdinate, bedded phosphorite occurrences (Boggiani, 1998). Theolostones are overlain by the Tamengo Formation, comprisinglack limestones, marls and shales (Almeida, 1965, 1984; Boggiani,998; Trompette et al., 1998; Alvarenga et al., 2000; Gaucher et al.,003). Thick siltstones and shales of the Guaicurus Formation over-

ie the Tamengo Formation, representing the youngest unit of theorumbá Group (Fig. 1).

Whereas the Puga, Cadiueus and Cerradinho formations are
ssociated with the rift stage of basin evolution, the Bocaina,amengo, and Guaicurus formations represent the post-rift torift stage. The evolution to open shelf sedimentation is stronglyuggested by phosphorite deposition at the top of the Bocaina For-ation and the presence of the cosmopolitan fossil Cloudina in
esearch 182 (2010) 382–401

the Tamengo Formation and acritarch assemblages similar to otherlate Ediacaran occurrences (Boggiani, 1998; Gaucher et al., 2003).During the rift to drift evolution, widespread carbonate depositionof the Tamengo Formation represents an important transgressionover the Apa River Block, which is also observed in Paraguayan ter-ritory (Itapucumí Group), where Cloudina fossils were also found(Boggiani and Gaucher, 2004). No foreland basin deposits overliethe Corumbá Group, highlighting the different basin evolution com-pared to the northern Paraguay Belt, as suggested by Alvarenga et al.(in press). Gaucher et al. (2003, 2009b) suggested that this may beexplained if the southern Paraguay Belt marks the eastern boundaryof the Río de la Plata Craton, as opposed to the northern ParaguayBelt, which is clearly associated with the Amazonian Craton.

2.2. Age of the Corumbá Group and associated units

Despite the absence of more accurate geochronological data,diamictites of the Puga Formation in both the northern and south-ern Paraguay Belt were interpreted as late Cryogenian (“Marinoan”;Alvarenga and Trompette, 1992; Alvarenga et al., 2004; Babinskiet al., 2006; Nogueira et al., 2007; Riccomini et al., 2007). Thisinterpretation is based only on the fact that diamictites are over-lain by Cloudina-bearing carbonates. However, this relationship isobserved only in the southern Paraguay Belt (Beurlen and Sommer,1957; Zaine and Fairchild, 1985; Gaucher et al., 2003), and not inits northern part. In the latter, the best available age constraint is aPb-Pb isochron of 633 ± 25 Ma for cap carbonates at the base of theAraras Group (Alvarenga et al., 2009), which is in good agreementwith an end-Cryogenian age for the underlying diamictites. In thesouthern Paraguay Belt, the youngest detrital zircon in the PugaFormation yielded U-Pb SHRIMP zircon ages of 706 Ma, providinga maximum age constraint for the unit (Babinski et al., 2008b).

In the Puerto Suárez region (Bolivia), a quartz-porphyry occur-ring at the basis of the Jacadigo/Boqui Group yielded a K-Ar age of623 ± 15 Ma (Shaw and O’Connor, 1986 in Berrangé and Litherland,1982; Avila Salinas, 1992). The mentioned age favors an Edi-acaran age for the Puga Formation. However, subsequent surveysconducted in Bolivia failed to identify the mentioned volcanicrocks. Metabasites were mentioned by Almeida (1965), and, subse-quently, reported from the northern Paraguay Belt (Cuiabá Group).Nogueira et al. (1978) described metabasites and possible volcanicashes in the eastern Serra da Bodoquena. Metabasite samples werecollected in the Serra da Bodoquena but no zircons could be sepa-rated, and the strong weathering precluded other geochronologicalstudies. Finally, in the Nova Xavantina region, in the eastern portionof the northern Paraguay Belt, a metavolcanosedimentary succes-sion was described as the Araés sequence (Dantas and Martinelli,2003). This sequence is considered part of the Cuiabá Group, whichin turn is interpreted as the distal equivalent of the glaciogenicPuga Formation (Alvarenga and Trompette, 1992). An alternativeinterpretation of the volcanic rocks would be that they representa separate, basal unit. Volcanic tuffs interbedded with Cloudina-bearing carbonates of the upper Tamengo Formation at Corcal Mine(Boggiani et al., 2005; Fig. 6) yielded an U-Pb SHRIMP zircon age of543 ± 3 Ma (Babinski et al., 2008a), which is within 1 Myr of the Edi-acaran – Cambrian boundary (Amthor et al., 2003). However, thewhole Tamengo Formation and at least the lower Guaicurus Forma-tion is Ediacaran in age, as shown by their fossil content (Gaucheret al., 2003 and references therein).

Within the rich fossil record of the Tamengo Formation the fol-lowing groups are biostratigraphically significant:

(a) Cloudina lucianoi occurs as event-accumulations in grainstoneof the Tamengo Formation (Zaine and Fairchild, 1985; Hahn andPflug, 1985; Gaucher et al., 2003). Cloudina is an index fossil ofthe late Ediacaran (e.g. Grant, 1990), and defines the Cloudina

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b) Corumbella werneri co-occurs with Cloudina in the Tamengo For-mation, and has been interpreted as a cnidarian, possibly relatedto the Scyphozoa (Hahn et al., 1982; Walde et al., 1982; Babcocket al., 2005).

(c) Titanotheca coimbrae occurs in phosphorites of the Bocaina For-mation (Gaucher et al., 2003). Titanotheca has been interpretedas an agglutinated foraminifer, occurring in late Ediacaran(ca. 570–550 Ma) successions from South America and south-ern Africa (Gaucher and Sprechmann, 1999; Gaucher, 2000;Teixeira and Gaucher, 2004; Gaucher et al., 2005; Gaucher andGerms, 2009; Fig. 11).

d) A low-diversity acritarch assemblage dominated by Bavlinellafaveolata occurs in the Tamengo Formation (Zaine and Fairchild,1987; Zaine, 1991; Gaucher et al., 2003), similar to other lateEdiacaran successions worldwide (Vidal and Moczydłowska-Vidal, 1997; Gaucher and Sprechmann, 2009).

e) Vendotaenid algae including Vendotaenia antiqua, Tawuia sp.and Eoholynia corumbensis occur in the upper Corumbá Group(Gaucher et al., 2003), the latter species occurring only insiltstones of the lower Guaicurus Formation (Gaucher et al.,2003). An algoflora characterized by Eoholynia occurs in theRedkino Horizon of the East European Platform, and is con-strained between 558 and 555 Ma by U-Pb zircon ages of ashbeds (Fedonkin et al., 2007 and references therein).

Thus, both biostratigraphic and radiometric data support a latediacaran age for the Tamengo Formation.

. Materials and methods

Carbon and oxygen isotope analyses of carbonate samples fromhe Laginha, Saladeiro, Calbon and Baía das Garcas Farm sectionsere performed at the Stable Isotope Laboratory (LABISE) of the

ederal University of Pernambuco (UFPE) at Recife, Brazil. Only leastltered portions of samples were micro-drilled with a 1 mm diam-ter drill bit. CO2 was extracted from these carbonate samples on aigh vacuum line after reaction with phosphoric acid at 25 ◦C dur-

ng three days, and cryogenically cleaned, according to the methodescribed by Craig (1957). Released CO2 gas was analyzed for O andisotopes in a double inlet, triple collector mass spectrometer (VG

sotech SIRA II), using the BSC reference gas (Borborema skarn cal-ite) that was calibrated against NBS-18, NBS-19 and NBS-20, and
as the ı18O value of −11.3‰PDB and ı13C = 8.6‰PDB. The exter-al precision based on multiple standard measurements of NBS-19as better than 0.1‰ for carbon and oxygen. Isotope analyses are
xpressed in the ı-notation in parts per thousand in relation to thePDB scale.

Fig. 2. Idealized cross section of the Corumbá shelf, showing the Tamengo

esearch 182 (2010) 382–401 385

Carbon and oxygen isotope analyses on limestones from theCorcal and Horii Mines were carried out at the Stable Isotope Lab-oratory of the Geochronological Research Center, University of SãoPaulo. Powders were recovered from rock slabs using a carbide den-tal burr. Analyses were performed after CO2 gas extraction from thecarbonates in a high vacuum line after reaction with 100% H3PO4at room temperature for 24 h. Following cryogenic cleaning, thereleased CO2 was analyzed in a EUROPA GEO 20–20 mass spec-trometer, using IAEA standards as well as a secondary standard.Results are reported in the conventional notation in per mil (‰)relative to VPDB standard. Uncertainties are 0.1‰ for both carbonand oxygen isotope results.

Whole-rock chemical analyses were carried out on fused beadsby X-ray Fluorescence at the LABISE (UFPE), in a RIX 3000 RIGAKUunit that is equipped with Rh tube (Laginha section) and at theX-ray fluorescence at IGC-USP (Corcal, Horii and Calbon sections).

4. Lithostratigraphy and facies of the Tamengo Formation

The Tamengo Formation is characterized by black limestonesand marls 100–200 m in thickness. The unit overlies the BocainaFormation with erosional unconformity. This is best shown by thebasal breccia of the Tamengo Formation (Boggiani, 1998), whichcomprises clasts of stromatolitic dolostone and phosphorite fromthe Bocaina Formation, as well as granite, rhyolite, chert and schistclasts. The matrix of the typically unstratified (chaotic) breccia ismainly composed of dolomite, apatite and occasional fluorite, thelatter possibly late diagenetic/hydrothermal in origin. At LaginhaMine, the breccia exceeds 30 m in thickness, and is concordantlyoverlain by mudstone and grainstone (Figs. 3 and 10). A packageof black, organic-rich marls 20 m in thickness occurs up section,representing a marker horizon (Fig. 3). The latter are overlain inturn by ooid grainstones and intercalated rudstones, up to 75 min thickness (Figs. 3 and 4). Grainstones of the Tamengo Formationoften show cross bedding, including hummocky cross bedding, andwavy structures (Boggiani, 1998). Cloudina-shell beds (event accu-mulations) mostly occur within this interval, as can be observedat Corcal Quarry nearby Corumbá (Boggiani, 1998; Gaucher et al.,2003). Grey siltstones of the Guaicurus Formation conformablyoverlie the Tamengo Formation. The contact displays low-reliefirregularities that may be related to exposure and karstification ofthe shelf prior to deposition of the Guaicurus Formation. Diamic-tites with limestone clasts up to 2 m in diameter derived from theTamengo Formation occur in the lower Guaicurus Formation (Lag-
inha diamictite), 5–15 m above the top of the former unit (Boggianiet al., 2004; Gaucher and Poiré, 2009; Fig. 10). This also showsthat shallow-water carbonates were eroded prior or concomitantto deposition of the lower Guaicurus Formation. Clasts in the Lag-inha diamictite often deform the underlying siltstones, allowing
and Guaicurus Formations and the location of the studied profiles.

386 P.C. Boggiani et al. / Precambrian Research 182 (2010) 382–401

Table 1Isotopic and geochemical data of studied samples of the Tamengo Formation.

Sample ıC13PDB ıO18PDB

Laginha westernTMG 4 −0.11 −7.96TMG 5 −0.43 −3.48TMG 8 −3.32 −9.32TMG 11 +0.27 −7.53TMG 12 +0.42 −7.88TMG 13 +4.59 −7.15TMG 15 +1.30 −7.32TMG 16 +1.87 −8.83TMG 17 +2.60 −7.01TMG 18 +3.15 −7.91TMG 19 +3.11 −6.67TMG 20 +1.39 −10.74TMG 21 +2.75 −9.33

Laginha western base05.LG. 2A −3.64 −5.5805.LG. 2B −3.23 −6.5805.LG. 2C −0.71 −8.8705.LG. 2D −0.56 −8.9805.LG. 2E −1.12 −11.4805.LG. 2F −0.31 −9.8405.LG. 2G −0.29 −9.7705.LG. 2H −0.40 −4.6805.LG. 2I −1.38 −6.0305.LG. 2J −0.95 −4.8205.LG. 2K −1.46 −5.3105.LG. 2L −1.13 −5.7105.LG. 2M −1.07 −6.1605.LG. 2N −1.10 −6.9105.LG. 2O −1.28 −6.5105.LG. 2P −1.34 −5.4305.LG. 2Q −1.35 −5.3505.LG. 2R −1.51 −5.9705.LG. 2S −1.89 −8.3405.LG. 2T −1.10 −8.5605.LG. 2U −0.36 −10.0305.LG. 2V +0.45 −8.8305.LG. 2X +0.13 −9.5505.LG. 2Y +0.97 −6.3905.LG. 2Z −0.56 −8.1005.LG. 2A1 +0.34 −9.3705.LG. 2B1 +0.56 −7.6105.LG. 2C1 +0.54 −7.9505.LG. 2D1 +3.18 −7.40

Sample ıC13PDB ıO18PDB Sr (ppm) Mn (ppm) Fe (ppm) Mn/Sr Fe/Sr

Laginha easternLAG 01 −0.21 −6.37 1610 79 874 0.049 0.54LAG 02 +0.26 −8.24LAG 03 +4.04 −5.91LAG 04 +3.94 −7.04LAG 05 +2.70 −7.05 2232 172 7010 0.077 3.14LAG 06 +4.03 −7.01LAG 07 +5.32 −7.12LAG 08 +5.29 −7.25LAG 09 +4.19 −6.10LAG 10 +5.43 −7.69 5453 91 4615 0.017 0.85LAG 11 +5.53 −4.48LAG 12 +1.62 −7.02LAG 13 +2.18 −6.68LAG 14 +3.06 −5.74LAG 15 +3.12 −6.71 1864 23 773 0.012 0.41LAG 16 +3.10 −6.17LAG 17 +3.32 −6.14LAG 18 +3.30 −6.06LAG 19 +3.35 −6.18LAG 20 +3.38 −6.38 2024 25 917 0.012 0.45LAG 21 +3.33 −5.74LAG 22 +3.24 −6.18LAG 23 +2.84 −6.46LAG 24 +2.60 −6.40LAG 25 +2.83 −6.99 1610 25 749 0.015 0.46LAG 26 +2.76 −7.10LAG 27 +2.19 −7.66

P.C. Boggiani et al. / Precambrian Research 182 (2010) 382–401 387

Table 1 (Continued )

Sample ıC13PDB ıO18PDB Sr (ppm) Mn (ppm) Fe (ppm) Mn/Sr Fe/Sr

LAG 28 +2.90 −7.05LAG 29 +3.02 −6.81LAG 30 +2.70 −6.99 1392 27 934 0.019 0.67LAG 31 +2.48 −7.30LAG 32 +2.71 −7.18LAG 33 +2.51 −7.20LAG 34 +2.21 −6.80LAG 35 +3.08 −5.94 2334 29 850 0.012 0.36LAG 36 +3.09 −6.34LAG 37 +3.03 −6.29LAG 38 +2.90 −6.00LAG 39 +2.56 −6.67LAG 40 +2.56 −6.25 1543 29 829 0.019 0.54LAG 41 +2.83 −7.07LAG 42 +2.46 −6.40LAG 43 +3.07 −6.42LAG 44 +3.12 −6.49LAG 45 +3.07 −6.78 1775 17 1112 0.009 0.63LAG 46 +2.58 −6.49LAG 47 +2.48 −7.23LAG 48 +2.43 −7.69LAG 49 +2.54 −7.29LAG 50 +2.61 −6.51 1560 14 890 0.009 0.57LAG 51 +2.82 −7.24LAG 52 +3.14 −7.15LAG 53 +3.23 −6.15LAG 54 +3.33 −6.02LAG 55 +3.20 −6.60 2161 10 1143 0.005 0.53LAG 56 +3.29 6.51LAG 57 +3.19 −5.96LAG 58LAG 59 +3.27 −5.99LAG 60 +3.16 −5.53 2787 10 1016 0.004 0.36LAG 61 +3.18 −7.01LAG 62 +3.19 −6.44LAG 63 +3.23 −5.34LAG 64 +3.21 −6.64LAG 65 +3.23 −6.40 2686 17 1304 0.006 0.48LAG 66 +3.26 −6.28LAG 67 +3.16 −6.40LAG 68 +3.03 −7.17LAG 69 +2.70 −7.21LAG 70 +2.88 −6.33 2605 23 1000 0.009 0.38LAG 71 +3.06 −7.69LAG 72 +2.56 −7.90LAG 73 +2.82 −6.61LAG 74 +2.90 −7.78LAG 75 +2.72 −7.92 1537 55 1063 0.03 0.69LAG 76 +2.39 −8.14LAG 77 +2.52 −7.59LAG 78 +5.76 −7.07LAG 79 +2.15 −7.98LAG 80 +1.78 −8.51LAG 81 +1.80 −8.41LAG 82 +1.71 −10.02LAG 83 +1.55 −9.07LAG 84 +2.15 −9.26LAG 85 +2.15 −9.25LAG 86 +1.85 −9.58


SaladeiroCO 46 C +2.87 −10.30CO 46 D +4.59 −7.18CO 46 E +4.38 −7.93CO 46 F +4.47 −7.97CO 46 G +4.61 −7.90CO 46 I +5.10 −6.53CO 46 J +3.93 −8.38CO 46 K +4.35 −6.29CO 46 L +2.02 −9.84CO 46 M +3.66 −7.07



Sample ıC13PDB ıO18PDB Sr (ppm) Mn (%) Fe (%) Mn/Sr Fe/Sr

CorcalY-A 4.4 −6.8 2381 0.013 0.077 0.55 0.32Y-B 2394Y-C 3.8 −5.9 2525 0.004 0.045 0.15 0.18Y-D 4.1 −7 2067 0.010 0.189 0.49 0.91Y-E 3.2 −5 2625 0.005 0.042 0.21 0.16Y-F 5.5 −4.2 2845 0.005 0.031 0.16 0.11Y-G 5.2 −4.1 2409 0.004 0.049 0.16 0.20Y-H 4.8 −7.3 2480 0.004 0.028 0.16 0.11Y-I 2417Y-J 5.6 −7.5 2151 0.004 0.028 0.18 0.13Y-Ia 1859Y-Jb 5.4 −6.4 2171 0.003 0.045 0.14 0.21Y-Kc 2056Y-L 2173Y-M 5.1 −7.7 2668 0.005 0.042 0.17 0.16Y-P 5.6 −5.8 3161 0.005 0.021 0.15 0.07Y-Q 5.2 −8.3 2241Y-R 5.1 −8.3 2464Y-S 5.2 −9.2 1944Y-T 1698 0.005 0.049 0.27 0.29Y-U 5 −9.3 1749 0.005 0.059 0.27 0.34Y-V 4.8 −9.6 1862 0.004 0.052 0.21 0.28Y-Xa -0.3 −10.1 1777 0.131 0.496 7.37 2.79Y-Z 685Z-B 4.9 −8.8 1827 0.005 0.056 0.28 0.31Z-C 4.4 −8.4 1557Z-E 4.3 −4.4 2055 0.005 0.045 0.26 0.22Z-G 3.8 −7.8 1421Z-H 1705Z-I 2210Z-K 4.1 −7.6 2671 0.009 0.080 0.32 0.30Z-L 4.6 −7.4 2510Z-M 5.2 −6.7 2962 0.008 0.056 0.26 0.19Z-O 5.3 −5.6 2632Z-R 5.3 −7.1 2895 0.007 0.035 0.24 0.12Z-S 3272Z-T 4.6 −6.3 2421Z-V 5 −6.9 2505 0.004 0.066 0.15 0.27Z-X 2866Z-Y 3.5 −6.3 2063 0.006 0.063 0.30 0.31


Baía das Garcas FarmBG-01 +2.77 −6.67BG-02 +3.31 −6.05BG-03 +1.57 −3.64BG-04 +3.21 −5.51BG-05BG-06 +3.64 −4.68BG-07 +2.99 −5.86BG-08 +3.37 −5.70BG-09 +2.48 −6.52BG-10 +1.50 −5.84BG-11 +3.98 −5.13


HoriiMH-04 2.5 −5 3792 0.002 0.017 0.04 0.05MH-07 3.0 −4.6 2536 0.002 0.003 0.09 0.01MH-12 1.9 −6.2 3669 0.002 0.007 0.04 0.02MH-15 3.0 −3.9 4321 0.002 0.003 0.04 0.01MH-17 2.8 −3.4 3067MH-20 2.7 −3.9 2772 0.002 0.003 0.07 0.01MH-32 3.0 −4.6 7383 0.003 0.017 0.04 0.02MH-45 2.7 −4.3 3119MH-49 3365 0.002 0.003 0.07 0.01MH-50 2.8 −4.2 3286MH-51 3.0 −4 3261 0.004 0.017 0.12 0.05MH-52 3.0 −6.2 2603MH-54 2.9 −4.2 2934MH-58 3.0 −4.3 3524 0.002 0.003 0.04 0.01MH-59 2.8 −3.9 3604MH-65 2.9 −4.5 4110 0.002 0.003 0.05 0.01MH-74 3.1 −3.4 3583 0.002 0.003 0.04 0.01




MH-79 10,289 0.005 0.122 0.05 0.12MH-86 3.0 −4.2 3180 0.002 0.014 0.07 0.04MH-92 3.1 −4.5 2553 0.002 0.003 0.06 0.01

Sample ıC13PDB ıO18PDB SiO2 MnO Fe2O3

CalbonCALB-02 3.98 −1.93CALB-03 3.65 −1.83 4.61 0.009 0.37CALB-04 4.59 −2.36CALB 5s 4.95 −3.24 3.16 0.017 0.29CALB 5inf 4.88 −2.79 3.16 0.017 0.29CALB-06 5.22 −3.65CALB-07 4.32 −1.95 28.48 0.009 0.34CALB-08 4.14 −1.03CALB-11 3.65 −4.12 2.32 0.017 0.45CALB-12 4.44 −1.54

aelr

dfAt

tt(b

tsG

pbSdodtaCa

5

5

aTo

hTtttd

CALB-15a −0.15 0.79CALB-15b 5.06 −0.12CALB-18 5.28 −0.62

n interpretation as dropstones (Gaucher and Poiré, 2009). How-ver, slumped, convoluted siltstone beds were observed at the sameevel, and an origin related to gravitational processes cannot beuled out (Boggiani et al., 2004).

The Tamengo Formation is best exposed at active and aban-oned mines around Corumbá. However its greatest expression isound in the Serra da Bodoquena, occurring either as cover on thepa River Block in the west, or part of the thrust-and fold belt to

he east.In previous works, deformed carbonates in the eastern por-

ion of the Paraguay Belt, considered in this work as belonging tohe Tamengo Formation, were assigned to the older Cuiabá GroupCorrêa et al., 1979; Nogueira et al., 1978; Araújo et al., 1982), solelyecause they are more deformed.

In the Corumbá region, stratigraphic sections were measured inhe Laginha, Saladeiro and Corcal Mines. In the Serra da Bodoquenaections were studied at Horii and Calbon mines and at Baía dasarcas Farm (Fig. 2).

Sections around Corumbá (Laginha, Saladeiro, and Corcal Mines)resent sedimentary facies typical of a moderately-deep shelfetween storm and fair-weather wave base. The sections in theerra da Bodoquena (Horii and Calbon Mines) were deposited ineeper environments (Fig. 2). Cloudina and Corumbella were foundnly in the Saladeiro and Corcal Mines in Corumbá, and not in theeeper sections. Cloudina was probably associated with stroma-olitic and thrombolitic reefs (cf. Grotzinger et al., 2000), whichgrees well with the bathymetric distribution of these fossils in theorumbá Group, where they occur in the shallower sections andre current-reworked.

. Carbon-isotope chemostratigraphy

.1. Laginha Mine section

At Laginha Mine, located 16 km to the south of Corumbá citylong the Campo Grande-Corumbá highway (BR-262), the entireamengo Formation is exposed. The unit occurs there in the flankf a syncline, bedding dipping 50–60◦ to the southeast (Fig. 10).

With the advance of mining operations, the presence of low- toigh-angle reverse faults and duplex structures could be observed.
he faults likely promoted layer duplication in the western side ofhe mine (Fig. 4). Despite these faults, it is possible to reconstructhe original stratigraphy, which shows at the base polymic-ic, carbonatic breccias. They are composed of centimeter- toecimeter-sized clasts of typical lithological types of the subjacent
0.027 0.570.79 0.027 0.57

Bocaina Formation (dolomite, chert, phosphorite), and granite-gneiss basement blocks showing that before deposition of theTamengo Formation, erosion of the Bocaina Formation and its base-ment must have occurred, possibly related to a significant eustaticdebasement.

In the western part of the mine, polymictic breccias arecovered by 43 m of wackestone and mudstone-marl rhythmites(Figs. 3 and 10). Negative ı13CVPDB values down to −3.5‰ werefound only in this package (20–30 m thick), which is not exposed inthe eastern part of the mine due to fault truncation. We could docu-ment the upper part of this negative excursion especially well in the“western Laginha base” section, showing a gradual increase from−3.5‰ to positive values (Table 1; Fig. 3, right column). Velásquez etal. (2008) report several negative ı13C values between −2 and −1‰from this interval as well. Up section, a marked black mudstone-marl rhythmite package occurs, in which a crossover from negativeto positive ı13C values of up to +5.5‰ is recorded (Fig. 3). At the topof the Tamengo Formation in the western Laginha section, a thickand homogeneous package of wackstones with intercalated rud-stones occurs, showing rather homogeneous ı13C values around+3‰ (Fig. 3). A gently declining trend between +3 and +2‰ isobserved in this interval (Fig. 3; Velásquez et al., 2008), and is alsoevident in the high-resolution ı13C curve obtained in the easternLaginha Mine (Fig. 4).

87Sr/86Sr values of 0.7086 were obtained for limestones of themiddle and upper Tamengo Formation at Laginha Mine (Boggiani,1998; Babinski et al., 2008a; Fig. 3).

The contact of the Tamengo Formation with siltstones and shalesof the overlying Guaicurus Formation is conformable and irreg-ular. As mentioned above, diamictites comprising large blocks ofTamengo limestones occur at the base of the Guaicurus Formation(Fig. 10), and were interpreted as originated by gravitational flows(Boggiani et al., 2004) or as a result of glacioeustatic sea-level fall(Gaucher and Poiré, 2009).

The occurrence of rare Cloudina and Corumbella fragmentshas been mentioned for the Tamengo Formation at LaginhaMine in the package of rhythmite (mudstone-carbonaceous marl:Zaine, 1991) above the negative C-isotope excursion. FurthermoreE. corumbensis occurs at the base of the Guaicurus Formation(Gaucher, 2000; Gaucher et al., 2003). A Bavlinella-dominated
acritarch assemblage characterizes the carbonaceous marls (Zaine,1991; Gaucher et al., 2003), and is possibly related to high-bioproductivity intervals (Gaucher, 2000; Velásquez et al., 2008).A Leiosphaeridia–Soldadophycus acritarch assemblage, similar toother late Ediacaran successions in SW Gondwana (Gaucher and


F n in ths

S(

5

eAfMIaHZ

wdprphr2sbms

ig. 3. Stratigraphic column and chemostratigraphic data of the Tamengo Formatioection showing mainly the base of the Tamengo Formation.

prechmann, 2009), occurs in the lower Guaicurus FormationGaucher et al., 2003).

.2. Saladeiro and Corcal Mine sections

The Saladeiro and Corcal Mines are located in the Corumbáscarpment, on the shore of the Rio Paraguay (Tamengo channel).t both sections the Tamengo Formation is less deformed, except

or the presence of folds and flexures at Corcal Mine. The Saladeiroine is located to the east of Corcal Mine, nearby the Cimento

taú plant. These exposures provided the best preserved and mostbundant Cloudina and Corumbella (e.g. Beurlen and Sommer, 1957;ahn et al., 1982; Hahn and Pflug, 1985; Zaine and Fairchild, 1987;aine, 1991; Gaucher et al., 2003).

Grainstones predominate at both sections, and are interbeddedith black mudstone and marls. The latter show a characteristicecoloration if weathered. Decimetric grainstone beds often dis-lay low-angle cross-lamination, and at least in some cases theyepresent hummocky cross-stratification. Symmetrical (wave) rip-les are common. Concentrations of Cloudina shells often showummocky cross-stratification, especially at Corcal Mine, thus rep-esenting bioclastic storm deposits (tempestites; Gaucher et al.,
003). Corumbellla occurs in marls, with concentrations of wholepecimens restricted to cm-thick beds. At Corcal Mine, the contactetween the Tamengo Formation and the overlying Guaicurus For-ation (siltstones and shales) is well exposed, showing that the
ection corresponds to the top of the Tamengo Formation.

e western part of Laginha Mine. Left: whole section. Right: parallel, more detailed

The Corcal and Saladeiro sections are interpreted as deposited inrelatively shallow water, probably at storm wave level, and clearlyshallower than the section at Laginha Mine. As mentioned above,this explains the rarity of metazoan fossils in the latter section.

At Saladeiro Mine ı13C values vary between +4 and +5‰ for mostof the section, decreasing to +2‰ at the top (Fig. 5). 87Sr/86Sr ratiosof 0.7085 were reported from these outcrops by Boggiani (1998).

The Corcal Mine section was surveyed in more detail. The valuesof ı13C are remarkably invariant for most of the section, hoveringaround +5‰ (Fig. 6). Slightly lighter ı13C values between +3 and+4‰ occur both at the base and top of the section (Fig. 6). Mea-sured 87Sr/86Sr ratios range between 0.7084 and 0.7085 (Babinskiet al., 2008a; Table 1). Although no diagnostic textures havebeen observed in thin section, distinct fine-grained layers interca-lated with the grainstones and marls were identified as volcanictuffs. These layers are calcretized, and euhedral zircons yieldeda weighted mean 238U/206Pb SHRIMP age of 543 ± 3 Ma (n = 17;95% confidence), which is interpreted as the depositional age ofthe upper Tamengo Formation (Babinski et al., 2008a). These beds,although they are not typical pyroclastic rocks, are probably theresult of volcanic ash deposition and local reworking by currents.

5.3. Baía das Garcas Farm section

In Serra da Bodoquena, to the west of Bonito town, a 100 m thick,key stratigraphic section is exposed at Baía das Garcas Farm. There,basement gneisses and granites are overlain by gently-dipping,


F amenb ng.

wfipGspsCt

ig. 4. Stratigraphic column and high-resolution chemostratigraphic data of the Treccias and overlying carbonates is faulted, the lower Tamengo Formation is missi

ell-sorted, graded subarkoses of the Cerradinho Formation. Aning- and thinning-, deepening-upward succession follows, com-rising siltstones and intercalated sandstones (Boggiani, 1998;aucher, 2000). Near the top of the Cerradinho Formation, green
iltstones are interbedded with marls and limestone beds. Theyass up section into black grainstone showing hummocky cross-tratification (Fig. 10) and mudstones, previously assigned to theerradinho Formation (Boggiani, 1998). Gaucher (2000) arguedhat these carbonates are lithologically identical to limestones of
go Formation in the eastern part of Laginha Mine. The contact between the basal

the Tamengo Formation and should be included in that unit. Thevalues of ı13C obtained in this study for carbonates at the Baía dasGarcas section are all positive and mostly around +3‰ (n = 9), reach-ing +4‰ (Fig. 7), thus reinforcing an assignment to the Tamengo
Formation. Moreover, thrombolites have been recently identifiedin these carbonates (Fig. 10), similar to other occurrences in thesouthern Paraguay Belt (Itapucumí Group).
A Bavlinella–Soldadophycus acritarch assemblage and the vendo-taenid V. antiqua were reported from green siltstones of the upper


hic da

Cjt

oFFtmCp2t

Fig. 5. Stratigraphic column, chemo- and biostratigrap

erradinho Formation (Gaucher et al., 2003). The microfossils occurust at the boundary between the Cerradinho and Tamengo forma-ions.

An important question is whether the positive ı13C valuesbtained from the lower Tamengo Formation at Baía das Garcasarm correlate with the positive peak in the middle Tamengoormation at Laginha Mine and elsewhere (e.g. Fig. 3), or ifhey pre-date the negative excursion in the lower Tamengo For-

ation. Gaucher (2000) argued that the contact between theerradinho and Tamengo formations is transitional, which is sup-orted by their similar acritarch assemblages (Gaucher et al.,003). If a lateral equivalence between the Bocaina and part ofhe Tamengo Formation is assumed (e.g. Gaucher et al., 2003),

ta of the upper Tamengo Formation at Saladeiro Mine.

then limestones at the Baía das Garcas section actually repre-sent the base of the latter unit, being older than the brecciasat the base of the Laginha Mine section (Fig. 3). This in turnmeans that a positive ı13C excursion to at least +4‰ separatesthe post-glacial negative excursion above the Puga Formation andthe negative excursion above the breccias of the lower TamengoFormation. The alternative would be to assign the positive ı13Cvalues at the Baía das Garcas section to the middle Tamengo
positive excursion, implying that there is a hiatus between theCerradinho and Tamengo Formations, because negative ı13C aremissing at that section. Interbedded siltstones and limestones sug-gest, however, that the contact between both units is transitional.Pending future confirmation, we mantain as a working hypoth-


F o Formi

ef

5

Beof

ig. 6. Stratigraphic column, bio- and chemostratigraphic data of the upper Tamengs not shown. U-Pb SHRIMP zircon age from Babinski et al. (2008b).

sis a concordant contact between the Cerradinho and Tamengoormations.

.4. Horii Mine section

In the fold-and-thrust belt domain of the southern Paraguayelt, the Tamengo Formation is exposed at Horii Mine, in the north-rn part of the Serra da Bodoquena (Figs. 1 and 2), nearby the townf Bodoquena. Although the succession there is affected by a thrustault, deformation is low and the polarity of the strata is interpreted

Fig. 7. Stratigraphic column and chemostratigraphic data of t

ation at Corcal Mine. The contact with shales of the overlying Guaicurus Formation

as normal. The structure is an open fold with axis striking N45E andaxis plunge 30◦ to the NE.

Carbonates at the Horii Mine section are characterized bydecimeter-to-metric layers of ooid grainstones, with sporadic cm-thick intercalations of gray to black marls and massive mudstones.
The contact with shales of the Guaicurus Formation is not exposedin the mine, and no metazoan fossils were found in this section.
C-isotope values are very homogeneous and around +3‰ for thewhole section (Fig. 8). 87Sr/86Sr ratios are fairly constant between0.7083 and 0.7085 (Babinski et al., 2008a, Table 1).

he lower Tamengo Formation at Baía das Garcas Farm.


Fig. 8. Stratigraphic column and chemostratigraphic data of the upper Tamengo Formation at Horii Mine, previously considered part of the Cuiabá Group and here assignedto the Corumbá Group. The contact with shales of the overlying Guaicurus Formation is not shown.


F matio

AGtfwe

5

dGa

Mac

teirroima

ig. 9. Stratigraphic column and chemostratigraphic data of the upper Tamengo For

In the available geological maps of the area (Corrêa et al., 1979;raújo et al., 1982), these limestones are assigned to the Cuiabároup. However, the petrographic, faciological and – especially –

he chemostratigraphic features allow interpreting them as distalacies of the Tamengo Formation, possibly of its upper part. As weill show below, carbonates exposed at Calbon Mine, further to the

ast, probably represent even a deeper water environment.

.5. Calbon Mine section

In the more deformed part of the southern Paraguay Belt, meta-olostones crop out which were mapped as part of the Cuiabároup, but their sedimentological and isotopic features allowssigning them to the Tamengo Formation (i.e. Corumbá Group).

Representative exposures of these carbonates occur at Calbonine, nearby Miranda (Fig. 1). In this location, bedding strikes N20E

nd dip 20◦ to the SE. A 23 m section was surveyed with sampleollection at intervals of 1m (Fig. 9).

In the lower part, dolomitic mudstone and grainstone intercala-ions occur, passing up section into graphitic-phyllites. Dolomitesxhibit plane-parallel lamination, and sedimentary structuresndicative of shallow water were not observed. Under the pet-ographic microscope, carbonates appear strongly deformed and
ecrystallized, as observed in the field. Despite recrystallization, theriginal lamination is preserved, in spite of some tectonic stretch-ng. Besides deformation, a high amount of quartz and organic
atter in some samples was verified. Quartz has been remobilizednd precipitated as lenses concordant with foliation.

n at Calbon Mine, where carbonates were previously assigned to the Cuiabá Group.

Samples are recrystallized and partly silicified, but cathodolu-minescence and petrographic analyses allowed extracting the leastaltered portions with a millimetric drill for C and O isotope analysis.The values of ı13C are quite homogeneous, ranging between +3.7and +5.3‰, thus similar to part of the Tamengo Formation. Cor-responding ı18OVPDB values are quite heavy, between −0.12 and−4.1‰. This may be interpreted as due to alteration, but becauseMnO values are less than 0.050%, the possibility of alteration isrendered unlikely. Dolostones usually exhibit heavier ı18O ratiosdue to the fractionation factor between dolomite and co-existingcalcite, which has been determined as 2.6‰ (Vasconcelos et al.,2005). This factor largely explains the offset of ı18O values betweenlimestones and dolostones of the Tamengo Formation.

Thus, on the basis of litho- and chemostratigraphy, we assigncarbonates exposed at Calbon Mine to the Tamengo Formation,representing a distal, deep-water setting.

6. Discussion

6.1. Nature of isotopic signals

The primary nature of �13C and 87Sr/86Sr values presented hereis supported by the following facts:

(1) Mn/Sr values vary between 0.04 and 0.55 for limestones ofthe Tamengo Formation (Table 1), thus within the “unaltered”field (Marshall, 1992; Jacobsen and Kaufman, 1999). Dolostonesyielded higher Mn/Sr ratios, of up to 7.4 (Table 1), but this is due


Fig. 10. Photographs of representative lithologic types and sedimentary structures of the Corumbá Group.(a) Columnar stromatolites of the Bocaina Formation atPorto Morrinhos, in the vicinity of cap carbonates of the Morro do Puga (length of pen: 10 cm). (b) Stromatolitic “tubestones” at the same locality as the previous; (c)Carbonate breccia with abundant phosphate clasts (black) at the base of the Tamengo Formation at Laginha Mine. (d) Contact between the basal breccias (left) and overlyingrhythmites (carbonaceous marl-calcisiltite) of the lower Tamengo Formation, Laginha Mine. (e) Ash bed intercalated with limestones of the upper Tamengo Formation at Cor-cal Mine (length of pen: 10 cm). (f) Laginha diamictite at the base of the Guaicurus Formation, with limestone lonestones derived from the Tamengo Formation. (g) Hummocky

rian R

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(

(

rc

6

bdt

Cg

(

(

(

(

(

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cF


to dolomite accepting more Mn and less Sr in the lattice, anddoes not imply higher post-depositional alteration (Kah et al.,1999; Gaucher et al., 2007).

2) Sr concentrations in limestones are high, between 685 and10,300 ppm (Table 1), suggesting an aragonitic precursor.

3) The values of ı18O in limestones are higher than −10‰ for thegreat majority of samples (all except 5 samples, Table 1), whichis generally considered as the boundary between altered andunaltered limestones (Marshall, 1992; Jacobsen and Kaufman,1999).

4) Biomarker (hopanes, steranes, gammacerane) and Rock Evalpyrolysis data from the Tamengo Formation (Velásquez et al.,2008), suggest that the degree of thermal alteration is low.

Thus, the C-isotope curves from the Tamengo Formation likelyepresent secular variations of coeval seawater values, and can beompared to other successions worldwide.

.2. Chemostratigraphic correlation and the global ı13C curve

Chemostratigraphic data support the hypothesis that many car-onate occurrences in the more deformed Paraguay Belt are in factistal, deeper water carbonates belonging to the Tamengo Forma-ion and not to the Cuiabá Group.

C-isotope secular variations in the different sections of theorumbá Group allow identifying the following chemostrati-raphic features (Fig. 11):

1) A ubiquitous feature in the upper Tamengo Formation is rep-resented by a positive ı13C plateau around +3 to +5‰, gentlydecreasing toward the contact with the overlying GuaicurusFormation. This positive interval is associated with all knownoccurrences of Cloudina and Corumbella (e.g. Figs. 5 and 6).

2) Below the previous plateau, a sharp ı13C positive excursionto >+5‰ is recorded at both sections in the Laginha Mine(Figs. 3 and 4).

3) Also at Laginha Mine, a negative ı13C excursion to −3.5‰ char-acterizes carbonates at the base of the Tamengo Formationoverlying polymictic breccias and related to a regression fol-lowed by a subsequent transgression (Fig. 3). This feature wasnot found in the other sections studied, but the latter are incom-plete and do not show the base of the Tamengo Formation.

4) Boggiani (1998) reported positive ı13C values for dolostonesof the Bocaina Formation immediately beneath the breccias ofthe lower Tamengo Formation (Boggiani, 1998). Limestones ofthe lower Tamengo Formation at Baía das Garcas section alsoyielded positive values (Fig. 7). We envisage that this positiveı13C excursion (up to +4‰) pre-dates the breccias and its asso-ciated negative anomaly of the lower Tamengo Formation.

5) Negative ı13C values down to −5‰ characterize cap carbonatesoverlying the arguably glaciogenic Puga Formation in its typearea (Boggiani et al., 2003; Fig. 11).

The reported U-Pb SHRIMP age of 543 Ma for an ash bedccurring at the top of the positive ı13C plateau in the Tamengoormation, near the contact to the overlying Guaicurus FormationBabinski et al., 2008a; Fig. 11), deserves some consideration here.
n Oman, ash beds dated by U-Pb TIMS at 542 ± 0.3 Ma occur withinhe negative ı13C Ediacaran–Cambrian boundary anomaly, whichmmediately overlies a ı13C positive plateau and Cloudina-bearingarbonates (Fig. 11; Amthor et al., 2003; Bowring et al., 2007). Theost parsimonious explanation is that the bio- and chemostrati-
ross-stratification in calcarenites of the lower Tamengo Formation, Baía das Garcas Formation, Baía das Garcas Farm. Note clotted fabric. Length of scale: 8 cm.

esearch 182 (2010) 382–401 397

graphic context of the upper Tamengo ash bed is consistent with a543 ± 3 Ma depositional age. In the Nama Group, U-Pb TIMS zirconages of 543 ± 1 Ma and 545 ± 1 Ma for ash beds occurring withinthe upper part of the latest Ediacaran positive ı13C plateau werereported (Fig. 11; Grotzinger et al., 1995, 2000). Two negativeexcursions in the Nama Group are, respectively, immediately aboveand considerably below an ash bed dated U-Pb TIMS on zircon at549 ± 1 Ma (Fig. 11), showing that at least 6 Myr separated the neg-ative ı13C excursion from the upper part of the positive plateau(543 ± 1 Ma). Likewise, in southern China, the end of the last Edi-acaran negative excursion is constrained by a U-Pb TIMS zircon ageof 551 ± 0.7 Ma (Fig. 11; Condon et al., 2005; Zhou and Xiao, 2007).Therefore, the time elapsed between the lower Tamengo negativeexcursion and the Tamengo-Guaicurus boundary can be estimatedbetween 6 and 8 Myr, which implies reasonable sedimentationrates of 17–13 m/Myr, comparable to other ancient carbonate plat-forms (Tucker and Wright, 1990).

In the following we attempt to correlate the Corumbá Group,and the Tamengo Formation in particular, to other successionsworldwide. Maybe the most ubiquitous chemostratigraphic featureocurring in the measured sections of the Tamengo Formation is thepositive ı13C plateau at the top of the unit. It is geochronologi-cally constrained by both the occurrence of Cloudina and a U-PbSHRIMP zircon age of 543 ± 3 Ma at its top (Fig. 11). Exactly thesame relationships are observed in the Schwarzrand Subgroup ofthe Nama Group (Namibia, Grotzinger et al., 1995; Germs et al.,2009), the upper Nafun and lower Ara groups in Oman (Amthor etal., 2003) and the Dengying Formation in southern China (Zhou andXiao, 2007). According to the data from southern China, the plateaustarts with a positive ı13C excursion at 551.1 ± 0.7 Ma (Condon etal., 2005; Zhou and Xiao, 2007; Sawaki et al., 2010), located atthe boundary between the Doushantuo and overlying Dengyingformations (Fig. 11). The high-resolution ı13C curve obtained forthe eastern Laginha Mine (Fig. 4) is almost identical to the curvereported for the Dengying Formation.

The curve at the eastern Laginha Mine is punctuated by lowerı13C values (+1‰) separating the lower positive excursion (>+5‰)from the overlying plateau (Fig. 4). This feature, which is not appar-ent in the Chinese sections, is well recorded both in the Nama Group(Kuibis-Schwarzrand boundary) and at the boundary between theNafun and Ara groups in Oman (Grotzinger et al., 1995, 2000;Amthor et al., 2003; Fig. 11). In the Nama Group, this minor nega-tive excursion is associated to evidence of at least local glaciation(Germs, 1995; Germs et al., 2009), and has been recently correlatedto karstification in coeval carbonates in the Saldania Belt (Praekeltet al., 2008).

A positive excursion to >+5‰ is recorded in the middle TamengoFormation. A similar positive excursion to ca. +5‰ is well con-strained in the Nama Group by an ash bed in its upper section datedU-Pb TIMS on zircon at 549 ± 1 Ma (Grotzinger et al., 1995; Fig. 11).In Oman, an age of ca. 547 Ma has been reported for the positiveexcursion of the Buah Group (Bowring et al., 2007), thus withinerror of the ages from the Nama Group. In China, a positive excur-sion to +5‰ in the lower Dengying Formation (EP3: Fig. 11) beginsat the level of an ash bed dated U-Pb at 551.1 ± 0.7 Ma (Condon et al.,2005; Zhou and Xiao, 2007; Sawaki et al., 2010). Therefore, we sug-gest that the positive excursion of the middle Tamengo Formation
reflect a global event between 551 and ca. 548 Ma.
Bearing the above correlations in mind, it follows that the neg-ative excursion in the lower Tamengo Formation matches (a) thenegative excursion EN3 in the upper Doushantuo Formation (Zhouand Xiao, 2007; Sawaki et al., 2010), (b) the negative excursion at

arm. (h) Sheet-like thrombolites interbedded with calcarenites, lower Tamengo


Fig. 11. Composite ı13C curve of the Corumbá Group and corresponding 87Sr/86Sr, biostratigraphic and radiochronological data, and comparison to other coeval successionsworldwide. Sources of data: (1) Gaucher et al. (2004, 2009c and references therein), Blanco et al. (2009), Oyhantcabal et al. (2009), (2) Grotzinger et al. (1995, 2000), Föllingand Frimmel (2002), (3) this work, Gaucher et al. (2003); (4) Zhou and Xiao (2007), Condon et al. (2005); (5) Brasier et al. (2000), Amthor et al. (2003), Bowring et al., 2007.A ; (2) PZ

t2iTtv2(rsI((

(tsa

pdl2ismT

bbreviations: (1) BN: Barriga Negra, CSF: Cerros San Francisco, CV: Cerro Victoriahuiaq.: Zhujiaqing Formation, Zh.: Zhongyicun Member.

he base of the Kuibis Subgroup (Saylor et al., 1998; Grotzinger et al.,000), and (c) the Shuram negative excursion of the Nafun Group

n Oman (Brasier et al., 2000; Le Guerroué and Cozzi, 2010; Fig. 11).he Shuram negative anomaly, however, is more pronounced thanhe lower Tamengo negative excursion, the former reaching ı13Calues as low as −12‰ (e.g. Brasier et al., 2000; Le Guerroué et al.,006). However, this has been interpreted by Bristow and Kennedy2008) as a local or diagenetic signal. Isotopic and geochemical dataeported by Le Guerroué and Cozzi (2010) may be interpreted asuggesting that ı13C values below −6‰ are diagenetically altered.n the lower Nama Group, ı13C values do not drop below −5‰Grotzinger et al., 2000), similar to the lower Tamengo FormationFig. 11).

The positive excursion recorded at the Baía das Garcas sectionFig. 7), here assigned to the lower Tamengo Formation and thoughto pre-date the negative excursion found at Laginha Mine, matchesimilar values in member III of the Doushantuo Formation (Zhound Xiao, 2007; Sawaki et al., 2010; Fig. 11).

The ı13C curve obtained for the Corumbá Group can be com-ared to other Ediacaran successions in South America. The Arroyoel Soldado Group, deposited on the eastern margin of the Río de

a Plata Craton, shows a similar ı13C curve (Gaucher et al., 2004,
009c; Fig. 11), as already noted by Gaucher et al. (2003). The pos-
tive ı13C excursion that characterizes the transition between theiliciclastic Yerbal Formation and carbonates of the Polanco For-ation parallels the lower Tamengo positive excursion (Fig. 11).

he significant ı13C negative excursion in the lower Polanco For-

.Nolloth: Port Nolloth Group, Nu.: Numees Formation, Ho.: Holgat Formation; (4)

mation (unit B: Gaucher et al., 2004, 2009c) matches the negativeexcursion associated with carbonates overlying the breccias inthe lower Tamengo Formation. In both cases, a sea-level falland reworking of shelf sediments followed by renewed flood-ing is evidenced, which has been related to glacioeustasy duringa non-global glaciation (Gaucher et al., 2004, 2009c; Gaucherand Poiré, 2009). Above this negative excursion, a positive ı13Cinterval characterizes the upper Polanco Formation (Fig. 11),and could be related to the positive ı13C values characteristicof the upper Tamengo Formation. Interestingly, Cloudina occursbeneath and within the negative excursion that characterizes unitB of the Polanco Formation (Gaucher and Sprechmann, 1999;Gaucher et al., 2003, 2009c; Gaucher and Germs, 2009), thus strati-graphically lower than in the Tamengo Formation. This may bemore related to preservational factors rather than to evolutionarytrends.

It is worth noting that, despite their geographical proximity,ı13C curves obtained in the northern Paraguay Belt (Araras Group;Alvarenga et al., 2004, 2009; Nogueira et al., 2007; Riccomini etal., 2007) differ markedly from that of the southern Paraguay Belt.The Araras Group is possibly older than the Corumbá Group, as sug-gested by a Pb-Pb carbonate age of 633 ± 25 Ma for the lower Araras
Group (Alvarenga et al., 2009).
As mentioned above, a marked sea-level fall followed byrenewed transgression associated with a ı13C negative excursionis evidenced in the lower Tamengo Formation. The occurrence ofclasts from the subjacent Bocaina Formation, and even granite-

rian R

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neiss basement clasts, may be associated with considerableustatic sea-level fall (e.g. Gaucher and Poiré, 2009). An alternativexplanation can be related to the transition from rift to drift, whichould make this regression-transgression a local phenomenon.owever, the fact that the same regression is recorded in the Arroyoel Soldado and Sierras Bayas groups (Gaucher and Poiré, 2009),resently 2000–3000 km to the south, suggests that it is not just a

ocal feature.

. Conclusions

The Tamengo Formation, and consequently the Corumbá Group,rops out over an area larger than previously accepted. Several car-onate occurrences, formerly assigned to the Cuiabá Group, areere considered distal facies of the Tamengo Formation. Two exam-les are the Calbon and Horii Mines, which yielded ı13C valuesetween +2 and +5‰ VPDB and 87Sr/86Sr ratios between 0.7083nd 0.7085, typical for the upper Tamengo Formation. Limestonesverlying the Cerradinho Formation at Baía das Garcas Farm aressigned to the base of the Tamengo Formation, recording a posi-ive ı13C excursion to +4‰ VPDB. Up section, breccias in the loweramengo Formation record a significant regression followed byubsequent transgression, associated with negative ı13C valuesown to −3.5‰. The upper Tamengo Formation is characterizedy a positive ı13C excursion to +5‰ VPDB, followed by a plateaut +3‰. Corresponding 87Sr/86Sr ratios vary between 0.7084 and.7086. An ash bed near the top of the positive plateau yielded a U-b zircon age of 543 ± 3 Ma, which is consistent with similar agesrom Namibia and Oman for this interval.

The integrated calibration of the Ediacaran carbon-isotopeurve allows identifying a more complex scenario than previouslyccepted by most compilations. Following the Gaskiers glaciation at83 Ma, three negative excursions have been identified, one endingt ca. 551 Ma (Shuram–Wonoka anomaly), a second at ca. 547 Maeparating the Kuibis and Schwarzrand subgroups in the Namaroup and the third at the Ediacaran–Cambrian boundary. The twolder negative excursions are probably recorded in the Tamengoormation. These negative excursions are–at least in part- relatedo glacial events, possibly of high-latitude and not global nature.he most parsimonious interpretation for sedimentary brecciasccurring in the lower Tamengo Formation is a glacioeustatic sea-evel fall, as also postulated for correlative strata in the Arroyo deloldado Group (Uruguay), but a tectonic interpretation cannot beompletely ruled out.

cknowledgements

This work was supported by FAPESP (Proc. 04/01233-0 grantedo PCB and Proc. 06/58688-1 to MB), by VITAE (B11487/10B003)ranted to VPF, and by PROSUL/CNPq (AC-38) granted to ANS. This iscontribution to IGCP 478 (Neoproterozoic-Early Palaeozoic Events

n southwestern Gondwana) and to IGCP 512 (Neoproterozoic Iceges). Thanks are also given to anonymous referees by the reviewsf the manuscript.

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